C '
THE DECENNIAL PUBLICATI(3NS OF
THE UNIVERSITY OF CHICAGO
THE DECENNIAL PUBLICATIONS
ISSUED IN COMMEMORATION OF THE COMPLETION OF THE FIRST TEN
YEARS OF THE UNIVERSITY'S EXISTENCE
AUTHORIZED BY THE BOARD OP TRUSTEES ON THE RECOMMENDATION
OP THE PRESIDENT AND SENATE
EDITED BY A COMMITTEE APPOINTED BY THE SENATE
EDWARD CAPPS
STARE WILLARD CUTTING ROLLIN D. SALISBURY
JAMES ROWLAND ANGELL WILLIAM I. THOMAS SHAILER MATHEWS
CARL DARLING BUCK FREDERIC IVES CARPENTER OSKAR BOLZA
JULIUS 8TIEGL1TZ JACQUES LOEB
THESE VOLUMES ARE DEDICATED
TO THE MEN AND WOMEN
OF OUR TIME AND COUNTRY WHO BY WISE AND GENEROUS GIVING
HAVE ENCOURAGED THE SEARCH AFTER TRUTH
IN ALL DEPARTMENTS OF KNOWLEDGE
THE STUDY OF STELLAR EVOLUTION
PLATE I
The Great Nebula in Andromeda
Photogrraphed with the 24-incli reflecting telescope of the Yerkes Observatory (Ritchey)
THE STUDY OF STELLAR
EVOLUTION
AN ACCOUNT OF SOME RECENT METHODS
OF ASTROPHYSICAL RESEARCH
GEORGE ELLERY HALE
FORMEBLT OF THE DEPARTMENT OF ASTRONOMY AND ASTBOPHTSICS
NOW DIRECTOR OF THE MOUNT WILSON SOLAR OBSERVATORY
*?)
THE DECENNIAL PUBLICATIONS
SECOND SERIES VOLUME X
CHICAGO
THE XJNIVERSITY OF CHICAGO PRESS
1908
Copyright 190S by
The Univeesity of Chicago
Entered at Stationers^ Hall
Publisbed May 1908
Composed and Printed By
The University of Cliicago Press
Chicago, Illinois, U. S. A.
PREFACE
As first planned, this book was intended to serve as a
handbook to the Yerkes Observatory. Many inquiries
regarding the observatory's work, made by the numerous
visitors received there annually, seemed to call for a printed
explanation of the purposes in view and the observational
methods employed. Removal to California and new duties
connected with the organization of the Mount Wilson Solar
Observatory caused a modification of the project. I finally
adopted the plan of describing a connected series of inves-
tigations, laying special stress on the observational methods
employed, in the hope of explaining clearly how the problem
of stellar evolution is studied. The advantage of using con-
crete illustrations drawn, in large part, from jjei-sonal experi-
ence, and the desire that the book should be of special service
to visitors at the Yerkes and Mount Wilson Observatories,
are sufficient reasons, I trust, for the otherwise undue pro-
portion of space devoted to these institutions.
The omission of such important subjects as the theories
of temporary and variable stars; Sir George Darwin's dis-
cussions of evolution as affected by tidal friction; Vogels
and Pickering's photometric and spectroscopic studies, and
the researches of the latter on the distribution of stars of
various types: Campbell's investigations of stellar spectra,
and, to mention no other work, his development of the spectro-
graphic method of determining radial velocities, sufficiently
indicate that I have made no attempt to deal with the general
problem of stellar evolution, or to offer anything approaching
an adequate description of the observational methods of astro-
physics. The various researches described are chosen rather
arbitrarily, in some cases with more regard for my personal
X Peeface
acquaintance with the facts than because of their intrinsic
importance. I trust, however, that although this method of
treatment has necessarily resulted in a fragmentary exposi-
tion of the subject, the book will serve to show how the
problem of stellar evolution is attacked along converging
lines, leading from solar, stellar, and laboratory investiga-
tions.
I wish to express my thanks to Sir William Huggins ; to
Messrs. Adams, Ellerman, Olmsted, and Ritchey of the
Mount Wilson Solar Observatory; to Professors Barnard,
Burnham, and Frost of the Yerkes Observatory ; to Professor
Campbell of the Lick Observatory; to Professor Pickering
of Harvard College Observatory ; to Mr. Abbot of the Smith-
sonian Astrophysical Observatory; to Professor Lord of the
Emerson McMillin Observatory; and to Professor Ames,
Mr. Jewell of Johns Hopkins University, for photographs
which appear in the plates. I am also indebted to the
Astrophysical Journal and the Publications of the Yerkes
Observatory for many cuts, and to Messrs. Ticknor & Co.
for permission to reproduce Langley's drawing of a typical
sun-spot. I am under special obligations to my colleague,
Mr, Ellerman, to whom are due most of the photographs
of instruments, buildings, and landscapes which appear in
the plates, besides many of the solar and stellar photographs
taken from our joint papers in the Astrophysical Journal
and elsewhere.
G. E. H.
Pasadena, California
November, 1907
CONTEXTS
I. The Problem of Stellar Evolution - - - 1
II. The Student of the Xew Astroxomt - - - 9
III. The Sun as a Typical Star ----- 15
IV. I arge and Sjxall Telescopes - - - - 20
V. Astronomical Photography with Camera Lenses - 27
VI. Development of the Reflecting Telescope - 38
VII. Elementary Principles of Spectrum Analysis - 46
VIII. Grating Spectroscopes and the Chemical Compo-
sition OF THE Sun ------- 56
IX. Phenomena of the Sun's Surface - - - - 67
X. The Sun's Surroundings ----- 73
XI. The Spectroheliograph ------ 82
XII. The Yerkes Observatory ----- 97
XIII. Astronomical Advantages of High Altitudes - 111
XIV. The Mount Wilson Solar Observatory - - 121
XV. The Snow Telescope -.--.. 131
XVI. Some Uses of Spectroheliograph Plates - - 139
X.VII. A Study of Sun-Spots ------ 151
XVIII. Stellar Temperatures -----. 165
XIX. The Xebular Hypothesis ----- 175
XX. Stellar Development ------ ig7
XXI. The Meteoritic and Planetesimal Hypotheses - 204
XXII. Does the Solar Heat Vary? - - - . 212
XXIII. The Construction of a Large Reflecting Tele-
scope --------- 219
XXIV. Some Possibilities of Xew Instruments - - 230
XXV. Opportunities for Amateur Observers - - - 243
Index ----------- 251
CHAPTER I
THE PROBLEM OF STELLAR EVOLUTION
It is not too much to say that the attitude of scientific
investigators toward research has undergone a radical change
since the publication of the Origin of Species. This is true
not only of biological research, but to some degree in the
domain of the physical sciences. Investigators who were
formerly content to study isolated phenomena, with little
regard to their larger relationships, have been led to take
a wider view. As a consequence, the attractive qualities of
scientific research have been greatly multiplied. Many a
student, who could see in a museum only a wilderness of dry
bones, now finds each fragment of profound interest if the
part it plays in a general scheme of evolution can be made
clear. The color and structure of any animal or plant, the
minute modifications which distinguish one variety from
another, take on new significance when considered as evi-
dences of development. Their appeal to the microscopist,
or to anyone who finds delight in intricacy of structure or
beauty of form, is quite as great as before. But to the stu-
dent whose interest is not aroused by such details, perhaps
from lack of technical knowledge, or from the feeling that
these matters are trivial as compared with the larger problems
^of science, such minor |)eculiarities must appear in a new
light. Their true significance becomes apparent, and the
importance of studying them, once perhaps underestimated,
now requires no demonstration.
In astronomy the idea of evolution goes back to a very
early period. In a crude and grotesque form the traditions
of the earliest peoples invariably struggle to account for the
1
Stellar Evolution
origin of the Earth and its inhabitants. On a much higher
plane stand the speculations of the Greek philosophers and
of those who have followed them in the centuries preceding
our own time. All schools of astronomers, dealing in some
instances with purely philosophical and theoretical considera-
tions, and in others basing their conclusions upon known
facts of observation, have sought in their turn to explain the
origin of the solar system and the larger relationships that
obtain in the universe as a whole. In the eighteenth century
these speculations reached their climax in the nebular hy-
pothesis of Laplace, which still remains as the most serious
attempt to exhibit the development of the solar system.
Attacked on many grounds, and showing signs of weakness
that seem to demand radical modification of Laplace's original
ideas, it nevertheless presents a picture of the solar system
which has served to connect in a general way a mass of indi-
vidual phenomena, and to give significance to apparently
isolated facts that offer little of interest without the illumina-
tion of this governing principle.
It will be seen, therefore, that the idea of evolution and
development is by no means new to the astronomer. But it
may nevertheless be maintained that it has occupied a more
important position since Darwin published his great work.
In 1859, the very year of the publication of the Origin of
Species, Kirchhoff first succeeded in determining the chemical
composition of the Sun by the aid of the spectroscope. His
fundamental discovery marked the entrance of this instru-
ment into the field of astronomical research and established
on a firm basis the new science of astrophysics. The impor-
tance of spectroscopic investigations in their relationship to
evolution was soon made clear. Within a single decade the
study of stellar spectra by Huggins, Rutherfurd, and Secchi
had shown that the stars may be divided into several classes,
characterized by distinctive peculiarities in their luminous
The Problem of Stellar Evolution 3
emission and marking definite stages in an orderly process
of development. Following close upon this pioneer work
came the capital discovery by Huggins of the gaseous nature
of the nebulae, and the relationship of these celestial clouds
to the stars which they enshroud. In these filmy masses of
luminous gas it appeared probable that the stars had their
origin, taking form after long ages of condensation, through
processes regarding which our ideas are still vague and ill
defined. Belief in such a mode of development has been
greatly strengthened through the results of recent investiga-
tions, and especially through the discovery by Keeler that
of 120,000 nebulae strewn over the heavens fully one-half are
distinctly spiral in form. This far-reaching conclusion,
coming at the end of the nineteenth century, is furnishing
materials for those who seek, through modification of the
nebular hypothesis, to provide a sound and sufficient explana-
tion of the development of suns like our own.
We are now in a position to regard the study of evolution
as that of a single great problem, beginning with the origin
of the stars in the nebulae and culminating in those difficult
and complex sciences that endeavor to account, not merely
for the phenomena of life, but for the laws which control a
society composed of human beings. Any such consideration
of all natural phenomena as elements in a single problem
must begin with a study of the Sun, the only star lying near
enough the Earth to permit of detailed investigation. The
knowledge thus derived may then be applied in researches
on the nebulae, and in the elucidation of spectroscopic obser-
vations of those stars which represent the early period of
stellar existence. According to present views, the state of
development attained by the Sun is that of maturity, if not
of decline. After it come the red stars, which represent the
last stages of luminous stellar life. Even the extinction of
light due to continued cooling is not sufficient to exclude
Stellar Evolution
altogether from the astrophysicist's study those dying stars
which represent a condition lying between that of a glowing
sun and a dead planet like the Earth or the Moon. Through
one of its many remarkable properties, the spectroscope
enables us to detect the presence, and sometimes to deter-
mine the dimensions, of vast bodies which have resulted from
the. cooling of former suns. It will be the object of this
book to show how the student of astrophysics attacks this
problem of stellar evolution, through the development of
special instruments and methods of research, and the accu-
mulation and discussion of observations.
It must not be forgotten that such a study comprises only
the earliest and simplest elements in the general problem of
evolution. The province of the student of astrophysics may
be said to end with an understanding of the production of a
planet like the Earth. It remains for the geologist to explain
the changes which the surface of the Earth has undergone
since the constructive process left it a rocky crust. The con-
ditions which brought about the formation of the oceans, the
effects of the long-continued action of winds and waves, and
the vast changes in surface structure that have resulted from
internal disturbances and the operation of volcanic phenom- '
ena, afPord limitless opportunity to the student of evolution
in this other aspect. Closely related to these changes, and
presenting difficulties far greater than those experienced by
the astrophysicist, comes the problem of accounting for the
origin and development of plant and animal life. The pres-
ervation of the earlier forms of life, principally through the
agency of sedimentary deposits, affords the paleontologist the
means of connecting the links in the evolutionary chain.
Thus we are brought to our own era, where countless living
objects continue to supply material for new inquiries. Both
in the examination of existing species and their relationships,
and in those experimental researches on variation which offer
The Problem of Stellar Evolution 5
such promising opportunities to the investigator, the evolu-
tionist may secure data for further advances. Outside the
immediate domain of the natural sciences, in regions of
activity where still greater complexity prevails, the student
mav seek to trace out evidences of unity and development in
the mental and moral relationships of the peoples of many
countries and of many generations.
It is a noteworthy fact, of prime significance to all investi-
gators who find special interest in attempting to enter new
and unoccupied fields, that some of the most important devel-
opments of recent years have taken place in those regions
which lie between the boundaries of the old established
sciences. Thus the union of physics and chemistry has
opened up the extensive field of physical chemistry, where
advances of the greatest value are being made. In the same
way the application of physical methods and the principles
of physical chemistry to the experimental study of physiology
has resulted so successfully as to give hope for even more
remarkable developments in the near future. In astronomy,
the introduction of physical methods has revolutionized the
observatory, transforming it from a simple observing station
into a laboratory, where the most diverse means are employed
in the solution of cosmical problems. The fact that physics
is common to these and other intermediate branches of
science affords striking proof of its fundamental importance.
An investigator who has been confined to the traditional
methods of a department of science where physics has as yet
played little part, may therefore find in physical methods a
powerful means of advancing his subject.
The suggestive value, to investigators in other depart-
ments, of any species of scientific research which involves new
methods and principles is perhaps greater at the present
time than ever before. Even those methods of research
which can find no direct application in other subjects are
6 Stellar Evolution
frequently capable of suggesting modifications or adaptations
involving related principles. The development and use of
new methods is quite as likely to advance a subject as the
prosecution of extensive investigations by existing means.
For this reason the investigator is ever on the alert to seize
and utilize suggestions derived from any source.
The interest of the student of astrophysics is no longer
confined simply to celestial phenomena. For astrophysics
has become, in its most modern aspect, almost an experi-
mental science, in which some of the fundamental problems
of physics and chemistry may find their solution. The stars
may be regarded as enormous crucibles, in some of which
terrestrial elements are subjected to temperatures and pres-
sures far transcending those obtainable by artificial means.
In the Sun, which appears to us not merely as a point of
light like the stars, but as a vast globe whose every detail
can be studied in its relationship to the general problem of
the solar constitution, the immense scale of the phenomena
always open to observation, the rapidity of the changes, and
the enormous masses of material involved, provide the means
for researches which could never be undertaken in terrestrial
laboratories. Hence it is that astrophysics may equally well
be regarded as a branch of physics or as a branch of astron-
omy. A telescope, may be defined as an instrument for
revealing celestial phenomena, or it may be likened to the
lens which the physicist uses in his laboratory to concentrate
the light of an electric spark on the slit of his spectroscope.
To the student of astrophysics whose interests are not con-
fined to a single branch of science, the subject is likely to
make a double appeal, no less strong on the physical and
chemical than on the astronomical side.
In entering upon our consideration of the study of stellar
development, we may think of the subject in either one of
two ways. Some will prefer to regard it as the general prob-
The Problem of Stellab Evolution 7
lem of stellar evolution, in its broad application to the uni-
verse at large. But others will find it easier to conceive of
the question as an investigation of the Sun, tracing it,
through analogies afforded by stars in earlier stages of
growth, from its origin in a nebula to those final chapters
which, though not yet wi-itten for the Sun itself, may be read
in the life-histories of the red stars. Viewed from which-
ever standpoint, the task of the investigator remains the
same, since in either case it is concerned with stellar origin,
development, and decay.
It must, of course, be remembered that the processes of
stellar development ordinarily advance so slowly that a life-
time would be far too short to permit any permanent change
to be observed in a star. Temporary stars flash into view and
fade rapidly away; but these represent an abnormal condi-
tion, typical of some catastrophe rather than of a natural
course of change. The spiral nebulae, though their appear-
ance leaves little doubt of extremely rapid motion and con-
stant change of form, are so far removed, and constructed on
so vast a scale, that no actual differences in structure have
been detected in photographs of the same object, taken at
intervals of many years. In the processes of creation a thou-
sand years is but a day, and we must be content to base our
stellar histories upon analogy.
Fortunately, the data needed for the construction of these
histories are easily found. Our problem is like that of one
who enters a forest of oaks, and desires to learn through
what stages the trees have passed in reaching their present
condition. He cannot wait long enough to see any single
tree go through its long cycle of change. But on the ground
he may find acorns, some unbroken and some sprouting.
Others have given rise to rapidly growing shoots, and sap-
lings are at hand to show the next stage of growth. From
saplings to trees is an easy step. Then may be found, in the
Stellar Evolution
form of dead limbs and branches, the first evidences of decay,
reaching its full in fallen trunks, where the hard wood is
wasting to powder.
Scattered over the heavens are millions of stars, each
representing a certain degree of development. The cloud
forms of the nebulae tell us of stellar origins; the white,
yellow, and red stars illustrate the rise and decline of stellar
life; and the Earth itself affords a picture of what may
remain after light and heat have been extinofuished.
CHAPTER II
THE STUDENT OP THE NEW ASTRONOMY
The traditional conception of the astronomer, while still
applicable (with sundry limitations) in certain modern
instances, does not accurately apply to the student of stellar
evolution. According to the old view, the astronomer, soon
after the setting: of the Sun, retires to a loftv tower, from
whose summit he gazes at the heavens throughout the
long watches of the night. His eve, fixed to the end of a
telescope tube, perceives wonders untold, while his mind
sweeps with his vision through the very confines of the uni-
verse. The lineal descendant of the seers and soothsayers
of the Chaldeans, he dwells apart, finding little of interest in
the ordinary concerns of the world, so occupied are his
thoughts with celestial mysteries.
Now there can be no doubt that the study of stellar evo-
lution brings a degree of pleasure and enthusiasm which it
would be difficult to surpass. The joys of the pioneer, the
excitement that comes to him who looks for the first time
upon an unknown land, the intense satisfaction of discovery,
all belong to the successful investigator. Moreover, mere
gazing through a telescope, as distinguished from the pains-
taking work of modern astronomei-s with micrometer or
photographic plate, is still competent to reveal new or chan-
ging phenomena, and important discoveries are yet to come
to the alert and careful observer. It is pleasant to picture
the surprise and delight of Galileo when he first perceived
spots on the supposedly immaculate surface of the Sun. His
little instrument, much less perfect than a modem spy -glass,
could reveal none of that intricate structure and exquisite
9
10 Stellar Evolution
detail that are at once the joy and the despair of present-day
sun-spot observers. But he had discovered a new and
important fact, the basic principle of the science of astro-
physics: he had shown that with suitable optical aid the
physical structure of the heavenly bodies might be investi-
gated. Prior to this time astronomy had concerned itself
only with the positions and motions of the stars; now it
became evident that each of these luminaries might present
peculiar and distinguishing phenomena worthy of the most
searching investigation. Discovery followed discovery in
rapid sequence. The mottled face of the Moon, formerly
without meaning, was suddenly revealed in unsuspected
landscapes of valley, plain, and mountain, resembling, in
curious degree, the variegated surface of the Earth. Jupiter,
who had seemed to travel alone through the heavens, was
found to possess four companions, whose revolutions about
him forcibly suggested the revolutions of the planets about
the Sun. The mysterious ansae, inclosing between them
the globe of Saturn, were soon made out to be the more
conspicuous elements of a vast incircling ring, unlike any-
thing of earlier experience. With the growth of the tele-
scope more marvels were brought to light, until it seemed, in
sound reason, as though the universe would never cease to
yield new knowledge to the explorer of its boundless wastes.
Thus was established that conception of the astronomer
that still persists, long after a new astronomy has come into
being. Gazing through a telescope, as has been said, is still
competent to bring discoveries ; for change is the very essence
of celestial phenomena, and persistent watching must detect
important facts, on which broad generalizations may be
founded. But the eye and the telescope have been supple-
mented by various instrumental aids which, in their multi-
plication, have transformed the occupations of the astronomer.
The micrometer, in its application to the accurate measure-
The Student of the Xew Astronomy 11
ment of place and form, permits changes to be detected which
are beyond the perception of the eye. The photometer, in
its precise determinations of brightness, has shown that stars
whose light never varies are rather the exception than the mle.
The photographic plate, used in conjunction with the tele-
scope, has proved itself to be more sensitive than the human
retina, in that it is capable of adding up into a visible record
the invisible radiations received during an exposure of many
hours. Finally, to mention but one more of the telescope's
new adjuncts, the spectroscope has introduced a new and
revolutionary principle into astronomy, permitting the chemi-
cal and physical analysis of the most distant stars.
Hence it is that the present-day student of astrophysics
does not correspond to the traditional idea of the astronomer.
His work at the telescope is largely confined to such tasks as
keeping a star at the precise intersection of two cross-hairs,
or on the narrow slit of a spectrograph, in order that stars
and nebulae, or their spectra, may be sharply recorded upon
the photographic plate. His most interesting work is done,
and most of his discoveries are made, when the plates have
been developed, and are subjected to long study and measure-
ment under the microscope. His problems of devising new
methods of calculation or reduction are as fascinatingr as the
invention of new instruments of observation. Much of his
time may be spent in the laboratory, imitating, with the means
placed at his disposal by the physicist and chemist, the vari-
ous conditions of temperature and pressure encountered in
the stars, and watching the behavior of metals and gases in
these uncommon environments. If, in the conviction that new
and promising means of research are always awaiting applica-
tion, he would advance into still unoccupied fields, he must
devote himself to the design and construction of new instru-
ments, to supplement the old. Kept thus in touch with the
newest phases of physical and chemical investigation, the
12 Stellar Evolution
countless applications of electricity, the methods of modern
engineering, and the practical details of workshop practice,
his interest in these things of the world is likely to be quite
as broad as that of the average man. His sympathy with
research in every branch of science must increase and
strengthen as his conception of the great problem of evolu-
tion is developed by his own investigations of its earliest
phases. And the pleasure and enthusiasm derived from his
studies must become, not like the vague passion of the mys-
tic, whose inability to see clearly leads him to pursue strange
gods, but such as every successful searcher after truth must
experience, whether he deal with the vast dimensions and
distances of the heavenly bodies, or with the minute but no
less marvelous phenomena of microscopic life and form.
Now, while it cannot be too strongly emphasized that the
student of stellar evolution can have no sympathy with the
mystic, whose habit of thought must be the very antithesis
of his own, yet it is true that the imagination, when properly
exercised and controlled, is to be regarded as his best aid to
progress. The question of control is so important that it
may well be mentioned first. For nothing has done more
injury to science than the play of imaginations subject to no"
control, on the part of men who enjoy in the public press the
rank of scientific authorities. Thus great sun-spots become
the innocent cause of earthquakes or tornadoes, not to speak
of their effect upon the price of wheat. Comets, once the
unerring portents of war and pestilence, still carry the brands
of conflagration, and threaten at each apparition to destroy
the Earth. Mystic properties are ascribed to the center of
the universe, and a well-known planet, because it is incor-
rectly assumed to be stationed there, is dogmatically asserted
to be the only possible abode of human life. There is a fine
field here for humor and amusing speculation, as the author
of the "Moon Hoax,'' and other more recent writers, have
The Student of the New Astronomy 13
shown us. But humor is not always intended: the pronun-
ciamentos go forth in the name of science, and are so accepted
by a host of intelligent persons, who naturally believe that the
supjxtsed authorities have reached their conclusions by scien-
tific methods. Thus there arises a false conception of science,
and a popular demand for wonders, which is not easily
satisfied by acquaintance with the less sensational facts.
But though dano^erous when unrestrained, the imagina-
tion, when rightly exercised, is the best guide of the
astronomer. His dreams run far ahead of his accomplish-
ments, and his work of today is part of the development of
a plan projected years ago. He perceives that only a few
c^enerations hence manv of the instruments and methods of
his time are to be replaced by better ones, and he strains his
vision to obtain some glimpse, imperfect though it be, into
the obscurities of the future. As he sits in his laboratory,
surrounded by lenses and prisms, gratings and mirrors, and
the other elementary apparatus of a science that subsists on
light, he cannot fail to entertain the alluring thought that
the intelligent recognition of some well-known principle of
optics might suffice to construct, from these very elements,
new instruments of enormous power. He learns of some
advance in engineering or in the art of the glass-maker, and
dreams of new possibilities in its application to the construc-
tion of his telescopes or the equipment of his laboratory. He
reads of discoveries in physics or chemistry, and at once his
mind is busy in its endeavor to apply the new knowledge to
the solution of long-standing cosmical problems.
But here, again, we see the need of control ; for with such
a multiplicity of interests, and such constant stimulus to the
imagination, the danger of mere dilettantism is obvious.
With scores of problems suggesting themselves for solution,
and with attractions on everv hand, each rivaling the other
in its apparent possibilities of development, the chief difficulty
14 Stellar Evolution
is to choose wisely. It is not a question of searching for
something to do, but of picking out those things which are
most worthy of pursuit. Here the importance of having a
definite and logical plan of research becomes apparent. Such
a plan may involve a single investigation, continued along
systematic lines over a long period of years, or it may com-
prise several investigations, carried on simultaneously. In
a large observatory each piece of work acquires increased
importance if it is selected, not at random, or solely because
of its intrinsic value, but rather because of the part it plays
in a single logical scheme of research. Its intrinsic impor-
tance need not be in the least diminished by its relationship
to other work, while the illumination which its results cast on
the other investigations of the scheme can hardly fail to
improve them, and may even reveal the chief source of their
meaning. Moreover, the same research, if carried on else-
where, might prove of small value, in the absence of such
suggestions and modifications as are sure to come from the
related investigations. We shall have occasion to revert to
this question in discussing a plan of attack on the general
problem of stellar evolution.
CHAPTER III
THE SUN AS A TYPICAL STAR
Before proceeding to the more detailed portions of our
discussion, let us examine the present condition of the bodies
with which we are to deal, and briefly trace out those ele-
ments of relationship which it will be our purpose later to
describe more fully. Let us begin with the consideration of
a single object, which we may afterward compare with other
objects less easily observed because of their greater distance
from the Earth.
The photographic reproduction in Plate II represents the
Sun, as seen with an ordinary telescope. So far as could be
judged from this picture, the Sun might be described as a
luminous sphere, brighter in its central part than near its
circumference, and marked with dark spots, irregularly dis-
tributed over the surface. On closer examination it will also
be seen that there are certain bright regions, which are most
easily noticed near the edge of the Sun. The dark spots are
the well-known sun-spots, first discovered by Galileo, while
the bright recnons are the faculae, which have also been
known since the invention of the telescope. At times of total
eclipse, when the bright body of the Sun is covered by the
dark body of the Moon, shielding our atmosphere from the
usual brilliant illumination, red flames, sometimes reaching
heights of several hundred thousand miles, may be seen rising
from a continuous sea of flame, which completely incircles
the Sun. These are the prominences, and the continuous
mass of flame from which they rise is the chromosphere
( Plate III).' Extending far beyond these flames into space,
1 See the remarks on anomalous dispersion, p. 148.
15
16 Stellar Evolution
sometimes to a distance of millions of miles, is the corona,
which shines with a silvery luster somewhat inferior in bright-
ness to that of the full Moon (Fig. 2, Plate IV).
An analysis of the light of the Sun, made with the spectro-
scope, has shown the presence of the vapors of iron, sodium,
magnesium, calcium, hydrogen, and many other substances
known to us on the Earth. In fact, it has been remarked
that if the Earth were heated to the temperature of the Sun,
the light emitted by its vapors would resemble closely, when
analyzed with the spectroscope, the light emitted by the Sun.
Thus the chemical composition of the Earth and the Sun is
ifiuch the same, although we have evidence of the existence
in the Sun of a large number of substances not yet found on
the Earth. This same means of analysis has led to the dis-
covery that the chromosphere, and the prominences which
rise out of it, are composed of the vapor of calcium and of the
light gases helium and hydrogen. The sun-spots, too, have
also been found to have a characteristic chemical compo-
sition; while the corona emits rays which probably indicate
the presence in it of very light and tenuous gases.
Observations of the Sun, continued without interruption
for more than half a century, have shown that the spots are
not constant in number, but vary in a characteristic way in
a period of about eleven years. At times of sun-spot maxi-
mum the surface of the Sun is marked by large numbers
of spots, which are found on attentive observation to be the
scene of great activity, and frequently the source of the most
violent eruptions. At this period the prominences are large
and abundant, and testify to the general condition of disturb-
ance by exhibiting, from time to time, eruptive phenomena
on a very large scale, in which great masses of gas have
been known to shoot upward with velocities of hundreds of
miles a second. With the passage of time these evidences of
disturbance and activity become less and less marked, until
The Sun as a Typical Star 17
finally, during the minimum period, the surface of the Sun
for months together may be wholly devoid of sun-spots.
The prominences also become less numerous, and eruptive
phenomena, so common during the maximum period, are
rarelv to be observed at the minimum. Even the corona
undergoes changes in form which are {perfectly charac-
teristic, and show a definite connection with the sun-spot
period.
So much for the Sun and its more conspicuous phenomena.
We are now led to inquire whether it has any counterparts
among the other heavenly bodies. Let us suppose the Sun
removed to the distance of the nearest fixed stars. Its light
would then be reduced in so great a degree as to be sur-
passed by that of many of the brighter stars, though it
would still remain one of the more conspicuous objects in
the heavens. The planets of the solar system would be
wholly beyond the range of observation, even with the most
powerful telescopes. The light of the Sun would appear
yellowish, and it would be impossible to distinguish it from
certain stars which also shine with a yellowish light. Spec-
troscopic analysis of the light of these stars reveals the
presence in their atmospheres of elements familiar to us on
the Earth; indeed, the chemical composition of some of
them can be shown to be practically identical with that of the
Sun. On account of its immense distance, the Sun's disk
would be reduced to a minute point of light, as in the case of
the other stars, and the sun-spots, prominences, corona, and
other phenomena would be wholly invisible. For the same
reason, such phenomena, though undoubtedly present in
other stars, are hidden from observation. AYe may there-
fore conclude that the Sun is a star, practically identical in
chemical composition and in physical constitution with many
other stars in the heavens, and ranking in size below many
of these objects.
18 Stellar Evolution
A very casual acquaintance with the stars, based upon
naked-eye observations, is sufficient to make one familiar
with the fact that they differ from each other as much in
color as they do in brightness. Such objects as Sirius shine
with a bluish-white light, whereas Arcturus is yellowish like
the Sun. Antares, in the Scorpion, is a fine example of a
red star, and with the telescope smaller stars may be seen
of a deeper red color. Spectroscopic study of these various
classes of stars shows in the clearest way definitive peculiari-
ties, which may form the basis of a system of classification.
Indeed, we apparently find ourselves in the presence of stars
in every stage of growth, from the earliest, as represented
by the bluish-white objects, to the latest, typified by the red
stars' (Fig. 1, Plate IV). Intermediate in point of develop-
ment are yellowish stars like the Sun.
In various parts of the heavens clusters may be observed,
in some of which the stars are widely scattered, as in the
Pleiades, while in others they are densely packed together,
so closely that several thousand stars may sometimes be seen
within an area so small that to the naked eye they appear like
a single hazy star. Since we find clusters of every degree of
density, and since the stars in the heart of some of these
clusters are too close together to be separated by the tele-
scope, the question long ago arose whether the nebulae,
which seem to resemble luminous clouds in the heavens, are
to be regarded as star clusters so dense as to be beyond
telescopic resolution. It was not until the spectroscope had
been applied by Huggins (see p. 54) that this question was
finally settled. It then appeared that some of the nebulae, at
least, are vast masses of luminous gas, and that they are
therefore not composed of separate stars. It might then be
inquired what part in the scheme of evolution such nebulae
play. It will be shown in the course of this book that there
1 See the cautionary remarks on p. 198.
The Sun as a Typical Star 19
exists between stars and nebulae a relationship so intimate
as to leave little doubt that stars are condensed out of nebulae
through the long-continued action of gravitation. It thus
seems probable that the nebulae represent the stuff from
which stars are made, in its primitive and uncondensed
state.
CHAPTER IV
LARGE AND SMALL TELESCOPES
It must soon appear, to one who seeks in the heavens
with unaided vision for evidences of stellar evolution, that
but little progress can be made without powerful instrumental
means. When the nature of the problem is considered, and
it is remembered that all observations of the stars must be
made from the surface of a minute body moving through the
midst of the universe, the only cause for surprise will be that
instruments of sufficient power for our purpose can be con-
structed. The distances of the stars are so enormous that it
might seem hopeless ever to solve the problem of their phys-
ical constitution, or to analyze them as the chemist resolves
into its elements a substance in his laboratory.
Let us consider what must be accomplished before we may
even begin to study the subject of stellar evolution. In the
course of our work we must deal with stars which are not only
invisible to the naked eye, but are beyond the reach of any
except the most powerful telescopes. We must find the means
of collecting the light from such bodies, not only those rays
which, if intense enough, could be seen by the eye, but
also those which, because of the structure of the eye, are
wholly invisible. After collecting together such rays, we
must subject them to analysis by instruments which will per-
mit us to draw conclusions, both as to the nature of the
chemical elements present in the star's atmosphere and as to
the physical condition of these elements, illustrated by the
pressure and the temperature to which they are subjected.
Although we may never hope to see a star's actual disk,
even in the most powerful telescopes of the future, as other
20
Large and Small Telescopes 21
than a minute point of light, we must find means of differ-
entiating one part of the star from another and of determin-
ing, for example, whether the vapor of carbon lies above
or below that of iron or sodium in its atmosphere. If
luminous clouds, like those on the Sun, are strikingly char-
acteristic of the star under observation, we must be able to
detect their presence, though we may never see their form.
If, as in the case of temporary stars, vast temperatures or
pressures may produce great differences in physical condition
between the inner and outer parts of a stellar atmosphere,
we must learn a way of discovering such differences and of
ascribing them to their true cause. Incidentally, and as a
necessary precedent to these studies, we must be able to
determine whether the star is moving toward or away from
the Earth, and to measure its velocity in either direction with
great precision.
Moreover, our means of analysis must be so refined that
they shall enable us to investigate, not merely the general
physical and chemical properties of single stars, but also
those minute peculiarities of composition or of motion which
may relate them to other stars, and define their precise place
in some general scheme of stellar evolution. We must have
some means at hand which will brinor to lisrht the forms of
nebulae, even though they be invisible to a trained eye aided
by the most powerful telescope ever constructed. Being
given these forms, we must seek for evidences of relationship
between the cloudlike nebulae and the stellar points which
they surround. And the means of analysis which tells us of
the constitution of the stars must also tell us of the nature of
the nebulae, thus serving to establish relationships with stars
which no mere indications of position or of structure could
provide.
A refracting telescope consists of a lens (object-glass)
usually mounted at the end of a long tube, which is pointed
22 Stellar Evolution
at the object to be observed. In the present case we will
suppose this to be the Moon. The lens forms an image of
the Moon at the lower end of the tube, just as the lens of a
camera forms an image on the ground-glass. Indeed, a tele-
scope may be regarded as nothing more or less than a long
camera, in which a tube is substituted for the ordinary bellows.
By putting a plate at the point where the image is formed, and
giving a suitable exposure, the Moon may be photographed,
just as a landscape is photographed with the camera. For
eye observations, however, the image formed by the telescope
is looked at through a small lens called an eye-piece. The
image is magnified in the same way, and to the same extent,
as any object would be if looked at with the eye-piece, used
as an ordinary hand magnifier. The total magnifying power
of the telescope, however, of course depends not only upon
the magnifying power of the eye-piece, but also upon the size
of the image formed by the object-glass. The size of this
image is determined solely by the focal length, or distance
from the object-glass to the image. Suppose, for example,
we have two telescopes, with object-glasses of the same diam-
eter, but of different focal lengths. The one of longer focal
length will give the larger image. If the focal length is twic6
that of the other telescope, the image will be twice as large.
With the same eye-piece, therefore, the magnifying power of
the longer telescope will be twice that of the shorter one.
We thus see that the size of the image given by a tele-
scope does not depend upon the diameter of its object-glass.
The brightness of the image, however, evidently does depend
upon the amount of light concentrated in it, and this increases
with the diameter of the object-glass. If we double the
diameter of the object-glass, we get four times as much light
in the image of a star; for the amount of light collected
depends upon the area of the object-glass, and this increases
as the square of its diameter.
Large and Small Telescopes 23
These details are worth remembering, for they determine,
in great measure, the relative advantages of large and small
telescopes. There is another consideration, however, of the
first importance, which must not be overlooked. A small
telescope is limited, by the very nature of light, in its power
of separating two closely adjacent stars. If these stars are
less than a certain distance apart, no increase in the magnify-
ing power of the telescope, either through increase in its focal
length or through the use of a more powerful eye-piece, can
jx)ssibly show them as separate objects. The reason lies in
the fact that the image of a star in a telescope is a minute
disk, the diameter of the disk depending on the size of the
object-glass. The disk grows smaller as the object-glass
grows larger; so it is easy to see why a large telescope will
divide a close double star when a small one will not: the
star images, which are of sensible diameter and consequently
overlap, as seen in the small telescope, are reduced by the
high resolving power of the large telescope to such minute
dimensions that they appear distinct and separate.
Here, perhaps, a word of explanation may be useful; for
it is not at first sight obvious that a star should appear
smaller in a large telescope than in a small one. Such a
statement would not be true of the Sun, Moon, or planets.
These objects are all comparatively near the Earth, and even
a moderate magnifying power will show them (except the
most distant planets) as disks on which structural details are
visible. The stars, however, are so inconceivably remote
that no telescope, however powerful, can show their true
disks. They are mere points of light, brighter, and for this
reason apparently larger, in the case of the brilliant stars, but
always becoming more minute and pointlike under the most
favorable atmospheric conditions and with the most powerful
instruments.
The sjiurioHS disks, which would have no existence if
24 Stellar Evolution
light-waves were infinitely short, appear large in small tele-
scopes, but small in large ones. In the Yerkes telescope, for
example, stars separated by only a tenth of a second of arc
can be resolved under the best atmospheric conditions. A
four-inch telescope cannot separate stars that are less than a
second of arc apart, no matter what magnifying power be
applied. In such an instrument, therefore, the thousands of
double stars whose components are separated by less than a
second appear as single stars. In the same way, minute
markings, lying close together on the Sun, Moon, or planets,
are not separately distinguished in a small telescope, while
in a large one they may be seen as distinct objects, provided
the atmospheric conditions are sufficiently favorable.
We may sum up the preceding remarks by saying that in
all astronomical observations which involve the separate and
distinct recognition of very closely adjacent stars, or a knowl-
edge of the most minute structure of the Sun, Moon, or
planets, large telescopes must be employed under excellent
atmospheric conditions. Furthermore, if it is a question of
collecting sufficient light, either for eye observations, or for
photography, or for spectroscopic analysis, from an extremely
faint star, the great area of a large object-glass or mirror also
becomes essential. Nevertheless it will be shown that for
many important investigations small telescopes are equal or
even superior to large ones.
This brings us to the much-discussed question of the
relative advantages of large and small telescopes, regarding
which a great deal has been written. On the one hand, we
hear the amusing claims of the promoters of the great tele-
scope which was to be the clou of the last Paris Exposition.
This immense instrument — which does not seem to have been
completed, and is now lying unused — was to bring the Moon
within the observer's grasp — if he could reach a meter!
The light-heartedness of this claim is manifest when it is
Large and Small Telescopes 25
remembered that no existing telescope, under the best
atmospheric conditions, has ever shown the Moon as well as
it would appear to the unaided eye at a distance of fifty
miles.
On the other hand, it has been stated, with great insist-
ence, that it is absurd to use a telesco|:)e of more than four
inches' aperture east of the Mississippi River, or of more than
six inches' aperture in the better atmospheric conditions
west of it. This statement, although not so extreme as the
one which emanated from Paris, is entirely misleading and
unwarranted by the facts. It was probably intended to
emphasize a conviction that the atmospheric conditions in
the eastern part of the United States are very bad, and un-
suited for large telescopes. Now, it is quite true that atmos-
pheric disturbances are the bane of astronomers in all parts
of the world ; we shall have oc<?asion to discuss this question
in a future chapter. It is also true that the meteorological
conditions are, on the average, much more favorable for
astronomical observations in the southwestern part of the
United States than east of the Mississippi River. But it
cannot be denied that many of the valuable observations
turned out by our eastern observatories are directly due to
the fact that they are equipped with large telescopes. That
these telescopes would do more and better work under better
conditions goes without saying. Most of them would not
exist at all, however, if it had been a question of establishing
them some thousands of miles from the universities or col-
leges with which they are connected.
To those who have used both large and small telescopes,
the great advantages of large instruments for many kinds of
work are well known. I have heard a European astronomer
exclaim, when looking at Jupiter for the first time with the
forty-inch Yerkes telescope, that his years of study of this
planet with a small telescope seemed almost useless, so much
26 Stellar Evolution
more of detail could he perceive at a single glance. I have
seen minute structure on the Moon with this telescope, no
trace of which could be made out with a twelve-inch tele-
scope on the same evening. Countless fine bright lines in
the spectrum of the chromosphere, which could never be
detected with the twelve-inch, are easily seen with the
forty-inch. Burnham has separated double stars at the
theoretical limit of resolution of the Yerkes telescope, and
Barnard has observed the tiny fifth satellite of Jupiter when
it was beyond the reach of all but the largest existing instru-
ments. When I think of these observations and of Ritchey's
photographs of the Moon and star clusters, Frost's and
Adams' photographs of the spectra of faint stars, and the no
less important results obtained by Schlesinger, Parkhurst,
Ellerman, Fox, and others with the Yerkes telescope; and
when I remember that most of these observations or results
could not have been obtained with a small telescope, I see no
possible reason for denying the manifold advantages of large
instruments. My illustrations have been confined to obser-
vations made with the Yerkes telescope, because of personal
knowledge of them. But they could be greatly multiplied if
the remarkable work of the Lick telescope and of other large'
instruments were drawn upon for examples. In the next
chapter, through the aid of photography, some of the relative
advantages of large and small telescopes will be illustrated.
CHAPTER V
ASTRONOMICAL PHOTOGRAPHY WITH CAMERA LEXSES
The emphasis laid in the last chapter on the importance
of large telescopes must not be supposed to mean that small
telescopes are of little value. The single fact that Burnham
discovered i51 new double stars with a six-inch refractor
(Plate V) is sufficient evidence to the contrary. It is quite
true that small telescopes are not well adapted for certain
classes of work, in which large telescopes exceL But their
superiority over large telescopes is no less evident in other
fields. The equipment of an observatory recognizes this by
the provision of both large and small telescopes, each designed
for use in the investigations for which it is particularly suited.
In fact, the characteristic of a modem astrophysical observa-
tory which distinguishes it most clearly from the old observa-
tory of one or two instruments is the careful adaptation of a
multiplicity of special apparatus to certain narrowly defined
purposes. The day of the universal instrument has passed,
for conditions similar to those which have resulted in the
development of the innumerable special tools of the modern
machine shop obtain also in the observatory.
The amateur astronomer should keep this fact clearly in
mind. There is some reason to fear that the larore and
expensive equipments of modern observatories have tended
to discourage the worker with small instruments. As one
who has looked at the subject from both sides. I may be
}>ermitted to oppose this pessimistic view. Far from believ-
ing that recent developments have been detrimental to the
amateur. I am strongly of the opinion that his opportunities
for useful work have never before been so numerous. The
28 Stellar Evolution
importance of this subject, due to the high value of the
contributions to astronomy made by amateurs in the past,
has led me to devote a subsequent chapter to opportunities
for work with inexpensive instruments.
In considering the peculiar advantages of small telescopes
in certain fields of research, attention must be called at the
outset to the important part played by photography in the
astrophysical work of the present day. The photographic
plate, through its power of storing up impressions made by
feebly luminous rays, has in most cases an immense advan-
tage over the eye. The eye perceives almost at once as much
as can be seen by long gazing at a faint object. But the
photographic plate continues, hour after hour, and perhaps
night after night, to accumulate impressions, so that with
sufficiently long exposures, objects far too faint to be seen
by the eye with the same telescope are clearly and per-
manently recorded. Moreover, the photographic plate is
sensitive to light-waves which are too short to produce the
least effect upon the eye, and in this power of recording
objects which otherwise could never be rendered visible, no
matter what their intensity of radiation, the plate presents a
second great advantage. Because of these and other points '
of superiority, which far outweigh certain slight defects that
in some few instances still leave the plate inferior to the eye,
the photographic method is now exclusively employed for
many kinds of observations.
Some of the most important results of astronomy have
been derived from the use of an ordinary camera, having
just such a lens as is found in the possession of thousands
of amateur photographers. If we take an ordinary camera
and point it on a clear night toward the north pole, it will
be found, after an exposure of one or two hours, that the
stars which surround the pole have drawn arcs of circles
upon the plate (Plate VI). This is due to the fact that
Astronomical Photography 29
the Earth is rotating upon its axis at such a rate as to cause
every star in the sky to appear to travel through a complete
circle once in twenty-four hours. Since the pole is the place
in the sky toward which the Earth's axis is pointing, it is
easv to understand that the nearer the star lies to the pole,
the smaller does this circle become. As we move away from
the ^le we find the curvature of the star trails growing less
and less, until at the equator they appear as straight lines.
Just such photographs as these are frequently employed
in astrophysical investigations; e. g., for the purpose of
recording variations in a star's brightness, which would be
shown on the plate by changes in the strength of the trail.
But for most purposes it is desirable to have photographs of
stars in which they are represented as points of light rather
than as lines. To obtain these photographs it is necessary to
mount the camera in such a way that it can be turned about
an axis parallel to the Earth's axis, at a }>erfectly uniform
rate, once in twenty-four hours. A camera so mounted
becomes an equatorial photographic telescope, differing in no
important respect, save in the construction of its lens, from
the largest refractors.
Here, for example, is a photograph (Plate VII) of the
Bruce photographic telescope of the Yerkes Observatory.
This instrument has a compound lens ten inches in diameter,
made by Brashear from four lenses suitably combined, of
such curvature as to form an image at a point only fifty
inches distant from the optical center of the lens system. It
will be seen that such a lens must produce a very bright and
highly concentrated image, in which the various objects are
crowded close together because of the small scale of the
picture. If the same lens were so constructed as to form an
image ten times as far distant from the photographic plate,
the several elements of the picture would then be ten times
more widely separated, and a longer time would be required
30 Stellar Evolution
to photograph them, on account of the spreading of the same
amount of light over a larger surface. As will be seen from
the illustration, the tube which carries the lens and photo-
graphic plate is mounted in such a way that it may be turned
about an axis parallel to the axis of the Earth by means of a
driving-clock, placed in the upper part of the iron supporting
column. The same mounting carries not only the ten-inch
lens, but also the lens of a guiding telescope, through which
the observer watches a star during the entire period of
exposure, continued, perhaps, for many hours. He may
thus correct any slight irregularity in the motion of the tele-
scope by certain screws provided for the purpose, which per-
mit him to keep the star accurately at the intersection of two
illuminated cross- wires. The driving of the clock is so accu-
rate that this is accomplished almost automatically, though
small changes in atmospheric refraction and other causes
require minute displacements of the instrument to be made
from time to time, to insure the perfect immobility of the
stellar images upon the photographic plate.
Besides the ten-inch camera and the guiding telescope,
the Bruce telescope carries three other cameras, with lenses
of 6 inches, 3.4 inches, and 1.6 inches aperture respectively.'
Thus four photographs of the same part of the heavens, on
different scales, determined by the focal lengths of the
lenses, are obtained in a single operation. Our knowledge
of the structure of the vast girdle of stars that forms the
Milky Way is derived in very large part from a study of
photographs made with such an instrument. At the Lick
Observatory Barnard used the six-inch Willard lens to great
advantage in photographing these star clouds, and of late,
through the opportunity afforded by the Hooker Expedition
at the lower latitude of Mount Wilson, he has carried his
work farther south of the celestial equator. The Bruce tele-
scope, temporarily transferred from the Yerkes Observatory t( >
ASTBON'OMICAL PHOTOGRAPHY 31
Mount Wilson for use during the spring and summer of 1905,
has yielded some remarkably fine results in Barnard's hands.
The smallest of the four photographs made in a single opera-
tion is taken with an ordinary "magic -lantern" lens of 1.6
inches aperture and 6.4 inches focal length. This shows a
a rejjion about fifteen degrees' across within a circular area
about 1.7 inches in diameter on the photographic plate.
With the ten-inch lens the field of sharply defined images is
limited to about eight degrees, but it is still large enough
to include extensive star clouds and nebulae. The larger
scale, due to the greater focal length of the ten-inch lens,
brings out details of structure that are not visible on the
smaller photographs. Plates VIII and IX, reduced from the
originals in the same proportion, illustrate the relative scales
of the photographs made with the two lenses.
The Milky Way, as revealed by such photographs, is
a most extraordinary spectacle. The countless stars that
compose it are grouped in every conceivable manner, and
intertwined with long reaches of diffuse nebulous clouds.
Here and there vacant regions, sometimes apparently darker
than the background of the heavens, resemble vast lanes,
extendinor through the entire thickness of the star clouds, or
perhaps lead one to suspect that an obscuring medium may
be cutting off the light from immeasurablv distant bodies.
Again, a nebula of great extent, diffuse on one side and sharply
bounded on the other, may suggest the action of forces be-
yond our present means of investigation. The filmy veils
spread by certain nebulae seem to envelop the stars in mist,
though in most cases we cannot say with certainty whether
the stars are actually within the clouds, or remote from them
in the line of vision. The surest test of relationship between
stars and surrounding nebulae is afforded by the spectroscope,
> Readers who are not accastomed to an^rolar measure may be reminded that
the two " i>ointers " of the " Dipper " are about five degrees apart.
32 Stellar Evolution
as will be shown in a subsequent chapter. It has been found
that stars of different spectral types, which are ordinarily
assumed to indicate different degrees of development, are
not equally represented in the Milky Way. The connection
between these stars and surrounding nebulae, and the possible
relationship between spectral type and the grouping of the
stars in the cloudlike forms of the Galaxy, is one of the
important problems of the present time. Our knowledge of
the Milky Way and its structure is still very meager, but the
future is certain to bring great advances.
These illustrations may suffice to show the uses of the
ordinary camera lens in investigations bearing upon tl^e
general structure of the Milky Way. A simple compari-
son will serve to bring out both the advantages and dis-
advantages of large telescopes in studies of a similar kind.
Plate X shows the Milky Way in Ophiucus from one of Bar-
nard's photographs made with a portrait lens. It affords a
superb picture of this part of the sky, such as no visual
observations with any telescope could supply. If the same
region of the heavens were examined with a large telescope,
the field of view would be so restricted that no proper
impression could be obtained as to the character of the*
Milky Way or the distribution of the stars within it. It
would, of course, be possible to count one by one the hun-
dreds of stars included within a single field of view, and by
long and laborious measurements to map these stars and
ultimately to build up, from combination into a single picture
of the results thus obtained, a representation of the Milky
Way. However, such a task would occupy years of labor,
and the result would be less valuable, for many purposes,
than that illustrated in Plate X. This picture is an auto-
graphic record, showing not only the distribution of the
stars, but also their relative brightness on the date of the
exposure.
Astronomical Photography 33
Since such results are due to photography, the comparative
value of large telescopes should be judged by the same
means. Plate XI is a reproduction of a photograph of the
cluster Messier 11, which is represented in Plate VIII as a
small circular white dot in the upper part of the picture.
The short focal lenofth of the camera lens, which causes it to
form upon the plate a small-scale picture covering a large
region in the heavens, is not competent to separate out the
single stars of this cluster. The photograph reproduced in
Plate XI was made by Ritchey with the forty-inch Yerkes
telescope, which has a focal length of sixty-four feet, as com-
pared with the focal length of 6.4 inches of the camera lens
used for Barnard's photograph. The scale of the negative
obtained with the Yerkes telescope is therefore about 120
times as great as in the case of the camera lens. This
great scale, while disadvantageous so far as it bears upon
the question of the general structure of the Milky Way,
would be in the highest degree advantageous if the problem
under consideration involved the study of the individual stars
in the cluster Messier 11. With the camera lens these stars
are so close together upon the plate that their separate images
are confused. With the Yerkes telescope the images are
widely separated from one another, permitting the position
and the brightness of each star to be determined with great
precision. The Bruce lens gives an intermediate scale. If
Plate IX had been enlarged in the same proportion as Plate
XI, this cluster would be shown fairly well resolved. But
Messier 13 (Plate XIX) is far beyond the capacity of the
Bruce lens.
It may be of interest to include here another photograph
illustrative of the advantages of great focal length for certain
classes of work. Plate XII represents a photograph of the
Moon, made by Ritchey with the twelve-inch Kenwood tele-
scope, which is eighteen feet long. This picture gives an
34 Stellar Evolution
excellent general idea of the lunar topography. But if the
detailed structure of the lunar mountains is to be investigated,
such a picture as that reproduced in Plate XIII would evi-
dently be far more effective for the purpose. Theophilus, the
great ring mountain here represented, may be seen in Plate
XII on a smaller scale. The large-scale picture was obtained
by Ritchey with the forty-inch telescope, which, as already
remarked, has a focal length of sixty-four feet. The scale
of the original photograph was therefore about three and
one-half times as great as that of the photograph taken with
the Kenwood telescope. In consequence of the larger scale
of the Yerkes picture, it brings out many small details which
are entirely lacking on the Kenwood photograph.
These illustrations of the separating power of the large
telescope may lead us to an examination of the instrument
itself (Plate XIV) . Although so much larger, it differs in no
essential particulars from the Bruce photographic telescope,
also made by the firm of Warner & Swasey. The great
weight of the forty-inch lens, amounting with its cell to half
a ton, requires that the tube which carries it shall be of
immense rigidity and strength. This tube, sixty-four feet
in length, is supported at its middle point by the declination '
axis, which in its turn is carried by the polar axis,, ad justed
to accurate parallelism with the axis of the Earth. By
means of driving mechanism in the upper section of the iron
column, the whole instrument is turned about this polar
axis at such a rate that it would complete one revolution in
twenty-four hours. Although the moving parts weigh over
twenty tons, the telescope can be directed to any part of
the sky by hand, but this operation is much facilitated by
the use of electric motors provided for the purpose. When
once directed toward the object to be observed, it will fre-
quently happen that the lower end of the telescope is far
out of reach above the observer's head. For this reason the
Astronomical Photography 35
entire floor of the observing-room, seventy-five feet in diam-
eter, is constructed like an electric elevator, which, by moving
a lever, can be made to rise or fall through a distance of
twenty-three feet. Thus the lower end of the telescope is
rendered accessible even when the object is near the horizon
(Plate XV). In order that the observing slit may be di-
rected to any part of the sky, the dome, ninety feet in diameter
(Plate XVI), is mounted on wheels and can be turned to any
desired position by means of an electric motor controlled
from the rising-floor.
The telescope is used for a great variety of purposes in
conjunction with appropriate instruments, which are attached
to the lower end of the tube near the point where the image
is formed. We have already examined a photograph of a star
cluster taken with this telescope, but without describing the
process of making it. As a matter of fact, the forty-inch
object-glass was designed for visual observations, and its
maker, the late Alvan G. Clark, had no idea that it would ever
be employed for photography. Without dwelling upon the
distinguishing features of visual and photographic lenses, it
may be said that the former are so designed by the optician
as to unite into an image those rays of light, particularly the
yellow and the green, to which the eye is most sensitive.
With the only varieties of optical glass obtainable in large
pieces, it is impossible to unite into a single clearly defined
image all of the red, the yellow, the green, the blue, and the
violet rays that reach us from a star. Therefore, when the
optician decides to produce an image most suitable for eye
observations, he deliberately discards the blue and violet
rays, simply because they are less important to the eye than
the yellow and green rays. For this reason the image of a
./. star produced by a large visual refracting telescope is sur-
' rounded by a blue halo, containing the rays discarded by
the optician. These very rays, however, are the ones to which
36 Stellab Evolution
the ordinary photographic plate is most sensitive; hence in
a photographic telescope the blue and violet rays are united,
while the yellow and green rays are discarded.
The forty -inch telescope is of the first type, constructed
primarily for visual observations. In order to adapt it for
photography, Ritchey, then a member of the Yerkes Obser-
vatory staff, simply placed before the (isochromatic) plate a
thin screen of yellow glass, which cuts out the blue rays, but
allows the yellow and green rays to pass. As isochromatic
plates are sensitive to yellow and green light, there is no
difficulty in securing an image with the rays which the
object-glass unites into a perfect image. During the entire
time of the exposure some star lying just outside the region
to be photographed is observed through an eye-piece mag-
nifying 1,000 diameters. This eye-piece is attached to the
frame which carries the photographic plate, and is suscep-
tible of motion in two directions at right angles to one an-
other (Plate XVII). In the center of the eye-pieCe are two
very fine cross-lines of spider web, illuminated by a small
incandescent lamp. If the observer notices that through
some slight irregularity in the motion of the telescope, or
some change of refraction in the Earth's atmosphere, the'
star image is moving away from the point of intersection
of the cross-lines, he instantly brings it back by one or both
of the screws. As the plate moves with the eye-piece, it is
evident that this method furnishes a means of keeping the
star images exactly at the same position on the plate through-
out the entire exposure.
Many other comparisons of large and small telescopes
might be given, and some of these will be included in sub-
sequent chapters. They all serve to demonstrate that each
telescope has advantages and disadvantages peculiar to its
size and type of construction. For some purposes small
camera lenses are to be preferred to all other instruments.
Astronomical Photography 37
In fact, without their aid many investigations of the highest
importance could never be undertaken. For other investiga-
tions these short-focus instruments may be entirely unsuited,
while refracting telescopes of great focal length may give
excellent results. These larger telescopes also have their
limitations, and must yield to reflecting telescopes in certain
other kinds of work. The truth of this statement will be
brought out in the next chapter.
CHAPTEK VI
DEVELOPMENT OF THE REFLECTING TELESCOPE
On a night in April, 1845, while sweeping the sky in
the constellation of the Hunting Dogs, the observers with
the great Parsonstown reflector discovered a spiral nebula.
The instrument with which the discovery was made may well
be regarded as one of the most remarkable scientific achieve-
ments of the nineteenth century. With its immense mirror,
six feet in diameter, having a focal length of fifty-four feet,
the great telescope of Lord Rosse surpassed in size all others
ever constructed. Unfortunately for the progress of science,
the engineering methods of that day were inadequate to
provide a suitable mounting for this gigantic instrument.
All parts of the machinery had to be constructed on the spot,
with such tools as the period and the circumstances afforded.
It is no small credit to the Earl of Rosse that under these
conditions the telescope was ever erected, and kept in active ]
use by an able company of observers. Supported upon a ball-
and-socket joint at its lower end, the enormous tube, swung
in chains, was confined to observations within a short distance
of the meridian by two flanking stone walls. The observer,
mounted upon a platform far above the ground, saw the
image of an object as he looked down into the tube. To
the present-day astronomer, provided with every appliance
to facilitate the finding of an object, and with an accurate
driving-clock which moves the telescope so steadily and uni-
formly as to maintain the image in the field of view for hours
together, it is little short of marvelous that the observers
with the great Parsonstown reflector were able to obtain
results of value. But, in spite of the difficulties to be over-
38
Development of Reflecting Telescope 39
come, both in manipulating the telescope and in finding
opportunities for observation under the cloudy skies of
Ireland, Lord Rosse and his assistants recorded many val-
uable discoveries in their memoirs. Of all these discoveries
that of the spiral nebula in Canes Vencdici was perhaps
the most significant of the future (Plate LXXXVIII). Be-
fore this chapter is concluded we shall see how this beautiful
object, which once stood alone among the heavenly bodies
as the only visible representative of a distinctly spiral form,
has now come to be regarded, through the work of Keeler, as
a type of the most interesting and the most numerous class
of nebulae.
The history of the Parsonstown reflector has in some de-
gree resembled that of almost every reflecting telescope ever
built. The infinite care expended by Herschel and by others
who have followed him in the construction of mirrors for such
instruments has been in large part annulled by the imper-
fections of the mountings provided for the mirrors. In
the period that preceded the introduction of photographic
methods, these imperfections were far less serious than they
would be considered from our present point of view. It is
true that they hampered observation, and in the early days
rendered accurate measurement with the telescope practically
impossible. But the employment of the photographic plate
has imposed a new condition, rigorous and unyielding, upon
the constructors of telescope mountings. In order to secure
satisfactory photographs, which shall do full justice to the
optical qualities of the instrument, and show only such de-
fects as atmospheric disturbances may produce, it is necessary
that the mirrors be so rigidly supported, and so accurately
moved by the driving-clock, that a stellar image shall not
(K part, during exposures of many hours, by so much as
one-thousandth of an inch from a fixed position upon the
photographic plate.
40 Stellar Evolution
In view of the difficulties to be overcome, it will be under-
stood that to accomplish such a result is no small task. In
the first place, the mirror, which is so sensitive to deformation
that it will bend under its own weight unless supported by
special apparatus, must be" firmly mounted, yet without strain,
at the lower end of an open tube. In the second place, pro-
tection must be provided against currents of warm and
cold air, and even against the heat radiated from the
observer's body, on account of the great sensitiveness of the
mirror to heat, and of the light-rays to irregular refraction in
the telescope tube. These precautions having been taken,
the tube must be so mounted that it can be moved with per-
fect steadiness and uniformity about an axis parallel to the
axis of the Earth. This condition is imposed by the neces-
sity of counteracting the apparent motion of the star through
the heavens, due to the rotation of the Earth. But while
this rotation is uniform, the motion of the star is not, since
the displacement of its apparent position from its true posi-
tion in the heavens, due to the bending of its rays during
transmission through the Earth's atmosphere, varies with the
height of the star above the horizon. It therefore becomes
necessary, as previously explained, to supplement the uniform
motion of the driving-clock by corrections, accomplished by
the hands of an observer. All these obstacles having been
surmounted, there still remain serious sources of difficulty
in the shaking of the telescope by the wind, the changes of
temperature during the exposure, which alter the focal length
of the mirror, and finally, most serious of all, disturbances
in the atmosphere which tend to blur and confuse the image,
instead of leaving it, sharp and well defined, to make its
record upon the photographic plate. It should also be
remembered that the observer must be prepared to hold his
eye at the eye- piece, and correct every few seconds the posi-
tion of the plate, throughout exposures lasting several hours.
Development of Reflecting Telescope 41
in an open dome where the temperature may not infrequently
be below zero.
After the erection of Lord Rosse's great reflector, the
attention of opticians was confined mainly to the construction
of refracting telescopes, which grew rapidly in size, reaching
apertures of fifteen inches in the Harvard refractor (1845),
thirty-six inches in the Lick refractor (1889), and forty inches
in the Yerkes refractor (1897). In these instruments care-
ful attention was given to all details of mechanical construc-
tion, and the Lick and Yerkes telescopes are among the most
successful products of modern engineering.
The development of the reflecting telescope has been due
I mainly to amateurs, whereas refractors have been made by
professional opticians and mounted by experienced engineers.
j To the inadequate equipment of the amateur's workshop may
therefore be ascribed many of the deficiencies in the mount-
ings of reflecting telescopes. In some cases, however, re-
flectors of large aperture, figured and mounted by professional
opticians and engineers, have given results of little or no
value. In these cases, as in others, it appears that insutficient
attention was paid to the excessive sensitiveness of large
mirrors, which causes them to require much more careful
treatment than is amply sufficient to yield good images with
a lens.
In stellar spectroscopic work good results were obtained
with reflectors by Huggins and Draper at a comparatively
early period, but it was not until the last years of the nine-
teenth century that such telescopes were employed with any
^ considerable degree of success for the photography of nebulae.
The first photograph of a nebula was obtained with a refract-
ing telescope by Draper in 1881. Photographs of the Great
^ Nebula in Andromeda, made by Roberts in 1886 with a
f twenty-inch reflector, showed for the first time the truly
spiral form of this remarkable object, and thus indicated
42 Stellar Evolution
some of the great possibilities of investigating nebular
structure with instruments of this type. Briefly speaking,
their superiority to refractors lies in the fact that the light
is not weakened by passage through glass, but, after reflec-
tion from a surface of pure silver, all the rays, independently
of their color, are united in a common focus. With a
refractor many of the rays are completely cut off during
transmission through the glass of the lens, which is as
impervious as so much steel to the very short waves of the
ultra-violet spectrum. Furthermore, a lens does not unite
all the rays of different colors into a single focus, but forms
a series of images, corresponding to light of different wave-
lengths. In order to get a sharp photograph with a refract-
ing telescope, it is therefore necessary to discard some of
these rays, in the manner already described (p. 35). The
reflector, on the contrary, utilizes all of the light ^ — an advan-
tage which is clearly shown by the results obtained with this
type of telescope.
The photographic studies of nebulae made by Keeler with
the Crossley reflector of the Lick Observatory, mark a step
of the greatest importance in the development of the reflecting
telescope. The mounting of this instrument, constructed in
England some years previously, and presented to the Lick
Observatory by Mr. Crossley, was very poorly adapted to carry
the excellent mirror of three feet aperture. But through the
extraordinary efforts of Keeler, whose severe exertions in
carrying out this work hastened his death, the mounting
was so strengthened and improved as to permit remarkable
results to be obtained. In other hands, even after these
improvements had been made, it is doubtful whether such
exquisite photographs would have resulted. But, after many
unsuccessful efforts, Keeler learned how to overcome the
difficulties peculiar to the instrument, and in this he has been
1 Except a certain percentage lost in reflection.
Development of Reflecting Telescope 43
ably followed by Perrine,^ We shall have occasion later to
refer to their results.
The mounting of the two-foot reflecting telescope of the
Yerkes Observatory was designed express!}' for photographic
purposes, and no pains were spared to adapt the instrument
for the exacting requirements of such work. The mirror, 23i
inches in diameter and of 93 inches focal length, was con-
structed by Ritchey in 1895 at his home in Chicago. This
mirror is of the highest quality, meeting the most severe
optical tests that can be applied to it. The mounting of the
telescope, designed by Wadsworth, with modifications by
Ritchey, was constructed in the instrument shop of the Yerkes
Observatory, and is very heavy and rigid. In the photograph
(Plate XVIII) the mirror may be seen in position at the lower
end of the skeleton tube. At the upper end of this tube is
a small plane mirror, so supported that its face makes an
angle of 45° with the axis of the tube. The telescope is
therefore of the Newtonian type, the image being formed on
the photographic plate near the upper end of the tube, after
reflection of the cone of rays from the small mirror. The
double slide plate-carrier, which holds a plate 3^ X 4J inches
in size, is precisely similar to the plate-carrier employed with
the forty-inch refractor (Plate XVII).
A comparison of the results obtained with this instrument,
with those secured with the forty-inch Yerkes refractor, will
suffice to show the peculiar advantages of the reflector for
certain kinds of work. It should not be forcrotten that the
forty-inch refractor has other advantages, which tit it for
work that could not be done under any circumstances with
the two-foot reflector.* But in the photography of faint
• A new and satisfactory mounting has since been constructed for the Crossley
reflector.
- For example, the scale of the images given by the forty-inch is eight times that
i)f the two-foot reflector. Moreover, the former is well adapted for work on the Sun,
for which the latter cannot be used.
44 Stellar Evolution
stars, particularly in the photography of nebulae, the tv/o-
foot reflector is especially useful. It is possible with this
instrument, in an exposure of only forty minutes, to photo-
graph stars which are at the extreme limit of vision with the
forty-inch refractor. With longer exposures, countless stars,
which can never be seen or photographed with the large
refractor, are recorded on the plates. Compare, for example,
the photographs of the star cluster Messier 13, reproduced
in Plates XIX and XX. The principal advantage of the
reflector in such work, as already explained, is the con-
centration of the light-rays, irrespective of their color, in
a single focal image.
The photographs of nebulae obtained by Ritchey with the
two-foot reflector show in a remarkable way the beauty and
delicacy of structure which characterize these objects. It
will be seen from the illustrations in the plates that the nebulae
are of many types, although the spiral form predominates.
The Great Nebula in Orion (Plate XXI), which is the most
brilliant of the larger nebulae, is of irregular form, and
marked complexity of structure. Of very difiPerent pattern
is the beautiful nebula in Cijgnns, the delicate filamentous
structure of which is admirably shown by Ritchey's photo-
graph (Plate LXXXVII). In the nebulae which envelop the
stars of the Pleiades (Plate LXXXVI) two very different
types of structure are shown ; long parallel filaments predomi-
nate, but there may also be seen in the photograph a mass of
nebulosity resembling the flame of a torch blown by the wind.
But although, as we shall see, evidences may be found of the
relationship of the stars in these nebulae to the cloud-forms
themselves, the spiral nebulae certainly appeal most strongly
to the imagination. The largest of these, the Great Nebula in
Andromeda, is perhaps the most interesting object in the
heavens (Frontispiece). Persistent attempts to measure the
distance of this nebula from the Earth, made with the most
Development of Reflecting Telescope 45
powerful of modern instruments, have totally failed. We
mav therefore conclude that this distance is almost incon-
ceivablv £rreat, and that therefore the dimensions of the
nebula are so enormous as to be quite beyond comparison
with those of the solar system. In the beautifully defined
spiral character of this object, so clearly visible on the photo-
graph, although beyond recognition in visual observations,
we seem to see strong indications of motion with respect to
the center. But hitherto, in spite of the careful comparison
of photographs made many years apart, no evidence of such
motion has been detected. This fact would tend to confirm
what we already know from measurement, namely, that the
nebula is exceedinglv remote from the Earth, and that the
phenomena which it exhibits are on a gigantic scale. We
cannot doubt that the component parts are in motion, and
that in the course of time evidences of this motion will come
to light. But to detect them it is certain that the most
powerful instrumental means will be required, and that long
intervals of time must separate the photographs which are to
be compared.
The Great Nebula in Andromeda thus stands as the
largest representative of that great class of nebulae which
was first made known through Lord Rosse's discovery of the
spiral nebula in the Hiintiiuj Dogs. From some of Ritchey's
photographs we are fortunate in being able to illustrate other
spiral nebulae, which differ in various particulars, but in
all cases show clearly the spiral structure (Plates LXXXIX
and XC). As already stated, Keelers photographic investi-
gations with the Crossley reflector have shown that while large
. objects of this kind are comparatively few, the sky is scattered
I over with an immense number of small ones. The investi-
c-gation of these nebulae, with the great reflecting telescopes
* of the future, should lead to results of fundamental
importance.
CHAPTER VII
ELEMENTARY PRINCIPLES OF SPECTRUM ANALYSIS
The problem of determining the nature of the nebulae
seemed to be placed beyond solution by telescopic means
when it was found that star clusters exist in which the stars are
so densely packed that they cannot be separately distinguished
by any telescope. A photographic illustration of this is given
in Plate XIX. In Plate XI we see a cluster easily resolved
into its constituent stars. In the case of Messier 18, however,
the photograph here reproduced might leave some doubt on
the score of resolvability.' Visual observations, better com-
petent than photographic ones to settle this particular point,
remove the doubt in the present instance. But other clusters
are still more closely crowded, and it was easy to believe that
the unresolved nebulae might be objects of this nature. The
structure of such a nebula as that shown in Plate XC might
also be supposed to favor such a view. Sir William Herschel,
great not only as an observer, but as a philosopher who looked
deep into the nature of things, was not deceived by these
circumstances, and persisted in his belief that the nebulae
are masses of uncondensed gas, diifering essentially from
clusters of stars. As evidence of the uncertainty which
nevertheless existed, it must be added that Sir John Herschel,
though himself a great philosopher, was led to a contrary
conclusion. For him no nebula existed that could not be
resolved with a sufficiently powerful telescope into a congeries
of stars. Under these circumstances it is evident that some
additional means of analysis must be called upon to solve the
problem. For as telescopes increased in size the nebulae
remained unresolved, showing that either they were in their
1 Even the large-scale photograph in Plate XX does not separate the closest stars.
46
Principles of Spectrum Analysis 47
nature unresolvable, or that far more powerful instruments
would be required to reveal their constituent parts.
This was the condition of affairs when Spencer boldly
took issue with the astronomers. Convinced that the
principle of evolution must operate universally, and that
the stars must have their origin in the still unformed
masses of the nebulae, he ventured to question the con-
clusion that the resolution of nebulae into stars was only
a matter of telescopic power. He had not long to wait for
support, for at this juncture a new method of research, long
previously foreshadowed by Fraunhofer's analysis of sunlight
in the early part of the nineteenth century, suddenly pro-
claimed its power of accomplishing many surprising results.
It has been known since the time of Xewton that when
sunlight is passed through a prism, it is spread out into a
band containing all the colors of the rainbow. In Newton's
experiments the sunlight was admitted to the prism through
a circular hole, and he consequently failed to see in the
colored spectrum any of those breaks or dark lines that were
found in later years to be so significant. Fraunhofer, on the
contrary, examined sunlight which reached the prism from
a narrow slit, placed at a considerable distance. He was
rewarded by the discovery of a large number of dark lines,
differing greatly from one another in intensity, and irregularly
distributed through the spectrum. He measured the positions
,oi these lines in the spectrum with care, and designated the
more striking ones with the letters of the alphabet. His
designations are still retained, and the dark lines of the solar
spectrum are still called the Fraunhofer lines. But of the
origin of these lines Fraunhofer had no knowledge. He
found, indeed, that the lines seen in sunlight, while present
in the light of the planets, were replaced by different lines
in the spectra of some of the stars. But while he con-
cluded that the cause of the lines did not reside in the
48 Stellar Evolution
Earth's atmosphere, he nevertheless failed to discover their
true explanation, and thus did not perceive the possibili-
ties of the science of spectrum analysis.
Let us consider for a moment what happens when light
is passed through a prism. We may assume the light to be
derived from the glowing filament of an incandescent lamp,
placed just in front of a narrow slit. After passing through
the slit a (Fig. 1) the divergent rays fall upon the lens 6,
FIG. 1
Passage of Rays through a Prism
which renders them parallel, and is known as the collimating
lens. The parallel rays now meet the face of the prism c,
through which they are transmitted. After passing through
the prism the rays fall upon the lens d, precisely similar to
the collimating lens, which forms an image on the screen e.
Now, when light strikes a prism it is deviated from a
straight path, and the amount of its deviation depends upon
the color of the light. Yellow light, for example, is deflected
by a prism more than red light. Green light is deflected
more than yellow light, blue light suffers even a greater
change of direction, while violet light is deflected most of all.
It is thus evident that if the light from the incandescent
lamp were pure red, and contained no other color, we should
have a red image of the slit at R. If it were yellow, a yellow
image of the slit would be formed at Y. Green light would
form a green image of the slit at G, blue light a blue image
Peixciples of Spectrum Analysis 49
at B, and violet light a violet image at F. White light is
compounded of all these colors, and shows every intermedi-
ate gradation of tint. When passed through a prism it is
therefore dispersed into a colored spectrum, extending from
red at one end through yellow, green, and blue to violet. This
is called a continuous spectrum, and is produced when the
light from any white-hot solid body is analyzed by a prism.
Liquids, or even gases when sufficiently compressed, may
give a continuous spectrum when highly heated. But vapors
and gases, under ordinary conditions, produce characteristic
spectra of bright lines, by which they may be recognized.
For example, let us replace the incandescent lamp flame
by a non-luminous gas flame, such as is produced when gas
is burned after being thoroughly mixed with air. If we
introduce into this flame a little common salt, it will be
instantly colored a deep yellow. This yellow light, after
transmission through the slit and the prism, will produce
upon the screen a single yellow line at the point Y. A more
powerful instrument would resolve this line into two, placed
very close together on the screen. But for our present pur-
poses we may consider this to be a single line due to the
metal sodium, which in conjunction with chlorine constitutes
common salt. Wherever sodium is present in a state of vapor,
whether in a flame, or between the carbon poles of an elec-
tric arc, or in the atmosphere of the Sun, or in that of the
most distant star, it gives rise to this line, which always lies
at precisely the same point in the spectrum. With suffi-
ciently powerful instruments the line is always double, and
its presence, when accurately determined, is sufficient to
prove the existence of sodium in any luminous source (Fig. 1,
Plate XXII).
Most substances, when their vapors are caused to radiate
in this way, produce more than one colored image of the
slit upon the screen. Thus strontium, when introduced into
50 Stellar Evolution
the flame, gives two red. lines and a strong blue line. Potas-
sium gives a line in the extreme red and another in the
extreme violet. But the essential point to notice is that no
two substances give lines at precisely the same place in the
spectrum. From this we may conclude that the spectra are
entirely characteristic of the various elements, and therefore
that the presence of these elements in a state of vapor can
always be recognized by the detection of their peculiar lines.
The spectra of the elements are of all degrees of com-
plexity, ranging from only two or three lines up to several
thousand. Iron, for example, when turned into vapor in
the electric arc, shows, after analysis by the prism, several
thousand lines, irregularly distributed through all parts
of the spectrum (a few of these are shown in Fig. 2, Plate
XXII). It is evident, therefore, if the lines are to be clearly
distinguished from one another, and so accurately recognized
as to avoid confusing a line of iron, for example, with one
belonging to some other substance, that powerful dispersion
may be necessary; i. e., the various lines must be separated
from one another as far as possible by drawing out the spec-
trum to a great length. This can be done by passing the
light through several prisms in succession, rather than
through a single prism, as in the present instance.
So far we have referred to the spectra of metallic vapors,
rendered luminous in the gas flame or in the electric arc.
In order to obtain the characteristic spectrum of a gas, such
as hydrogen, it may be placed in a tube, and made lumi-
nous by an electric discharge. The best results are ob-
tained after the pressure in the tube has been reduced by
pumping out some of the gas, until the electric discharge
passes quietly and continuously, so that the whole interior
of the tube continues to glow with the light of its gaseous con-
tents. This light, when analyzed by a spectroscope like that
shown in Fig. 2, is found to give lines which are charactei-
Principles of Spectrum Analysis
51
istic of the gas employed. The light of hydrogen in a vacuum
tube, for example, gives precisely the same spectrum as the
light of hydrogen proceeding from one of the great flames
at the edge of the Sun.
We have now considered two types of spectra: (1) the
continuous spectrum, produced when a solid body, a liquid, or
FKt. 2
Kirchhoff's Spectroscope
a highly compressed gas, is rendered white-hot by sufficient
heat; and (2) a hright-line spectrum, consisting of bright
lines, irregularly distributed on a dark background, and de-
rived from the prismatic analysis of the light emitted by
luminous metallic vapors, or gases rendered incandescent
by electric discharges. One other type of spectrum remains
to be mentioned: a dark-line spectrum, such as Kirchhotf
( ibserved and explained when he eflFected his famous analysis
of sunlight at Heidelberg in 1859.
We have already remarked that Fraunhofer had noted
52 Stellar Evolution
the existence of dark lines in the continuous spectrum of the
Sun, and accurately measured their positions, though with-
out understanding their meaning. Kirchhoff, using the four-
prism spectroscope shown in Fig. 2, saw these same dark
lines in the solar spectrum, and succeeded in explaining their
origin. In the yellow part of the spectrum he observed two
strong dark lines, very close together. When the sunlight
was excluded from the spectroscope, and a gas flame contain-
ing sodium vapor was placed in front of the slit, two strong
bright lines, occupying exactly the same positions as the
dark lines of the solar spectrum, were seen in their place.
The flame was then copiously charged with sodium vapor
and retained in its position in front of the slit, the sunlight
being permitted to shine through it. It was immediately
noticed that the two dark lines in the solar spectrum were
considerably darker and more conspicuous when the sunlight
passed through the sodium flame than when it was observed
alone. Furthermore, it was found that when any white light,
producing a continuous spectrum without lines, was allowed
to shine through a flame containing sodium vapor, the effeet
of the flame was to produce two dark lines in the yellow, in
the precise position of this conspicuous pair of dark lines.
Iron, when transformed to luminous vapor in the electric
arc, gave an even more convincing proof that the true expla
nation of the solar spectrum had been found: the bright
lines observed in its spectrum by Kirchhotf and Bunsen
were seen to be represented in the solar spectrum by an
equal number of dark lines, precisely resembling them both
in position and in relative intensity. Magnesium, nickel,
calcium, and other substances gave similar results, and tln'
conclusion was irresistible that all of these substances exist
in the Sun in a state of vapor. It followed from these exper-
iments that the body of the Sun must be an intensely hot
mass, emitting white light, which, if it could be observed
Principles of Spectrum Analysis 53
alone, would give a continuous spectrum, crossed by no lines
of any kind. Surrounding this brilliant white sphere, the
observations proved the existence of a cooler atmosphere con-
taining, in a state of vapor, most of the metals known on the
Earth. These vapors, though cooler than the central body
of the Sun, are nevertheless intensely hot, their temperature
undoubtedly exceeding that of the most powerful electric arc.
Hence, if their light could be observed alone, they would be
seen to give a very complex spectrum of bright lines, in which
all of the lines characteristic of the different elements would
be present. It will be shown later that such a spectrum of
bright lines may be seen at the edge of the Sun, when the
apparatus is so adjusted as to admit only the light of the
chromosphere to the slit of the spectroscope, while excluding
all of the ligrht from the Sun's disk. The bright lines in
this spectrum are less brilliant than the continuous spectrum
due to the more highly heated body of the Sun. Hence,
when observed against the disk, the bright lines, appear
dark by comparison. The cooler metallic vapors were
shown by Kirchhoff's experiments to be capable of absorbing
the same rays which they themselves emit, and the feebler
radiations, emitted by the vapors themselves, produce the
dark lines of the solar spectrum.' It is important to notice
that these so-called dark lines are dark only by comparison,
since it will be explained later that photographs of the Sun
can be taken by the light of any of these lines with the spec-
troheliograph, showing the distribution of the corresponding
element in the solar atmosphere.
It immediately became CArident to students of astrophysics
that the method of analysis initiated by Kirchhoff must prove
immensely powerful in extending their researches. In 1862
Hucrcrins. Secchi, and Rutherfurd commenced their extensive
1 In Fig. 1, Plate XXII, the two bright lines are due to very hot sodium vapor at
the center of the arc. The cooler and less dense vapor in the outer arc produces, by
absorption, the narrow dark lines seen superposed on the bright ones.
54 Stellar Evolution
observations on the spectra of stars, and soon established a
system of types, based upon the examination of the spectra
of several thousand objects. This work has since been greatly
extended through the application of photographic methods,
introduced by Huggins, and applied with marked success by
Draper and many others. In 1868 the spectroscope was
used for the first time to analyze the red flames seen during
total eclipses of the Sun. Not only did it demonstrate their
gaseous nature, but a short time later, through the efforts of
Janssen, Lockyer, and Huggins, it was found possible to em-
ploy the spectroscope to observe the forms of the prominences
in full sunlight.
These and other applications of the spectroscope will be
more fully described in subsequent chapters. Our present
purpose is to explain how the new method, in the hands of
Huggins (Plate XXIII), finally proved beyond doubt that
certain nebulae are to be sharply distinguished from star
clusters.
Sir William Huggins' account of his first spectroscopic
examination of a nebula is recorded in the Publications of the
Tulse Hill Observato7^y, Vol. I:
On the evening of August 29, 1864, I directed the spectroscope
for the first time to a planetary nebula in Draco. I looked into the
spectroscope. No spectrum such as I had expected! A single
bright line only! At first I suspected some displacement of the
prism and that I was looking at a reflection of the illuminated slit
from one of its faces. This thought was scarcely more than
momentary; then the true interpretation flashed upon me. The
light of the nebula was monochromatic, and so, unlike any other
light I had as yet subjected to prismatic examination, could not be
extended out to form a complete spectrum. After passing through
the two prisms it remained concentrated into a single bright line,
having a width corresponding to the width of the slit, and occupy-
ing in the instrument a position at that part of the spectrum to
which its light belongs in refrangibility. A little closer looking
showed two other bright lines on the side toward the blue, all three
Principles of Spectrum Analysis 55
lines being separated by intervals relatively dark. The riddle of
the nebulae was solved. The answer, which had come to us in the
light itself, read: Not an aggregation of stars, but a luminous gas.
With this advance a new era of progress began. The
power of the spectroscope to distinguish between a glowing
gas and a starlike mass of partially condensed vapors estab-
lished it at once in the place it still holds as the chief instru-
ment of the student of stellar evolution. It became apparent
that the unformed nebulae might furnish the material from
which stars are made.
It must not be forgotten, however, that only a small
number of nebulae give a spectrum of bright lines, showing
them to be gaseous. Most of the nebulae, including the
very numerous spiral type, have a continuous spectrum, in
which no lines have yet been detected. As stars are almost
certainly formed from these "white" nebulae, as well as
from the "green" gaseous ones, the theory of stellar evolu-
tion must be broad enough to embrace both types.
CHAPTER VIII
GRATING SPECTROSCOPES AND THE CHEMICAL
COMPOSITION OF THE SUN
The general process employed by KirchhofiP to investi-
gate the chemical constitution of the Sun has already
been described, but it also seems desirable to give an
account of the perfected method used for this purpose in
a modern laboratory. In order to prove that a given sub-
stance exists in the Sun, its lines must be identified with
certainty in the solar spectrum. The spectrum of iron, for
example, contains thousands of lines, and it might easily
happen that through chance proximity many of these lines
would appear to coincide with some of the exceedingly
numerous lines of the solar spectrum. It is evident, there-
fore, that the method of comparison adopted must be such
as to permit of a high degree of precision in measuring the
positions of the lines. In other words, the dispersion of the
spectroscope must be so great as to give a very long spec-
trum, in which the lines are well separated from one anothei-.
Thus their positions can be accurately determined, and there is
no danger of confusion in the case of closely adjacent lines,
which in a less powerful instrument might be seen as one.
The recent great advances in spectroscopy have been due
in very large measure to the success of Rowland in rulin<jf
gratings of high resolving power. In a previous chapter it
was remarked that the dispersion of a spectroscope may be
increased by increasing the number of prisms through which
the light passes. This not only gives a longer spectrum ; it
also increases the resolving power of the instrument, or
its capacity of separating closely adjacent lines. But
56
Grating Spectroscopes 57
through the loss of light bj reflection and absorption, which
becomes very serious when many prisms are employed,
a limit is soon set to the increase in resolving power of
prism spectroscopes. It is for this reason that the grating
has played so large a part in the recent development of
the subject. For the resolving power of a perfect grating
depends only upon the total number of lines it contains, and
the light efficiency, per unit area, may be as great for a
large grating as for a small one.
The production of very powerful spectroscopes, through
the use of large and accurately ruled gratings, is what Row-
land succeeded in accomplishing in his epoch-making work
at the Johns Hopkins University. An optical grating con-
sists of a |X)lished metallic surface, on which many equidis-
tant lines are ruled with a diamond point. Tjhe perfection
of the spectra given by such a grating depends upon the
number of lines it contains and upon the accuracy of their
spacing. The difficulty of Rowland's task will be appre-
ciated when it is remembered that a orratincr must contain
from 10.000 to 20,000 lines per inch, and that errors in the
positions of the lines, amounting to a very small fraction of
the interval between them, would affect the performance of
the grating, tending to blur and confuse the spectra pro-
duced by it.
Gratings that gave very good results were made many
years ago by Rutherfurd, of New York, but it remained for
Rowland to surpass them, both in quality and in size. His
celebrated ruling-engines ( Plate XXIV ) , which are still in
regular use in the underground constant-temperature vaults
of the physical laboratory at the Johns Hopkins University,
depend for their success upon the fact that the screw, which
i< employed to move the grating- plate forward by about
1/15,000 of an inch between successive strokes of the dia-
mond, contains almost no errors. It cannot be said, of course.
58 Stellak Evolution
that the screw is entirely free from error, but the effect of
the exceedingly minute irregularities is almost wholly com-
pensated by ingenious devices that form a part of the rul-
ing-engine. The machine is automatic in its action, and
when set in motion the ruling of a large grating goes on
without interruption for six days and nights before it is
completed.
The gratings manufactured on Rowland's machine have
gone into observatories and laboratories in all parts of the
world, where they have been the principal agents of spectro-
scopic research during the last quarter of a century. Their
great efficiency has caused them to displace prisms from
nearly all spectroscopes in which very high resolving power
is required. As we shall see later, however, the prism still
remains of great importance to the spectroscopist, particularly
in work requiring moderate resolving power, where it gives
a much brighter spectrum than a grating.
Rowland's contributions to spectroscopy were by no means
confined to the manufacture and distribution of optical grat-
ings. In addition to his very extensive researches on the
solar spectrum, and on the spectra of the elements, he
invented the concave grating, which now forms an essential
part of the powerful spectroscopes found in many labora-
tories. Prior to Rowland's time the comparatively few
gratings which had been made were ruled on plane surfaces,
and employed with the ordinary collimator and telescope
of the laboratory spectroscope. That is to say, the prism of
an ordinary spectroscope was removed, and the grating sub-
stituted for it. In such an instrument the rays of light, after
passing through the slit, fall upon the collimator lens, which
renders them parallel. The parallel rays then meet the sur-
face of the grating, where they are diffracted and spread out
into a spectrum. This spectrum is observed or photographed
with the aid of a second lens, which forms its image on the
Grating Spectboscopes
59
retina or on a sensitive plate. A large spectroscope of this
kind, used with the 40-inch Yerkes telescope for spectroscopic
observations of the Sun, is illustrated in Plate XXX.
Rowland showed, from theoretical considerations, that,
if the grating were given a concave spherical surface, the
collimator lens, and the observ-
ing telescope as well, might be
entirely dispensed with. He
also devised the form of mount-
ing for a concave grating illus-
trated in Fig. 3. In the diagram,
a is the slit through which the
light enters, b the concave grat-
ing, and c the eye or photo-
graphic plate. It will be seen
that no lenses enter into the
construction of the apparatus;
for some classes of work this is
a point of great advantage. In
the largest gratings used by Rowland the radius of curvature
of the grating-plate, which is equal to the distance between
the grating b and the photographic plate c, is 21 feet. The
spectrum given by such a grating is many feet in length,
and a portion of the spectrum 20 inches long or longer can
be recorded by a single exposure on the photographic plate.
In order to pass from one part of the spectrum to another,
the grating-carriage b is moved along the rail ab, which
causes the plate-carriage c to move toward or away from the
slit on the rail ac. The whole apparatus is set up on piers
in a dark room, to which no light is admitted except that
which passes through the slit of the spectroscope.
It should be remarked that a grating, unlike a prism,
f produces not merely a single spectrum, but several spectra,
which can be observed successively by moving the carriage c
FIG. 3
Diagram of a Concave Grating
Moanting
60 Stellar Evolution
along the track away from the slit. The first-order spec-
trum lies nearest the slit. The second-order spectrum, twice
as long as the first, which it partially overlaps, lies farther
from the slit. The third and fourth orders, of increasingly
higher dispersion, lie still farther from the slit. Only a por-
tion of the fifth order can be observed with this instrument,
and the higher orders, also beyond reach, are usually too
faint to be of any service.
Let us suppose that we wish to determine with such a
spectroscope whether iron exists in the Sun. To accomplish
this, sunlight must be reflected from the mirror of a heliostat
(driven by clock-work, to maintain the beam in a fixed direc-
tion) to the slit. Between the slit and the heliostat a lens is
introduced, for the purpose of forming an image of the Sun
upon the slit. When the illumination is secured in this way,
the whole grating is filled with light from the diverging rays.
The grating then produces an image of the solar spectrum
upon the photographic plate, where it may be recorded by
giving a suitable exposure.
To facilitate an accurate comparison, the solar spectrum
is photographed side by side on the same plate with the
spectrum of the substance whose presence in the Sun is to
be determined. In order to accomplish this, one-half of the
slit is covered, and the sunlight is admitted through the
other half. Thus the solar spectrum is photographed on
one side of the plate. After this exposure is completed, the
sunlight is shut off, and the screen in front of the slit moved
so as to cover the half previously open, and to uncover the
other half. The image of the Sun on the slit of the spectro-
scope is then replaced by an image of an electric arc light,
burning between two poles of iron. The spectrum of the iron
vapor is thus produced on the plate, and a strip of this
spectrum is photographed beside the strip of solar spectrum.
This is illustrated in Fig. 2, Plate XXII, where the upper
Grating Spectroscopes 61
strip is a small part of the spectrum of iron. It will be seen
by a glance at this photograph that these bright lines of iron
are represented in the solar spectrum by corresponding dark
lines, which accurately match them in position. In Rowland's
work on the solar sj^ectrum thousands of lines were found to
correspond with iron lines given by the electric arc.
The same process can be employed to determine the pres-
ence of other substances in the Sun. In the case of metals,
the electric discharge may be caused to pass between two
metallic rods, or fragments of the metal may be placed in a
hole drilled in one of the carbons of an ordinary electric arc-
lamp. In the latter case the spectrum of carbon, and also of
the impurities which the carbon poles always contain, will
appear on the plate with the spectrum of the metal in ques-
tion. But these extra lines may always be identified, and
usually give no trouble. The identification of the solar
lines, however, is not always so simple as in the case of iron.
Many substances are doubtfully represented in the Sun by
only a small number of lines, and it is sometimes very
difficult to decide whether a few apparent coincidences are
sufficient to warrant one in drawinor definite conclusions.
The matter is usually determined by ascertaining whether
certain well-known groups of lines, which for various reasons
are considered to be especially characteristic of an element,
are actually represented. If these groups are absent, an
apparent coincidence with certain less characteristic lines
belonging to the same element should be regarded with
suspicion. In the case of gases, the comparison is effected
by the aid of vacuum tubes, in which the gas, usually at low
pressure, is illuminated by an electric discharge. Thus the
lines given by a hydrogen tube in the laboratory have been
shown to coincide in position with lines ascribed to hydrogen
in the Sun.
After many years of study of the solar si^ectrum by these
62 Stellar Evolution
methods, Rowland reached the conclusion that the chemical
composition of the Sun closely resembles that of the Earth.
Certain elements, such as gold and radium, iodine, sulphur,
and phosphorus, chlorine and nitrogen, have not been
detected in the Sun. But this does not prove that they are
certainly absent, as their level in the solar atmosphere, or
the low degree of their absorptive effects might prevent
them from being represented. On the other hand, various
substances, not yet found on the Earth, are shown by many
unidentified lines of the solar spectrum to be present in the
Sun. Some, if not all, of these, will probably be discovered
by chemists, just as helium was found by Ramsay in cleveite
(p. 78). Rowland remarked that if the Earth were heated
to a sufficiently high temperature, it would give a spectrum
closely resembling that of the Sun.
The most perfect maps of the solar spectrum are those of
Rowland and Higgs. These are enlarged from photographs
made with the concave grating, and contain an immense
number of lines. Both maps extend into the extreme ultra-
violet spectrum (the invisible region beyond the violet), and
that of Higgs includes a considerable region of the infra-red
(also invisible to the eye) where photographic plates sensi-
tized for red light with alizarin blue or other dyes must be
employed. Both maps are provided with scales of wave-
length, so that the approximate positions of the lines can
be read off at once. The precise positions of all solar lines
photographed by Rowland are given in his Preliminary
Table of Solar Spectrum Wave-Lengths, which records the
places of about 20,000 lines. This table, although known
to contain some small errors, is at present employed by all
spectroscopists as the standard of reference. It gives Row-
land's identifications of the solar lines, but about two-
thirds of the lines have not yet been referred to any known
element. Recent investigations of the spectra of various
, Grating Spectroscopes 63
metals will no doubt permit a considerable number of these
lines to be identified.
In any examination of the solar spectrum the observer
cannot fail to be struck by the changing appearance of the
lines in certain regions. In the yellow part of the spectrum,
for example, near the well-known D lines of sodium, the
most casual examination will show surprising variations in
the intensity of the countless lines which are frequently
conspicuous here. How great the change is may be seen
in Plate XXV, which is a reproduction of two photographs
of this part of the spectrum taken under different conditions.
The lines which thus change in intensity are called teUuric
lines, since they are due to the absorption of the gases in
the Earth's atmosphere. The region illustrated in Plate
XXV contains a large number of lines due to water vapor.
Since the amount of water vapor undergoes great variations,
it is natural that the intensities of the lines should change
accordingly.
All of the telluric lines are most conspicuous in the
sj^ectrum of the Sun when it is near the horizon, since in this
case the light traverses a very great depth of atmosphere
before it reaches the spectroscope. Photographs of the
spectrum of the high and low Sun might therefore be
expected to show marked differences in the intensity of the
telluric lines. This is actually the case, and the method
therefore affords one means of identifying lines due to the
absorption of our atmosphere. The oxygen in the air pro-
duces two similar groups (A and B in Fraunhofer's original
designation of the solar lines) which lie at the extreme red
end of the solar spectrum. Comu observed these same
'"'■nips in the spectrum of an electric light at the summit of
Eiffel Tower, as seen from the Ecole Polytechnique in
Paris, at a distance of about 2.7 miles.
An ingenious method was employed by Comu to distin-
64 Stellar Evolution
guish the telluric lines from those due to absorption in the
Sun's atmosphere. According to Doppler's principle, the
lines in the spectrum of the east limb of the Sun must be
displaced toward the violet (by motion of approach), and
those from the west limb toward the red (recession), since the
Sun is rotating on its axis in a period of about twenty-five
days. It occurred to Cornu that this fact might give a very
delicate means of picking out the telluric lines, since only the
lines of solar origin can be displaced by the Sun's rotation,
while those due to absorption in the Earth's atmosphere
remain in their normal positions. He formed a small image
of the Sun on the slit of his spectroscope, by means of a lens
which could be made to oscillate rapidly, thus causing the east
limb and the west limb of the Sun's image to fall alternately
upon the slit. If the spectrum is observed while the image
is oscillating, the lines of solar origin will be seen to move
rapidly to and fro through a short distance, while the telluric
lines will remain fixed. This method was successfully em-
ployed by Cornu in an important study of the telluric lines.
Other investigations of these lines, which have resulted in
the production of extensive maps, have been made by Thollon
(continued by Spee), Becker, and McClean. In these inves-
tigations the telluric lines were distinguished by observations
of the spectrum of the high and low Sun.
If passage of sunlight through our atmosphere is thus
competent to produce dark lines in the solar spectrum, it is [
evident that the sunlight reflected from a planet should show i
evidence of its double transmission through the planet's
atmosphere. This method is actually employed to determine
the presence and the composition of the atmospheres of the |
planets.
Remarkable as was Rowland's success in the manufacture ,
of gratings, and the measurement of wave-lengths with their
aid, it has recently been surpassed by Michelson. With the :
Gbatixg Spectroscopes 65
interferometer, an instrument of his invention, Michelson
has established the length of the standard meter of the Inter-
national Bureau of Weights and Measures, in terms of light-
waves. This fixes, with the greatest precision, the wave-
length of certain lines in the spectrum of cadmium, and these
wave-lengths were adopted at the Oxford meeting ( 1905) of
the International Union for Co-operation in Solar Research
as primary standards, on which a new system of wave-
lengths, to replace Rowland's system, will be based. Through
his invention of the echelon, Michelson has realized a new
form of grating, composed of a series of glass plates, precisely
equal in thickness, piled one on another like a flight of steps,
through which a parallel beam of light is transmitted. The
spectra thus produced are of a very high order, and the
resolving power surpasses that of Rowland's largest gratings.
The echelon spectroscope thus furnishes the means of analyz-
ing compound lines, the members of which lie so close to-
gether that they cannot be separated with other instruments.
The latest success achieved by Michelson, however, opens
up still greater possibilities in sj^ectroscopy. The echelon
can be used only for the study of narrow and sharply defined
lines; its application is therefore limited to certain special
problems. For more general work, both in the laboratory
and in the solar observatory, very large gratings, of high
resolving power, are required. Six-inch gratings (ruled on
a disk of speculum metal 6 inches in diameter ) were success-
fully made by Rowland. After several years of labor
Michelson has completed a ruling-machine with an almost
-perfect screw, on which he has already made 8-inch and 10-
inch gratings. He hopes to produce a llr-inch grating, the
largest for which his machine is designed. There is reason
believe that his plan for constructing a ruling-machine
with four screws, which should reduce the error to one-fourth
its amount in a sinsrle screw machine, would result in the
66 Stellar Evolution
production of good 20-iTich gratings. The enormous impor-
tance of such gratings, in their application to the study of tlie
Sun, will become clearer as we proceed. One difficulty to
be overcome will be recognized when it is remembered that
a 20-inch grating, having 12,500 lines to the inch, would
contain more than 2,000,000 lines, each about 10 inches long.
The microscopic diamond crystal, used to cut all these lines
in the hard surface of the speculum metal, must not break, or
change its form appreciably, during the entire period of the
work.
It is satisfactory to add that Jewell has recently coii-
structed a new ruling machine at the Johns Hopkins Univer-
sity which appears likely, from preliminary tests, to be supe-
rior to Rowland's. We thus have good reason to hope that
the best existing photographs of the solar spectrum will soon
be surpassed.
CHAPTER IX
PHENOMENA OF THE SUN'S SXJRFACE
The results described in the last chapter relate to the
light of the Sun as a whole, and not to the details of its sur-
face phenomena. In most of the investigations there de-
scribed similar results might be attained if the Sun were
removed to the distance of the nearer stars. In that case it
would no longer be possible, even with the most powerful
telescope, to detect an appreciable disk, and the solar image
would be reduced to a microscopic point, brilliant enough,
however, to afford suflBcient light for spectroscopic examina-
tion. But it has already been pointed out that investigations
of the Sun acquire their greatest importance through the
comparative proximity of this star to the Earth. All other
stars are so far away that no distinction can be drawn be-
tween the radiations characteristic of different parts of their
disks. The spectroseopist must therefore be content to
observe in such cases a composite spectrum, produced by the
superposition of the spectra of the various surface phenom-
ena. The Sun, on the other hand, is so near us that its
image at the focus of a powerful telescope may have a diam-
eter as great as 7 inches, or even greater.^ Consequently,
the light from any point in this image, corresponding to a
small area of the solar surface, can be studied by itself. Our
extensive knowledge of the Sun, except that which has been
derived from an examination of its light as a whole, is based
upon this fact.
The appearance of the Sun in a telescope is illustrated by
Plate II, which is a reproduction of a direct photograph.
1 The actual diameter of the Sun is aboat 860.000 miles.
67
68 Stellar Evolution
The Sun's light is too brilliant to permit of visual observa-
tion without some method of reducing its intensity. The best
means of accomplishing this is by the aid of the polarizing
helioscope, which is attached just in front of the eye-
piece of the telescope. The cone of .light from the object-
glass meets a plane surface in the helioscope, from which it
is reflected at an angle such as to polarize the rays. As is
well known, the amount of plane polarized light which can
be reflected from a second surface depends upon the angle
at which the rays meet this surface. Consequently, by
rotating the reflecting prism the amount of light which
reaches the eye can be varied at will, thus producing an
image of any desired brightness. When protected by this
device, the eye of the observer of solar phenomena is sub-
jected to even less strain than is frequently experienced in
work on fainter objects.
A casual glance at the solar image is sufiicient to show-
that it is much darker near the circumference than at the cen-
tral part of the disk. This falling-off in brightness toward
the limb is probably due to the absorption of a smoke-like
envelope, which completely incloses the Sun. The absorj)-
tion is so marked that near the circumference of the Sun only
about 13 per cent, of the violet rays escape. For the blue,
green, and yellow rays the percentage of transmitted light
increases progressively, until it amounts to about 30 i)er cent,
for the red. It has therefore been concluded that, if this
absorbing atmosphere were removed, the color of the Sun
would appear blue, since the intensity of the violet rays
would be about two and one-half times as great as at present,
while the red rays would be only about half again as bright
as they are now.
The visible phenomena of the Sun's disk include the sun-
spots and the faculae. The general appearance of sun-spots,
when seen with a low magnifying power, is shown in Plate II.
Phenomena of the Sun's Surface 69
- Under perfect atmospheric conditions, a large sun-spot, when
observed with a {X)werful telescope, would more closely
resemble Plate XXVI, which is reproduced from a drawing
made bv Langley. The best solar observers agree that this
drawing is one of the most perfect representations of spot
structure yet obtained. The long narrow filaments, which
constitute the penumbra of the spot, reach in toward a dark
central region, called the umbra. It must be remembered
that the darkness of the umbra is only relative: if observed
alone, and not in contrast with the more brilliant surround-
ings of the photosphere, the great brilliancy of the umbra,
surpassing that of the most powerful electric arc-light, would
be evident. Knowledge of this fact has been quite suf-
ficient to set at rest the old notion that sun-spots are
merely rents in a brilliant cloud-covering of the Sun, through
which a dark and cool interior may be seen.
According to Langley "s view, the filaments which, taken
together, constitute the penumbra are everywhere present
on the solar surface. He reofarded them as resemblinof the
stalks of a wheat-field, seen on end in the undisturbed pho-
tosphere, and revealing more of their true characteristics in
the penumbra, where they are bent over and drawn out
toward the central part of the spot. Langley believed that
we are observing columns of luminous vapors rising from the
Sun's interior, the seats of convection currents which brins:
>; to the surface the immense supplies of heat radiated by the
Sun into space. Separating these luminous columns are
darker regions, characterized by a lower degree of radiation.
Such minute details can be recorded only with the greatest
difficulty. Under ordinary atmospheric conditions the solar
'; image is not seen as a sharp and well-defined object, but its
i;' details are continually blurred by the effect of irregularly
f heated currents in our atmosphere. Even under the best
conditions the moments of very sharp definition are few. and
70 Stellae Evolution
the greatest patience and perseverance are reqiiired. on the
part of an observer who would record his impressions of the
solar structure. At the best, drawings based upon visual
observations must be unsatisfactory, since even the skilled
hands of Langley could not secure the perfect precision
which is so desirable. It accordingly might be hoped that
here, as in other departments of solar research, photography
would afford the necessary means of securing results unat-
tainable by the eye. Unfortunately, however, this hope has
been only partially realized, as a brief consideration of the
best results in this field will show.
It is a comparatively simple matter to make a direct
photograph of the Sun. It is only necessary to form a solar
image, considerably enlarged, upon a "slow" photographic
plate, and then to give an excessively short exposure by
means of a shutter containing a narrow slit, which is shot
across just in front of the plate at very high speed. The light
from any part of the Sun reaches the plate only during the
brief interval in which the slit is passing the corresponding
part of the image. The exposure for any point may thus
amount to no more than rirroirF ^^ ^ second. The photo-
graph reproduced in Plate II was taken in this way.
In order to obtain photographs showing the smaller
details of the photosphere, it is desirable to use a solar
image enlarged to a diameter of from 15 to 30 inches, with
photographic plates particularly adapted for this class of work.
The best direct photographs hitherto made are those taken
by Janssen at the Observatory of Meudon, near Paris, A
portion of one of these pictures, representing the great Sun-
spot of June 22, 1885, is reproduced in Plate XXVII. The
penumbra is not very well shown, since the exposure required
for the brighter regions of the surrounding photosphere is
too short to bring out its fainter details. Even with sufficient
exposure, however, such photographs do not reveal the more
Phenomena of the Sun's Surface 71
delicate details recorded in Langley's drawings. But they
do show, with considerable success, the minute structure of
the photosphere, as Plate XXVII illustrates. Here may be
seen, autographically recorded, the photospheric "grains"
which Langley believed to be the extremities of long fila-
ments reaching down toward the interior of the Sun. Jans-
sen holds a different view, since he regards the brigrht o^rains
to be small spherical masses of luminous vapor, separated by
vacant regions. In chap, xi it will be shown that the results
of recent investigations with the spectroheliograph tend to
bear out Langley's view.
There can be little doubt that direct photographs of the
Sun, showing smaller details than have vet been registered,
will ultimately be obtained. Janssen's photographs have all
been secured with a small instrument, used in an atmosphere
where the conditions are not particularly favorable for work
of this character. It is therefore to be hoped that, with much
more powerful apparatus, employed in a better atmosphere,
the results would be still more satisfactory.
In spite of long study and much discussion, it remains
uncertain whether sun-spots are to be regarded as cavities or
as elevated regions of the photosphere. At one time it was
supposed, mainly as the result of observations made by
Wilson, of Glasgow, in the eighteenth century, that sun-spots
were saucer-shaped cavities, the penumbra representing the
sloping edge of the saucer, with the umbra at the center.
More recent investigations, however, have failed to confirm
Wilson's observations, though there can be little doubt that
the umbra lies below the level of the faculae that usually sur-
njund spots. Faculae are elevated regions of the photo-
sphere, and the question remains open whether the level of the
umbra is above or below the average level of the photosphere,
outside of the faculae.
The only other phenomena visible in direct observations
72 Stellar Evolution
of the Sun are the faculae. They are usually most numerous
in the vicinity of Sun-spots, and near the Sun's limb they
are sometimes very conspicuous brilliant objects, covering
large areas. Near the center of the Sun, however, they
are practically invisible, though faint traces of them can
sometimes be made out on photographs taken with a suitable
exposure. This increase of brightness toward the Sun's limb
is assumed to be due to the elevation of the faculae above the
photospheric level, and their escape from a considerable part
of that absorption which so materially reduces the brightness
of the photosphere. Rising above the denser part of the
absorbing veil, and thus suffering but little diminution of
light, they appear near the Sun's limb as bright objects
on a less luminous background.
Janssen's photographs tend to bear out the assumption
that the faculae resemble the rest of the photosphere, differ-
ing mainly in their greater altitude. They are shown by
these photographs to be resolved into granular elements
similar to those that constitute the photosphere. It will be
seen later, however, that the faculae play a very important
r6le, since they are the regions from which immense masses of
vapors rise to the solar surface. These vapors are invisible
to the eye, and no trace of them is shown on photographs
taken in the manner described above. But they may be
photographed with the spectroheliograph, by the method
explained in chap. xi.
CHAPTER X
THE SUNS SURROUXDIXGS
The first observations of the Sun's surroundings date back
to an early period. On the occasion of a total eclipse the
dark body of the Moon covers the solar disk, cuts off the
sunlight which at other times illuminates our atmosphere,
and reveals phenomena ordinarily hidden by its glare. It is
well known that, if our atmosphere were absent, there would
Ije no such scattering: of the sunlight, and the skv would be
as dark during the day as it now appears at night. In such
a case the stars would be visible by day, as well as the solar
corona. Formerly, when no artificial means of reducing this
brilliant illumination of the atmosphere were known, all
knowledge of celestial phenomena in the immediate vicinity
of the Sun was of necessity obtained during total eclipses.
The solar corona was thus discovered, and likewise the red
flames, or prominences, which do not extend so far from the
Sun's ~;urface.
The corona still remains a mysterious phenomenon, since
no means has yet been discovered of observing it without an
eclipse. Our knowledge is thus confined to the results of
observations made during the very brief periods when the
Moon shields our atmosphere from illumination by the sun-
light. The general appearance of the corona, as seen at
the eclipse of May 28, 1900, is illustrated in Plate IV,
reproduced from a photograph made by Barnard and
Ritchey. It may be described as a faintly luminous veil
of light, extending outward in long streamers from the sur-
face of the photosphere to distances of several millions of
miles, and exceeded in brilliancy, even in its brightest
73
74 Stellar Evolution
parts, by the full Moon. In many ways its streamers re-
semble those of the aurora borealis, and it is indeed possible
that their origin may be ascribed to some similar electrical
cause. During the few minutes of a total eclipse they are
not seen to undergo change of form, but the outline of the
corona does vary greatly from year to year, in sympathy
with the general variation of the solar activity described in
another chapter. At times of minimum sun-spots the form of
the corona resembles that shown in Plate IV. This minimum
type is marked by great winglike extensions along the solar
equator, and by much shorter streamers, diverging like fans
near the Sun's pole. At times of maximum sun-spots the
corona is much more extensive in the polar regions, the
streamers equaling in length those of the equatorial zone.
Spectroscopic observations have shown that the corona con-
sists mainly of gases unknown to the chemist. That is to
say, the lines in its spectrum do not coincide in position with
the lines of any terrestrial element. Whether these gases,
which are probably very light, will ultimately be found on
the Earth cannot be predicted. Like helium, fii-st known in
the Sun, they may eventually be encountered, in minute
quantities, in some mineral, where they have hitherto escaped
the chemist's analysis. The fact that the lower part of the
corona gives a continuous spectrum, with a feeble solar spec-
trum superposed upon it, indicates that minute incandescent
particles are present, which are hot enough to radiate white
light, and which scatter enough sunlight to account for the
presence of the solar spectrum.
It may now be of interest to explain how the solar corona
is photographed during a total eclipse of the Sun, especially
as the same means are employed during eclipses in photo-
graphing the solar prominences, and also because we shall
have occasion in a subsequent chapter to refer more at
length to the general type of telescope here represented.
The Sun's Scbboundixgs 75
It has already been explained that the size of an image of
the Sun given by a telescoj^ie depends directly upon the focal
length of the lens employed. In order to show as distinctly
as possible the more minute phenomena of the corona, it is
therefore desirable to obtain large-scale photographs of it
with a telescope of great focal length. Obviously such an
instrument as the 40-inch Yerkes refractor could not easily
be transported to the more or less remote regions of the
Earth where the passing shadow of the Moon may render a
total eclipse visible. Fortunately, however, the size of the
focal image does not depend upon the diameter of a lens, but
merely upon its focal length. Hence the desired result can
be obtained by using a long-focus lens of comparatively
small diameter. In some cases such lenses are jxjinted
directly at the Sun, and the motion of the solar image,
caused by the Earths rotation, is compensated for by a cor-
responding motion of the photographic plate on which the
image falls. Another method, which offers various points of
advantage, is illustrated in Plate XXVIII, reproduced from
a photograph of the horizontal telescope used by the eclipse
party of the Yerkes Observatory at Wadesboro. North Caro-
lina, on May 28, 1900.
The essential feature of this instrument is a plane mirror,
12 inches in diameter, which reflects the Sun's rays hori-
zontally through a long tube. The plane of the mirror is
parallel to the Earth's axis, and, by means of an accurate
driving-clock, the mirror is made to complete a rotation once
in 48 hours. Such a motion of the mirror is just sufficient to
counteract the effect of the Earth's rotation, and thus to keep
the Sun's rays reflected in the same direction for an indefinite
. period. After leaving the coelostat mirror, the rays fall upon
> a 6-inch photographic lens, which forms an image upon a
f sensitive plate at its focus. 61i feet away. Through the
rotation of the mirror the imagfe is maintained at a fixed
76 Stellar Evolution
position upon the photographic plate, so that any desired
exposure may be given.
With this apparatus some remarkably fine photographs of
the corona and prominences were obtained by Barnard and
Ritchey, of the Yerkes Observatory party (Plate XXIX).
During the 87 seconds of the eclipse seven exposures were
made, ranging in length from ^ second to 30 seconds. Several
of the photographic plates used were 25 X 30 inches in size.
To facilitate rapid handling, they were mounted on a wooden
carrier 15 feet long, free to move on ball bearings on steel
rails extending at right angles to the tube through the en-
tire length of the photographic house. A catch, operated by
hand, served to stop the plate -carrier at the proper position
for each exposure.
The solar prominences are seen at total eclipses of the
Sun, projecting like red flames beyond the dark edge of the
Moon (Plate XXIX). With our present knowledge of these
phenomena, it seems hardly possible that just prior to the
middle of the last century they were regarded by sonic
observers as the illuminated summits of lunar mountains.
Their truly solar origin was conclusively demonstrated in
1860, when they were photographed by Secchi and de la
Rue, and were shown not to share the motion of the Moon.
At that time, however, no conclusions could be drawn as to
their chemical composition, and it was not until 1868 that
their gaseous nature and their connection with the Sun
became known through the' use of the spectroscope. It
was then found that these immense masses of hydrogen and
helium gas rise from a sea of flame (the chromosphere, which
completely envelops the Sun), and sometimes attain eleva-
tions of hundreds of thousands of miles.
The rarity and brief duration of total eclipses would have
limited greatly our knowledge of the prominences, had not
Janssen, Lockyer, and Huggins devised an epoch-making
The Sun's Surroundings
77
method by which they can be observed on any clear day, in
spite of the glare of our atmosphere near the Sun. The
instrument which permits this result to be accomplished is
the spectroscope, used
in conjunction with a
telescope. The prin-
ciple of the method is
simple and easily un-
derstood. The white
light of the sky, when
passed through the
spectroscope, is drawn
out into a long rain-
bow band, and there-
by greatly reduced in
intensity. The light
of the prominences,
on the contrary, is
concentrated in the radiations characteristic of hydrogen and
helium gas. and the dispersing |X)wer of the spectroscope
merely separates more and more widely the colored images
which correspond to these radiations, withont seriously en-
feebling them. With the spectroscope they therefore become
visible, since their images are brighter than the highly dis-
persed background of skylight on which they lie.
Plate XXX shows a solar spectroscope suitable for ob-
serving the spectra or the forms of the prominences in full
sunlight.' This spectroscope consists essentially of a slit
(s in the accompanying diagram, Fig. 4), which may beset
tangentially or radially upon the Sun's limb; a collimating
. lens. /. which renders parallel the rays coming to it from
,lthe slit; a plane grating, g, ruled with about 15,000 lines to
FIG. 4
Dia^am of Solar Spectroscope
• This spectroscope, here shown attached to the Yerkes telescope, was formerly
used as a spectrohelio^apb with the Kenwood telescope (Plate XXXlVj.
78 Stellar Evolution
the inch; and a second lens and eye-piece, / and e, which
form the observing telescope. The grating diffracts the light
which reaches it from the coUimating lens, and produces a
spectrum, an image of which is formed by the lens t, in the
focal plane of the eye-piece e. If it is desired to photograph
the spectrum, the eye-piece may be replaced by a sensitive
plate.
If we wish, for example, to observe the spectrum of the
chromosphere with this instrument, the slit, about 1/1,000
of an inch wide, is made exactly tangential to the solar image.
Under these circumstances the observer at the instrument
will see the spectrum of the bright sky near the Sun, which
is of course merely the spectrum of reflected sunlight, and is
therefore crossed by all of the dark Fraunhofer lines. In the
case of substances which are present in the chromosphere, the
lines of the spectrum will be reversed from dark to bright in
regions which correspond to the section of the chromosphere
lying upon the slit. The most conspicuous bright lines to be
observed in this way are the hydrogen lines Ha (red), H(3
(blue-green), Hj (blue), and H8 (violet), and the brilliant
yellow helium line D^.
The history of this helium line affords an interesting
illustration of the. intimacy of the relationship which now
unites terrestrial and solar chemistry. In his first observa-
tions of the spectrum of the prominences, made in 18G8,
Lockyer was attracted by the presence of a bright line in the
yellow, not far from the position of the D, and Dg lines of
sodium. This line was designated as D.^, but all attempts to
identify it among the lines of known elements were unsuc-
cessful. Accordingly, it was assumed to represent a new gas,
probably very light, on account of its association with hydro-
gen at great elevations above the solar surface. Lockyer
gave the name "helium" to this gas, because of its solar
origin. In 1895 Ramsay, while engaged in an analysis of
The Sun's Subboundings 79
I the mineral cleveite, discovered an unknown gas, which he
found to give a yellow line near the position of D.j. The
spectroscope he employed was not powerful enough to deter-
mine the position of the line with great accuracy, but Runge
proved beyond a doubt, a short time later, that the line was
actually in the position of D3. However, he detected a faint
companion to this line, on the side toward the red, which
had never been observed in the solar prominences. An exam-
ination of D3 in the prominence spectrum, made at the Ken-
wood Observatory immediately upon the receipt of Runge's
description of his laboratory results, showed the undoubted
presence of a similar companion, which was found by repeated
measures to agree well in position with Runge's determina-
tions. The companion was so faint that it would easily escape
observations made without knowledge of its existence. It
may be said, however, that this first observation was greatly
facilitated by the presence of a very bright prominence, in
which D3 was beautifully shown. Huggins and others de-
tected the duplicity of the line about the same time.
As was anticipated by its behavior in the Sun, helium was
found to be the lightest of all known gases, except hydrogen.
Further study of the spectrum showed Dg to be only one of
a series of lines, other members of which are also represented
in the chromosphere. Many lines characteristic of the spectra
of "Orion" stars, which had not been identified before Ram-
say's discovery, are also due to helium. It is extremely prob-
able that other new elements, not yet discovered on the Earth,
are represented by some of the unknown lines of the solar
spectrum.
While the lines of hydrogen and helium are more bril-
liant and conspicuous than all others in the visible spectrum
of the chromosphere, it is nevertheless true that a very large
f number of lines due to other elements can be seen on any
good day with a powerful telescope and spectroscope. The
80 Stellab Evolution
vapors of magnesium, iron, and several other substances are
conspicuously represented by bright lines in the chromo-
spheric spectrum; but these lines are shorter than those of
hydrogen and helium, since the vapors do not rise to so great
a height. With the Yerkes telescope it is even possible to
observe a multitude of fine bright lines due to the vapor of
carbon, which lies in close contact with the photosphere.
The layer of carbon vapor is so thin that the least motion
of the solar image, or a very slight disturbance of the atmos-
phere, are sufficient to render the lines invisible.
A total eclipse affords a most favorable opportunity to
determine photographically the depths of the several layers.
The simplest way of accomplishing this is to place a prism
over the object-glass of a telescope, which is directed toward
the Sun, When, at the moment of totality, the Moon covers
the photosphere, arcs of the chromosphere are left projecting
beyond the Moon's edge. After passing through the prism,
the image formed on the photographic plate will appear like
that reproduced in Plate XXXI, which was taken by Lord at
the eclipse of 1900. The arcs represented here correspond
to the various lines in the spectrum of the chromosphere.
In this case, however, since no slit was used in the spectro-
scope, the form of each arc is defined by the distribution of
the corresponding vapor. If a prominence is present at any
point, its image will be repeated in each of the arcs re[)re-
senting the element it contains.' Of course, this "spectrum
of the flash," first observed by Young, and so called on account
of its brief duration, can be photographed only during the few
seconds while the Moon's edge is passing over the chromo-
sphere.
It will now be seen more clearly how the forms as well as
the spectra of the prominences can be observed by the spec-
1 The prominence group shown in Plate XXIX is faintly represented here
(reversed in position) in each of the two stronger arcs.
The Sun's Subbouxdings 81
troscopic method without an eclipse. So long as a narrow
slit is employed, the spectrum will consist of narrow lines,
havintr the same form as the slit. That is, if the slit be
straigrht. the lines will be short, straight sections of the
chromosphere or prominences, corresponding in width to the
slit. If the slit be curved, the lines will have a corresponding
curvature. In other words, the lines are simply monochro-
matic images of the slit. Hence, if the slit be widely opened,
the lines will assume the form of that ix)rtion of the chromo-
sphere or prominence which happens to lie across it. It is
as though one were looking out through a narrow window
u|X)n a mass of great flames.
The application of the spectroscopic method to the study
of the chromosphere and prominences marked a new era in
solar research. Daily observations were inaugurated with
great enthusiasm by Lockyer, Young. Janssen, and other
astronomers in Europe and America. It was found that the
prominences could be divided into two classes — quiescent,
or cloudlike, and eruptive. The former are much the more
numerous, and may always be seen, in larger or smaller num-
bers, at the Sun's limb. They change slowly in form, and
sometimes persist for days, or until carried out of view by
the solar rotation. When seen under excellent atmospheric
conditions, the complex details of their structure resemble
those of the clouds in our own atmosphere. The eruptive
prominences change very rapidly in appearance, sometimes
shooting up to elevations of over two hundred thousand miles
in a few minutes (see Plates XXXV and XXXVI). Like the
quiescent forms, they are most numerous at times of greatest
sun-spot activity. They are never observed in very high
latitudes, though the quiescent prominences appear at all
parts of the solar circumference. The photographic study
of these phenomena will be described in the next chapter.
CHAPTER XI
THE SPECTROHELIOGRAPH
The spectroscopic method, as applied by astrophysicists
in various parts of the world, has yielded a nearly continuous
record of the solar prominences extending back over more
than "thirty years. For many purposes such a record is
entirely satisfactory, and permits important conclusions to
be drawn. But the process of observation is not only slow
and painstaking : it is subject to the errors and uncertainties
that necessarily attend the hand delineation of any object,
seen through a fluctuating atmosphere. Moreover, changes
in the forms of eruptive prominences are frequently so rapid
that the draughtsman cannot record them. It was principally
in the hope of simplifying the process of observation, and of
rendering it more rapid and more accurate, that the spectro-
heliograph was devised at the Kenwood Observatory in
1889.'
The principle of this instrument is very simple. Its
object is to build up on a photographic plate a picture of
the solar flames, by recording side by side images of the
bright spectral lines which characterize the luminous gases.
In the first place, an image of the Sun is formed by a tele-
scope on the slit of a spectroscope. The light of the Sun, after
transmission through the spectroscope, is spread out into a
long band of color, crossed by lines representing the various
elements. At points where the slit of the spectroscope hap-
pens to intersect a gaseous prominence, the bright lines of
hydrogen and helium may be seen extending from the base
1 It was subsequently learned that the method embodied in the spectrohelio-
j?raph had been suggested by Janssen as early as 1869, reinvented by Braun of
Kalocsa, and actually tried by Lohse at Potsdam. But it had not proved a success.
82
The Spectboheliogbaph 83
of the proipinence to its outer boundary. If a series of such
lines, corresponding to different positions of the slit on the
image of the prominence, were registered side by side on a
photographic plate, it is obvious that they would give a rep-
resentation of the form of the prominence itself. To accom-
plish this result, it is necessary to cause the solar image to
move at a uniform rate across the first slit of the spectro-
scope, and, with the aid of a second slit (which occupies the
place of the ordinary eyepiece of the spectroscope) , to isolate
one of the lines, permitting the light from this line, and
from no other portion of the spectrum, to pass through the
second slit to a photographic plate. If the plate be moved
at the same speed with which the solar image passes across
the first slit, an image of the prominence will be recorded
upon it. The principle of the instrument thus lies in photo-
graphing the prominence through a narrow slit, from which
all light is excluded except that which is characteristic of the
prominence itself. It is evidently immaterial whether the
solar image and photographic plate are moved with respect
to the spectroheliograph slits, or the slits with respect to a
fixed solar image and plate.
This method, when first tried at the Harvard Observa-
tory in 1890, proved unsuccessful. The lack of success
was pa tly due to the fact that a line of hydrogen was
employed. This line, though fairly suitable for the pho-
tography of prominences with the perfected spectrohelio-
graph of the present day, was too faint for successful use
amidst the difficulties which surrounded the first experi-
ments. Accordingly, when the work was resumed a year
later at the Kenwood Observatory in Chicago (Plate XXXIII)
an attempt was made, through a photographic investigation
of the violet and ultra-violet regions of the prominence spec-
trum, to discover other lines better fitted for future experi-
ments. In the extreme violet region, in the midst of two
84 Stellar Evolution
broad dark bands which form the most striking feature of the
solar spectrum, two bright lines (H and K) were found and
attributed to the vapor of calcium. They had previously
been seen visually by Young, but, on account of the insensi-
tiveness of the eye for light of this color, they could not be
observed satisfactorily. A careful study soon showed them
to be present in every prominence examined, at elevations
above the solar surface equaling or exceeding those attained
by hydrogen itself (Plate XXXII, «). Their suitability for
the purpose of prominence photography is due to several
causes, among which may be mentioned their exceptional
brilliancy, their presence at the center of broad dark bands
which greatly diminish the brightness of the sky spectrum,
and the comparatively high sensitiveness of photographic
plates for light of this wave-length.
While fairly efficient from an optical point of view, the
spectroheliograph of the preceding year had possessed many
mechanical defects. It sufficed to give photographs of
individual prominences, but they were not very satisfactory.
In a new instrument, devised for use with the 12-inch Ken-
wood telescope, the principal defects were overcome, and
means of securing the necessary conditions of the experi-
ment were provided. The Kenwood spectroheliograph is
shown in Plate XXXIV. In this instrument the solar image
and photographic plate were fixed, while the first and second
slits were made to move across them by means of a system
of levers, set in motion by hydraulic power. The first trials
of the instrument, made in January, 1892, were entirely
successful, and the chromosphere and prominences surround-
ing the Sun's disk were easily and rapidly recorded (Plates
III, XXXV, and XXXVI). The details of their structure
were shown with the sharpness and precision characteristic
of the best eclipse photographs. And the opportunity for
making such records, previously limited to the brief dura-
The Spectboheliograph 85
tion, never exceeding seven minutes, of a total eclipse, was
at once indefinitely extended. Thus it became possible to
study photographically the slowly varying forms of the qui-
escent, cloudlike prominences, and, to particular advantage,
the rapid changes of violent eruptions.
But even before this primary purpose of the work had
been accomplished, the possibility of making another and
much more important application of the instrument had
presented itself. A photographic study of the spectrum of
various portions of the Sun's surface had shown the existence
at many points of great clouds of calcium vapor, luminous
enough to render their existence evident through the produc-
tion of bright H and K lines on the solar disk (Plate XXXII,
h and c). Some of these calcium clouds had, indeed, been
known to exist through the important visual observations of
Young:, who had observed the briojht H and K lines in the
vicinity of sun-spots. But the vast extent and the charac-
teristic forms of the phenomena could not be ascertained
by such means. What was required was such a repre-
sentation of the solar disk as the spectroheliograph had been
designed to give in the case of the prominences. From a
consideration of the results obtained in the spectroscopic
study of the disk, it appeared probable that an imjxirtant
application of the spectroheliograph might be made in this
new direction.
Before describing this second application of the instru-
ment, it may be well to recall the appearance of the Sun
when seen with a telescope, or when photographed in the
ordinary manner without a spectroheliograph. From photo-
graphs like that reproduced in Plate II, we see that the
. most conspicuous features of the solar surface, at least so far
> as the eye can detect, are the well-known sun-spots. The
^ bright faculae, which rise above the photosphere, are con-
spicuous when near the edge of the Sun, but practically
86 Stellar Evolution
invisible when they liappen to lie near the center of the disk.
The bright H and K lines, referred to in the last paragraph,
were found in close association with the faculae, and it
appeared probable that much of the highly heated calcium
vapor, to which these bright lines are due, rises from the
interior of the Sun through the faculae. It was therefore
to be expected that a successful application of the spectro-
heliograph to the photography of the luminous calcium
clouds would give bright forms resembling those of the
faculae. Furthermore, it was to be hoped that these brilliant
clouds could be recorded, not only near the limb of the Sun,
but also in the central part of the disk, since the bright
reversals of the H and K lines were equally well photo-
graphed in all parts of the image.
The results of the first experiments, which were made at
the beginning of 1892, were such as to justify fully the
expectations that had been entertained. It was at once found
possible to record the forms, not only of the brilliant clouds
of calcium vapor associated with the faculae, and occurring
in the vicinity of sun-spots, but also of a reticulated struc-
ture extending over the entire surface of the Sun. The
earliest use of the method was made in the study of the
great sun-spot of February, 1892, which, through the great
scale of the phenomena it exhibited and the rapid changes
that resulted from its exceptional activity, afforded the very
conditions required to bring out the peculiar advantages of
the spectroheliograph. In the systematic use of the instru-
ment continued at the Kenwood Observatory through the
following years, a great variety of solar phenomena were
recorded, and the changes which they underwent from day
to day — sometimes, in the more violent eruptions, from
minute to minute — were registered in permanent form.
During this period, which ended with the transfer of the
Kenwood instruments to the Yerkes Observatory, over 3,000
The Spectboheliograph 87
photographs of solar phenomena were secured. From a
systematic study of these negatives, in the course of which
the heliographie latitude and longitude of the calcium clouds
(subsequently named the flocculi) in many parts of the Sun's
disk were measured from day to day (by Fox), a new deter-
mination of the rate of the solar rotation in various latitudes
has been made. This shows that the calcium flocculi, like
the sun-spots, complete a rotation in much shorter time at the
solar equator than at points nearer the poles. In other
words, the Sun does not rotate as a solid body would do. but
rather like a ball of vapor, subject to laws which are not yet
understood.
In this first period of its career the spectroheliograph
had therefore permitted the accomplishment of two principal
objects. It had provided a simple and accurate means of
photographing the solar prominences in full sunlight, which
gave results hardly inferior to those obtained during the
brief moments of a total eclipse. It had also given a means
of recording a new class of phenomena, known previously to
exist only through glimpses of the bright calcium lines in the
vicinity of sun spots, but wholly invisible to observation,
either visually or on photographs taken by ordinary methods.
It was not difficult to see. however, that the possibilities of
the new method were much greater than had been indicated
by the work so far accomplished. It seemed probable that
our knowledge of the finer details of the calcium flocculi
would be greatly increased if provision could be made for
photographing a much larger solar image with a spectro-
heliograph of improved design. And it was furthermore
evident that other applications of the instrument, involving
the use of different spectral lines, and the employment of
principles which had not been thoroughly, tested in the
earlier work, might reasonably be hoped for. Attempts
were, indeed, made to photograph the Sun's disk with the
Stellar Evolution
dark lines of hydrogen, but the Kenwood spectroheliograph
was not well adapted for this purpose.
The 40-inch telescope of the Yerkes Observatory pro-
vided the first requisite for the new work — namely, a large
solar image, having a diameter of 7 inches as compared with
the 2-inch image given by the Kenwood telescope. The
construction of a spectroheliograph large enough to photo-
graph such an image of the Sun involved serious difficulties,
but these were finally overcome. The Rumford spectro-
heliograph, designed to meet the special conditions of the
new work, was built in the instrument shop of the Yerkes
Observatory, and is now in daily use with the 40-inch
telescope (Plate XXXVII).
In this instrument the solar image is caused to move
across the first slit by means of an electric motor, which
gives the entire telescope a slow and uniform motion in
declination. The sunlight, after passing through the first
slit, is rendered parallel by a large lens at the lower end of
the collimator tube. The parallel rays from this lens fall
upon a silvered glass mirror, from which they are reflected to
the first of two prisms, by which they are dispersed into a
spectrum (Plate XLI, Fig. 1). After passing through the
prisms, the light, which has now been deflected through an
angle of 180°, falls upon a second large lens at the lower
end of the camera tube. This forms an image of the
spectrum at the upper end of the tube, where the second
slit is placed. Any line in the spectrum may be made to
fall upon this slit, by properly adjusting the mirror and
prisms. Above the slit, and nearly in contact with it, the
photographic plate is mounted in a carriage, which runs on
rails at right angles to the length of the slit. The rails are
covered by a light-tight camera box, so that no light can
reach the plate except that which passes through the second
slit. While the solar image is moving across the first slit.
The Spectboheliogbaph 89
the plate is moved at the same rate across the second slit, by
a shaft leading down the tube from the electric motor, and
connected, by means of belting, with screws that drive the
plate-carriage.
Photographs of the solar disk taken with this instrument
under good atmospheric conditions reveal a multiplicity of fine
details (Plate XXXVIII). The entire surface of the Sun is
shown by these plates to be covered by minute luminous
clouds of calcium vapor, only about a second of arc in
diameter, separated by darker spaces, and closely resembling
in api^earance the well-known granulation of the solar photo-
sphere ( Plate XXXIX ) . A sharp distinction must, however,
be drawn between this apj^earance, which is wholly invisible to
the eye at the telescope, and the granulation of the photosphere.
In accordance with Langley's view, the grains into which the
solar surface is resolved under ofood conditions of visual obser-
vation are the extremities of columns of vapor rising from the
Sun's interior. They seem to mark the regions at which
convection currents, proceeding from within the Sun, bring
up highly heated vajxjrs to a height where the temperature
becomes low enough to permit them to condense. It might
be anticipated that out of the summits of these condensed
columns, other vapors, less easily condensed, would continue
to rise, and that the granulated appearance obtained with
the spectroheliograph may represent the calcium clouds
thus ascending from the columns (Plate XL). We might,
indeed, go a step farther, and imagine the larger and higher
calcium clouds to be constituted of similar vaporous columns,
passing upward through the chromosphere, and perhaps at
times extending out into the prominences themselves. A
means of research now to be described, which represents
another application of the spectroheliograph, involving a
new principle, seems competent to throw some light on this
question.
90 Stellas Evolution
Mention has already been made of the faculae, which are
simply regions in the photosphere that rise above the ordi-
nary level. Near the edge of the Sun their summits lie
above the lower and denser part of that absorbing atmos-
phere which so greatly reduces the Sun's light near the
limb, and in this region the faculae may be seen visually.
At times they may be traced to considerable distances from
the limb, but as a rule they are inconspicuous or wholly
invisible toward the central part of the solar disk. The
Kenwood experiments had shown that the calcium vapor
coincides closely in form and position with the faculae, and
hence the calcium clouds were long spoken of under this
name. In the new work at the Yerkes Observatory the dif-
ferences between the calcium clouds and the underlying
faculae became so marked that a distinctive name for the
vaporous clouds appeared necessary. They were therefore
designated Ji occult, a name chosen without reference to their
particular nature, but suggested by the flocculent appearance
of the photographs.
In order to analyze these flocculi and to determine their
true structure, a method was desired which would permit
sections of them at different heights above the photosphere to'
be photographed. Fortunately there is a simple means (first
described by Deslandres) which appears to accomplish this
apparently difficult object. At the base of the flocculi the
calcium vapor, just rising from the Sun's interior, is com-
paratively dense. As it passes upward through the flocculi
it reaches a region of much lower pressure, and during the
ascent it might be expected to expand, and therefore to
become less dense. Now we know from experiments in the
laboratory that dense calcium vapor produces very broad
spectral bands, and that, as the density of the vapor is
decreased, these bands narrow down into fine, sharp lines
(Plate XLI, Fig. 2). An examination of the solar spectrum
The Spectboheliograph 91
will show that the H and K lines of calcium give evidence of
the occurrence of this substance under widely different densi-
ties in the Sun. The broad dark bands, which for convenience
we designate H, and K,, are due to the low-lying, dense
calcium vajx)r (Plate XXXII). At their middle points (over
flocculi) are seen two bright lines, which are much narrower
and better defined. These lines, designated H, and K„ are
the ones ordinarily employed in photographing the flocculi
with the spectroheliograph. Superposed upon these bright
lines are still narrower dark lines, due to the absorption of
cooler calcium vapor at higher elevations (H3, Kj). It will
be seen that the evidence of the existence of calcium vapor
at various densities in the Sun is apparently complete, and
that we may here find a way of photographing the vapor at
low levels without admitting to the photographic plate any
light that comes from the rarer vapors at higher levels. It is
simply necessary to set the second slit of the spectrohelio-
graph near the edge of the broad H , or K , bands, in order
to obtain a picture showing only that vapor which is dense
enough to produce a band of width sufficient to reach this
position of the slit. No light from the rarer vapors above
can enter the second slit under these circumstances, since
they are incapable of producing a band of the necessary
width.'
The great sun-spot of October, 1903, afforded an oppor-
tunity to try this method in a very satisfactory manner.
Sections of the calcium vapor in the neighborhood of this
spot-group, corresponding to the two different levels photo-
graphed on October 9, are shown in Figs. 1 and 2, Plate
' The bright regions photographed in this way resemble the faculae very closely,
and may be regarded as essentiaUy identical with them, since the white light from
the continuous spectrum of the faculae contributes in an important degree to the
formation of the photographic images. However, any dense calcium vapor which
extends beyond the boundaries of the faculae will be recorded on the photograph.
In any case we should expect the dense calcium vapor, supposed to be rising from
the faculae, to correspond closely with them in form.
92 Stellar Evolution
XLII.' The manner in which the vapor at the Hj level over-
hangs the edge of the sun-spot is very striking, and thorough
study should throw some light on the conditions which exist
in such regions. For it is possible, not only to photograph
sections of the vapor at various levels, but also to ascertain,
by the displacement of the H2 or H3 line, as photographed by
a powerful spectrograph, the direction and velocity of motion
of the vapor which constitutes the fiocculi. It is commonly
found that the vapor is moving upward at the rate of about
one kilometer per second, though the velocity varies con-
siderably at different points and under different conditions.
The photographs occasionally show the existence of fiocculi
remarkable for their great brilliancy. In these regions active
eruptions are in progress. The vapor, rendered highly
luminous by intense heat or other causes, is shot out from
the Sun's interior with great velocity. Consequently there
are rapid changes in the forms of these brilliant regions,
whereas the ordinary fiocculi change slowly, and represent
a much less highly disturbed condition of affairs. The
brilliant eruptive fiocculi always occur in active regions of
the solar surface, and probably correspond with the erup-
tive prominences sometimes photographed projecting from
the Sun's limb. A remarkable instance was recorded on
the Kenwood photographs, which showed four successive
stages of an eruption of calcium vapor on an enormous scale.
A vast cloud thrown out from the Sun's interior completely
blotted from view a large sun-spot, and spread out in a few-
minutes so as to cover an area of four hundred millions of
square miles.
1 Although these photographs have been arranged for comparison with the
stereoscope, it is to be understood that no stereoscopic effect in the ordinary sense
will be obtained in examining them. The purpose of using the stereoscope is simply
to allow the images to be superposed, thus permitting them to be seen at the same
point in rapid succession by moving a card so as to cover alternately the two lenses
of the stereoscope. Thus the manner in which the calcium fiocculi overhang the
penumbra, and sometimes the umbra, of spots can be observed.
The Spectroheliogeaph 93
Although the eruptive flocculi probably correspond in
many instances with eruptive prominences, it must not be
concluded that the quiescent calcium flocculi correspond
with the quiescent, cloudlike prominences. As a matter of
fact, we have gfood evidence for the belief that the flocculi
shown in these photographs represent in most instances
comparatively low-lying vapors, while the prominences,
which extend above the level of the chromosphere, do not
ordinarily reveal themselves as bright objects in projection
aofaiust the disk.
So far, we have considered the photography of the Sun
with the light of the H and K lines of calcium. But it
must naturally occur to anyone familiar with the solar
spectrum that it should be possible to take photographs
corresponding to other lines, and thus representing the
vapors of other substances. For the darkness of the lines
is only relative; if they could be seen apart from the bright
background of continuous spectrum on which they lie, these
lines would shine with great brilliancy. It is thus evident
that, if all light except that which comes from one of these
dark lines can be excluded from the photographic plate by
means of the second slit of the spectroheliograph, it should be
possible to obtain a photograph showing the distribution of
the vapors corresponding to the line in question.
At this point attention should be called to the extreme
sensitiveness of the spectroheliograph in recording minute
variations in the intensity of a line — variations so sliofht that
no trace of them can be seen in a spectrum photograph
showing only the line itself. A well-known physiological
effect is here concerned, for it is common experience that
the eye cannot detect minute differences of intensity in
various parts of an extremely narrow line, whereas these
would become conspicuous if the line were widened oat
into a band of considerable width. The spectroheliograph
94 Stellar Evolution
records side by side upon the photographic plate a great
number of images of a line which, taken together, build up
the form of the region from which the light proceeds. In
this way the full benefit of the physiological principle is
derived, and very minute differences of intensity at various
parts of the solar disk are clearly registered upon the
plate.
It is obviously essential in photographing with the dark
lines to exclude completely the light from the continuous
spectrum on either side of the line employed. The admis-
sion of even a small quantity of this light might completely
nullify the slight differences of intensity recorded by the
aid of the comparatively faint light of the dark line. As
the second slit cannot be narrowed beyond a certain point,
it is evident that for successful photography with the dark
lines their width must be increased by dispersion in the
spectroheliograph to such a degree as to make them wider
than the second slit.
The first satisfactory photographs obtained with dark lines
were made with the Rumford spectroheliograph in May, 1903.
The lines of hydrogen were chosen for this purpose, on
account of their considerable breadth, and because of the
prominent part played by this gas in the chromosphere and
prominences. In order to secure sufficient width of the
lines, the mirror of the spectroheliograph was replaced by a
large plane grating having 20,000 lines to the inch. After
leaving the grating the diffracted light enters the prisms,
where it is still further dispersed before the image of the
spectrum is formed upon the second slit. The effect of the
prisms is not only to give additional dispersion, but also to
reduce the intensity of the diffuse light from the grating —
a most important matter in work of this nature. The hydro-
gen lines employed were i//3, H'y, or i/5, in the green-blue,
blue, and violet, respectively.
The Spectroheliograph 95
On developing the first plate it was surprising to find
evidences of a mottled structure covering the Sun's disk,
resembling in a general way the structure of the calcium
flocculi, but differing in the important fact that, whereas the
calcium flocculi are brigrht. those of hvdrogen are dark
(Plate XLIII). This result was confirmed by subsequent
photographs, and it was found that in general the hydrogen
flocculi are dark, although in certain disturbed regions bright
hydrogen flocculi appear. Some of these are eruptive in
character, and corresjxjnd closely with the brilliant eruptive
calcium flocculi. But in other cases, in regions where no
violent eruptive disturbances seem to be present, the hydrogen
flocculi frequently appear bright instead of dark (Plate
LXXII), Such regions are usually in the immediate vicinity
of active sun-spots, where it is probable that the temperature
of the hydrogen is considerably higher than in the surround-
ing regions. Since a higher temperature would undoubtedly
produce increased brightness, the spectroheliograph thus
seems to afford a method of distinoruishincr between reofions
of higher and lower temjjerature — an additional property
which should prove of great value in investigations on the
vapors associated with sun-spots. It is possible, of course,
that the increased brightness is due, not merely to an
increase of temperature, but to other causes, perhaps of a
chemical or electrical nature, which are not yet understood.
But the assumption that increased temperature is the effective
cause may be provisionally accepted as very probable.
The comparative darkness of the ordinary hydrogen flocculi
evidently indicates that this gas in the flocculi for some rea-
son radiates less light than the hydrogen gas which, probably
after diffusing from the flocculi, has spread in a nearly uni-
form mass over the entire surface of the Sun. The simplest
^ hypothesis is to assume that the diminished brightness of
the flocculi is due to the reduced temperature in the upper
96 Stellar Evolution
chromosphere, where the absorption probably occurs. The
results of work at Mount Wilson, described in chap, xvi, seem
to render this view probable. It should be emphasized at
this point, however, that the explanation of spectroheliograph
results offered in this chapter is merely an hypothesis, which
subsequent investigation may not prove to be correct.
According to Julius, the flocculi are not luminous clouds, but
the effects of anomalous dispersion of light passing out from
the Sun's interior through vapors of unequal density (see
p. 148).
The Rumford spectroheliograph was also used to secure
photographs with some of the stronger dark lines of iron
and other substances. But even with the grating the disper-
sion was insufficient to give thoroughly trustworthy results,
except in a very few cases. It was evident that much greater
dispersion must be employed in order to realize the full advan- I
tages of the method in future work. Subsequent progress
in the development of the spectroheliograph is described in
chap. xvi.
Within a short time after the first work at the Kenwood
Observatory the spectroheliograph came into general use.
Evershed constructed and successfully used one of thes6
instruments in England, and a year later Deslandres, whose
admirable work on the spectra of the flocculi was contempo-
raneous with the investigations at the Kenwood Observatory,
undertook systematic research with a spectroheliograph at
the Paris Observatory. His contributions to the develop-
ment of the instrument have been very valuable. Other
spectroheliographs are now used daily in India, Sicily, Spain,
Germany, England, and the United States.
CHAPTER XII
THE YERKES OBSERVATORY
The formulation of the theory of natural selection by
Darwin was the result of an extensiye series of closely cor-
related inyestigations, covering a broad field. His object
was not merely to bring together a great collection of plants
or animals, describe their peculiarities, and confer upon them
appropriate names. To Darwin each of these plants and
animals might be of great interest. But brilliant plumage,
unusual form, and other distinctive peculiarities were of
importance to him mainly because of their bearing upon the
question of development, or the possible relationship of the
particular specimen to others. It is obvious that a study of
such relationships must greatly enhance, rather than dimin-
ish, the interest of the investigator in the peculiarities which
distinguish species. Having in mind a governing principle,
he may detect, through the aid of delicate markings or minute
modifications of form which might otherwise be inappre-
ciable, the evidences of development which constitute the
prime object of his search.
Similar tendencies toward unification and correlation have
shown themselves in every department of science. Co-opera-
tive undertakinofs on a large scale, which have enlisted the
best efforts of scientific men in all parts of the world, are
common at the present time. It may confidently be pre-
dicted that the future will see such work greatly extended,
and that the various agencies which can thus be employed
to advance science will be utilized in an increasingly effec-
tive manner.
In astronomical and astrophysical research the opportu-
97
98 Stellar Evolution
nities for co-operation and correlation are unusually good,
and have yielded many important results. The impossibility
of completing at any one observatory the extensive investi-
gations required for the solution of large cosmical problems,
and the advantages which may result from the discussion of
observations made simultaneously or at stated intervals from
stations difiPering widely in geographic position, altitude, or
climatic conditions, render co-operation essential in many
cases. Plans for international co-operation in solar research
are mentioned elsewhere. An attempt to provide for the
closest possible correlation of work within a single observa-
tory is also described in this book.
In establishing an observatory, either one of two policies,
both represented in existing institutions, may be adopted.
On the one hand, attention may be directed to the prosecu-
tion of individual researches or extensive routine investiga-
tions, not necessarily closely related to one another, but each
constituting an important contribution to knowledge. On
the other hand, a single large problem may be chosen, and
all individual investigations planned so as to lead as directly
as possible toward its solution. The observations required
may be very diverse, and cover a broad field. Each, how-
ever, to be most effective for its purpose, must be chosen with
special reference to the existing needs, and the general pro-
gramme must be revised from time to time, in the light of
every important advance.
The Yerkes Observatory may serve as an example of an
institution in which extensive individual investigations,
differing widely in character, comprise the programme of
research.' Its scheme of work was based on a deliberate
intention to realize the fullest possible advantages of the
40-inch refractor in the diverse researches for which it is
1 In the astrophysical work, however, an effort was made to correlate the solar,
stellar, and laboratory investigations.
The Yebkes Observatory 99
peculiarly adapted. The object of the Mount Wilson Solar
Observatory of the Carnegie Institution, however, is to con-
centrate its entire attention upon the study of the Sun and
the problem of stellar evolution.
After the si^ectroheliograph had been tested at the Ken-
wood Observatory, it seemed certain that this method was
capable of further extension, and the desirability of securing
better instrumental facilities accordingly presented itself.
The establishment of the new University of Chicago appeared
to offer the best prosj^ects in this direction. The opportunity
of purchasing two disks of glass for the objective of a
■40-inch refractor was encountered in 1893. This glass had
been ordered three years before for a telescope to be erected
on Mount Wilson in southern California^ — an odd coinci-
dence in the light of subsequent events. As funds were not
available for the completion of the California project the
glass disks, then in the hands of Alvan Clark & Sons, were
obtainable. The opportunity was an unusual one, since the
disks were of the largest size and of the most perfect optical
glass. After several unsuccessful attempts to secure the
funds from other sources, the matter was placed before Mr.
Charles T. Yerkes by President Harper. He promptly sig-
nified his desire to provide for the construction of a 40-inch
refractor. The glass was purchased, a contract arranged
with Clark to complete the object-glass, and the mounting
ordered from Warner & Swasey. The construction of the
Yerkes Observatory was undertaken in 1895 and completed
in 1897.
The gift which provided for the Yerkes Observatory was
i. made before the University of Chicago had opened its doors
i to students. In fact, the original idea of establishing a col-
^ lege, rather than a university, had hardly been outgrown,
and the question of the recognition to be accorded to research
was still a cause of concern to the members of the rapidly
100 Stellar Evolution
enlarging faculty. A narrow view of the future on the part
of the trustees might have led to the erection of the obser-
vatory in Chicago, and its use for the purposes of instruction
rather than for those of research. Fortunately, a different
policy prevailed. It was recognized that the iO-inch tele-
scope should be exclusively devoted to investigation, and that
a site in the immediate neighborhood of the university
grounds would prevent its effective use. It was accord-
ingly decided to secure a site in the most favorable location
within a reasonable distance of Chicago, and a tract of
land in Wisconsin, on the shore of Lake Geneva, was finally
selected.
The plan of the building shows the influence of the Lick
Observatory and the Astrophysical Observatory of Potsdam,
both of which embody many admirable features. The adopted
form of a Roman cross permitted the three domes to be sepa-
rated to such an extent that they practically do not interfere
in the least with one another (Plate XLIV). The desire of
the donor for an ornate structure, and the decision of the
architect to introduce rather florid embellishments of terra-
cotta, led to the use of brick as a building material. This was
quite in accordance with convention, but in conflict with the
condition, well known to astronomers, that the temperature
within an observing-room should be as nearly as possible the
same as the temperature of the outer air. The massive brick
wall of the great tower in which the 40-inch telescope is
mounted is therefore decidedly inferior to a light steel con-
struction, with a thin metallic wall, shielded from the Sun by
an outer wall of similar type. Architectural considerations,
however, have weighed as heavily in nearly all of the world's
largest observatories, and the complete freedom of action,
subsequently experienced at Mount Wilson, had not yet been
attained.
The engineering problems presented by the great size
The Yebkes Obsebvatobt 101
of the Yerkes telescope, and of the dome under which it was
mounted, were such as to tax the efforts of even so skilful a
firm as that of Warner & Swasey, to whom the work was
intrusted. The admirable qualities of the mounting of the
Yerkes telescope show the advantage of the experience gained
by them in constructing the Lick telescope. The dome and
rising-floor, after several faults of design and construction
had been remedied, also performed very well. Thoroughly
tested by continuous use, by night and by day, for a period of
ten years, the entire plant may certainly be considered to
reflect much credit ujx)n these well-known engineers.
The -tO-inch telescope, and other instruments of the
Yerkes Observatory, have already been described in previous
chapters, but a few additional details may be of interest.
The object-glass, which was put in place only a few weeks
before the death of Alvan G. Clark, the last member of the
celebrated firm of Alvan Clark & Sons, is made up of two
lenses. The outer lens, made of crown glass, is double con-
vex in form (Plate XLVV The inner lens, separated from
the other by a distance of about eight inches, is plano-con-
cave, and made of flint glass. The total weight of the class
in the two lenses is about 500 pounds. The rough o^lass
disks, from which the lenses were fashioned by the Clarks.
were made by Mantois. of Paris. The glass is of extra-
ordinary purity and transparency, but in spite of this fact it
ai)Sorbs much light, on account of its considerable thickness
^^ about three inches in all). The conditions are verv dif-
ferent from those of a reflecting telescope, where much less
rfect glass is required, since in the latter case the lio^ht is
tlected from a layer of pure silver on the front surface and
iierefore suffers no absorption in transmission (though some
light is lost in reflection). It has already been pointed
out that refracting and reflecting telescopes have their own
peculiar advantages and defects. The choice of the one or
102 Stellar Evolution
the other must depend upon the needs of the work for which
it is required.
In order to direct the 40-inch telescope to a faint star,
the sidereal time, as well as the right ascension and declina-
tion of the star, must be known. After the opening in the
dome has been turned toward the proper quarter of the
heavens, the telescope is moved in right ascension (i. e.,
around the polar axis, which is parallel to the Earth's axis)
until the hour circle, attached to this axis, indicates the
proper reading. This reading is determined by taking the
difference between the sidereal time and the rigfht ascension
of the star. The result gives the distance of the star from
the meridian, expressed in hours and minutes of time. The
motion of the telescope in right ascension is produced by
means of an electric motor, controlled by a rope running down
the north face of the iron column and easily reached from
the rising-floor. The next operation is to move the telescope
in declination (i. e., around an axis at right angles to the
polar axis) until the declination circle indicates the proper
reading, so many degrees north or south of the equator. If
the eye-end of the telescope is then too high to be reached
by the observer on the rising-floor, the floor is raised by
means of an electric motor, controlled by a switch near the
telescope column. An adjoining switch controls the motor
which turns the dome. On looking into the eye-piece the
star will be found in the field, provided the setting has been
accurately made. The telescope is next clamped in right
ascension and declination. It will then be carried by the
driving-clock, which causes the polar axis to rotate through
a complete revolution in twenty-four hours. The apparent
motion of the star in the heavens is thus counteracted, and
the image remains fixed in the field of view, where it may
be studied in any way desired.
If, for example, the observer wishes to measure the posi-
The Yebkes Observatory 103
tion of the star with respect to other stars in its neighbor-
hood, this is accomplished by means of a position micrometer,
in which a fine spider line can be moved through the neces-
sary distance by a micrometer screw. The value of one divi-
sion of the micrometer head, in seconds of arc, is previously
determined by measuring the distance between two known
stars, whose positions have been accurately fixed by means
of a meridian circle. Burnham's admirable observations
of double stars with the -tO-inch telescope have all involved
the accurate micrometric measurement of the distance sepa-
rating the stars of each pair. The position angle of the line
joining the two stars, with reference to a north-and-south
line in the heavens, is also measured in each case with the
aid of a divided circle attached to the micrometer. On
account of the large aperture of the telescope, it is possible
to separate with it stars about one-tenth of a second of arc
apart, provided the atmospheric conditions are sufficiently
good for the purpose. As the distance between the two
images in the principal focus of the telescope would, in this
case, amount to but little over one three-thousandth part of
an inch, it is obvious that the best of conditions are required
for such exacting work.
Barnard's observations with the Yerkes telescope have also
involved the constant use of the micrometer. The difficulty of
the work, and the patience required to pursue it, can be ima-
gined when it is remembered that Barnard has measured the
positions of hundreds of stars in such a closely crowded cluster
as that illustrated in Plate XIX, In such work as this the
observer remains standing throughout the entire night. It
sliould also be remembered that in the open dome the tem-
])erature sometimes falls to — 20" F. in the rigorous Wiscon-
sin winters. It is evident that only the greatest interest and
ilevotion on the part of the observer can permit him to make
accurate measures, night after night, under such conditions.
104 Stellar Evolution
We have already seen (in chap, xi) how the Rumford
spectroheliograpli is used with the Yerkes telescope! As the
spectroheliograph weighs about 700 pounds, and must be
attached each morning and taken off at night, special arrange-
ments are required to facilitate this work. Each heavy
instrument used in conjunction with the telescope is mounted
on a carriage, which stands on the rising-floor. When the
change is to be made from one attachment to another, the
floor is raised to its highest position and the telescope tube
firmly anchored to it by means of a steel bar. This is to
obviate any danger of accident when the balance of the tube
is temporarily disturbed. The carriage bearing the spectro-
heliograph is brought to the eye-end of the telescope, the
spectroheliograph clamped to its supporting ring, and over
700 pounds of iron weights removed from the telescope
tube. This restores the balance, which must be adjusted
to a nicety.
The Bruce spectrograph (Plates XL VI and LXXVIII)
is used by Frost for the photographic study of stellar spectra.
The image of a star is formed on the slit of the spectrograph,
which is about one-thousandth of an inch in width. The
light then passes to a collimator lens, which renders the rays
parallel. Three large prisms, next traversed by the rays,
bend them through an angle of 180° and disperse them
into a spectrum. The camera lens forms an image of the
spectrum upon the photographic plate. Throughout the
exposure, which may be continued several hours, the ob-
server watches the star image and keeps it accurately on
the slit, any imperfections in the driving of the telescope
being corrected by means of electric slow motions. In order
to eliminate the effect of the changing temperature in the
open dome, the spectrograph is inclosed in a tight-fitting
case, the interior of which is maintained at a uniform tem-
perature by electric-heating coils.
The Yerkes Obsebvatoby 105
In order to determine the position of the lines in a
spectrum, a suitable comparison spectrum is required. This
is obtained by passing an electric spark between poles of
titanium or iron and photographing the spectrum of the
spark on each side of that of the star. An enlargement of
one of Frost and Adams' photographs of ?; Leonis, made in
this way, is reproduced in Plate XL VII. It will be seen that
the lines of the comparison spectrum are shifted a slight
distance toward the red (right), with reference to the corre-
sponding lines in the star. This shift is due to the motion
of the star away from the Earth, which in this instance
amounts to 28 kilometers per second. On account of its
orbital motion, the Earth was moving toward the star on
this date at the rate of 26 kilometers per second. Hence the
velocity of rj Leonis with respect to the Sun was — 2 kilo-
meters per second.
Such displacements of the lines provide the only means
of determining whether a star is approaching or receding
from the Earth. This method, tirst tried visuallv bv Hug-
gins, was successfully adopted for photographic work by
Vogel, and subsequently greatly refined by Campbell, who
applied it with remarkable success at the Lick Observatory.
In the hands of Campbell. Frost, and others, it has resulted in
the discovery of many "spectroscopic binaries" — double stars
in which the component members are revolving at such great
- velocities that they periodically displace the lines in their
spectra. In most of these binaries one of the components
is a dark star. Our only clue to their duplicity is thus fur-
nished by the fact that the lines move back and forth with
. respect to the comparison lines, the displacement being
i' toward the violet when the star is approaching, and toward
, the red when it is receding from the Earth. In a subsequent
' chapter it will appear how photographs of stellar spectra
are used in the studv of stellar evolution.
106 Stellar Evolution
The Rumford spectrolieliograph and the Bruce spectro-
graph were constructed in the instrument shop of the Yerkes
Observatory. It had long been customary for observatories
to provide means of repairing their own instruments, but the
work of construction had, as a rule, been left to the profes-
sional instrument-makers. At the Yerkes Observatory a well-
equipped shop was not only a convenience, but a necessity.
The funds given for the establishment of the observatory did
not provide for a general equipment of minor instruments.
In the absence of the means of purchasing instruments, the
only alternative was to construct them. Fortunately, a
number of machine tools had formed part of the equipment
of the Kenwood Observatory and were immediately available.
The appropriations of the University of Chicago permitted a
skilled instrument-maker to be regularly employed, and spe-
cial gifts, received from various sources in subsequent years,
sometimes enabled us to keep several men at work. The
instrument shop, at first under the direction of Wadsworth
and subsequently under Ritchey (who was in charge of the
optical shop from the beginning), proved to be indispensable
to the success of the Observatory's work. Not only the instru-
ments already mentioned, but also the 2-foot reflector, the
Snow telescope, a 3^-inch transit instrument, spectroscopic
and other apparatus used in the laboratory, and many special
instruments and appliances employed with the 40-inch tele-
scope and in other departments of the work, came from this
source. It may be said that in a large astrophysical observa-
tory, where new types of instruments are constantly being
devised, a well-equipped instrument shop is essential if the
best results are to be obtained. This is largely because of
the advantage of having the instruments constructed under
the immediate supervision of the men who are responsible for
their design.
The optical shop was another feature of the Yerkes
The Yebkes Observatory 107
Observatory which contributed in a most important manner
to its work. Here Ritchey made numerous mirrors — plane,
concave, and convex — for use in the Snow telescope, the
2-foot reflector, and other instruments, and here also he did
a large part of the work on the 60-inch mirror, which was
subsequently transferred to the Solar Observatory. As the
methods employed in grinding and polishing this mirror are
described in chap, xxiii, no further mention will be made of
them here. It may be said, however, that many special
investigations set on foot at the Yerkes Observatory could
not have been undertaken without the unique advantages
afforded by the optical shop.
Still another feature of the Yerkes Observatory, which
was subsequently repeated, in improved form, at Mount
Wilson, is the spectroscopic laboratory, in which various
i solar and stellar phenomena are imitated experimentally.
Apparatus for producing sparks between metallic poles in
air, in liquids, and in compressed gases is arranged on the
circumference of a circular table. Low-voltage arcs are also
provided, the purpose of the equipment being to furnish
means of varying, between wide limits, the conditions of
temperature and pressure, and of gaseous or liquid environ-
ment, in which the metallic vapors emit their characteristic
radiations. By setting at the proper angle a plane mirror,
mounted at the center of the table, light from any source
can be reflected to a concave mirror, which forms an image of
the source on the slit of a large concave grating spectrograph.
The most extensive single investioration made in this labora-
tory was a study of the spectrum of the spark in liquids and
compressed gases, to test Wilsing's pressure theory of
^ temporary stars.
;> In the diversified work of the Yerkes Observatory the
* desire to attack the problem of stellar evolution in the most
effective manner was not forgotten. Experience with the
108 Stellar Evolution
large concave grating of the Kenwood Observatory had
furnished convincing evidence of the advantages of fixed
instruments mounted on piers, and the beautiful resolution of
the solar spectrum with this apparatus made observations of
stellar spectra with small prism spectroscopes seem unsatis-
factory. It was felt from the first that every effort should be
made to devise a telescope capable of bringing a large and
well-defined solar image, or a sharp and brilliant stellar j
image, into a laboratory, where it could be observed to the
best possible advantage, with appliances too large or too
heavy for use with moving telescopes. It seemed clear that,
if this desire could be realized, and if the full advantages <
of reflecting telescopes for astrophysical research could be
attained, the means thus provided should render possible a
well-directed attack on the problem in mind.
The work of the Rumford spectroheliograph showed that
the further development of this instrument must involve a
considerable increase in dispersion, so as to permit the use
of the narrower dark lines. This meant an instrument of I
large dimensions, necessarily to be mounted in a fixed posi- ]
tion, since it could not be attached to a moving telescope :
tube. Another piece of work pointed to the same require-
ment. At the Kenwood Observatory attempts were made to
photograph the spectra of sun-spots, and negatives were
secured showing a few of the more conspicuous widened lines.
The need of a larger solar image for this work was met by
the Yerkes telescope. A marked improvement in the photo- I
graphs resulted. However, it was clear that photographs of
spot spectra suitable for the most refined investigations could
not be obtained without the use of a spectrograph of much
higher dispersion. For satisfactory results a spectrograj^h
of at least 10 feet focal length was needed, and this could
not be attached to the moving telescope tube. Here, again,
was another argument for the fixed type of telescope.
The Yebkes Obsebvatobt 109
The work of constructinof such an instrument was accord-
ingly taken up. The original purpose of building a heliostat
was modified, through the recognition of the superior advan-
tages of the coelostat, introduced by Turner for eclipse
observations. A 30-inch coelostat, designed by Ritchey,
was constructed in the instrument shop of the Yerkes
Observatory. This was destroyed by fire, but a gift from
Miss Snow of Chicago, in memory of her father, provided
the funds required for the Snow telescope. In the prelimi-
narv tests of this instrument at the Yerkes Observatorv the
images were not very satisfactory, but it subsequently gave
admirable results at Mount Wilson.
In establishing the Carneorie Institution at Washington.
Mr. Carnegie gave expression to his appreciation of the fact
that some of the most fundamental needs of scientific research
could not be supplied by existing agencies. As a rule, a
university must build its observatory or biological laboratory
near at hand, rather than at a site chosen because of atmos-
pheric advantages or the richness of the local fauna and flora.
Its funds, usually given for specific purposes, are likely to be
unavailable, or perhaps inadequate, to provide a suificiently
large corps of investigators, devoted to research. If, through
the efforts of one of its faculty, a new and promising instru-
ment is projected, the trustees may not be in a position to
supply the financial means required to construct it. Such
conditions result from the very nature of a university's
work, and consequently affect, in some degree, the policy
of even so progressive an institution as the University of
Chicago, where the authorities strongly favor original in-
vestigation. The Carnegie Institution, devoted exclusively
to the furtherance of research, is not thus hampered. It
therefore came about that this new Institution, with the
cordial co-operation of the University of Chicago, made pro-
vision for the continuation and development of the work set
110 Stellar Evolution
on foot at the Kenwood and Yerkes Observatories. A com-
mittee, appointed to report on the advisability of establishing
an observatory for solar research, and another observatory for
observations of the southern heavens, favored both of these
projects. A careful test of various sites in the United States
and in Australia, made at the request of the committee by
Hussey, led to the provisional selection of Mount Wilson
(5,886 feet), near Pasadena in southern California, as the
site for the proposed solar observatory. An appropriation,
granted by the Carnegie Institution in 1904, furnished the
means of sending an expedition from the Yerkes Observatory
to Mount Wilson. The Snow telescope was erected on the
mountain, in a new type of house especially designed for it.
An instrument shop was established in Pasadena for the
construction of the spectroheliographs and other apparatus
required for use with the Snow telescope. In December,
1901, the Carnegie Institution decided to establish a solar
observatory of its own on Mount Wilson. Through the
courtesy of the authorities of the Yerkes Observatory and
the University of Chicago, the Snow telescope was retained
on the mountain, and has since been purchased by the Solar
Observatory. The optical work on the 60-inch mirror, which
was also acquired by the Solar Observatory, was resumed
by Ritchey in the optical shop at Pasadena. He also
designed the mounting for this telescope, and the work of
constructins it was soon undertaken.
CHAPTER XIII
ASTRONOMICAL ADVANTAGES OF HIGH ALTITUDES
The recognition of the advantages of making astronomical
observations at high altitudes goes back to the time of New-
ton, who wrote as follows in his Opticks (third edition, p. 98) :
If the Theory of making Telescopes could at length l^e fully
brought into practice, yet there would l^e certain Boimds Ijeyond
which Telescopes could not perform. For the Air through which
we look upon the Stars, is in a perpetual Tremor; as may be seen
by the tremulous Motion of Shadows cast from high Towers, and by
the twinkling of the fix'd stars. * * * The only remedy is a most
serene and quiet Air, such as may perhaps be fovmd on the tops of
the highest Mountains above the grosser Clouds.
It will be observed from these remarks that a clear and
transparent sky is not the only need of the astronomer. In
their passage through our atmosphere the rays which are
united by a telescope to form the image of a star traverse
different paths, depending upon their color. For air, like
water or grlass, thoucrh in a less degrree, is a refracting medi-
um; i. e., a ray of light entering it is bent from its straight
course, and the amount of its bending depends upon the
color of the ray, just as in the case of a prism. Violet light
suffers the orreatest refraction, and red light the least. Ob-
viously, then, rays of different colors coming to a telescope
from a star do not pursue the same path. Since the degree
of refraction depends upon the temperature of the air. and
since, under ordinary conditions, the temperature is chan-
ging in an irregular manner, we thus see why a star twinkles
and undergoes rapid change of color. For the red rays may
be momentarily reduced in brightness, through a change in
111
112 Stellar Evolution
refraction of the air through which they pass. The star
would thus appear blue for the time being. The next
instant the intensity of the blue light might be reduced,
causing the star to seem red. Since the length of the light-
path and the degree of refraction increase toward the hori-
zon, the twinkling of stars, which frequently disappears alto-
gether at the zenith, is most apparent at low altitudes.
As the effect of twinkling is so apparent to the eye, it is
easy to see that it may be greatly magnified in a telescope
and produce serious interference with observations. The
star image, instead of being a minute, sharply defined point,
usually appears in the telescope enlarged, confused, and
tremulous. The component members of close double stars,
though easily within the resolving power of the telescope,
under such conditions may overlap and appear as one. Simi-
larly the minute surface details of the Moon or planets may
be entirely obliterated by atmospheric disturbance. It is as
though the astronomer were forced to observe the heavenly
bodies from the bottom of an ocean, not calm and tranquil
throughout its mass, but constantly disturbed by currents of
various directions and at different depths, and by irregu-
larities of density arising from unequal temperatures.
It sometimes happens that excellent definition of tele-
scopic images is obtained through smoke or haze, under cir-
cumstances which might appear to be wholly unsuitable for
astronomical work. For certain kinds of observations, where
perfect definition is all important and brightness of the image
of less consequence, the lack of transparency occasioned by
hazy air does no harm. But in most classes of work particles
suspended in the atmosphere not only reduce the inten-
sity of the light, but produce serious interference through
scattering of the rays. The brightness of the sky near the
Sun, for example, increases greatly with the number of dust
or smoke particles in the air. In visual observations of the
Advantages of High Altitudes 113
details of sun-spots this might not be harmful : but the visi-
bility of the prominences is seriously reduced when they are
seen against a brilliant background of sky. The brightness
of stars is also much affected by haziness of the atmosphere.
Even on a clear and transparent night the stars are less
brilliant at sea level than when seen from the summit of a
high mountain. For the air itself is a powerful absorbing
medium and reduces, more than we ordinarily realize, the
brightness of objects seen through it. Illustrations of the
relative advantages of photographing stars at altitudes of
' 1,200 and 6,000 feet respectively is given in Plates LVI
and LYII.
The difficulties in astronomical observations arising from
atmospheric disturbances increase with the aperture of the
telescope employed. This is because the rays falling on
opjwsite sides of a large object-glass traverse more widely
separated paths than those united by a small object-glass.
They are thus liable to greater atmospheric disturbance, on
account of the difference in the conditions governing the
refraction of the light along the two paths. The disturbances
of the air take the form of more or less regular waves. With
an aperture which is small compared with the length of one
of these waves, the effect on the image might not be great.
If, however, several waves were included within the aperture,
the confusion might be very marked indeed. Hence large
telescopes require better conditions than small ones.
In selecting the site of the Yerkes Observatory, practical
considerations necessarily limited the choice. It was essential
that the observatory should be situated within easy reach of
the university, and this fact rendered it impossible to consider
. seriously the favorable mountain regions which were known
J, to exist in the extreme western part of the United States.
* The chosen site has many advantages over }X)iuts in the
immediate neighborhood of Chicago. The absence of smoke
114 Stellak Evolution
and the brilliant illumination of the sky produced in large
cities by electric lights, the freedom from vibration arising
from railways and the heavy traffic of a large city, and the
facilities for quiet study afforded by the tranquil life of the
country, were important recommendations of the Lake
Geneva site. The observational work of the Yerkes Obser-
vatory has been sufficient in amount and quality to show
that more valuable material can be secured under such
atmospheric conditions than can be adequately discussed
without a far larger staff of computers than the observatory
has ever been able to employ. It goes without saying, how-
ever, that a better site would have been preferable.
But it must not be supposed, from what has been said,
that all mountain peaks would make good observing stations.
It is true that by ascending into the upper atmosphere the
astronomer may escape the strong absorption exercised by
the dense air of lower levels. As one goes up, the stars
become brighter and brighter, especially near the horizon,
since the decrease in length of path is much greater in this
region than near the zenith. Blue and violet light suffer
more from atmospheric absorption than the red, yellow, and
green rays. For this reason, the advantages of high eleva-
tions, so far as transparency is concerned, are more apparent
in photographic than in visual observations, since the blue
and violet rays are principally concerned in the production
of the photographic image.
Thus, from the standpoint of atmospheric transparency,
mountain sites may always be considered to possess advan-
tages for astronomical work. But transparency is almost
invariably a much less important consideration than sharp-
ness of definition, which does not, by any means, depend
merely upon altitude. In the first place, the geographic
location of the mountain in question is a most important
factor. Long periods of continuous clear weather, enjoyed
Advantages of High Altitudes 115
in certain favored regions, are accompanied by a uniformity
of atmospheric conditions unknown in countries where storms
usually prevail. It is not merely that clouds and rain are
less common; for, if this were the only important considera-
tion, a clear night in one part of the world might be as good
for astronomical purposes as an equally clear night in an-
other. In a region of storms the disturbances follow one
another so rapidly that during the intervening periods of
clear weather the atmosphere rarely has time to settle down
to a calm, homoofeneous state. In southern California, for
example, the sky is almost constantly clear for many months
in the year, and the uniformity of the atmosphere is shown
by the steadiness of the barometer and the low average wind
velocity. Durins: the rainv season, however, when storms
may recur in rapid succession, the atmosphere in such a
reofion is disturbed, and the conditions for astronomical work
on the beautifully transparent nights that intervene between
storms are frequently no better than in the eastern part of
the United States.
Pike's Peak (14.147 feet) affords an example of a moun-
tain site poorly adapted for astronomical purposes. In June
and July of 1S93 I spent two weeks there, in company
with Keeler, engaged in an attempt to photograph the solar
corona without an eclipse. Under normal conditions the
sky, as seen from the peak, is of a deep blue by day, and
I very transparent by night. The conditions, therefore, are
favorable for work in which transparency is the only important
consideration. Thus Pike's Peak might serve very well for
[the measurement of the solar radiation, were it not for the
fact that during the summer months (always the most
important season for solar work), the mountain is frequently
capped by clouds through a considerable part of the day.
On many of the nights during our stay the sky was perfectly
clear, and remained so until about nine o'clock in the morn-
116 Stellar Evolution
ing. Then small cumulus clouds would begin to form imme-
diately around the peak, and by noon a thunderstorm would
be raging, frequently accompanied by a light fall of snow.
In these storms the wind rose to a tremendous velocity, some-
times as great as seventy miles an hour, and the electrical
phenomena were very remarkable. The frequency with which ;
these storms cut off all solar observations, except in the early
morning, illustrates the fact that even for work on the solar
radiation, which requires a clear and transparent sky through
the greater part of the day. Pike's Peak would serve but i
poorly, at least daring this season of the year. As many of '■
these storms were confined to the immediate summit of the
mountain, a station several thousand feet below would prob-
ably offer more opportunities for work than the peak itself.
But this is not all. The definition of the Sun or stars is
rarely good on Pike's Peak. This is probably due, not merely
to frequent storms and high wind velocities, but also in part
to the fact that the summit of the mountain is bare and rocky,
so that heated currents of air rise from the surface and ruin
the definition of the solar image. At this altitude mountain
sickness is also very common, and would undoubtedly inter-
fere, in some degree, with the operation of an observatory.
The observers at that time stationed there by the Weather
Bureau informed us that they could not remain on the
mountain for long periods without impairment of health and
energy. Two-thirds of the tourists who came to the summit,
by the railway or on foot, were visibly affected by the high
altitude. Another cause of difficulty at the time was forest
fires in the mountains surrounding the peak, which sent
volumes of smoke into the air. This rose to a great altitude
and destroyed the deep blue of the sky.
The unsuccessful attempts to photograph the corona were
renewed on Mount Etna in July, 1891, through the kindness
of Professor Ricc6, director of the Bellini Observatories
Advantages of High Altitudes 117
of Catania and Mount Etna. Our party, consisting of
Professor Ricco, Signorina Ricco, Antonino Capra, mecha-
nician of the observatories, Mrs. Hale, and myself, left
Catania on July 7. After a drive of three hours we arrived
at Nicolosi, where we spent the night. The following extracts
from my diary relate mainly to the atmospheric conditions
encountered :
July 8. Left Nicolosi at 6 a. m. Arrived at Casa del Bosco
(4,760 feet) at 8^ 30™. Examined sky frequently, and found slight
decrease of white as we ascended. Crossed lava stream of 1892,
and had excellent view of the craters of that year, the latest of which
still emits vapor. Arrived at the observatory (9,650 feet) at 1^ 3h^.
The temperature had fallen to 9" C, and the sky was nearly covered
with clouds. Half an hour later we were enveloped in cloud, which
surrounded us imtil evening, when sky was whitish, with marked
halo around Moon. Stars unsteady, even in zenith.
July 9. Sky clear, with strong wind blowing the smoke from
the great crater (which rose behind the observatory to an altitude
of 10,900 feet) away from the direction of the Sun. Half the island
of Sicily was dimly visible from the observatory through a great
brown bank of thick haze, the upper surface of which seemed to
be nearly on a level with us. Cumulus clouds commenced to form
at 9^, and soon the sky was nearly covered. At 12'i the Sun was seen
between passing clouds to be surrounded by a bright halo. Wind
changed to west in the afternoon, and sky became much whiter.
July 10. Wind blew smoke of great crater over Sun, making
sky very white. Observed Sun with Professor Ricc5.by projection
with 12-inch telescope. Image rather better than at Catania, but
became unsteady later. At 10 ^^ some small cumulus clouds had
formed, and Sun was surrounded by bright halo. Clouds of insects
were also noticed in direction of Sun, as on Pike's Peak. Observed
prominences with Professor Ricco, but images were no better than
at Catania. At sunset watched shadow of Etna from the Torre del
Filosofo. Whole sky covered with dense haze.
July 11. Sky very white, bright ring around Sun. Observed
atmospheric lines with direct-vision spectroscope. Balanced tele-
j^cope, and oK'^rved Sun by projection. Seeing excellent; granu-
lation, spots, and faculae well defined. Strong odor of sulphur. At
118 Stellar Evolution
sunset visited Valle del Bove. Sky filled with haze, and almost too
bright for the eye 10° from Sun.
July 12. Sky very white. Wind still blowing smoke from crater
over Sun. Bank of haze above level of observatory. Observed Sun!
by projection with Professor Ricc6; image unsteady. Climbed to
top of crater, and found sky in zenith of deeper blue than when seen
from observatory. Whole island enveloped in haze. Descended
to observatory by moonlight; double halo around Moon. Observed
Moon, Saturn, and several stars with the 12-inch, using powers up
to 430. Seeing magnificent; images almost perfectly steady with
highest power. Both Moon and Saturn were very low, but images
were remarkably good. With naked eye scintillation was hardly
perceptible in stars higher than 30°.
July IB. Wind blowing from direction of crater, but sky best
since July 9: cloudless and generally whitish, but increase in bright-
ness toward Sun was gradual. Much dust. Telescope in use until
gh 40™ by Professor Ricc5 for daily record of chromosphere. Prom-
inences very well seen. At Q^^ 50™ broad and brilliant ring of
whiteness around Sun, making it useless to try for corona. Smoke
blowing directly over Sun, and diffusing through entire sky. Solar
image observed by projection; definition very poor. At 11 ^ sky had
improved, and preparations were made to photograph corona, but
five minutes later more smoke blew over Sun, and sky became very
white. Mirror found to be dewed, and surface badly tarnished by
the sulphurous fumes, though it had been tightly covered every
moment it was not in use. Sky around Sun remained bright, and
wind was so violent that no photographs could be made. Strong
sulphurous odor.
July 14. Smoke blowing across sun. Strong sulphurous odor.
Whole eastern sky white. Prominences fairly well seen at 7 ^ 45™.
Left observatory at 3 h, and arrived at Catania about midnight.
As I was assured by Tacchini and Ricco that the sky is
frequently very clear on Etna, it may safely be concluded
that the difficulties we encountered were exceptional. During
the entire time of our stay in southern Italy and Sicily the
atmosphere was very hazy, and the sky was rarely of a deep
blue. I was told by Galvagno, the custodian of the Etna
Observatory, that the smoke this year was much more notice-
Advantages of High Altitudes 119
able than usual. If the wind had blown it away from, instead
of toward, us, the sky would probably have been pure, though
hardly as blue as when seen from Pike's Peak during the
first part of our visit there.*
So much for the results of brief personal experience in
Sicily and the Rocky Mountains. From the standpoint of
a solar observer requiring fine definition, they do not appear
very encouraging. Moreover, conclusions reached by other
astronomers have been equally unfavorable to Colorado air;
and we find Piazzi Smith, in his book Teneriffe: An Astron-
omer's Experiment, reporting but very little good solar
definition at altitudes up to 10,700 feet on a tropical island.
His expedition to Teneriffe in 1856, made for the express
purpose of testing the atmospheric conditions on a mountain-
peak, was the first serious study of this kind. The trans-
parency of the air and the definition of the stars by night
were found to be excellent; but high winds, dust in the
upper atmosphere, and unsteady solar images were also
encountered.
However, good solar definition is experienced on Mont
Blanc (15.780 feet), at the Kodaikanal Solar Observatory
in India (7,700 feet), and at the Pic-du-Midi in France.
There is obviously no incompatibility between high altitudes
and good solar definition. The poor definition reported by
various observers on mountain-peaks is due either to the
prevalence of storms or to local disturbances, caused by
warm air rising from the heated summits of mountain-tops
protected by little or no foliage. At Mount Hamilton, where
the night conditions are so favorable, the slopes immediately
around the summit are composed of bare rock, which becomes
intensely heated and necessarily affects the solar definition.
This is a matter of no special consequence to the Lick
1 The attempts to photograph the coroaa were oontinned by Riced ander better
conditions, but neither this method nor any other has yet proved snccessfol.
120 Stellar Evolution
Observatory (Plate XLVIII), since the work is confined to
night observations. The great number of admirable results,
many of them requiring the finest definition, which have
been obtained at the Lick Observatory, afford the best of
evidence that its site was well chosen.
The results of experience in various parts of the world
would seem to indicate that a mountain observatory, if it is
to enjoy good conditions both by night and by day, should
be situated in a climate where the sky is clear continuously
for periods of several weeks or months, and the average wind
velocity is low. The summit of the mountain, as well as its
slopes, should be covered with foliage, to protect it from the
heat of the Sun. Finally, the elevation should be sufficient
to escape the dust which diffuses itself through the air in
the dry season, and the low-lying fogs and clouds frequently
encountered in regions near the sea.
CHAPTER XIV
TEE MOUNT WILSON SOLAR OBSERVATORY
From the preceding chapters, it will be seen how the plan
of research of the Solar Observatory was developed. At
Kenwood a programme of solar observations, involving the
use of the spectroheliograph, the photographic study of the
spectra of Sun-spots and other solar phenomena, and the fullest
possible application of laboratory methods in astrophysical
research, was instituted. At the Yerkes Observatory this
programme was broadened and extended, in the hope of
providing ultimately for the general study of stellar evolu-
tion; the possibilities of the spectroheliograph were more
fully realized, through the advantages offered by the 4:0-inch
refractor; and instruments better adapted than the large
refractor for the further prosecution of the work, such as the
Snow telescope for solar reseai-ch, and the 60-inch reflector
for stellar investigations, were designed and partially or
wholly constructed. After this period of preparation, devoted
in large part to the development of plans and methods, the
Mount Wilson Solar Observatory was organized for the study
of stellar evolution, at a station enjoying the best climatic
advantages.
In brief, the scheme of research of the Solar Observatory
comprises: (1) solar investigations, to contribute toward our
knowledge of the Sun («) as a typical star and (6) as the
central body of the solar system; (2) photographic and
spectroscopic studies of stars and nebulae, bearing directly
upon the physical nature of these bodies, with special refer-
ence to their development; (3) laboratory investigations, for
the interpretation of solar and stellar phenomena. With
121
122 Stellar Evolution
the central problem in mind, each successive research is
designed to occupy a logical place in a concentrated attack,
proceeding along these converging lines.
The variety of the problems connected with the establish-
ment of the Solar Observatory on Mount Wilson affords a
good illustration of the diversified work of an astronomer.
It was necessary, in the first place, to test the atmospheric
conditions by means of telescopic and meteorological observa-
tions extending over a considerable period of time, in order
to make certain that the site would prove suitable. In the
second place, since the summit could be reached only by a
narrow mountain trail, it was evident from the outset that
the question of transporting building materials and the parts
of heavy instruments would not be an easy one to solve.
Again, since one of the prime purposes of the new observatory
was to take advantage of the possibilities of improved instru-
ments, the design and construction of the telescopes, spectro-
scopes, and other appliances would require the solution of
many instrumental and engineering problems, and much work
of experiment. It was known, for example, that glass mirrors
change their form decidedly when exposed to the Sun's rays.
For this reason it was to be feared that they might not give
good solar images. This is a matter of fundamental impor-
tance, since the fixed telescope for solar observations neces-
sarily involves the employment of mirrors. In addition to
these questions, many others, very diverse in character, pre-
sented themselves. These included the preparation of a
programme of research, adapted for the special requirements
of the new observatory, in which all the investigations in
progress were to be closely correlated; the consideration of
the best methods of discussing and interpreting the photo-
graphs made with the spectroheliograph and other instru-
ments; the invention and construction of special measuring
and computing machines, etc.
Mount Wilson Solab Obsebvatoby 123
From a meteorological standpoint, the state of California
may be divided into three i^arts. In the northern region
the rainfall is very considerable, much cloudiness prevails,
and in almost all respects the conditions are unfavorable for
astronomical work. The central region, which may be con-
sidered to extend as far south as Point Conception, is favored
with much better weather conditions, best exemplified at the
Lick Observatory, on Mount Hamilton, where a high average
of night-seeing is maintained during a large part of the year.
In the southern part of California the climatic conditions are
different from those which prevail in the two other sections
of the state. The lighter rainfall is naturally associated with
fewer clouds, a remarkably steady barometer, and very light
winds.
There can be no doubt that the character of the country
immediately adjoining an observatory site affects the condi-
tions for astronomical work to an important degree. For this
reason it became desirable to make preliminary tests of a con-
siderable number of points in southern California. Similar
tests might have been desirable in Arizona, were it not for
the thunderstorms that prevail during the summer months in
the vicinity of Flagstaff, and other promising localities, which
would interfere so seriously with solar work as to put this
reofion almost entirelv out of consideration. As there were
other serious objections to Arizona sites, and as Hussey's tests
at Flagstaff did not indicate that the conditions were as favor-
able as in California, attention was concentrated on the rela-
tive claims of various mountains in southern California.
Hussey's tests in this region included Echo Mountain,
Mount Lowe, and Mount Wilson, in the Sierra Madre range,
and Cuyamaca and Palomar, much farther to the south.
His observations seemed to leave no doubt that Mount Wilson
would prove to be the best site for the purposes of a solar
observatory.
124 Stellae Evolution
Mount Wilson is one of many mountains that form the
southern boundary of the Sierra Madre range (Plate XLIX).
Standing at a distance of thirty miles from the ocean, it rises
abruptly from the valley floor, flanked only by a few spurs of
lesser elevation, of which Mount Harvard is the highest.
Except for a narrow saddle, Mount Wilson is separated from
Mount Harvard by a deep canon, the walls of which are very
precipitous. Farther to the west, beyond the saddle leading
to Mount Harvard, the ridge of Mount Wilson forms the upper
extremity of Eaton Canon, which leads directly to the San
Gabriel Valley. East and north of Mount Wilson lies the
deep canon through which flows the west fork of the San
Gabriel River, and beyond this rise a constant succession of
mountains, most of them higher than Mount Wilson, which
extend in a broken mass to the Mojave Desert. The Sierra
Madre range forms the northern boundary of the San
Gabriel Valley, which is further protected toward the east
from the desert by the high peaks of the San Bernardino
range.
The view from the summit of Mount Wilson is most exten-
sive, embracing the whole of southern California, and reach-
ing out over the Pacific to islands nearly one hundred miles
distant. Cuyamaca, about 130 miles to the south, not far
from the Mexican boundary, is easily visible. San Bernardino
and San Jacinto peaks, the latter 90 miles away, are so dis-
tinctly seen under normal conditions that a station might
easily be established on either of them, for experiments in
measuring the velocity of light from Mount Wilson. Mount
San Antonio (10,080 feet), 25 miles away, has already served
as a station for certain observations of the solar radiation,
supplementing the work of the Smithsonian Expedition at
Mount Wilson (Plate L).
During a part of the year, particularly from April to
August, fog rolls in from the ocean and covers much of the
Mount Wilson Solar Observatory 125
San Gabriel Valley during the night ( Plate LI). But these
fog-clouds rarely attain elevations exceeding 3.000 feet. The
mountains of the Sierra Madre range rise high above the
fog, and during many months of the year they enjoy practi-
cally continuous sunshine. In summer the sea breeze blows
for a part of the day, but it attains only a low velocity,
which decreases in passing from the valley to the moun-
tain tops.
Mount Wilson is reached from the San Gabriel Valley by
either one of two trails. One of these, known as the "Wilson
Trail," ascends from Sierra Madre, and is steep and irregular.
The other, called the "Xew Trail," rises from the foot of
Eaton Canon, about 6^ miles from Pasadena, and is about
9^ miles long. When our work commenced, it was but little
over two feet in width at its narrowest parts. It has an average
grade of about 10 per cent., and is much better adapted for
transportation purposes than the old Wilson Trail.
Some hundreds of tons of building material for the
observatory have been taken over the New Trail, on the backs
of mules or "burros" (donkeys) (Plate LII). The heavier
parts of instruments, which could not be taken up in this
way. were carried on a special truck built for the purpose
(Plate LIII). The running-gear consists of four automobile
wheels with rubber tires. The bodv of the truck is hunsr bv
wrought-iron yokes from the running-gear, with its lower
surface at a height of only six inches above the ground.
Steering-gear, of the type used on automobiles, is provided
for both pairs of wheels. A man riding on the load steers
the forward wheels, while the rear wheels are steered with a
tiller by a man walking behind the carriage. A single large
horse pulls a load of a thousand pounds on this carriage with-
out difficulty. With two horses, used in relays, the trip from
the lower end of the trail to the summit and return is com-
pleted with such a load in about fifteen hours. About sixty
126 Stellar Evolution
round trips were made with this truck for the purpose of
carrying the mirrors, lenses, and heavy castings of the Snow
and Bruce telescopes, the parts of a 15-H. P. gas engine,
and other heavy machines, as well as the 4-inch pipe columns
used in constructing the steel skeleton of the Snow telescope
house.
During the first two years, it was hoped that a railway
would be constructed to the summit of the mountain, where
a hotel had already been erected. When it finally appeared
that this hope must be abandoned, we were compelled to
adopt the alternative of widening the New Trail into a wagon-
road (Plate LIV). This work, which was done during the
autumn and spring of 1906 and 1907, was considerably ham-
pered by unprecedented storms in December and January.
The snow on the summit of Mount Wilson (Plate LV) was
five feet deep on a level, and the torrential rains, below the
snow line, brought down thousands of tons of earth and rocks
from the steep slopes of the mountain. When these difficul-
ties had been overcome, the transportation problem was so
far solved as to permit the structural steel for the building
and dome of the 60-inch reflector to be hauled to their
destination.
Our systematic tests of the atmospheric conditions on
Mount Wilson began in March, 1904. An old log cabin,
which had been in a state of partial ruin, was rendered
habitable and occupied until the "Monastery" was com-
pleted, in the following December. Frequent tests of tht'
solar definition were made with a 3|^-inch refracting tel(>-
scope, supplemented by meteorological observations.
The specific requirements of a site for an observatory to
be devoted to solar research and the study of stellar evolu-
tion are as follows:
1. Excellent definition of the solar image, on many days
of the year.
Mount Wilson Solar Obsekvatoby 127
2. Excellent definition by night, so as to permit reflecting
telescopes of large aperture to be used for the most exacting
work.
3. Great transparency of the day and night sky, essential
for accurate determinations of the "solar constant" (the
total heat radiation of the Sun, at a point outside of the
Earth's atmosphere), and the photography of stars and
nebulae requiring very long exposures.
4. Continuous clear weather for periods of many weeks,
rendering possible daily observations of changing phenom-
ena, of which an imperfect or erroneous idea might be derived
from scattered observations.
5. A low average wind velocity, especially during the
best observing season, to insure freedom from vibration of
telescopes employed for photographic work.
It is easy to see why the definition of the Sun's image is
usually much inferior to that of the stars or planets. The
heating of the earth, caused by the Sun's rays, produces
currents of warm air, which rise and mix with the cooler air
above. It has already been explained that poor definition is
produced by irregular refraction in the atmosphere, and that
this is caused by irregularities in the temperature of the air
through which the light rays pass. In this respect a moun-
tain peak may have some disadvantages as compared with an
extensive level area, because the rising currents of warm air
follow the mountain sides and tend to prouuce marked dis-
turbances in the images observed from the summit. It is
evident that this effect will be greatly enhanced if the moun-
tain is bare and rocky, instead of having its slopes covered
with trees and bushes. As the latter condition prevails on
most of the slopes of Mount Wilson, the heating of the air
is much less pronounced than in the case of many other
mountains. It is nevertheless very noticeable, and for this
reason the best observations of the Sun are made one or two
128 Stellar Evolution
hours after sunrise, and about the same time before sunset.
It is true that great depths of atmosphere must be traversed-
by the Sun's rays when it is so near the horizon. Neverthe-
less, the image on a large number of days in the summer
season is wonderfully sharp and distinct, permitting the
finest details of structure to be observed. The conclusions
based upon observations made with the 3^-incli refractor
were afterward confirmed with the large aperture of the
Snow telescope, leaving no doubt that with respect to solar
definition Mount Wilson offers very exceptional advantages.
The tests of the night definition, and of the transparency
of the night sky, were made by Barnard, during the work
of the Hooker Expedition. In chap, v a description has
been given of the Bruce 10-inch photographic telescope
of the Yerkes Observatory, used by Barnard in his studies
of the Milky Way. In order to extend farther south the
work previously done with an instrument of 6 inches aper-
ture on Mount Hamilton, Barnard brought the Bruce tele-
scope to Mount Wilson and made with it a remarkable series
of photographs. Mount Wilson (latitude + 34° 13') is 8°
south of the Yerkes Observatory, and 3° south of the Lick OU-
servatory. This fact, combined with the great transparency
of the sky, permitted Barnard to photograph regions of the
Milky Way which had been out of reach in his earlier work.
The best way of comparing the transparency of the sky
at Lake Geneva and Mount Wilson is by taking two photo-
graphs of the same region of the heavens, with the same
exposure time, on photographic plates of the same sensitive-
ness, used with the same telescope, by the same observer.
Such a comparison is illustrated in Plate LVI, which repre-
sents the cluster Messier 35. The difference in the number of
stars included on the photograph is a striking illustration of
the advantages of Mount Wilson. Indeed, if this result were
not confirmed by many others, and regarded by Barnard
Mount Wilson Solar Observatory 129
as representing a fair relative test, it might be supposed
that some difference in the mode of development or in the
sensitiveness of the plate had entered. The night on which
the Mount Wilson photograph was made was an average sum-
mer nigfht. while in the case of the Yerkes Observatorv
photograph the transparency was possibly higher than the
average there.
An illustration of the same sort is given in Plate LVII,
which shows the Pleiades as photographed by Barnard with
the Bruce telescope, with an exposure of 3 hours and 48
minutes at Mount Wilson and 9 hours and 47 minutes at
the Yerkes Observatory. It will be seen that the first photo-
graph shows quite as many stars as the second, and also
has a great advantage in sharpness, as indicated by the
much larc^er amount of detail broug^ht out in the nebulae.
This is due to the fact that the j^reater diffusion of liorht in
the Wisconsin sky tends to obliterate the finer details of the
photograph. It is interesting to conjecture what advantages
will result from the use of the 60-inch reflector under these
fine conditions.
During the long exposures Barnard kept a star on a pair
of cross-hairs in the eye-piece of a 5-inch refractor, attached
to the Bruce telescope. In this way he observed the defini-
tion of the stellar imaofes on a large number of nicfhts. As
previously explained, the definition of a star does not depend
in any considerable degree upon the transparency of the
atmosphere, but rather upon the absence of irregular refrac-
tion. Barnard found the average night '"seeing" to be
remarkably good, and this conclusion has also been con-
firmed with the large aperture of the Snow telescope.
The transparency of the sky by day has been most
: thoroughly tested by Abbot, in his studies of the "solar
constant" of radiation, which are described in chap. xxii.
As compared with Washington, where the previous work
130 Stellab Evolution
of the Smithsonian Astrophysical Observatory has been
done, the advantages of Mount Wilson are very marked.
Of equal importance for this work is the fact that the
observations can.be made day after day, with practically no
interruption, for a period of many v/eeks. In Washington,
during the same period, it might be possible to obtain onl v
two or three trustworthy determinations. Thus the manner
in which the solar radiation varies can be shown, in the one
case, by its daily fluctuations, while in the other it might be
wholly concealed.
Finally, the average wind velocity in the dry season
proved to be extraordinarily low, not only for an exposed
mountain-peak, but as compared with a station at any level.
During the rainy season, when there is much cloudy weather,
violent storms, accompanied by high winds, are not uncom-
mon. But in the dry season an almost dead calm frequently
prevails at night, and also during the early morning solar
observations. In the later hours of the day there is usually
a light breeze. The typical condition on Mount Wilson
during the dry season may be described as a perfectly cloud-
less sky, and so little breeze that the leaves are hardly
stirred by it.
It would be tedious to discuss the other conditions, such
as the heavy growth of foliage, the presence of abundant
springs of water, the neighborhood of large cities, etc., which
contribute toward the advantages of Mount Wilson as an
observatory site. The astronomical tests have been described
in detail because they illustrate the practical bearing of
atmospheric conditions on astronomical observations.
CHAPTER XV
THE SNOW TELESCOPE
Leon Foucal lt appears to have been the first to appre-
ciate the advantages of a fixed telescope, capable of forming
a solar or stellar image within a laboratory. A large sidero-
staf, constructed by Eichens after his designs, was completed
in 1868, the year of Foucault's death. It remained at the
Paris Observatory, where it was subsequently employed by
Deslandres for solar photography. For small images of the
Sun this instrument gave good results, although the imper-
fection of the driving caused the image to wander more or
less from a fixed position. This difficulty has been inherent
in almost all types of fixed telescojjes. It is coupled with
the inconvenience that the solar image produced by the
siderostat or heliostat rotates in an irregular manner, which
would cause distortion in long-exposure photography with a
fixed spectroheliograph.
It is not easy to see why the heliostat. in some of its
forms, was not more rapidly developed. With a few excep-
tions, its practical application has been confined to small
heliostats of various types, used to reflect a beam of sunlight
into the laboratory, but not to produce a large image of the
Sun. In other words, the heliostat was not developed into
an instrument of precision, capable of giving a large and
well-defined solar image, and maintaining it accurately fixed
in position, until the coelostat was revived for eclipse pur-
poses, about ten years ago.' This instrument had been
invented long before, but its great advantages, due to the
1 The great equatorial coud4 of the Paris Observatory is an admirable example
of a fixed telescope, but I do not think it has been tested for solar obserrationB.
131
132 Stellar Evolution
simplicity of its construction, the ease of driving it with a
precision as great as in the case of the equatorial refractor,
and, above all, the fact that the solar image produced by it
does not rotate, had been overlooked. At Turner's sugges-
tion it was employed for eclipse purposes, at first by some
of the English parties, and subsequently by astronomers in
all parts of the world.
However, the conditions under which eclipse observations
are made are very different from those that obtain in ordi-
nary solar work. A defect of the coelostat is that the direc-
tion of the beam of light reflected horizontally from the.
mirror varies with the declination of the Sun. During the
few minutes of a total eclipse the Sun's declination does not
change appreciably, and the telescope into which the light
is reflected by the coelostat stands fixed in position. But.
in solar observations continued throughout the year the
direction of the reflected beam is constantly changing, as
the Sun moves north or south of the equator. As it would
be inconvenient to swing the long telescope tube, which is
pointed at the coelostat, around through the necessarily large
angle, a second mirror must be introduced to receive the
light reflected from the coelostat mirror and send it in any
desired direction. About once a week, or sufficiently often
to cause no appreciable loss of sunlight, the second mirror
is moved a short distance, so that it may continue to receive
all the light of the reflected beam, in the changed position
given it by the variation in the Sun's declination.
Another difficulty of the coelostat, which is common to
all forms of heliostat, but plays no part in total-eclipse work,
is the distortion of the mirrors by sunlight. This obstacle
is really the only serious one presented by this form of
telescope. How it has been met at the Solar Observatory
is explained below.
The Snow telescope, the optical and mechanical parts of
The Snow Telescope
133
which were constructed under Kitchey's supervision in the
shops of the Yerkes Observatory, is illustrated in Plate
LVIII. This photograph shows the coelostat and the adjust-
able second mirror, whence the light is reflected to a concave
mirror of 60 feet focal length, which forms the solar image.
The general arrangement of the telescope, as established on
Mount Wilson, is indicated in Fior. 5. The coelostat stands
'%^\]
FIG. 5
Plan and EleTation of Snow Telescope House on Monnt Wilson
on a carriage, which can be moved east or west along the
line aa. On account of the configuration of the ground,
which falls rapidly toward the north, it was necessary to
make the long axis of the building run 15" east of north,
instead of being: exactlv in the meridian. For the same
reason this axis is not horizontal, but inclined downward 5°
toward the north. Without these adaptations of the plan to
the conditions of the site, the height of the northern part of
the building would have been very great, involving serious
increase of expense. The rails 66, on which the carriage
:" bearing the second mirror slides, are parallel to the optical
axis. The coelostat mirror, 30 inches in diameter, and the
second mirror, 24 inches in diameter, have plane surfaces,
134 . Stellar Evolution
and serve merely for bringing the sunlight into the telescope
house. The plane of the coelostat mirror is parallel to the
Earth's axis, and the mirror can be rotated around this axis
once in forty-eight hours, by means of a driving-clock. This
exactly counteracts the motion of the beam due to the Sun's
apparent motion through the heavens.
From the second mirror the light passes to either one of
two concave mirrors, each 24 inches in diameter (Plate LIX) .
One of these, which has a focal length of 60 feet, is supported
on a carriage so that it can be moved (for focusing) along
the rails cc, which are mounted on the small pier shown near
the middle of Fig. 5. This mirror produces an image
of the Sun about 6.7 inches in diameter, at a position in the
spectroscope house determined by the angle which the con-
cave mirror makes with the incident beam of sunlight. If
the mirror stood normal to the beam, the sunlight would be
reflected directly back upon itself toward the second mirror.
If, however, the concave mirror is turned slightly to one side,
the solar image can be formed at the end of the pier /, where
the 5-foot spectroheliograph stands. By moving the mirror
back toward the north, along the rails on which it slides, the
image can be brought to a focus on the pier i, where the slit
and photographic plate of a Littrow spectrograph, of 18 feet
focal length, are mounted. Again, by rotating the concave
mirror so as to return the beam in a somewhat different direc-
tion, the solar image can be sent into the constant tempera-
ture room III, where the holographic apparatus, for studying
the heat radiation of different parts of the Sun, is mounted
on the massive triangular pier kkk.
If a larger solar image is required, the mirror of 60 feet
focal length is moved out of the way, and the beam from the
second mirror allowed to pass to a concave mirror of 143
feet focal length, mounted on a pier at the extreme north
end of the telescope house. The image of the Sun is then
The Snow Telescope 135
formed at the pier g, 143 feet from the concave mirror.
This imaofe is 16 inches in diameter, and is used for the
sj^ecial study of solar details, for which a large scale is
required.
The remarkable convenience of such a telescope, when
cx)ntrasted with a ffreat movable refractor like the Yerkes
telescope, is immediately evident. Instead of attaching each
heavy instrument, one by one, to the end of a moving tele-
scope tube, it is set up once for all on a pier, where its
adjustments need never be disturbed. It is thus possible to
pass rapidly from one instrument to another, photographing
the forms of the calcium flocculi, for example, with the
s])ectroheliograph, and their spectra, only a moment later,
with the powerful Littrow spectrograph. In view of the
importance of studying solar phenomena nearly simulta-
neously by various methods, and of closely correlating the
observations, the advantages afforded by such a telescope
will be easily recognized.
The peculiar form of house in which the Snow telescope
is mounted calls for a word of explanation. In previous
experiments, some of which were made on Mount Wilson in
the spring of 1904, the conclusion was reached that the dis-
turbance of the definition caused by warm air rising from
the ground in the immediate neighborhood of the heliostat
could be appreciably reduced by mounting the instrument
at a considerable heiffht. Observations made with a tele-
seo})e supported in a tree, at various heights up to seventy
feet, seemed to leave no doubt regarding this point. A
second consideration, the importance of which had been
- particularly emphasized by experience with a smaller coelo-
stat telescope, having a closed tube not provided with means
'; of ventilation, was the necessity of designing a house so that
the temperature within would be at all times as nearly as
possible the same as that of the outer air. It is evident that.
136 Stellar Evolution
if this condition is not met, the mixture of air of different
temperatures at the open end of the house, through which
the beam enters, will cause irregular refraction and conse-
quent disturbance of the image.
Plate LX shows the pier on which the coelostat is
mounted, at a height of nearly 30 feet above the ground.
Since the parallel rays from the coelostat to the concave
mirror pass through a closed house, it is not essential that that
part of the building should stand high above the ground. It
is important, however, that disturbances due to heating of
the walls, caused by sunlight falling upon them, be obviated.
For this reason all parts of the building, including the
movable shelter, the spectroscopic laboratory, and the long
narrow house extending north from the laboratory, have an
inner wall and ceiling of canvas, and an outer wall composed
of canvas louvers, very completely ventilated. The roof is
also ventilated, by wooden louvers at the ridge throughout
the entire length of the movable shelter and the north exten-
sion, and at the peak of the laboratory. Rain and snow are
prevented from entering the roof louvers by means of canvas
guards, which can be raised or lowered at will. The house,
extending north from the laboratory has a floor of canvas,
with a space below, through which the air may pass freely.
The louvers surrounding the coelostat pier are intended
to protect the pier from vibration caused by the wind, and
from heating by the Sun. The steel structure does not touch
the pier at any point, and is therefore made rigid enough to
support itself in high winds. When not in use, the coelostat
and second mirror are covered by a house on wheels, closed
at both ends by walls of heavy canvas. These may be opened,
so that when the house is moved to the north the coelostat
stands fully exposed. The movable shelter then fits closely
against the south wall of the laboratory, and forms a part of
the tube through which the beam passes.
The Snow Telescope 137
In the preliminary tests of the Snow telescope at the
Yerkes Observatory the results were rather disappointing,
though good images were sometimes obtained. There was
evidence of distortion of the mirrors by the Sun's heat, and
in the first experiments on Mount Wilson similar difficulty
was experienced. Soon after the exposure of the mirrors to
the Sun it was seen that the focal length was increasing, and,
as the focus changed, evidence of astigmatism, due to the
distortion of the plane mirrors, made itself apparent in the
appearance of the image inside and outside of the focal
plane. It was soon found that the focus changed much more
rapidly after the mirrors had been silvered for some time,
because of the greater absorption of heat by the slightly tar-
nished surfaces. Moreover, the change was less on a day
with a cool breeze than on a day with no wind. The question
then arose whether this difficulty could be remedied.
In the early morning, when, as before stated, the defini-
tion of the Sun is best, the heating is much less marked than
later in the day. If the mirrors are shielded from sunlight
between the exposures of photographs, and if the exposures
are made as short as possible, excellent results can be
obtained at this time, and in the late afternoon, not long be-
fore sunset. It has been found advantageous to direct a
strong blast of air on the surfaces of the mirrors, bv means
of electric fans, during the exposures of the photographs and
the intervals between them.
It must be understood that the precautions mentioned are
necessary only when it is desired to secure the finest possible
definition of the solar image. When such precautions are
used, the average photographs taken during the summer in
the early morning with the Snow telescope and a temporary
spectroheliograph are but little inferior to the best photo-
graphs, secured on only a few days in the year, with the 40-
inch Yerkes telescope and the Rumford spectroheliograph.
138 Stellar Evolution
The best photographs taken on Mount Wilson are distinctly
superior to the best secured in our work with the Rumford
spectroheliograph. It must not be supposed that no work
can be done with the Snow telescope except under the con-
ditions stated. As a matter of fact, very fair photographs
can be obtained with the spectroheliograph at almost any
time during a cool day, and in the early morning and late
afternoon hours of a hot day without wind. It is only
necessary to arrange the daily programme of observations so
that the spectroheliograph, which requires the finest defini-
tion, is used during the period when the seeing is best.
Photographic work on the spectra of sun-spots follows, and
after this is completed the conditions are entirely satisfactory
for various other observations, such as bolographic work on
the absorption of the solar atmosphere, etc. Some of the
results obtained with the Snow telescope will be illustrated
in subsequent chapters.
From laboratory tests, it appears that the distortion of
mirrors in sunlight is chiefly due to actual bending of the
glass, the front surface, expanded by the heat, becoming
convex and the rear surface concave. Radiation from an
electric heating coil, placed a short distance behind a mirror",
restores its figure, but not perfectly. A much better way of
heating the back of a mirror is by reflecting sunlight upon
it. Perhaps the best plan, however, is merely to increase
the thickness of the glass mirrors (p. 235).
CHAPTER XVI
SOME USES OF SPECTROHELIOGRAPH PLATES
The necessity of designing tlie Rumford spectroheliograph
for use as an attachment of the Yerkes telescope interfered
somewhat with its efficiency. Under good conditions it gives
excellent results, but the limitations of aperture, and the
difficulty of securing perfect equality in motion of plate and
solar image, are sometimes apparent in the photographs ob-
tained with it. Fortunately, the case was different with the
Snow telescope. It was possible here to adopt the most
satisfactory form of spectroheliograph. in which the instru-
ment is moved as a whole, while the image of the Sun and
the photographic plate are stationary. The first spectrohelio-
graph of this type was constructed in 1893 and employed
in attempts to photograph the solar corona without an
eclipse, from the summit of Mount Etna. For all instru-
ments of moderate dimensions, motion of the spectrohelio-
graph as a whole appears to be preferable to any mechanical
contrivance for moving the plate and solar image in syn-
chronism.
A photograph of the spectroheliograph, mounted for use
with the Snow telescope, is reproduced in Plate LXI. A better
idea of the general design may be obtained from Plate LXII,
which shows the spectroheliograph in our instrument shop
before it was completed. It consists essentially of a massive
cast-iron base, bearing four short V-rails at its four corners, on
which the moving part of the instrument is carried by four
^ steel balls. The cast-iron platform which bears the slits and
optical parts has four inverted A-rails. which rest on the
steel balls, but almost its entire weight is supported by
139
140 Stellar Evolution
mercury, in three tanks formed by subdivisions in the base
casting. Wooden floats extend from the lower surface of
the iron platform into these tanks, reducing to a minimum
the amount of mercury (about 560 pounds) required to up-
hold the instrument. The motion of this platform with re-
spect to the fixed solar image and photographic plate is pro-
duced by either one of two screws of different pitch, driven
by an electric motor arranged to give a perfectly uniform
speed.
The collimator slit, on which the solar image is formed,
is shown on the right of Plate LXI. On account of the large
size of the solar image, which is about 6.7 inches in diameter,
the slit is 8^ inches long. After passing through the slit
the light falls upon a large collimating lens 8 inches in diam-
eter, which renders the rays parallel. They then meet a
silvered glass mirror, from which they are reflected to the two
prisms, of 63^° angle. After being dispersed by the prisms
the rays strike the 8-inch camera lens, which forms an image
of the spectrum on' the camera slit (shown near the center of
Plate LXI). The optical train thus resembles that of the
Rumford spectroheliograph, but the lenses and prisms are
so much larger that no light is lost from the circumference
of the solar image.
On account of the great curvature of the spectral lines
produced by such prisms, it would be necessary to employ a
highly curved camera slit, in case an ordinary straight slit
were used to admit the light from the Sun. In this event
the resulting photograph would be greatly distorted, because
points lying along a straight line on the Sun would ap{)ear
along a curved line in the photographs. Thus the image,
instead of being circular, would be shaped somewhat like
an apple, greatly flattened on one side. By dividing the
curvature evenly between the two slits the distortion is
eliminated and the photograph is made circular.
Uses of Specteoheliogbaph Plates 141
The actual operations in making a photograph of the
flocculi with one of the calcimn lines are as follows: An
electric arc, the carbons of which have been moistened with
a solution of calcium chloride, is mounted in front of the
collimator slit. The brig^ht H and K lines are easilv visible
in the spectrum of such an arc, although the same region of
the solar sj3ectrum is difficult to see distinctly. By means of
a micrometer screw, the camera slit is made to coincide with
one of the lines. Thus the only light which can reach the
photographic plate is that of calcium vapor. Up to this time
the mirror of the coelostat has been shielded from the Sun
by a canvas screen, in order to protect it from distortion.
After the photographic plate has been placed in position in
its support in front of the camera slit, the canvas screen is
removed and the solar image brought to a sharp focus on the
collimator slit, by moving the concave mirror of the Snow
telescope. The slide of the plate-holder is then drawn and
the electric motor started. The screw, driven by the electric
motor, then causes the entire spectroheliograph to move at a
slow and uniform rate, so that the collimator slit passes over
the solar image and the camera slit moves across the photo-
graphic plate.
If it is desired to take a photograph with a hydrogen line,
instead of a calcium line, the prisms and mirror are adjusted
until the line in question falls upon the camera slit, when
the exposure is made as before.
In the daily programme of observations at least one photo-
graph with the Hj line of calcium, showing the faculae and
low level calcium vapor; one with the H., line of calcium,
. showing the flocculi at a higher level : one with the H'y line
.of hydrogen: and one with an iron line, are made in the
^? early morning and again, if circumstances permit, in the late
' afternoon (Plates LXIII-LXVII). Since the weather is
clear dav after dav throucjh the summer and autumn months
1-4:2 Stellar Evolution
(on 112 consecutive days in the summer of 1907), and not
infrequently during the rainy season, the instrument thus
yields a large number of plates, suitable for the comparative
study of the flocculi.
Photographs of the prominences are also made daily, when
circumstances permit. These are used to determine the
changes in number and total area of the prominences during
the Sun-spot period.
In the establishment of an observatory much remains to
be done after successful photographs of astronomical phe-
nomena have been obtained. Indeed, although the work of
organization must be far advanced before photographs can
be secured, the most important steps are still to be taken.
For an astronomical photograph, while it may yield much
new information from casual examination, is to be regarded
as a document of great value, worthy of prolonged investi-
gation. Every photograph of the Sun, for example, repre-
sents its changing phenomena as they were at the moment
of the exposure, under conditions which will never be exactly
repeated. The best methods of obtaining from photographs
all the knowledge they are capable of conveying are to be.
arrived at only after the fullest consideration of the possi-
bilities.
In chap, xi the most striking characteristics of the floc-
culi have been explained and illustrated. We must now
consider how these objects may be systematically studied, in
such a way as to contribute to our knowledge of the solar
constitution. The most obvious peculiarity of the flocculi,
apart from their change in form, is their motion across the
Sun's disk. This is due to the solar rotation, which was
first discovered through the daily motion of sun-spots. It
is remarkable that the spots do not move as they would if
they were fixed to the surface of a solid sphere. Spots in
different latitudes move with different angular velocities, and
Uses of Spectboheliogbaph Plates 143
exhibit wliat is called the "equatorial acceleration:" i. e.,
s|x)ts near the Sun's equator complete a revolution in much
shorter time than those in higher latitudes. At the equator
the rotation period is about twenty-five days. At 10° north
or south latitude the j^eriod is several hours longer, and at
45' it is about twenty-seven and a half days. The faculae,
according to results obtained by Stratonoff and others, follow
the same general law. Spectroscopic observations, based on
an application of Dopplers principle show that the motion
is not confined to the spots and faculae, but is also shared
by the layer of metallic vapors (the "reversing layer") which
lies just above the photosphere, and produces the dark lines
of the solar spectrum by absorption of the white light
coming through it from below. It thus becomes interesting
to inquire whether the calcium flocculi, which we suppose to
be clouds of luminous vapor lying at an elevation of several
thousand miles above the photosphere, show a similar law
of rotation.
The method employed to determine the rotation period of
the spots is to measure their latitude and longitude, referred
to the center of the Sun, on plates taken at intervals of one
or more days, and in this way to ascertain the change of
longitude of the same spot in twenty-four hours. By thus
obtaining the velocities of spots in different latitudes the law
of rotation can be derived. In considering the methods of
measuring the latitude and longitude of a spot, we must
remember that the plane of the Sun's rotation is inclined at
an angle of about T" with that of the Earth's orbit. The
Earth passes through the nodes (the intersection of this
plane with the ecliptic) about June 3 and December 5. and
only on these dates do the sjx)ts appear to move in straight
I lines across the disk. The angle between the Sun's axis and
the north and south line in the sky (called the "position
angle'' of the Sun's axis) varies about 53 "" in the course of
144 Stellar Evolution
the year — about 26^° each side of zero. It is thus evident
that in determining the latitude and longitude of a spot by
ordinary methods of measurement considerable calculation
will be required. The process employed at Greenwich, on
the direct photographs of the Sun obtained there, is to meas-
ure the distance of the spot from the center, and the angle
between the Sun's axis and the line joining the spot with
the center of the disk. As the inclination of the Sun's axis
is known for every day in the year, it then becomes possible
to calculate the latitude and longitude of the spot.
This method is very satisfactory when a comparatively
small number of objects are to be measured on each plate,
which is the case with sun-spots. But the flocculi are so
numerous, and offer so many points suitable for measure-
ment, that the calculations required for each spectrohelio-
graph plate would be very extensive. In seeking to find
some simple method of abridging these calculations, it
appeared that the solar photograph might be projected
upon the surface of a globe ruled with meridians and
parallels 1° apart. The axis of the globe being set at the
inclination corresponding to the date of the photograph, it
should then be possible to read off the latitude and longitude
directly, by estimating the position, in tenths of a degree,
of the flocculus in question, with reference to the nearest
meridian and parallel (Plate LXIX). As the longitude of
the center of the Sun's disk is tabulated for each day in the
year, no calculations would be necessary, except to add or
subtract this longitude in the case of each of the readings.
This method proved so satisfactory, when used at the
Yerkes Observatory in measuring the Kenwood photographs,
that it was afterward adopted, in perfected form, in the
Computing Division of the Solar Observatory. The new
globe-measuring machine, or "heliomicrometer," is illus-
trated in Plate LXX. Two 4-inch telescopes, shown in the
Uses of Spectroheliogbaph Plates 145
upper part of the cut, are pointed toward two plane silvered
glass mirrors thirty feet away. One of these mirrors receives
light from the spectroheliograph plate, which is mounted
immediately under the right-hand telescope and illuminated
by incandescent lamps from behind. The other receives
light from a globe, mounted below the left-hand telescope
and illuminated on its front surface. The images of globe
and plate, given by the two telescopes, are brought together
in a single eye-piece, so that the observer sees them super-
posed. If, then, the surface of the globe is ruled with
meridians and parallels, as in the instrument previously
described, the positions of the flocculi can be read oflf by
estimation. However, it is desired in this case to attain a
higher degree of precision in the measurements, and to see
small and faint flocculi to better advantage than would be
possible if they were observed in projection against the illu-
minated surface of the globe. Accordingly, a pair of cross-
hairs, which can be moved over the plate in a horizontal or
vertical direction by the observer at the eye-piece, is made
to coincide with the object to be measured. The globe is
then illuminated, and rotated in latitude and longitude until
t a point corresponding to the intersection of the equator and
the central meridian falls exactly upon the cross-hairs. A
circle, which can be read by the observer at the eye-piece,
then shows the angle through which the globe has been
turned in latitude, A second circle gives the distance in
longitude from the center of the Sun. It is, of course, to be
understood that the axis about which the globe is turned in
measuring longitudes is set at the proj^er inclination for the
date of the photograph. For less precise measurements, the
position of the cross-hairs may be estimated with reference
;to the rulings on the (fixed) globe,
f This instrument, which was constructed in the shop of
the Solar Observatory, has proved very satisfactory in prac-
146 Stellar Evolution
tice. It has been found that the latitudes and longitudes,
thus read off directly, are as accurate as when determined
by measuring the plate in an ordinary measuring-machiiic
and performing the necessary calculations. Since the meas-
urements can be be made quite as rapidly on the heliomi-
crometer as on the other machine, all the time required to
make the calculations is saved. Thus one observer can
measure a great number of flocculi, and the services of sev-
eral computers are rendered unnecessary.
A discussion of the measurements made in this way shows
that the flocculi follow a law of rotation similar to that which
governs the spots and faculae. It will require some time to
learn whether the velocities of the flocculi differ appreciably
from those of the spots. It appears probable, however, from
results thus far obtained, that the flocculi move with about
the same velocity as the faculae. This would be a natural
result, since, as already explained, the vapors of the flocculi
probably rise from the faculae, and lie immediately above
them.
The importance of providing for the closest possible cor-
relation between all of the investigations of the Solar Obser-
vatory has already been mentioned. For this reason studies
of the solar rotation should be made with reference to other
solar work. The motions of individual flocculi frequently
differ considerably from the average motions of the flocculi in
the same latitude. Such differences, in many instances, are
doubtless similar to those observed in the case of sun-spots,
where they are related to the spot's activity, which varies
greatly during the course of its development. However, the
daily motion of a flocculus may also depend iipon its height
above the photosphere, and this may vary from day to day.
It thusbecomes desirable to learn whether differences in the
height of flocculi can be detected and actually measured.
For example, do the hydrogen flocculi lie at an average level
Uses of Spectroheliograph Plates 147
in the solar atmosphere above or below that of the calcium
flocculi, and, if so, do they show differences of rotational
velocity that may de{.)end upon this fact?
It has already been explained, in chap, xi, that the cal-
cium flocculi photographed when the bright H^, or K., line
is employed probably lie above the bright objects of similar
form, but somewhat smaller area, which are photographed
when the slit is set on the broad H, or K, band. It is not so
obvious, however, that the average level of the hydrogen floc-
culi is above that of the H , and K, calcium flocculi, but this can
be determined by accurate measurements. The forms of the
dark hydrogen flocculi. as already remarked, closely resemble
those of the bright calcium flocculi. though in manv cases
there are im|X)rtant differences (Plates LXXI and LXXII).'
With the aid of the stereocomparator, an instrument manu-
factured by the Zeiss Optical Company for the purpose of
making accurate comparisons of photographs, it is possible
to observe a hydrogen photograph in superposition upon a
calcium photograph, taken within so short an interval of time
that no appreciable change occurred on the Sun between the
exposures. With the monocular eye-piece of the instrument
the two photographs, in precise superposition, are observed in
quick succession. For this purpose a device is used which
permits the eye to see one of the plates, and, immediately
afterward, the other. If a micrometer wire is set on a cal-
cium flocculus Iving near the edge of the Sun, and the image •
of the corresjxjnding hydrogen flocculus is then brought into
view, it is found to be displaced slightly away from the center
of the disk. This is not true of all the hydrogen floc<?uli.
On the average, however, these dark hydrogen clouds seem to
' These photographs were separated by an interval of 2h 26'u, daring which time
the changes in the forms of the flocculi would not ordinarily be snfliciently marked
to interfere with the general comparison of the more conspicuous features. In this
case, however, the changes may have been rapid, since the numerous bright flocculi
near the spot indicate great eruptive activity. For the accurate comparison of
details, the photographs must be taken simultaneously.
148 Stellar Evolution
be displaced in this way, by an amount representing a heiglit
of some 1,500 miles above the corresponding calcium clouds.'
It will therefore be interesting to determine at some future
time whether the rotational velocity of the hydrogen flocculi
differs appreciably from that of the calcium flocculi. I
An important step in the interpretation of spectrohelio-
graph plates will be made when it can be ascertained whethei
anomalous dispersion plays any part in producing the phe-
nomena recorded by them. Our present views as to th<
nature of Sun-spots, prominences, and other solar phenomena
are based on the assumption that their light reaches us along
nearly straight lines. If the pressure in the region through
which the rays pass is low, this may be essentially true for
white light. But we know that light of about the same
wave-length as that of an absorption line in the spectrum, is
bent far out of a straight path when it passes through the
vapor to whose absorption the line is due. The conse-
quences of this fact have led Julius to develop a new solar
theory, based on the supposition that all metallic vapors at
any given distance from the Sun's center are completely
mixed, but not of uniform density throughout. Under these
circumstances the chromosphere, prominences, and flocculi
would not exist as we see them, but such appearances might
be caused by anomalous dispersion of light passing out
through the vapors from the interior of the Sun. A series
of investigations, involving solar, stellar, and laboratory work,
is being carried out on Mount Wilson for the purpose of
testing this theory.
The rotation periods of sun-spots may depend upon their
level, and this raises the old question as to the position of
these objects with respect to the photosphere. According
to the common view sun-spots are saucer-shaped cavities in
the photosphere. This idea is based upon the observations
1 Tliis result must be checked on photographs taken simultaneously.
Uses of 8pectroheliogbaph Plates 149
)f Wilson, who found that when a spot is carried toward the
[imb by the solar rotation, the penumbra, on the side toward
the center of the disk, is reduced in apparent width, as it
would be, on account of its inclination to the line of vision, if
it sloped downward toward the umbra. The best modern
results do not offer any certain confirmation of this view, and
thus render necessary an appeal to some independent test of
the question. Ten years ago it was pointed out by Frost
that the heat radiation of a spot, as compared with that of the
neighboring photosphere, increases as the S|X)t approaches
the limb. From this it was naturally concluded that the spot
must lie above the photosphere, at such a level as to escape
the influence of the low-lying absorbing veil, which so greatly
reduces the intensity of the photospheric light at the solar
circumference. It has recently been found, however, that
sun-spots radiate a much smaller proportion of violet light
than the photosphere. As violet light is always reduced by
an absorbing atmosphere in much larger proportion than
liorht of lonorer wave-length, it follows that the observed
effect would be seen in the case of sun-spots, even if they
were at the same level as the photosphere. To remove
the diflSculty it is only necessary to confine the comparative
measures to a single color, rather than to use the total radia-
tion, comprising light of all wave-lengths.
The S[jectroheliograph affords a simple means of accom-
plishing this. It is employed to make photographs of a sun-
six)t and the surrounding photosphere on various dates, corre-
sponding to the changing position of the spot on the solar
disk. In making these photographs the camera slit is set, not
, on any of the spectral lines, but on a space between the lines,
, preferably in the yellow or red, since the influence of extra-
/neous light will be least marked in this region. On account
of the darkness of the spot, which would require an exposure
about six times as long as that for the photosphere to give a
150 Stellar Evolution
photograph of equal intensity, it is desirable to decrease the
intensity of the photospheric light by a dark glass, placed over
the slit, but so arranged as not to reduce the light from the
sun-spot. In this way the spot and photospheric light can be
compared from day to day, by means of photometric meas-
urements. The same method can be employed to measure
the level of the flocculi. Such work is now in progress at the
Solar Observatory, in conjunction with the other investiga-
tions already mentioned. Since the level of a spot may
aflFect its temperature, and therefore its spectrum, an attempt
will be made to correlate this work, not only with determina-
tions of the spot's motion, but also with the spectroscopic
observations described in chap. xvii.
These few examples may suffice to give an idea of the
character of the work done with the spectroheliograph of the
Snow telescope. The vertical motion of the calcium vapor
in the flocculi ; the manner in which it flows horizontally
over sun-spots ; the relationship, in point of development, of
flocculi to spots ; and other similar matters, are also studied
systematically. It may also be added that the area of the
flocculi is measured on each day's plates, since it serves as an
index to the Sun's activity, which may prove important when
considered in its bearing on possible variations of the solar
radiation and their effect on terrestrial phenomena.
CHAPTER XVII
A STUDY OF SUN-SPOTS
It has already been remarked (p. 69) that sun-spots,
though apparently much darker than the photosphere, are, in
reality, brilliantly luminous objects. Though they thus ap-
pear dark merely by contrast, the cause of their reduced
brilliancy has given rise to much discussion. Some of the
most recent theories have maintained that sun-spots are so
much hotter than other parts of the solar surface that the
photospheric clouds, due to condensation of the vapors rising
from the Sun's interior, cannot form at these points. One of
Lockyer's arguments in support of his hypothesis that the
terrestrial elements are dissociated at the high temperature
of the stars is based upon the view that at times of sun-spot
maxima the spots are too hot to permit certain of the terres-
trial elements to exist in them. This conclusion was founded
ujxin a long series of observations of certain lines in the
spectra of sun-spots. The spot spectrum diflFers from the
solar spectrum in the fact that some of the solar lines are
strengthened or widened, some are weakened, and many are
unchanged (Plate LXXIV). The number of lines whose
intensities are thus altered amounts to many hundreds;
indeed, if the fainter lines are taken into account, to several
t]i()usands. Lockyers observations consist in recording, on
ry clear day, the "twelve most widened lines" in the
ctra of spots then visible on the Sun. His results
- med to indicate that at sun-spot minima the most widened
lilies represent known substances; while at sun-spot maxima
many of these give place to unknown lines, which he attrib-
uted to unknown substances produced by dissociation of the
151
152 Stellar Evolution
elements at the high temperature assumed to be character-
istic of periods of greatest solar activity. Some of his later
papers favor the view that sun-spots' possess a lower tem-
perature than would thus be indicated, and he may therefore
have decided to abandon the conclusions based on his earlier
spectroscopic observations.
In spite of these results, and of all the theories which
attribute high temperature to sun-spots, the more common
opinion has been that they are regions of reduced tempera-
ture. This view has been based partly upon their decreased
brightness, as compared with the photosphere, and partly
upon the presence in their spectra of certain bands which,
though unidentified, were supposed to represent molecules
that cannot exist at the high temperature of the Sun. Accu-
rate knowledge of these bands, however, was almost entirely
lacking, on account of their faintness and the extreme difii-
culty of observing them visually.
It seemed probable that progress in this department of
solar research might be expected to result from the success-
ful application of photography to the study of spot spectra.
Experiments made with this object in view at the Kenwood
Observatory showed some of the principal widened lines,
but failed to give the details needed for satisfactory work.
These results were surpassed by photographs made by Young
with the 23-inch Princeton refractor, but here also the need
of more powerful instrumental means seemed to be apparent.
The Kenwood experiments were continued with the 40-inch
Yerkes telescope, and some of the "band lines," first observed
visually by Maunder, were photographed, in addition to many
of the widened lines. However, there was reason to believe
that much better results could be obtained with the aid of a
long-focus grating spectrograph, capable of photographing
• Because of the great strength of the titanium lines in Arcturiis.
A Study of Sun-Spots 153
the sj)ectrum on a large scale. Further work was therefore
deferred until it could be taken up with the Snow telescoj^e
and a powerful Littrow or auto-coUimating spectrograph.
This spectrograph is of a very simple type. The image of
the Sun is formed on a slit, s, through which the light passes to
a 6-inch collimating objective, o. of 18 feet focal length, which
renders the rays parallel (Fig. 6). The rays then fall upon
a plane grating, g, which diffracts them into a series of
spectra. Light from a jxjrtion of one of these spectra returns
to the objective, o. which forms an image of the spectrum on
(^i • ^
FIG. 6
Path of Bays in Littrow Spectrograph
a photographic plate, p, standing just above the slit. In
order to form the image at this point the grating must be
slightly inclined backward, so as to send the beam npward.
This instrument, as mounted for use with the Snow tele-
scope, is shown in Plate LXXIII. As the tube of the spec-
trograph stands immediately above the spectroheliograph, a
section of it can be rotated out of the way. to permit access
to the prism-train of the latter instrument. When the
spectrograph is to be used instead of the spectroheliograph,
the concave telescope mirror is moved north through a suffi-
cient distance to transfer the focal plane from the spectro-
heliograph slit to the spectrograph slit. Then, by inclining
- the mirror backward through a small angle, the solar image
is raised to the proper height. After final focusing, a sun-
spot is brought exactly upon the slit with the aid of slow-
^ motion electric motors, connected with the concave mirror
and controlled from a point near the focal plane.
In photographing the spectrum of a sun-spot, all light is
154 Stellar Evolution
excluded from the spectrograph except that which comes from
the umbra. This is done by covering all of the slit except
a small portion at the center. The dispersion of the second
or third order of the grating is usually employed. After this
exposure has been completed, the centei* of the slit is covered
and light from the photosphere admitted on each side. This
gives a narrow photograph of the spot spectrum between
two strips of solar spectra (Fig. 1, Plate LXXIV).
Casual examination of the spot spectra thus recorded is
sufficient to show that the problem of interpreting them is
not a simple one. If we consider, for example, the lines of
some single element represented in the spot, we find that they
are not all affected alike. Some are greatly strengthened,
or perhaps attended by broad, faint wings. The former effect
is so very pronounced, in certain cases, that lines wholly
invisible in the solar spectrum are among the most conspi-
cuous of the spot lines. Some of the solar lines, on the con-
trary, are greatly weakened, or entirely absent in the spot
spectrum. Finally, there are many spot lines of unchanged
intensity. Examples of most of these phenomena are illus-
trated in Plates LXXIV and LXXVI.
In order to interpret such results, it is necessary to hav(»
recourse to laboratory experiments. It might be supposed
that the required knowledge of terrestrial spectra would be
available in the literature of spectroscopy. This, however,
is not the case. It is true that the lines in the spectra of
most of the elements have been measured, and many experi-
ments have been made on the changes in spectra produced
by varying the conditions under which the vapors emit their
radiations. It usually happens, however, when one attempts
to apply published results to the interpretation of solar
phenomena, that the data required for the solution of the
particular problem in hand are lacking. Pressure, for
example, is known to displace spectral lines toward the red,
A Study of Sun-Spots 155
and the actual sliifts of certain lines of several different ele-
ments have been measured. But these form a very small per-
centage of the total number of lines in the spectra of these
substances, and the shifts of any lines that happen to be
under investigation are rarely found in the published tables.
The same may be said of the effect of temjierature on spectra.
It has long been known that a reduction of temperature
increases the relative brightness of certain lines, decreases
that of others, and is without effect on the rest of the spec-
trum. Indeed, it was even known that some of the iron lines
which are prominent at low temj^eratures are among the more
conspicuous lines of spot spectra. But these instances were
so few and scattered that no safe inferences could be based
u}X)n them. Moreover, it had not been definitely proved that
these changes of relative intensity could actually be produced
by temperature alone. Most of the exj^eriments showing
variations of spectra have involved the use of electric dis-
charges, where causes are at work which might have a far
greater influence than temj^erature change on the character
of the spectra. Examples might easily be multiplied to show
that the study of solar and stellar physics cannot be carried
on effectively without a constant appeal to laboratory experi-
ments, planned with special reference to the needs of the
particular problem under investigation.
For this reason much stress has been laid in the equip-
ment of the Solar Observatory upon the provision of suitable
laboratory facilities. It seemed essential, in designingf the
spectroscopic laboratory on Mount Wilson, not only to in-
clude a considerable number of light-sources, which could
be examined under various conditions of temj^rature, pres-
sure, etc., but also to arrange them in such a way that the
^ appeal to one or the other condition could be made without
the delays ordinarily exj^erienced when apparatus must be
specially set up for a certain investigation. In the desired
156 Stellar Evolution
plan the apparatus must be always ready, needing only the
turning-on of an electric current, or the adjustment of a
mirror, to bring it into action. It is not so much a question
of the saving of time, which the provision of these means
undoubtedly offers, as it is of the greatly increased efficiency
of the working programme thus rendered possible. The
immediate imitation in the laboratory, under experimental
conditions subject to easy trial, of solar and stellar phe-
nomena, not only tends to clear up obscure points, but pre-
pares the way for the development along logical lines of
the train of reasoning started by the astronomical work.
Questions are constantly arising which, if partially or wholly
answered by suitable laboratory experiments, may modify
in an important way the daily programme of astronomical
observations.
The arrangement of the apparatus in the spectroscopic
laboratory of the Yerkes Observatory has already been
described^ ( p. 107). At the Solar Observatory an improved
plan has been adopted. Instead of a circular wooden table,
an annular concrete pier is employed, giving space on the
inner wall for the various switches used to control the cur-
rent supplied to the different sources, and also permitting
the observer to inspect any light-source from the direction
of the plane mirror at the center of the pier. Instead of a
single plane mirror, two are provided, capable of rotating
independently of one another about the same vertical axis.
When the Littrow spectrograph is used to photograph the
spectrum of any of the light-sources, only the lower plane
mirror is in action. By setting this at the proper angle,
light from any source on the annular pier can be sent to
a concave mirror (seen near the middle of Plate LXXY
which forms an image on the slit of the Littrow spectro-
graph. If low dispersion, rather than high dispersion, is
required, a one-prism quartz spectrograph is used. Again,
A Study of Sux-Spots 157
for the sjiecial study of certain lines under the highest resolv-
ing }X)wer, particularly in investigations of the Zeeraan effect,
an echelon spectroscope is used. In either case the concave
mirror is tipped back at a small angle, so as to return the
light to the upper plane mirror, from which it is reflected to
the slit of one of these instruments. In Plate LXXV the
quartz spectrograph may be seen just above the concave
mirror, while the echelon spectroscope stands on the extreme
right, near the end of the room. The Littrow spectrograph,
which is ordinarily employed, is similar in type to the spec-
trograph used with the Snow telescope. The rectangular box
which carries the slit and plate-holder of this instrument is
shown on the pier in the lower left corner of Plate LXXV.
The following apparatus stands on the annular pier: the
first instrument on the right is a powerful electro-magnet,
used for the study of the Zeeman effect — i. e., the influence
of a magnetic iield in separating spectral lines into several
components. For example, in the spectrum of a spark passing
between iron terminals most of the lines appear single, even
when observed with the great resolving power of an echelon
spectroscope. If, however, the spark is placed between the
poles of a powerful magnet, the effect of the magnetic field
is to break each line up into several components. It would
take us too far away from our immediate subject to discuss
the theoretical questions which underlie these phenomena.
It may be said, however, that by observing whether certain
lines behave similarly under the influence of a magnetic
field, we can tell whether they would be expected to act
together in the Sun. It is not a question here of detecting
magnetic phenomena in the Sun, since most careful study
has not revealed any evidence of solar magnetic fields capable
of affecting the appearance of the spectral lines. Neverthe-
less, the method provides an arbitrary means of [ucking out
certain groups of lines, which may be so intimately related
158 Stellar Evolution
to one another that we should expect them always to behave
alike when observed in the Sun or stars.
In the illustration a mercury tube is suspended between
the poles of the magnet and connected by heavy pressure
tubing with a duplex vacuum pump, by which the pressure
of the mercury vapor, illuminated by the discharge of an
induction coil, can be reduced as desired. The current
required for the magnet is supplied from a large storage
battery in an adjoining building. This battery is the prin-
cipal source of current for most of the apparatus on the
annular pier; an alternating current, required for certain
experiments, is obtained from a generator in the power-house.
It would be tedious to describe in detail all of the appa-
ratus. It includes arrangements for studying the spark
spectra of metals in air and in liquids; arc spectra in gases
at high or low pressure; flame spectra, for which a Bunsen
burner and an oxyhydrogen blow-pipe are required; vacuum
tube spectra; etc. A small electric furnace permits the phe-
nomena of anomalous dispersion to be observed in the vapors
of sodium and other metals which melt at low temperatures.
The auxiliary apparatus includes a special pump capable of
compressing gases up to pressures of three thousand pounds
to the square inch; an induction coil, giving a 16-inch
spark; X-ray apparatus for the study of the effect of X-rays
on the radiation of gases and vapors; a small heliostat, to
supply sunlight; etc. All of the work on the solar image
is done in the Snow telescope house, but sunlight' is fre-
quently required in the laboratory, to give a solar spectrum
for comparison with the laboratory spectra.
Let us now return to the problem of explaining the
strengthening and weakening of the solar lines in sun-spot
spectra. As already remarked, there was reason to suspect
that reduced temperature might be the effective cause of
these changes. Accordingly, the spectrum of iron was
A Study of Sun-Spots 159
photographed by Gale and Adams in the electric arc, first
with a large current (15 amperes), and then with a small
current (2 amperes). It was found that most of the lines
that are strengthened in spots are relatively strengthened in
the 2-ampere arc, while most of the lines that are weakened
in spots are also weakened in this arc. Furthermore, the
majority of the lines showed no change of intensity, which
is also the case with most of the iron lines in sun-spots.
Similar results were obtained with titanium, vanadium, chro-
mium, manganese, and other metals represented in spots.
The next question was to determine whether the metallic
vapors in the 2-ampere arc are certainly cooler than in the
30-amf)ere arc. This is by no means an easy thing to decide,
on account of various complicating elements that may not
appear at first sight. However, it was a simple matter to
compare the spectrum of the long flame which extends out
from the arc with that of the core of the arc between the
carbon poles. As the outer part of the flame is undoubtedly
much cooler than the core of the arc, the effect of decreased
temperature should be apparent here. The results confirmed,
in the most complete manner, those obtained by reducing
the current. In other words, in passing from the hot core
of the arc to the cooler flame, changes in the relative inten-
sities of the lines of the various metals, similar to those
observed in comparing the solar spectrum with the sun-spot
si^ectrum, were found. It thus seemed probable that the
modified relative intensities of the lines in spots might be the
result of a local reduction in temperature of the solar vapors.
However, it is not known precisely what part electrical
phenomena in the arc may play in producing the character-
istic radiations of the vapors. Indeed, opinions have differed
so much on this subject that some of the ablest physicists
ascribe the observed line intensities entirely to the electrical
conditions of the arc, and do not admit that temperature
160 Stellak Evolution
changes can have any influence upon them. Thus the
results so far obtained would not be accepted as proof that
the spot vapors are at a lower temperature than the corre-
sponding vapors in the Sun's reversing layer. It remained
to be seen whether simple reduction of temperature, under
conditions which excluded any possible influence of electrical
effects, would be competent to change the relative intensities
of the lines in the same way as passage from the core to the
flame of the arc had done.
The simplest way of testing this was to inclose the metal
in question within a carbon or graphite tube (chosen because
of its power to withstand very high temperatures), and to
heat this tube by a powerful electric arc playing on its outer
walls. Under these conditions, since the vapors are not
observed within the electric arc, but are separated from the
flame of the arc by the walls of the carbon tube, it should
be possible to determine the effect of change of temperature
on the relative intensities of the lines.
As the dynamo on Mount Wilson was not adequate to
supply the electric power (50 kilowatts) desired for this
work, the furnace was erected in the Pasadena laboratory
of the Solar Observatory. As in an electric furnace used
by Moissan, the arc was produced between two large carbon
poles, in a box with carbon walls, surrounded by a large mass
of magnesite, inclosed in a sheet-iron case. Running longi-
tudinally through the carbon box, and between the poles of
the arc, a carbon tube containing the metal was placed. This
tube extended out through the walls of the furnace, so that
light from the hot vapors seen through its open end could
be focused on the slit of a Littrow spectrograph of 18 feet
focal length (similar to the one used with the Snow telescope
in photographing spot spectra).
With this furnace it did not prove to be possible to
vaporize titanium and vanadium, but the test was made for
I
A Study of Sun-Spots 161
chromium and iron. The relative intensities of the lines of
these metals were found to be very nearly the same as in
I the flame of the arc. In other words, the lines which are
strengthened in passing from the cpre of the arc to the flame
are also strengthened in passing from the core of the arc to the
electric furnace. Moreover, even after the arc which heated
the carbon tube in the furnace had been extinguished, the
still glowing vapors continued to give a spectrum in which
the lines strengthened in sun-spots were relatively strong.
But the proof is not yet complete. For, with the facili-
ties available, it was not possible to vary the tem})erature in
the furnace through a sufficient range to produce undoubted
changes in the relative intensities of the lines. Therefore it
might be argued that the increased intensity in the core of
the arc of some lines, and the decreased intensity of others,
are due to electrical phenomena, and not to increased tem-
perature. The inference was strong that reduced tempera-
ture was the deciding factor in determining the relative
intensities of the lines, since it is common to the flame of
the arc and to the furnace, and since electrical effects were
excluded in the latter. But the laboratory work cannot
furnish an absolute proof, unless it should become possible,
through increase in the temperature of the furnace, to pro-
duce spectra in which the relative intensities of the lines are
the same as in the case of sun-spots. Experiments are now
in progress with this end in view.
Fortunately, however, there are other sources of infor-
mation to which we may appeal. In the reversing layer,
oxygen exists in the presence of such substances as iron and
titanium. Now, it is well known that this can be true only
under conditions of very high temperature. Hence, if the
^metallic va|X)rs in sun-spots are actually cooler than the
vapors outside of spots, the reduction in temj^erature may
be sufficient to permit the oxygen to enter into combination
162 Stellae Evolution
with some of the metals present. Titanium oxide, in partic-
ular, is capable of resisting a very high temperature, which
would immediately dissociate an oxide of iron. Is there any
evidence, then, that titanium oxide exists in sun-spots?
Thanks to the excellent photographs of spot spectra ob-
tained with the aid of the Snow telescope, this question is
easily answered. Titanium oxide gives a very characteristic
fluted spectrum, consisting of bands in which the numerous
lines lie closer and closer together until they terminate in
definite "heads." Fig. 2, Plate LXXIV, shows some of
these titanium oxide flutings in the extreme red end of the
spectrum, as photographed (on specially sensitized plates)
in the outer flame of the electric arc. The photograph is
a negative; i. e., the lines which are bright in the arc are
shown dark, to facilitate comparison with the dark lines
in the photograph of the spot spectrum, shown just above
the titanium oxide spectrum. It will be seen at a glance
that each of the heads of the fluting is represented in
the spot, and that a great number of the fine lines which
make up the fluting also agree in position with correspond-
ing spot lines. The spot spectrum contains many lines
not represented in the arc, which are due to substances
other than titanium oxide. The arc spectrum also contains
a few lines due to impurities, which are not present in the
spot. Nevertheless, the general agreement is so perfect that
the presence of the titanium oxide bands in spot spectra
cannot be doubted. Several other bands belonging to the
same substance are also represented in our photographs of
spot spectra.
The identification of these bands by Adams would seem
to leave no doubt as to the reduced temperature of the spot
vapors. The objection might be made, it is true, that some
question exists as to whether these bands are actually due to
the oxide, since there is some reason to suppose that titanium
A Study of Sun-Spots 163
itself is capable of producing them. However, the molecule
which gives them rise undoubtedly differs from the atom
which produces the line spectrum of titanium. In laboratory
experiments the flutings become more and more conspicuous
as the temperature is reduced, suggesting that the molecule
is broken up at high temperatures. The absence of the flut-
ings from the spectrum of the Sun sustains this inference.
Moreover, Fowler, in London, has since found some of the
green flutings in the Mount Wilson photographic map of
the spot spectrum to be due to magnesium hydride, and
Olmsted, on Mount Wilson, has identified some of the red
flutings with those of calcium hydride.
It therefore appears to be true that the vapors which con-
stitute the umbra of a sun-spot are cooler than the corre-
sponding vapors in other parts of the Sun. This would
readily account for the relative intensities of the spectral
lines and for the comparative darkness of sun-spots. But the
cause of such a reduction of temperature is yet to be deter-
mined. Knowledge of the comparatively low^ temperature of
spot vapors at once permits us, however, to discard various
spot theories which postulate very high temperatures, and to
attack the question of the true meaning of sun-spots in an
intelligent manner.
In order to facilitate the spectroscopic study of sun-
spots, a preliminary photographic map of the spot spectrum
has been issued by the Solar Observatory. This consists of
twenty-six sections, each covering one hundred Angstrom
units of the spectrum, the whole map extending from wave-
length 4600 to wave-length 7200. In enlarging the original
negatives, Ellerman photographed each section on a sensitive
plate, moved up and down (in the direction of the spectral
jines) during the exposure. This process widened the narrow
'spot spectrum, and rendered visible many slight changes in
the relative intensities of lines which would otherwise escape
164 Stellar Evolution
notice. Beside each strip of the spot spectrum the normal
solar spectrum is given for comparison (Plate LXXVI).
The information derived, as explained above, from solar
and laboratory investigations applies not only to the Sun.
If, by cooling in some degree the vapors lying within a
limited area on the solar surface, the spectrum is changed
in the manner illustrated in sun-spots, it should follow that
if the entire Sun, or a star like the Sun, were cooled in the
same degree, its spectrum would resemble that of a sun-spot.
Our ideas of stellar evolution are based on the belief that
stars exist in all stages of development and differing greatly
in temperature. If our inference be correct, we should find,
among the stars which have passed by continued cooling
beyond the solar stage, some in whose spectra spot lines
appear. The next chapter explains how this test has been
applied.
CHAPTER XVIII
STELLAR TEMPERATURES
The advantages of great resolving power in spectroscopic
work have been mentioned in previous chapters. In the case
of the Sun the amount of light at our disposal is so abundant
that grating spectroscopes of very high dispersion can be
used without difficulty. The degree in which the light is
weakened by dispersion will be appreciated when it is remem-
bered that the light entering the spectroscope through a slit
one-thousandth of an inch in width is spread out into a spec-
trum many feet in length. In the case of the stars, however,
only a small amount of light is at our disposal, and for this
reason the spectroscopes employed have always been much
inferior in dispersion to those used for solar research. The
interpretation of stellar spectra is thus rendered difficult,
since several closely adjacent lines may be compressed into
one. If, then, we are to learn the true relative intensities of
stellar lines, in order, for example, to make certain of any
apparent analogy with sun-spots, we must find means of
studying stellar spectra with a dispersion as great as that
used for solar observations.
A difficulty which does not exist in visual observations
has an important bearing on the nature of the spectroscopes
required for such work. If it were jx)ssible to see the s})ec-
trum of a star to good advantage, a high resolving power
could be obtained with a spectroscope of moderate dimen-
sions, supplied with a powerful grating. But, for two principal
treasons, photographic methods are almost exclusively used
in stellar spectroscopy. In the first place, except in the case
of a few of the bricfhtest stars, the smaller details of stellar
165
16G Stellar Evolution
spectra cannot be seen, on account of the faintness of the
light. In the second place, the unsteadiness of the image,
due to atmospheric disturbances, causes the extremely narrow
spectrum to flicker so seriously as to prevent any refined
work. This flickering, however, has no effect upon the photo-
graphic plate, which merely sums up all of the light it
receives during the exposure. Moreover, by prolonging the
exposure, a spectrum too faint to be seen can be recorded
photographically. In all modern work of precision, there-
fore, photographs of stellar spectra are substituted for visual
observations.
But the photographic plate has a granular structure, due
to the fact that it is made up of silver grains, which can be
separately distinguished with a microscope. On account of
this granular structure of the plate the details of the image
are imperfectly recorded, so that no advantage results from
the use of high powers when examining the plate. If the
visual image could be well seen at the spectroscope, an
increase of magnification (attained by the use of a suitable
eye-piece) would separate all lines within the resolving power
of the prisms or grating. On the photographic plate, how-
ever, the images of these lines may lie so close together that
they appear as one, and cannot be separated by magnification.
What is needed, in order to realize photographically the full
resolving power of the prisms or grating employed, is a spec-
troscope of such length that the closest lines that could be
distinguished visually are so far separated as to be independ-
ently recorded, in spite of the effect of the silver grains.
The powerful grating spectrograph used by Rowland in
his study of the solar spectrum has a focal length of 21 feet.
Photographs made with a spectrograph of this size show
nearly all the lines that can be separately distinguished in
visual observations with the same instrument. Obviously it '
would be out of the question to attach such a spectrograph,
Stellar Temperatures 167
or even an equally powerful one of the more compact Littrow
type, to the end of a movable telescoj^e tube. Moreover,
the very high dispersion would demand, in the case of
stars, exposures prolonged for many nights. Temperature
changes, or the slightest flexure of the apparatus during the
exposure, would shift the position of the lines on the plate
and thus destroy, by producing a blurred image, all the
advantages afforded by large spectrographs.
Such instruments as the three-prism spectrograph of the
Potsdam Astrophysical Observatory, the Mills spectrograph
of the Lick Observatory, and the Bruce spectrograph of the
Yerkes Observatory, give beautifully defined photographs of
stellar spectra, from the measurement of which the motions
of stars in the line of sight are determined with great pre-
cision. For most classes of work such spectrographs could
hardly be surpassed. Nevertheless, the necessary limitations
of resolving |X)wer and focal length in these instruments
prevents them from separating many of the lines resolved by
Rowland in his studies of the solar spectrum. It is evidently
to be greatly desired that the spectra of a few of the brightest
stars, at least, be photographed with spectrographs as power-
ful as Rowland's. In order to accomplish this the spectro-
graph must be fixed in position on a massive pier, and main-
tained at a constant temperature throughout the exposure.
To test the feasibility of this, and to decide whether a
spectrograph of high dispersion could advantageously be
used with a 60-inch reflecting telescope (p. 228), a grating
spectrograph of 13 feet focal length has been tried with
the Snow telescope. This instrument was mounted on the
triangular stone pier (p. 133, Fig. 5) in the spectroscope
house of the Snow telescope. The pier is inclosed in a room
'so constructed that the fluctuations of temperature within
it are very slight. The 6-inch Rowland plane grating was
mounted so as to form the front wall of a cubical metallic
168 Stellar Evolution
box containing water. An extremely delicate thermostat, con-
sisting of a bulb containing saturated ether vapor immersed
in the water, caused a column of mecury to make or break
an electric circuit if the temperature of the water varied as
much as a hundredth of a degree. When the temperature fell
by this amount, a relay turned on the current of two incan-
descent lamps immersed in the liquid. The heating produced
by the lamps raised the temperature, and the current was
then automatically cut off. The water was constantly stirred
by small propellers driven by an electric motor. In this way
the grating, which is, of course, the most sensitive part of the
apparatus, was kept at an almost perfectly constant tempera-
ture throughout the exposure.
Ai'cturus, on account of its yellowish color and the charac-
ter of its spectrum, has long been considered to represent a
stage of stellar development somewhat advanced beyond that
of the Sun. As its spectral lines show its chemical com-
position to be practically the same as that of the Sun, a
reduction in temperature, due to cooling continued beyond
the solar stage, should, on the hypothesis developed in the
last chapter, cause its spectrum to resemble that of a sun-
spot. Accordingly, the spectrum of Arcturus was photo-
graphed with the Snow telescope and the grating spectro-
graph.
Because of the great dispersion, an exposure of five hours,
which was all that could be given on a single night, was
entirely insufficient. In fact, an exposure continued for five
nights in succession, and aggregating twenty-three hours,
was required. During all this time it was essential that the
temperature of the grating remain practically constant, and
that none of the parts of the spectrograph be displaced by
any cause. For this reason the observer did not enter the
constant-temperature room after the exposure was started,
but merely brought the star to the slit of the spectrograph
Stellar Temperatures 169
each night, and maintained it there, by watching the star
image reflected from the slit jaws, and correcting any slight
deviations in its |>osition. The same process was repeated
from night to night, until the exposure was completed.
In these first experiments the possibilities of the method
were not fairly tested, on account of some imperfections in
the apparatus. The Snow telescope was designed for solar
work, and is not well adapted for stellar observations. More-
over, work in progress on the telescope house caused some
vibration of the piers, which doubtless affected the definition.
Nevertheless, the resulting photographs are sufficiently good
to show that this method, when properly carried out with
the 60-inch reflector, should give a few stellar spectra not
essentially inferior to the best obtained in solar work. The
60-inch reflector will collect about six times as much light
as the Snow telescope, and the exposure time, for the same
dispersion, will be decreased in about this ratio. Thus the
spectrum of Arcturus should be photographed with the
grating used for the present work in about four hours. As
subsequent experiments with the Snow telescope showed that
large prisms can be used to much better advantage than the
grating for stellar spectra, this exposure time, for the same
dispersion, will be still further reduced. In the case of the
60-inch reflector, the dispersion will be increased sufficiently
to make the scale of the spectrum about the same as that of
Rowland's solar spectrum photographs.
Plate LXXVII shows a portion of the Ai'cturus spectrum
thus photographed, in comparison with spot and solar spectra.
Barring some exceptions, which require further study, it will
be noticed that the spectrum resembles the spot spectrum
more closely than it does the solar spectrum.' On account
1 In comparing these spectra, changes of intensity should be noted with refer nee
to adjoining (unaffected) lines in the same spectrum. Unavoidable differences of
absolute intensity in the photographs prevent a satisfactory comparison, unless this
precaution be observed.
170 Stellar Evolution
of the imperfections of the Arcturus photograph, many of
the lines are shown with less contrast than they would ex-i
exhibit in a really good negative. However, the illustration
should be sufficient to indicate the important bearing of spot
spectra on the question of stellar temperatures.
The earliest classification of stellar spectra was that of
Secchi, who distinguished four principal types: I, spectra
of white and bluish-white stars, like Sirius, which contain
broad and strong hydrogen and calcium lines, and but few
lines, narrow and comparatively faint, of other elements; II,
spectra of yellowish-white stars, like the Sun; III, spectra
of red stars, containing a very characteristic series of bands,
not identified by Secchi; IV, spectra of another class of red
stars, containing the strongly marked bands of carbon. The
bands in the spectra of stars of Secchi's third type were
finally identified by Fowler, who showed that they are due
to titanium oxide. In view of the presence of these same
bands in spot spectra (Fig. 2, Plate LXXIV), it becomes
interesting to inquire whether the lines in stellar spectra of
this type also resemble those in sun-spots.
The brilliant red star Betelgeuze (a Orioiu's) which pre-
sents so striking a contrast with the bluish star Rigel, in
the constellation of Orion, is a good representative of the
third type. It was accordingly selected to test the question.
A dense flint glass prism belonging to the 5-foot spectrohelio-
graph was substituted for the grating in the large stellar
spectrograph of the Snow telescope, and the thermostat was
modified so as to control the temperature of the air surround-
ing the prism. In this way the spectrum of a Orionis was
photographed by Adams, with a total exposure of seven hours
on two consecutive nights. The work was done during the
rainy season, and clouds, followed by continuous bad weather,
cut short the exposure on the second night, and prevented the
observations from being continued. The plate, while not
Stellab Temperatures 171
strong enough to be of the best quality, is nevertheless
sufficiently good to serve for the purjwse of a general com-
parison. It was found that essentially all of the lines are
stroucrer than in the Sun. and that lines which are streno^th-
ened in spots are much more decidedly strengthened in a
Orionis than lines unaffected in sjx)ts. In fact, the relative
strengthening is much more marked in the case of this star
than in the spots themselves, probably indicating that its
temperature is lower. As the titanium oxide flutings form a
conspicuous feature of the spectrum of a Orionis, and are also
present in sun-spots, the evidence appears to be practically
complete. More detailed investigations will undoubtedly
reveal various discrepancies, due to differences in chemical
composition or physical condition. Nevertheless, it may be
said, in general, that the resemblance between the spectra of
sun-spots and those of third-ty[>e stars is so close as to indi-
cate that the same cause is controlling the relative intensities
of many lines in both instances. This cause, as the laboratory
work indicates, is to be regarded as reduced temperature.
Thus we have been led, through the study of certain
phenomena of our typical star, the Sun, and through their
interpretation by laboratory experiments, to the considera-
tion of the general question of stellar temperatures. Let us
now inquire whether other independent methods can be
applied to determine these temperatures, dealing first with
the ixjssibility of measuring directly the heat radiation of
stars.
The early experiments of Huggins and Stone failed, for
lack of suitable apparatus, to detect the exceedingly small
degrees of heat which reach us from stellar sources. Even
Boys was no more successful in 1888, though he concen-
trated the stellar radiations on his newly invented radio-
micrometer, which would show ^ ^^^^^^^ of a degree rise of
temperature. With the sensitiveness used, fs^wwo^ o^ *h^
172 Stellar Evolution
heat received by his telescope mirror from the full Moon could
be detected. Yet the brightest stars produced no certain
effect. As the result of this work, Boys was convinced that
no star sends us as much heat as would be received from a
candle at a distance of 1.7 miles, if there were no atmos-
pheric absorption.
The subject of stellar heat was investigated by E. F.
Nichols at the Yerkes Observatory, in 1898 and 1900. The
radiometer employed as the heat-measuring apparatus con-
sisted of two circular vanes of mica, each about one-twelfth
of an inch in diameter, attached to the opposite ends of a
delicate cross-arm of drawn glass, cemented to a whip of fine
drawn glass about one and one-quarter inches long. To the
lower end of this system a minute mirror, made by silvering
a fragment of very thin microscope cover-glass, was attached,
and the whole was suspended by a very fine quartz fiber in
a vacuum chamber. This radiometer was mounted on a pier
in the coelostat room of the Yerkes Observatory. A coelostat
reflected the starlight to a 24:-inch mirror of 8 feet focal
length, which concentrated the stellar rays upon one of the
vanes, after entering the radiometer case through a window
of fluorite, which is very transparent to heat radiations. By
observing a scale reflected in the small mirror attached to
the radiometer suspension, the deflection of the vane, which
indicated the heating effect of the stellar rays, could be
measured. In this way it was found that Arcfuriis sends us
about as much heat as would be received from a candle six
miles away, if there were no absorption in the atmosphere.
Vega has less than half the thermal intensity of Arcttirus.
The extraordinary sensitiveness of the apparatus employed
may be illustrated by some observations of a candle 2,500
feet from the observatory. Heat from this candle, when
concentrated on the radiometer vane of the 24:-inch mirror,
gave a deflection of about sixty-two scale divisions. On one
Stellar Tempebatukes 173
occasion the assistant extinguished the candle and placed
his head in front of it when the signal was given, instead of
uncovering the flame. The deflection caused by the heat
radiation of his face, at a distance of 2,500 feet, was twenty-
five scale divisions! With no atmospheric absorption, the
number of candles in a group at a distance of sixteen miles
could be determined from the average of a series of meas-
urements of their total heat radiation.
As Arctiirus and Vega appear about equally bright to the
eye, the greater heat radiation of the former star indicates
that it sends out a larger proportion of the long (red) waves.
If neither star possessed an absorbing atmosphere, it might
then be concluded that A returns is cooler than Vega, but so
much larofer in anovular diameter, when seen from the Earth,
as to be fully as bright as Vega, and to send us more than
twice as much heat. However, since we know that the
absorbing atmosphere of stars like ^4/*c/»rj/8 is much denser
than that of stars like Vega, this conclusion would not
hold. We are therefore not in a position to judge from
these experiments as to the relative temperatures of these
stars.
Lockyer has recently endeavored to determine the relative
tem})eratures of stars by comparing their spectra, when
photographed under similar conditions, in order to learn
which of two stars sends us the greater proportion of violet
light. In accordance with a well-known law, the proportion
of violet light emitted by a luminous body increases as the
temj>erature rises. By measuring the position of maximum
intensity in the spectrum of a star, it should thus be possible
to determine its temi^erature. Unfortunately, however, as
already remarked, no absolutely safe conclusions can be based
u{i(jn a test of this kind. Stars with dense atmospheres must
appear red in color, no matter what their temperature, as com-
pared with stars whose atmospheres ar« much less dense. For
174 Stellar Evolution
we have here just such a condition of things as we observe in
the setting Sun, which appears red simply because the violet
rays are more highly absorbed by our atmosphere than the
red rays. It seems to be true that the older and cooler stars
have denser atmospheres than the younger and hotter ones.
It is thus probable that the stars whose spectra contain the
greater proportion of red light actually are cooler than those
in which the violet light is relatively stronger. But the
fact remains that we are not warranted in basing determina-
tions of stellar temperatures on measurements which so
obviously depend upon the effect of atmospheres of unknown
density. We will return to this question of stellar tempera-
tures in a further consideration of the classification of stars
(chap. XX ).
CHAPTER XIX
THE NEBULAR HYPOTHESIS
In the preceding chapters we have seen how the study of
stellar evolution depends primarily upon the most accurate
knowledge we can obtain of the Sun, regarded as a typical
star. We have also examined certain methods of observing
solar, stellar and laboratory phenomena, and have taken
advantage of the opportunity afforded by the peculiarities of
Sun-spot spectra to illustrate the mutual dependence of these
various means of research. In passing to certain of the more
general considerations underlying our subject, we may now
examine some of the prHcipal hypotheses which have been
offered to account for the development of solar and stellar
systems.
Passing over the important speculations of Kant, and the
conclusions drawn by Herschel from his extensive observa-
tions, we reach the nebular hypothesis of Laplace. This cele-
brated explanation of the origin of the solar system has domi-
nated the world's thought since the very date of its publication.
The eminence of its author, and the unique value of his
great work on celestial mechanics, led to the immediate
acceptance of his ideas, even when advanced in speculative
form and without the support of mathematical analysis.
The greatest physicists and astronomers of the nineteenth
century have given the weight of their approval to the
nebular hypothesis, and all calculations as to the age of the
Sun have been based upon it. When viewing it in the light
pf recent destructive criticism, we must not forget the value
of Laplace's speculations in directing thought and in seek-
ing to account, by a single generalization, for a host of
175
176 Stellar Evolution
observed phenomena. Nor must we overlook his remark
that the hypothesis was presented "with the distrust which
should be inspired by everything that is not the result of
observation or calculation." The widespread and favoral)le
influence exerted by the hypothesis on the intellectual life
of the nineteenth century cannot be destroyed by recent
developments. In the same way, the beneficial effect of
Darwin's work on organic evolution would remain, even if
the hypothesis of natural selection were forced from its phuf
by that of mutation.
As the original statement of the nebular hypothesis is
not easily accessible to every reader, it seems desirable tc
include here a free translation of Note VII, at the end of
Laplace's Exposition dii systdme du monde. A few para-
graphs, dealing with more technical details, are omitted, but
all of the essential features are retained.
In seeking to trace the cause of the original motions of tlie
planetary systems, the following five phenomena, enumerated in
the last chapter (of Laplace's book), are available: the motions of
the planets in the same direction and nearly in the same plane; tlic
motions of the satellites in the same direction as the planets; the "
motions of rotation of these different bodies and of the Sun in the
same direction as their orbital motions, and in but slightly diffeicnt
planes; the small eccentricity of the orbits of planets and satellitts;
finally, the great eccentricity of comets' orbits, as though tlicii-
inclination had been left to chance.
So far as I am aware, Buff on is the only one who has endeavored.
since the discovery of the true system of the world, to trace the
origin of the planets and their satellites. He supposes that a
comet, falling upon the Sun, drove from it a torrent of matt( i,
which reunited at a distance in several globes, varying in size and
in distance from the Sun; these globes, having become opaque and
solid by cooling, are the planets and their satellites.
Laplace then goes on to show that, although this hypotli-
esis might account for the first of the five phenomena
The Nebular Hypothesis 177
mentioned above, the others could not be explained by
it. In seeking to discern their true cause, he continues as
Follows :
Whatever be its nature, since it has produced or directs! the
motions of the planets, it must have embraced all of these bodies,
and. in view of the prodigious distances that separate them, it
could only have been a fluid of immense extent. In order to give
them a nearly circular motion about the Sim. in the same direction,
the fluid must have surrounded this body like an atmosphere. The
consideration of planetary motions thus leads us to think that, as
the result of excessive heat, the solar system originally extended
beyond the orbits of all the planets, and that it contracted by suc-
cessive steps to its present limits.
In the assumed primitive condition of the Sun, it resembled
those nebulae which are shown by the telescope to be composed of
a more or less brilliant nucleus, surrounded by nebulosity which,
in condensing toward the sm-face of the nucleus, transforms it into
a star. If. by analogy, we conceive of all the stars being formed
in this manner, we may imagine their earher nebular state, itself
preceded by other states, in which the nebular matter was more
and more diffuse, the nucleus being less and less luminous. By
^oiug Ixick as far as jjossible, we thus arrive at a nebulosity so
diffuse that its existence could hardly be susjjected.
Philosophical observers have long been impressed with the
peculiar distribution of certain stars visible to the naked eye.
Mitchel has remarked on the improbability that the stars of the
Pleiades, for example, could have been compressed within the
Qarrow limits which inclose them by mere chance, and he has
bence concluded that this group of stars, and similar groups in the
heavens, are the effects of an original cause or of a general law of
oatm-e. These groups are the necessary result of the condensation
of nebulae having several nuclei; for it is evident that, if the nebu-
lar matter were continually attracted by these various nuclei, they
srould ultimately form a group of stars like that of the Pleiades.
The condensation of nebulae having two nuclei will similarly form
itars lying very close together, and revolving about one another,
like the double stars whose motions have already been observed.
But how has the solar atmosphere determined the motions of
rotation and of revolution of the planets and satellites? If these
178 Stellar Evolution
bodies had penetrated deeply into this atmosphere, its resistance
would have caused them to fall upon the Sun. We may thus con-
jecture that the planets were formed at its successive limits, by the
condensation of zones of vapors which the Sun, in cooling, mnst
have abandoned in the plane of its equator.
Let us recall the results given in a preceding chapter. Tlie
atmosphere of the Sun could not have extended out indefinitely.
Its limit was the point where the centrifugal force, due to its
motion of rotation, balanced the attraction of gravitation. Now. as
cooling contracted the atmosphere and condensed at the surface of
the Sun the molecules lying near it, the motion of rotation acceler
ated. For, from the law of areas, the sum of the areas descrilxd
by the radius vector of each molecule of the Sun and of its atmos-
phere, when projected on the plane of its equator, being always
the same, the rotation must be more rapid when these molecules
approach the center of the Sun. The centrifugal force due to this
motion thus becoming greater, the point where it equals the weight
is nearer the Sun. If we then adopt the natural supposition that
the atmosphere extended, at some period, to an extreme limit, it
must have left behind, in cooling, the molecules situated at tliis
limit and at the successive limits produced by the acceleration of
the Sun's rotation. These abandoned molecules must have con-
tinued to revolve around the Sun, since their centrifugal force was
balanced by their weight. But since this equilibrium did not
obtain in the case of the atmospheric molecules in higher latitude^,
their weight caused them to approach the atmosphere as it con-
densed, and they did not cease to belong to it until this motion
brought them to the equator.
Let us now consider the zones of vapor successively left behind.
To all appearances these zones should form, by their condensation
and the mutual attraction of their molecules, various concentric
rings of vapor revolving around the Sun. The mutual friction of
the moleciiles of each ring should have accelerated some and
retarded others, until they had all acquired the same angular
velocity. Thus the linear velocities of the molecules farthest from
the center of the Svni must have been the greatest. The following
cause would also contribute toward the production of this difference
of velocity. The molecules farthest from the Sun, which, through
the effects of cooling and condensation, came together to form the
outer part of the ring, always described areas proportional to the
The Nebulab Hypothesis 179
time, since the central force which controlled them was constantly
directed toward the Siin. This constancy of areas requires that
the velocity increase as the molecules move inward. It is evident
that the same cause must have diminished the velocity of those
molecules which moved outward to form the inner edge of the ring.
If all the molecules of a ring of vapor continued to condense
without separating, they would finally form a liquid or solid ring.
But the uniformity which this formation demands in all parts of
the ring, and in their rate of cooling, must have rendered this
phenomenon extremely rare. Thus the solar system offers only a
single example of it, that of the rings of Saturn. In almost all
cases each ring of vapor must have broken into several masses
which, having only slightly different velocities, continued to revolve
at the same distance around the Sun. These masses must have
assumed a spheroidal form, with a motion of rotation correspond-
ing in direction with that of their revolution, since their inner
molecules had smaller linear velocities than their outer molecules;
they thus foraied as many planets in a vaporous state. But if one
of them had possessed sufficient power of attraction to bring all the
others successively together about its own center, the vaporous ring
would thus have been transformed into a single spheroidal mass of
vapor, revolving alxmt the Sun and rotating in a direction corre-
sponding to that of its revolution. This latter case has been the
most common one. Nevertheless, the solar system offers an
example of the first case in the four minor planets which lie between
Jupiter and Mars : unless we suppose, in agreement with M. Olbers,
that they originally formed a single planet broken up by a violent
explosion into several parts having different velocities.
Now, if we follow the changes which ultimate cooling must
have produced in the vaporous planets whose formation we have
just pictured, we shall witness the production, at the center of each,
of a nucleus which continues to develop through the condensation
of the atmosphere surrounding it. In this state the planet exactly
'.nbles the Sun in its primitive nebular condition. Cooling
must thus have produced, at the various limits of its atmosphere,
phenomena similar to those we have described ; that is to say, rings
; satellites revolving around its center in the direction of its
aon of rotation, and turning in the same direction upon them-
' selves. The symmetrical distribution of Satuni's rings about its
center and in the plane of its equator naturally results from this
180 Stellar Evolution
hypothesis, and would be inexplicable without it. These rings
seem to me ever-present proofs of the original extension of
Saturn's atmosphere and of its successive retreats. Thus the
singular phenomena of the slight eccentricity of the orbits of the
planets and satellites, the small inclination of these orbits to the
solar equator, the identity in direction of the motions of rotation
and revolution of all these bodies with that of the solar rotation :
flow from our hypothesis and give it great probability.
If the solar system had been formed with perfect regularity,
the orbits of the bodies which compose it wovild have been circles
whose planes, like those of the various equators and rings, woiild
have coincided with the plane of the solar equator. But it may be
conceived that the endless varieties which must have existed in the
temperature and density of the various parts of these great masses
produced the eccentricity of their orbits and the deviation of their
motions from the plane of this equator.
In our hypothesis, comets are strangers to the planetary system.
In considering them, as we have done, to be small nebulae wander-
ing from system to system, and formed by the condensation of
nebular matter distributed with such profusion throughout the
universe, we perceive that, when they arrive in the region of
space where the solar attraction is predominant, it forces them
to describe elliptical and hyperbolic orbits. But their motions
being equally possible in all directions, they must move indifferently
in all directions and at all inclinations to the ecliptic ; which is in-
agreement with observation. Thus the condensation of nebular
matter, by which we have just explained the motions of rotation
and revolution of the planets and satellites in the same direction,
and in planes differing but slightly, also explains why the motions
of comets do not agree with this general law.
Laplace, after discussing the great eccentricity of comets'
orbits, as bearing on the nebular hypothesis, continues as
follows:
If certain comets entered the atmospheres of the Sun and
planets during the formative period they nuist have fallen upon
these bodies, after pursuing spiral paths. The result of their fall
would be to cause the planes of the orbits and the equators of the
planets to deviate from the solar equator.
The Nebular Hypothesis 181
If in the zones left behind by the solar atmosphere there were
molecules too volatile to combine among themselves or with the
planets, they must have continued to revolve about the Sun. They
would thus give rise to such an appearance as that of the zodiacal
light, without oflFering appreciable resistance to the various bodies
of the planetary system, either because of their extreme rarity, or
because their motion is very nearly the same as that of the planets
which they encounter.
A close examination of all the details of the solar system adds
still further to the probability of our hypothesis. The original
fluidity of the planets is clearly indicated by the flattening of their
figure, in conformity with the laws of mutual attraction of their
molecules ; furthermore, it is proved in the case of the Earth by
the regular diminution of weight from the equator to the poles.
This condition of original fluidity, to which we are led by astronomi-
cal phenomena, should show itself in the phenomena of natiu*al
history. But, to perceive it there, it is necessary to take into
accoimt the immense variety of combinations formed by all ter-
restrial substances mingled together in a state of vapor, when the
reduction of temperature permitted their elements to unite among
themselves. It is also necessary to consider the enormous changes
that this fall of temperature must have brought about successively
within the Earth and upon its surface, in all formations, in the
constitution and the pressure of the atmosphere, in the ocean, and
in the bodies which it held in solution. Finally, consideration
should Ije given to violent disturbances, such as great volcanic
eruptions, which must have modified, at various epochs, the regu-
larity of these changes. Geology studied from this point of view,
which unites it to astronomy, will acquire precision and certainty
in many particulars.
Although the nebular hypothesis received almost universal
acceptance, objections and difficulties were brought forward
at various times during the nineteenth century. The criti-
cisms of Babiuet and Kirkwood were followed by the argu-
ments of Faye, who concluded that the planets, if developed
from the ring-system of Laplace, should rotate in the oppo-
site direction. Laplace had assumed that the rino-s which
were to form the planets revolved like solid bodies, their
182 Stellar Evolution
outer edge traveling faster than the inner one. This wouk
have involved forward rotation of the planets, as now observec
But such a condition of things could not have occurred — th
rings, split asunder by the forces acting upon them, mu
have followed Kepler's laws, which would require the inn
edge to move the faster. The rings of Saturn, held up l
Laplace as a striking illustration of his views, were show
by Maxwell in 1859 to be composed of small bodies lik
meteorites. This was the result of a mathematical demon-
stration that the rings, if solid, would fly to pieces. It was
confirmed by Keeler, in 1895, by one of the most beautiful
applications of the spectroscope ever made. According to
Doppler's principle, the position of a line in the spectrum
of a moving body depends upon the velocity of the mo-
tion. This is true, even when the light is reflected from
the surface of the moving body, after being received from
the Sun. If the inner edge of Saturn's ring is moving
faster than the outer edge, the lines in the spectrum of the
ring should be increasingly bent toward the violet (on the
approaching side of the planet) or toward the red (on
the receding side), in passing from the outer toward the
inner edge. Keeler's photograph of Satuni's spectrum
shows this to be the case. Thus we have certain proof that
Saturn's rings are made up of meteorites, each moving at the
velocity a satellite would have at the same distance from the
planet.
In spite of these and kindred objections, the nebular
hypothesis, at least in its general outlines, retained its com-
manding position until subjected to a searching test insti-
tuted by Chamberlin and Moulton. The principal arguments
brought together in Moulton's paper, entitled "An Attempt
to Test the Nebular Hypothesis by an Appeal to the Laws of
Dynamics," ' and in the discussion of the question in Volume
1 Astrophysical Journal, Vol. XI (1900), p. 103.
The Nebular Hypothesis 183
TI of Chamberlin and Salisbury's Geology, are briefly sum-
Qarized below.
In the pa{5er just referred to, Moulton defines the nebu-
ir hypothesis in much more general terms than Laplace
mployed. In other words, in order to make the test as
Dmplete as possible, he assumes that the original nebula
aiorht consist of a gas or of a swarm of meteorites, since
3arwin had proved mathematically that the properties of
gases may be fulfilled in a meteoroidal swarm. Moulton* s
discussion, moreover, does not insist upon the assumption of
a very high temj^erature, since the progress of knowledge
has shown that the present heat of the Sun may be accounted
for as a result of the contraction of a nebula oriffinallv at a
low temperature. Finally, the breaking-up of the nebula is
not limited to the abandonment of rings, but is considered
to include possible division by some fission process, the
separated portions having contracted to form the planets and
satellites.
The fact that the revolutions of certain satellites, such as
those of Uranus and Neptune, are in a retrograde direction,
while the planes of the orbits of the four satellites of Uranus
are almost perpendicular to the plane of the planet's orbit,
is an old argument against the nebular hypothesis. While
the former difficultv could easilv be overcome, the great
inclination of the orbits of these satellites and that of Xep-
fune seems to be directly opposed to Laplace's views. In the
-econd place, the masses of the various planets, as well as the
densities of the rings from which they are supposed to be
formed, are shown to be entirely out of harmony with what
the hypothesis would lead us to expect. Again, the inner
satellite revolves about Mars in a period less than a third of
the planet's rotation, while the hypothesis would require its
velocity to be much less than that of the planet's surface.
Darwin has shown that the friction of solar tides might have
184 Stellar Evolution
retarded the rotation of Mars, without affecting the satellite's
motion. But Moulton points out that the inner edge of
Saturii's ring completes a revolution in about half the time
of Saturn's rotation period. At this great distance from
the Sun, the very small tides could not have retarded suf-
ficiently the rotation period of Saturn, unless they have
been operating several thousand times as long as the Martian
tides. An attempt to ascribe the effect to the satellites of
Saturn proves equally futile.
Moulton next endeavors to answer the question whether
the supposed initial conditions could have developed into the
existing system. We know that the molecules of a gas are
moving about at velocities which increase with the tempera-
ture. Near the surface of the original Laplacian nebula the
velocities of the molecules, in the case of such light elements
as hydrogen, would be so great that the molecules would
overcome the power of gravitation and be dispersed in space.
It would, therefore, be difficult to account for the abundant
supplies of this gas now observed on the Sun. A stronger
objection is afforded through the application of these prin-
ciples to the planets. It is easy to calculate, through the
known velocities of gaseous molecules and the masses of the
planets, the power of each planet to retain an atmosphere.
It is also possible to determine with the spectroscope whether
atmospheres exist on the planets. Working in this way,
Moulton shows the improbability that the diffuse Earth -Moon
ring, with its low power of attraction, could have held any of
the atmospheric gases or water vapor, when such concentrated
bodies as the Moon and Mercury are unable at the present
time to hold atmospheres.
As we know the masses of the Sun and planets, the
average density of the original nebula, when it extended to
the orbit of Neptune, can be approximately calculated.
Moulton finds this to be about ^ g^xo oito¥o oir o ^^ ^^^^^ ^^ water.
The Nebulab Hypothesis 185
In this extraordinarily rare nebula, whether truly gaseous or
meteoroidal, it is shown that matter would have been left
behind continually and that the formation of separate rings
would be impossible — a conclusion reached by Kirk wood in
1869. Moulton thinks it equally certain that a large mass
could not have been detached by any fission process. Fur-
thermore, even if a ring had been formed, he shows it to be
utterly improbable that its matter could have been drawn
together into a planet.
Some of the above conclusions may perhaps be open to
question, but the final argument seems to be unanswerable.
It is a well-known principle of dynamics that the moment of
momentum of a system of bodies not under the action of
external forces is constant. The moment of momentum is
defined by the sum of the products of the masses of all the
particles by their velocities and by their distances from the
center of the system. This quantity should remain abso-
lutely unchanged, whether the system be in the form of a
nebula occupying the whole of Xeptutte's orbit, or a group
of planets revolving around the Sun. Making his assump-
tions in such a way as to be most favorable to the nebular
hypothesis, Moulton obtains the following results for the
moment of momentum:
When the nebula extended to Mars' orbit M=32.176
When the nebula extended to Jupiter's orbit M=13.250
When the nebula extended to the Earth's orbit M— 5.690
When the nebula extended to J/erc«r*/'s orbit M= 3.400
In the system at present M= 0.151
Thus, instead of remaining constant, the moment of
momentum is shown to decrease rapidly and irregularly.
In spite of the precautions taken to favor the nebular
hypothesis as much as possible, the moment of momentum
of the original system comes out 213 times that of the
present solar system.
186 Stellar Evolution
The papers of Chamberlin and Moulton contain other
serious criticisms based upon the study of the moment of
momentum of the system, and raise various additional difficul-
ties. Thus the attenuated state of the rock-forming substances
of the Earth in the Earth-Moon ring would probably have
resulted in their condensation into solid particles. Again, no
nebulae closely resembling the annulated solar nebula have
yet been discovered. Without going further into details, and
without necessarily admitting the finality of all the above
arguments, it can hardly be denied that Laplace's idea of the
development of the solar system must be reconstructed or
abandoned. It remains to be seen what can be substituted
for it. Two attempts in this direction will be described in a
later chapter.
CHAPTER XX
STELLAR DEVELOPMENT
The nebular hypothesis, as outlined in the last chapter,
presents a picture of the development of a planetary system
like our own. In testing it, recourse may be had both to
theoretical investiofations and to observations of various
kinds, particularly of nebulae, which may throw light on
the earlier stages of the process of condensation. It must
be remembered that planets comparable in size with the
members of our solar system would be quite invisible at
the distances of the stars. However, in the study of stellar
evolution we are concerned primarily with stars, rather than
with the planets that may accompany them. It is neverthe-
less evident that the two questions cannot be considered
independently, since the details of the processes that result
in the formation of planets must be of the highest impor-
tance in researches on the development of the central suns
of which they may have formed a part.
Herschel, whose mind was always occupied with the prob-
lem of the structure of the universe and the formation of its
individual members, thought he perceived in the nebulae
evidences of growth and development. He supposed that
the cloud forms, of irregular structure, which extend over
vast regions of the heavens, represent the earliest and most
rudimentary condition of stellar life. Condensation toward
a center, brought about by the action of gravity, would be
shown in such a cloud by increased brightness. Latest in the
line of nebular existence Herschel placed the planetary nebu-
lae, in whose symmetrical forms he saw illustrated some such
condition as Laplace postulated for the primitive solar system.
187
188 Stellar Evolution
The mystery of the planetary nebulae still remains un-
solved, but evidence is lacking that they represent a more
advanced state than such irregular cloud masses as the
Great Nebula in Orion. Indeed, it must be admitted that
the accumulation of observations, principally through the
aid of photography, has rendered the problem of nebular
development more complex than it appeared to Sir William
Herschel. Thousands of nebulae, entirely unknown to him,
have been brought to our knowledge through improvements
in telescope design and the aid of the sensitive plate. These
range in character from immense luminous tracts, such as
are shown, intermingled with stars, in photographs of the
Milky Way, to the definite outlines and highly suggestive
structure of the spiral nebulae. Of all objects in the heavens
these latter most strongly suggest the operation of some
process of development. But not a single object of this type
was known to Herschel, and even to this day their enormous
distance from the Earth has prevented the detection of any
changes in form, which might point to the explanation of
their origin.'
If we follow Herschel, and consider the simplest case of
nebular development, we may suppose that through loss of
heat by radiation a portion of a nebulous mass begins to
condense toward a center. Although still wholly gaseous,
and showing few points of difference from an ordinary nebula,
we may regard such an object as representing the first period
in the life of a star. In the heart of the Orion nebula, Plate
XXI, are four small stars, which constitute the well-known
Trapezium. Situated as they are in this enormous mass
of gas, it is not difficult to picture them as centers of con-
densation, toward which the play of gravitational forces
tends to concentrate the gases of the nebula. It might
therefore be expected that stars in this early stage of growth
1 See chap. xxi.
Stellar Development 189
would show, through the spectroscopic analysis of their light,
some evidence of relationship with the surrounding nebula.
Now, this is precisely what the spectroscope has demon-
strated. Not only these stars, but many others in the con-
stellation of Orion, are shown by the spectroscope to contain
the same gases that constitute the nebula. Moreover, they
also partake of its motion through space. Finally, Frost and
Adams have demonstrated the interesting fact that some of
these stars are actually moving in orbits about dark com-
panions situated in the very heart of the nebula. Since the
orbital velocities of the moving stars are very high, it thus
seems probable that the matter which constitutes the Great
Nebula in Orion is exceedingly tenuous, offering little resist-
ance to motion within it.
Other examples of direct relationship between stars and
surrounding masses of nebulae might be mentioned, but
this one will suffice for our present purpose. We must now
consider what changes in color and in s[)ectrum accompany
the further development of the star as it continues to lose
heat throuorh radiation.
Fraunhofer was the first, in the opening years of the
nineteenth century, to observe the spectra of the stars. The
simple method he employed, which consisted in placing a
prism over the object-glass of a telescope, has since become,
through the skill and energy of Pickering, a wonderfully
effective agent for the wholesale study of stellar s^^ctra.
To Fraunhofer the differences he perceived when comparing
the spectra of different stars were of no meaning, since
the work of Kirchhoff had not yet been done. But
the photographs made under Pickering's direction at the
Harvard CoUecre Observatorv now tell a remarkable storv
to the initiated. In making these photographs, a large
prism is mounted in front of the object-glass of a
(refracting) telescope, which is directed to a field of stars
190 Stellae Evolution
and made to follow its apparent motion by a driving-clock.
Under these conditions, each star-image in the field of the
telescope is drawn out into a spectrum, which falls upon a
photographic plate at the focus. If the rate of the driving-
clock were perfect, each of these spectra would be extremely
narrow, and the "lines" which cross it might not be per-
ceptible. To give the spectra the necessary width, the
prism is set with its refracting edge parallel to the diurnal
motion, so that the spectra would drift on the photographic
plate, if the telescope were at rest, in a direction at right
angles to their length. In making the photographs, the rate
of the driving-clock is slightly altered, so that the drift of
the spectra during the exposure is just sufficient to give them
the desired width. Without this drift, each "line" would
be merely a point in the spectrum. Plate LXXIX illustrates
how admirably the spectra of the various stars in the field
are recorded, and brings before us evidence of the spectral
diversity which is supposed to characterize the different
stages of stellar growth.
As already indicated (chap, xviii), the spectra of stars
increase in complexity as the cooling process continues. The
gaseous nebulae contain a few bright lines in their spectra,
the most conspicuous one of which belongs to a gas ("nebu-
lum") not yet discovered on the Earth. The other nebular
lines are due to hydrogen and helium. Those stars of the
^'' Orion type" which appear to be earliest in order of devel-
opment contain no lines except those of hydrogen and helium,
which are faint and very broad and difipuse. As these gases
are found in the gaseous nebulae, and as the relationshij)
of these stars to surrounding nebulous matter is otherwise
apparent, there is every reason to believe that they represent
the earliest phase of stellar life. The stars of the Trapezium,
which have already been mentioned as organically related to
the Great Nebula of Orion, are of this type. '■'■Orion'''' stars
Stellar Development 191
which appear to be somewhat further developed, show lines
of maoTiesium. silicon, oxvofen, and nitroo^en, in addition to
those of hydrogen and helium (Plate LXXX).
Next in order of evolution appear to be the white, or
bluish-white, stars like Sirius (Fig. 1, Plate LXXXI). The
spectrum of Sirius is marked by broad and conspicuous
hvdroo^en lines, associated with narrow and faint lines of
iron, sodium, magnesium, etc. It has been shown by investi-
gations of certain pairs of stars, in which the two components
are in rapid rotation about their common center of gravity,
that stars like Sirius are much less dense than the Sun, their
specific gravity not exceeding that of water. This, of course,
is exactly what would be expected in an early stage of transi-
tion from a gaseous nebula to a highly condensed star.
It is well known through mathematical demonstration
that the condensation of such a mass of gas as that in which
the Sun originated must involve the production of a vast
amount of heat. Indeed, the present solar radiation may be
accounted for by supposing that the Sun's diameter decreases
about 400 feet in the course of a year. The seemingly
paradoxical fact that a gaseous mass, through loss of heat by
radiation, will actually grow hotter as long as it remains in a
gaseous condition, was demonstrated by Lane in 1870. The
point is that the heat produced by shrinkage is more than
sufficient to compensate for the loss by radiation. Conse-
quently, the shrinking mass grows hotter as long as it remains
purely gaseous. The time finally comes, however, when its
outer parts, which radiate freely into space and are not pro-
tected from loss by outlying masses of heated matter, are
cooled to the point of condensation. That is to say, certain
metallic elements present in a state of vapor condense into
clouds made up of minute liquid drops, thus resembling our
terrestrial clouds, which are caused by the condensation of
water vapor.
192 Stellab Evolution
When this point is reached, radiation must take place
mainly from the surface of the star. This would result in
very rapid surface cooling, were it not for convection cur-
rents, which rise from the interior and supply the heat lost
by radiation. In the Sun we have strong evidence of the
existence of such currents, which are represented by the
bright filaments that constitute the granulated surface
(chap, xi), and by the minute fiocculi illustrated in Plate
XXXIX. The darker spaces (pores) between the granulations
probably represent the cooler descending vapors. The denser
vapors, which perhaps occupy these darker regions, apparently
lie below the general photospheric level, for it has recently
been found at Mount Wilson that the spectrum of the Sun's
disk, at points very near the limb, differs decidedly from the
spectrum at the center. The hazy w4ngs, which may be seen
on either side of many lines photographed at the Sun's cen-
ter, and are still more conspicuous in sun-spots, are greatly
reduced in intensity at the limb (Plate LXXXII). This
would seem to indicate that at the center of the Sun we are
looking down into the regions between the granulations, to a
level where the vapor is dense enough to produce the winged
lines. Near the edge of the Sun, on the contrary, we look
across the tops of the bright filaments, and therefore fail to
receive light from the denser vapors below. The absorption
of the higher and cooler vapors should produce a change in the
relative intensities of the lines such as takes place in sun-
spots (p. 159), but in much smaller degree. Observation
shows this to be the case, but there is by no means a strict
parallel between the two classes of phenomena, and judg-
ment must be reserved for the present. One of the next
steps will be to photograph the spectrum of a pore, if so
minute an object can be separately observed. The inves-
tigation, when completely worked out, should furnish a
searching criterion as to the validity of the hypothesis of
Stellar Development 193
reduced temperature in spots, and as to the cause of certain
phenomena in the Sun and stars.
We have already seen (p. 173) that increased density of
the absorbing atmosphere tends to reduce the proportion of
violet and ultra-violet rays, and thus to introduce a yellowish
or reddish tinore into the star's liorht. Such stars as Sinus
do not possess dense absorbing atmospheres, and because
of this fact and of their extremely high temperature, their
spectra extend far into the ultra-violet.
In passing from these white stars to the yellowish stars,
which constitute the solar class, the continued process of
condensation is accompanied by the production of an absorb-
ing atmosphere similar to that of the Sun. Beginning in
the ultra-violet, the absorption becomes more and more
appreciable as the solar type of star is approached. The
decrease of intensity, while most marked in the ultra-Wolet
region, is also manifest in the blue and violet part of the
spectrum, whereas the red, yellow, and green are not greatly
affected. The natural result is a change of color, throusrh
a deficiency in blue light. For this reason, stars of the
solar class are yellowish in hue. Langley has pointed
out that the Sun would appear bluish-white, if its absorbing
atmosphere were removed. Ac<;ompanying this change of
color we have the decreasing strength of the hydrogen lines,
and the increasing strength of the metallic lines, which
become very numerous in Procyon (Plate LXXXI ) and still
more so in the Sun.
As already remarked, this gradual increase of atmospheric
absorption prevents us from basing conclusions as to relative
stellar temperatures on the position of the maximum of
intensity in the spectrum. We may fall back, however,
upon comparisons of the relative intensities of certain
lines, just as was done in the study of sun-spots described
in chap. xvii. This method of classifying stars according
194: Stellar Evolution
to their temperature was applied by Lockyer many years
ago. He found that the "enhanced lines," which are
brightest in the spark spectrum of a metal, exist alone in
certain stars. In other words, the arc lines of the same
metal, which are strong at the lower temperature of the arc,
and feeble or absent in the spark, are so much reduced in
intensity in these stars as to be entirely invisible.
Lockyer's contention that these changes of relative inten-
sity afford a mode of classifying stellar spectra on a temper-
ature basis was denied by many spectroscopists, because of
the possibility that such changes might be produced in stars
by different electrical conditions rather than by differences
of temperature. The results obtained in our laboratory
imitation of sun-spot phenomena, however, seem to favor
the view that a temperature classification of stars, on the
basis of the relative intensities of lines, is perfectly pos-
sible. For in these experiments it was shown that
when all electrical phenomena are excluded, a decrease in
temperature of the radiating vapors is accompanied by an
increase in intensity of the lines that are strengthened in
sun-spots and in red stars. Since the spark lines are weak-
ened under the same conditions, and since conclusive evi-
dence of comparatively low temperature is afforded by the
presence in these spectra of flutings due to substances which
are broken up at the higher temperature of the Sun, the
temperature hypothesis may perhaps be taken as affording a
simple basis of classification. This statement is not made
without some reservations, however, as indicated by thi»
remarks at the end of this chapter, and by the discussion
of Lockyer's meteoritic hypothesis in chap. xxi. Moreover,
since this classification takes no account of the possible effect
of mass and environment on spectral type, it is hardly likely
to prove adequate.
Let us now consider the phenomena of declining stars,
Stellar Development 195
which have passed beyond the solar stage and are fading into
invisibility. It will be remembered that these stars are
orange or red in color and that they may be divided into two
classes, similar in appearance to the eye, but easily dis-
tinguishable with the aid of the spectroscope. The first of
these classes (Secchi's third type) includes certain bright
stars, such as the red Antares, which is a conspicuous
object in the southern heavens during the summer months.
The second class (Secchi's fourth type) has no brilliant
representative. Indeed, the brightest stars of this char-
acter are but barely discernible by the naked eye. while the
great majority are to be observed only with the aid of a
telescope.
In the spectroscope both classes show a spectrum vastly
more complicated than that of stars in an earlier stage of
growth. The broad lines of hydrogen, which are greatly
reduced in intensity in the Sun, are still further reduced in
the red stars. In fact, the dark hydrogen lines have in cer-
tain red stars given place to bright lines, especially in the
case of variable stars, whose light undergoes regular or
irregular fluctuations. The most characteristic feature of
the red stars, however, is the presence in their spectra of
dark bands or flutings. In third-type stars the sharp edges
of these bands lie toward the violet, while on the red side
the intensity gradually decreases. These bands have been
found by Fowler to be due to the oxide of titanium, which may
be broken up at the higher temperature of the Sun, but exists
in the sj^ectra of sun-spots (Plate LXXIV) . The bands of the
red stars of Secchi's fourth type face in the opposite direction,
J, with their sharply defined boundaries toward the red. These
, bands, as Plates LXXXIII and LXXXIV illustrate, are due
,?to carbon and cyanogen. Some of them are faintly present
in the Sun, but in the fourth-type stars they are much more
strongly developed.
196 Stellar Evolution
The extensive investigations of Vogel and Dun6r, made
visually, have given us much information regarding the
spectra of the red stars. However, the fourth-type stars are
so faint that only the bands in their spectra could be seen
with the telescopes used by these investigators, and their
numerous dark lines were beyond observation. The great
light-grasping power of the Yerkes telescope rendered a
photographic study of these spectra possible, with the results
shown in Plates LXXXIII, LXXXIV, and LXXXV. When
the fourth-type stars are ranged in a series, the gradual
change of spectrum from star to star is well illustrated (Fig. 2,
Plate LXXXIII). The carbon flutings become stronger and
stronger, until in a star like 152 Schjellerup they are so dense
that they cut out a considerable portion of the light. In the
Sun, the Yerkes telescope shows the existence of a very thin
layer of carbon vapor, lying in close contact with the photo-
sphere. In the fourth-type stars we may suppose that the
further process of condensation results in an increased
development of carbon vapor, the absorption of which becomes
the characteristic feature of the spectrum.
Another important point brought out by this investigation
is the close relationship existing between the line spectra of
third- and fourth-type stars. As will be seen by an exami-
nation of Plate LXXXV, the line spectra of /a Geminorum
and 74 Schjellerup seem to be almost precisely identical in
certain regions. The presence of titanium oxide bands in
the one case, and the carbon flutings in the other, compli-
cate the comparison of the line spectra in other regions,
though much is yet to be learned on this subject through
further study of these stars with spectrographs of the highest
dispersion.
In view of the resemblance of the line spectra, it is diffi-
cult to understand the diversity of the band spectra in the
two great classes of red stars. Among third-type stars all
Stellar Development 197
intermediate types of spectra may be found between the Sun
and the most advanced representative of the class. It might
thus seem, esjjecially in view of the close relationship
between sun-spot and third-type spectra, that the cooling of
the Sun would result in the formation of a third-type star.
However, although no such perfect continuity has been shown
to exist in the transition from solar to fourth-tyj:)e stars, it
seems possible that stars intermediate in character between
280 Schjelleriip (see Plate LXXXV ) and the Sun may yet be
discovered. Should this prove to be the case, and a more
rigorous test be found to confirm the observed resemblance
of the line spectra of the third and fourth types, the ques-
tion whether a star like the Sun will develop into a third- or
into a fourth-type star would be difficult to answer. This
problem, which is one of the most interesting of those con-
nected with the study of stellar evolution, will occupy a
prominent place in the working programme of the Solar
Observatory.
Although the red stai-s represent the last period of lumi-
nous stellar life, there remain to be considered the dark stars,
which have been discovered in spite of their complete invisi-
bility. Hundreds of these objects are already known to us
through spectroscopic observations. They are members of
double or triple systems, moving in orbits about a common
center of gravity. Their existence has been inferred from
measurements of the oscillation of the spectral lines, which
move back and forth toward the red or toward the violet, as
the star under observation recedes from and then approaches
the earth in its orbital motion. Obviously, it is the spectrum
of the \Hsible star which can be observed, but motion in an
orbit necessarily implies the existence of a companion star,
which may or mav not be luminous. If sufficiently brio-ht
to be visible, it may not be separated from its close neighbor
in the most powerful telescopes. But in the spectroscope
198 Stellar Evolution
the lines of the composite spectrum will appear double, twice
in each orbital revolution of the pair. If only one star is
bright enough to give a spectrum, its lines will simply oscil-
late to and fro.
Hitherto we have tacitly assumed, in harmony with cur-
rent views, that all stars are built on a single model, and that
each passes through the same stages of development in its
transition from the nebular condition to the solid state. It
should be pointed out here, however, that many circumstances
warn us against implicit acceptance of such a law of uni-
formity. The assumption that a given type of spectrum
represents a given stage of growth involves the idea that the
chemical composition of all stars is essentially the same, and
that the particular position of the star in the universe, and
other conditions which may obtain in individual cases, are
matters of no importance. While it is true that we have
strong reasons for belief in the universal distribution of most
of the chemical elements known on the Earth, and the uni-
versal operation of the law of gravitation, and of all other
laws which define terrestrial conditions, the assumption that
identically the same course is pursued by every star in
passing from its origin to its final decay is entirely un-
warranted. We must be prepared to meet widely diverse
conditions and to observe modifications in the process of
development which are determined directly by such con-
ditions.
Take the case of the Pleiades, for example (Plate
LXXXVI). Here we have a group of stars entangled in
nebulosity, and moving together through the heavens.
Every indication goes to show that this is an organic group,
whose members are of common origin. But the spectra of
practically all of these stars, irrespective of size and bright-
ness, are of Secchi's first type. How are we to believe that
widely different masses will pass through their evolutional
Stellab Development 199
steps with equal rapidity ? An appeal to double stars, whose
members are undoubtedly of common origin, does not help
matters. We invariably find that the fainter member of the
system, which might have been supposed to cool most rapidly
because of its smaller size, is yellow or red in color, while its
larger companion is more nearly white, or tinged with blue.
Huggins argues, however, that the greater surface gravity
possessed by stars of large mass may cause more rapid change
in spectral type. Thus a large star of low density may be no
farther advanced in spectral type than a smaller but more
highly condensed star. According to Huggins' views, the
early steps in evolution would be characterized by small
gravity at the surface, comparatively slow changes of tempera-
ture in passing outward from the interior, and convection
currents less violent than those observed in the Sun. If the
star were hot enough, hydrogen might be the only gas suf-
ficiently cool, with respect to the radiation from below, to
show itself by absorption lines. Vapors of greater density
would lie lower in the star, where their temperature might
be so nearly that of the region behind them that their lines
would not appear in the spectrum.
Schuster, who has done an important service in empha-
sizing the elements of weakness in the assumed law of uni-
formity, nevertheless believes that most of the s})ectral types
represent stages in the development of stars. Thus, while
he does not maintain that all stars pass through an identical
-• ries of chancres, he aorrees with the view that the greneral
. ourse of development lies along similar lines, though imjxir-
tant modifications may enter in particular cases. The order
( "f development which he favors is as follows :
(1) Helium or Orion stars.
(2) Hydrogen or SIn'an stars.
(3) Calcium or Procyon stars.
(4) Solar or CapeUan stars.
200 Stellar Evolution
In describing the process of condensation, Schuster points
out that the expansion caused by the rising temperature of
the gaseous bodies must at first result in the rejection of
helium, hydrogen, and other light gases, on the supposition
that the gravitation is not sufficient to retain them. These
light gases will thus be left to constitute diffuse nebulous
masses, as illustrated by the gaseous nebulae, particularly
by the nebulous regions in such a group as the Pleiades.
In the process of time, however, the star will have condensed
sufficiently to retain hydrogen and helium, and these gases
will then begin to diffuse into the interior, where they will
be absorbed at a rate which depends upon the star's mass.
Helium, which is denser than hydrogen, will be retained
first, thus giving rise to the helium, or Orioii, stars. As this
gas diffuses inward, its place will be taken by hydrogen,
which will thus become predominant in the spectrum. In
its turn, the hydrogen will diffuse into the star, and the
increasing convection currents will cause a more and more
complete stirring-up of the low-lying metallic vapors, which
will therefore play an increasingly prominent part in the
spectrum. Thus the solar stage will ultimately be reached.
An interesting point in this explanation is the consider-
able possibility of variation which stars of different mass and
in different environments may exhibit. If but little hydrogen
happens to be in the neighborhood, the process of conden-
sation may not result in the attraction of a sufficient
quantity of this gas to produce the hydrogen type of spec-
trum. Again, the star may be of such low density that it is
unable to attract hydrogen, and thus it may pass into the
solar stage without exhibiting strong hydrogen lines. There
may also be stars of such small mass that, in spite of having
condensed sufficiently to attract hydrogen, they are not able
to absorb it all, and therefore they may continue to exhibit
a spectrum of the first type without ever passing into the
Stellab Development 201
solar stao-e. Furthermore, in the case of two stars of equal
age but different mass, the larger may have passed to the
condition of Arcfurus (incipient red star), while the other is
still in the solar stage, because of its more rapid absorption
of hydrogen.
This theory gives an interesting explanation of the above-
mentioned spectral phenomena of double stars, since it indi-
cates that the larger star, through its power of absorbing
hvdrogen more rapidly and completely, may pass to the solar
stage, while the smaller one continues to give a spectrum of
the first type. Schuster agrees with Huggins that the small
mass would lose heat more rapidly than the larger one, but
believes that the type of spectrum may be more completely
controlled by the rapidity with which the hydrogen is
absorbed.
It is evident that the highly suggestive views of Schuster
should stimulate much research. The distribution of spectra
of different types through the heavens is a subject of great
interest, and doubtless has an important bearing on the
question of stellar evolution. Certain types of stars, for
example, tend to cluster thickly in the Milky Way, while
others show no such tendency. Pickering's work in photo-
graphing the spectra of an immense number of stars in the
northern and southern heavens offers most valuable material
for the study of this subject. The investigation may |)erhaps
be extended to fainter stars with the 60-inch Mount Wilson
reflector, through the use of a spectrograph having no slit
and so designed as to record photographically the spectra of
all stars lying within a certain field. But since this field
m hardly exceed 20' of arc in diameter, it would not be
reasible to photograph the entire heavens in this way.*
However, a most important scheme of co-operation has
been instituted by Kapteyn, for the purpose of obtaining
1 The objective prism photographs cover a field several degrees in diameter.
202 Stellar Evolution
data bearing upon the problem of the geometrical stucture
of the universe and the distribution of stars within it.
Through the impracticability of securing all necessary data
for stars distributed over the entire heavens, Kapteyn has
selected certain limited areas of the sky, so distributed as to
render it probable that conclusions based upon a complete
study of the stars within these areas will be likely to apply
to the heavens at large. The application of the 60-inch
reflector to the photography of stellar spectra by the above-
mentioned process will therefore be confined, for the most
part, to Kapteyn's areas, where many other observers are
already gathering information, in accordance with a plan
which allots to each institution the work for which its instru-
ments are best adapted. For Kapteyn's purposes, only the
general type of spectrum is required, since he is primarily
concerned with questions of distribution and structure, rather
than those which relate to the evolution of stars. The data he
desires include determinations of the brightness, distance,
and motions of the stars within the selected areas. The 60-
inch reflector, on account of its great light-gathering power,
can assist materially in those portions of this work which
relate to the faintest stars. The investigation of the motions
of these stars in the line of sight is necessary from the evo-
lutional standpoint, because community of motion may mean
organic relationship of stars in a group, as in the case of
the Pleiades. Photometric investigations and the study of
parallaxes are also required, since, when the distance and
brightness of a star are known, its mass can be determined,
if certain reasonable assumptions as to the surface brilliancy
are made. We have just seen how important a factor the
mass of a star may be in determining the course of its
evolution.
Enough has been said to indicate the nature of the work
which large telescopes may perform. The direct photography
Stellar Development 203
of nebulae may provide the means of detecting, in the course
of years, changes in their form bearing directly upon the
manner and rate of their condensation. The photographic
study of their spectra may help to explain why a few nebulae
show the bright line spectra of gases, while the very numerous
spiral nebulae appear to have merely a continuous spectrum.
With the high dis{>ersion of powerful spectrographs like the one
shown in the constant-temj^erature chamber in Plate XCVI,
the spectra of a few of the brightest stars in the heavens,
which include most of the spectral types, can be minutely
analyzed. In this way, and with the aid of smaller spectro-
graphs, the spectra of red stars of the third and fourth types
can be examined to much better advantage than previously,
with reference to their relationship to the Sun, to sun-spots,
and to one another. It is evident that these investigations,
with others on the spectra of stars of various classes, the
distribution of the different types of spectra within Kapteyn's
selected areas, and studies of the brightness and parallaxes
of the same stars, might well involve the co-operative use of
several telescopes of the largest size.
CHAPTER XXI
THE METEORITIC AND PLANETESIMAL HYPOTHESES
In even the briefest outline of the methods of studying
stellar evolution, reference must be made to two hypotheses
which are intended by their authors to take the place of the
nebular hypothesis of Laplace. In both of these, swarms
of meteorites, rather than matter in the gaseous state, are
supposed to afford the raw material of which stellar systems
are compounded. The nature of the swarms, however, is
unlike in the two cases. According to Lockyer, the meteorites
are to be regarded as analogous to the wandering mole-
cules of gases, in that they move indiscriminately in all direc-
tions and at widely different velocities. Sir George Darwin
has, indeed, demonstrated mathematically that a meteoritic
swarm, constituted in this way, is closely analogous to a
gas. The meteorites move rapidly about, colliding with one
another from time to time, just as the molecules of a gas are.
supposed to do, according to the kinetic theory. Chamberlin
and Moulton, on the contrary, assume their meteorites to be
revolving in well-defined orbits, and therefore suffering only
such collisions as may result from certain meteorites over-
taking others of lower velocity.'
The most characteristic nebular line is a brilliant one in
the green part of the spectrum, attributed to an unknown
gas, which has been called "nebulum." According to
Lockyer, this line is the remnant of a complicated fluting
in the spectrum of magnesium oxide, with the brightest
part of which he found it exactly to coincide. In his view
1 In his explanation of globular and spiral nebulae, and of certain other celestial
pbeijomena, Lockyer also assumee the meteorites to revolve in well-defined orbits.
204
Meteoritic and Planetesimal Hypotheses 205
the rest of the fluting is invisible only because of the faint-
ness of the nebular line. This has been completely dis-
proved, however, by Keeler's remarkably precise measures
of the chief nebular line at the Lick Observatory. His
observations show, not only that the chief nebular line does
not corresix)nd in position with the head of the magnesium
fluting, but also that it differs entirely from it in appearance.
It is therefore not possible to regard magnesium oxide as a
constituent of the nebulae. The green line may with far
greater probability be considered to represent a very light
gas, not yet discovered on the Earth.
Lockyer's conclusions as to the origin of the chief nebular
line play an important part in his meteoritic hypothesis.
He believes that the frequent collisions between meteorites
in the swarms produce sufficient heat to volatilize certain
constituents of the meteorites, which are rendered luminous,
so that their lines should appear in the nebular spectrum.
Lockyer tried the experiment of heating fragments of mete-
orites in a tube, from which the air had been partially
exhausted. He found that hydrogen, hydrocarbon vapors,
and the vapor of magnesium oxide were given off from the
meteorites. When an electric discharge was passed through
the gases in the tube at reduced pressure, the spectrum was
found to consist of the lines of hydrogen, the characteristic
flutings given by compounds of carbon, and the green fluting
of magnesium oxide, to which reference has been made. In
comets, which are known to be intimately associated with
meteorites, the flutings due to compounds of carbon form
the most characteristic feature of the s{)ectrum. But although
certain astronomers have believed these flutings to be present
in the spectrum of the nebulae, their conclusions are not
confirmed by the majority of observers, who can neither
see nor photograph any trace of the flutings. The only
remaining connection between the nebulae and the gases
206 Stellar Evolution
derived by Lockyer from meteorites therefore depends upon
the presence of hydrogen in both cases. But hydrogen is
so universally distributed among the celestial bodies that its
absence from nebulae would almost be regarded as an anom-
aly requiring explanation. It therefore cannot be said that
much weight is to be accorded to the experimental basis of
the meteoritic hypothesis.
It ought to be said, in favor of the hypothesis, that it
provides a simple way of accounting for the existence in
the nebulae of substances not represented in their spectra,
but which appear in stars evolved from nebulae. If a
nebula is to be regarded as a glowing gas, in which all
substances contained in stars exist in a state of vapor,
it remains to be shown why a very few gases manifest
their presence by the appearance of their bright lines in
the spectrum, whereas all the other elements produce no
lines, and therefore give no indication of their existence.
In this connection it must not be forgotten that in mix-
tures of various vapors the spectra of some of the vapors
appear when an electric discharge is passed through the
mixture, while the lines due to certain other vapors remain
invisible. Too little has been done, however, in this im-
portant field of research, to permit final conclusions to be
drawn. For this reason no one is at present able to say in
what form the iron, nickel, and other metals, which sub-
sequently make their appearance in the stars, can exist in
the nebulae.
This question is, indeed, but one of the many mysteries
which at present surround the nebulae (Plates LXXXVI-
XC). We have no knowledge, for example, why they glow
with a steady and unchanging light, since there is no direct
evidence that this light is produced either by heat or by
electrical excitation. It must not be forgotten that very few
nebulae are certainly known to be gaseous: thousands of
Meteobitic and Planetesimal Hypotheses 207
them seem to give a continuous spectrum, in which the bright
lines of gases do not ap}:>ear. Whether this is due to the
presence of solid or liquid matter, to pressure effects, or to
other causes, is not yet known. The process by which stars
are condensed out of nebulae is also not clearly understood.
It cannot depend wholly upon some action connected with the
spiral form, since, as already stated, we have in the Orion
nebula, which is not a spiral, one of the best -known examples
of direct relationship between stars and nebulae. It is now
rather commonly believed that, while the temperature of
small particles in the nebulae may be very high, the mean
temperature of the entire mass may nevertheless be very low,
since it has been pointed out by Huggins that the appear-
ance presented by the nebulae could be produced by widely
separated luminous particles. In view of all these facts, it
may therefore be said that much work remains to be done
on the nebulae, not only in photographing their forms, but in
investigating their spectra, and in interpreting them through
laboratory experiments.
Starting from the meteoritic hypothesis, and assuming
that the chemical elements, at the tem})erature of the hottest
stars, are dissociated into simpler substances, Lockyer has
developed a plan of stellar evolution which comprises a classi-
tication of stellar spectra on a temperature basis. He sup-
poses that the meteoritic swarms represented by the nebulae
gradually condense into stars, by processes whose details are
still uncertain. According to his classification, the gaseous
and bright-line stars, in which the temperature is supposed
to be higher than that of the less condensed nebulae, lie just
above the latter in point of development. Then come the
red stars of Secchi's third type : though it may appear to many
spectroscopists that the difficulty of tracing a connection
between their spectra and those of the stars placed just before
them would be altogether insuperable. Further conden-
208 Stellar Evolution
sation, still involving a rise of temperature, would produce
stars analogous to the Sun, but differing in the important
particular that, while their temperature is increasing, that
of the Sun is supposed to be decreasing. Finally, at the
point of maximum temperature, Lockyer places stars of
Secchi's first type. Here the meteorites, long since com-
pletely transformed into the gaseous state, have reached the
condition implied by Lane's law, at which the rise in super-
ficial temperature, due to continued condensation, is just
balanced by the loss resulting from radiation. The declining
period, then setting in, results in the development of stars
like the Sun, which can be only arbitrarily distinguished from
stars of equal, but rising, temperature, lying on the opposite
branch of the temperature curve. After the solar stars come
the red stars of Secchi's fourth type, and after these, final
extinction of light.
This system of classification, considered apart from the
hypotheses with which it is connected, has the advantage of
providing for both the ascending and descending branches
of the temperature curve. Unfortunately, we are perhaps not
yet in a position to distinguish clearly between stars of the
same surface temperature, in one of which the gain of heat
is more rapid than the loss, while in the other the reverse
is true. As already remarked, the assumption that the red
stars of Secchi's third type lie not far above the nebulae is also
a difficult one to admit. But the classification nevertheless
deserves careful consideration, and the most searching tests
that can be applied.
As the late Miss Gierke has well said, the complex struc-
ture of meteorites suggests a highly developed, rather than an
elementary, condition of existence. This, however, is hardly
to be taken as an objection to Lockyer's hypothesis, since
the manner in which the meteoritic swarms came into exist-
ence is not postulated. The planetesimal hypothesis, however.
Meteobitic and P.lanetesimal Hypotheses 209
begins with a fully organized sun, which is supposed, in its
motion through space, to come into the immediate neighbor-
hood of another sun, equal to or greater than itself. The
etfect of the attraction between the two bodies would be to
reduce the immense restraining power of the Sun along the
line of mutual attraction. /. e., in the direction of the other
sun, and in the opposite direction. Under certain conditions
the Sun is observed to shoot out prominences with velocities
approaching 300 miles per second. If the velocity exceeded
382 miles per second, the matter projected from the Sun
would escape the power of its attraction and move off into
space, never to return. If another great body were passing
near the Sun, the tendency toward eruptions would be greatly
augmented along the line joining the two bodies, and immense
protuberances would doubtless be projected at high velocities
from opposite ends of the solar diameter corresponding
with this line.
According to the planetesimal hypothesis, the two pro-
tuberances would be formed as the two suns were swinwinor
past one another around their common center of gravity.
The effect of mutual attraction would be to cause the two
great arms to assume a spiral form, in which the scattered
materials revolve about the central mass in elliptical orbits.
Moulton has shown, by rigorous mathematical tests, that just
such a result might actually occur, and that the forms of
the spiral nebulae may thus be closely imitated (Plates
LXXXVIII-XC). Although the matter shot out from the
Sun would necessarily be gaseous, the hypothesis assumes
that it wo aid rapidly cool down to a finely divided solid con-
dition.' The outer portions of the protuberances would
naturally be formed from the surface materials of the Sun,
while the inner extremities would come mainly from lower
1 How, it may be asked, can these small bodies remain brilliantly luminous for
many years? And why do we not discover incipient spirals, giving a bright line
spectrum?
210 Stellae Evolution
depths, where the heavier elements are found. This may
possibly explain the lightness of the outer planets of our solar
system, and the great relative weight of the inner ones. The
changing attraction of the neighboring star might also cause
a series of irregular outbursts, accounting for the knotty and
uneven distribution of the matter in the spirals (Plate XC).
Chamberlin points out that a very small fraction of the Sun's
mass, not exceeding 1 or 2 per cent., would be amply sufficient
to supply all of the matter required to form a planetary
system like our own.
In the further evolution of the system, the central mass
is supposed to form the sun, the knots to serve as the nuclei
about which the planetary materials gather, and the remain-
ing diffuse nebulous matter to be swept up by the nuclei or
absorbed by the sun. The building-up of the planets is not
supposed to take place, as in the nebular hypothesis, simply
through the gravitational attraction of the planetary nuclei
on the matter surrounding them. On the contrary, the main
agency is assumed to be a gradual accretion of the mass
through collisions of isolated planetesimals (meteorites)
resulting from the intersection of the individual orbits,
brought about periodically through the rotation of their line
of apsides. Thus it is held, according to this hypothesis,
that the Earth was never a molten mass, but that it was built
up by gradual accretions. Chamberlin was led to this view
of the condition of the Earth's interior from various geo-
logical considerations, which seem to him inconsistent with
the hypothesis of a fluid origin.
If this book were a treatise on stellar evolution, all of
these questions would require much fuller discussion and
criticism, and space would necessarily be devoted to the
remarkable phenomena of variable and temporary stars, the
tidal investigations of Darwin and their possible bearing on
the evolution of double-star systems, and many other subjects
Meteoritic and Planetesimal Hypotheses '211
which have not received consideration. Enough has been
said, however, to give an idea of the nature of the problems
which an observer concerned with stellar evolution is called
upon to attack, and the general character of some of the
observational methods required to solve them.
CHAPTER XXII
DOES THE SOLAR HEAT VARY ?
One does not often stop to think of the delicate balance
that determines the conditions of life on the Earth. But it
is obvious enough that a small change in the intensity of the
solar radiation would suffice to transform the climate of the
temperate zones to that of the equatorial or polar regions.
A greater change might soon result in the complete destruc-
tion of life.
It is therefore a matter of the most vital interest to
inquire into the source and constancy of the Sun's heat.
What fuel maintains the great fire that warms and lights us,
and supplies, through its beneficent influence on growing
crops, the food that we consume? Is the average daily
influx of solar rays constant and unchangeable, and are we
justified in our tacit belief in the inexhaustibility of the
supply? Such thoughts, seriously pondered by students of
solar physics, have led to extensive investigations, which
must go on for many years before these questions can be
finally answered.
As we have already seen, the contraction of a nebulous
mass to form a star, or a sun like our own, must result in
the liberation of much heat. Indeed, the total solar radia-
tion in the course of a year can be accounted for on the sup-
position that the Sun's diameter decreases about 250 feet in
this time. Since the discovery of radium, which possesses
the remarkable property of sending out heat, with little evi-
dence of exhaustion, for very long periods of time, it has
been suggested that this substance, if it exists in the Sun,
may be the source of part of its radiation. Radium has not
212
Does the Solab Heat Vabt? 213
yet been detected in the Sun with the spectroscope, bnt it
may lie at low levels, where its vapor would take no part in
the absorption that produces the lines of the solar s|)ectmni.
The abundance of helium in the Sun suggests that radium,
which gives off this gas during the disintegration process,
may perhaps exist within or beneath the photosphere.
If radium really supplies any considerable part of the
Sun's heat, its ultimate exhaustion would involve a decided
decrease in the solar radiation. As we are not yet certain,
however, that there is any radium in the Sun, the possibility
of such a contingency may be regarded as too remote for
profitable speculation.
We may take it for granted that the Sun will continue to
radiate heat, at practically the present average rate, for many
centuries to come. But do we know that the rate is abso-
lutely constant? May not fluctuations occur of sufficient
magnitude to affect our climate appreciably, and to be
reflected in the ebb and flow of crops and the price of wheat ?
Until a short time ago this question had been tested in
only the roughest way. It was known that sun-spots pass
through a regular cycle of change, occupying about eleven
years. A curve was accordingly drawn, showing the varying
number of sun-spots, and compared with a curve represent-
ing, for example, the varying price of wheat. As the two
were thought to show some correspondence in form, it was
held that the price of wheat is determined by the solar
activity, as measured by the number of spots.
But the correspondence of the two curves was far from
[f rfect, and might have resulted from mere chance. Rain-
fall and temperature curves have given results that appear
more satisfactory, but the whole question is still in its primi-
tive stages, and little that is absolutely definite and reliable
has been learned. The efforts now being made by the Solar
Commission of the International Meteorological Committee
214 Stellar Evolution
may be expected to help matters, but much will depend upon
the appliances used to measure the solar radiation, and to
determine the amount of heat lost by absorption in the
Earth's atmosphere.
The most elaborate study of this question yet made is
due to the late Secretary Langley, of the Smithsonian Insti-
tution. He long ago recognized that the chief difficulty of
the problem lies in the constantly varying absorption of the
air above us. If measures of the solar radiation could be
made from a point outside of our atmosphere, any observed
fluctuations would be due to the Sun itself. But near the
level of the sea the difficulties are very great.
To diminish them, Langley led an expedition to Mount
Whitney in California. Here, at an elevation of over 15,000
feet, the denser and more variable half of the atmosphere is
left below. The precision of the measures was thus greatly
increased, but the expedition was not able to remain long
enough to determine whether the so-called "solar constant"
of radiation is actually a constant, or undergoes changes of
an irregular or a periodic character.
Langley strongly felt the importance of continuing this
work with the greatly improved apparatus developed by
Abbot and others at the Smithsonian Astrophysical Obser-
vatory in Washington. He therefore recommended that the
Carnegie Institution make provision for further researches
of this nature at a mountain station. When the Solar Ob-
servatory was established, a co-operative arrangement with
the Smithsonian Institution was accordingly entered into,
and measures of the solar constant were made daily by Abbot
on Mount Wilson during the summers of 1905 and 190(5.
The apparatus used in this work is most ingenious. Two
independent operations are carried on simultaneously: the
direct measurement of the solar radiation with some form of
pyrheliometer ; and the determination of the atmospheric
Does the Solab Heat Vaby? 215
absorption, for all the colors of the spectram, with a bolometer
(Plate XCI).
The pyrheliometer, in the form used by Abbot, measures
the rise in temperature, in a given time, of a known volume
of liquid exposed to the Sun's rays. If there were no atmos-
phere, pvrheliometer measures alone would suffice to furnish
the desired information. But the heat of the Sun at noon
is far greater than shortly after sunrise, since the rays pass
through a much shorter air-path. Consequently, the obser-
vations must be repeated at regular intervals throughout the
morning.
The bolometer, invented by Langley, is so sensitive
to radiation that it will measure a rise in temperature of
less than one-millionth of a degree. It consists of two
very fine threads of platinum, about ^-5^00^ inch thick,
mounted side by side within a constant temperature chamber.
One of these is shielded, the other exposed to the radiation
to be measured. The platinum threads form two of the arms
of a " Wheatstone's bridge," and are connected with a stor-
age battery, so that a feeble current constantly passes through
them, A galvanometer of the most sensitive type is so bal-
anced in the circuit that its reading is zero when the currents
flowing through the two platinum threads are equal. The
moment the resistance of the exjx)sed strip is changed by
radiation falling inx)n it, the galvanometer is deflected by
an amount which measures the heating effect of the radiation.
In practice, the solar spectrum is caused to move slowly
across the exposed bolometer thread. The galvanometer
needle then swings back and forth, giving small deflections
when a dark line or absorption band is passing over the
bolometer, and large deflections when the full intensity of
the sjiectrum is being measured. To record the motions of
the needle a minute mirror, attached to it, is caused to reflect
a spot of light upon a photographic plate. The same mechan-
216 Stellar Evolution
ism that moves the spectrum across the bolometer causes
this plate to travel slowly downward. Thus the deflections
of the needle are photographically registered upon the plate.
With the aid of such curves the total atmospheric absorption,
measured separately for each region of the spectrum, is
accurately determined. The reduced pyrheliometer readings,
corrected in this way for absorption, give the value of the
solar constant.
With such highly developed instruments the systematic
study of the solar radiation was pursued in Washington. On
the best days, which came none too often, the refinement of
the method permitted the atmospheric absorption to be elimi-
nated, even at this station so near the level of the sea. It
was soon found that the values of the "solar constant" were
not constant, but variable. Indeed, differences as great as
10 per cent, of the whole were encountered. Was it safe to
conclude that the solar radiation undergoes variations of this
considerable amount?
On Mount W^ilson the escape from the denser air of the
valley, the purity of the upper sky, and the constant succes-
sion of perfectly clear days, permitted the question to be
put to the test. Day after day the Sun was followed through
the heavens, from a time soon after it rose above the eastern
mountains to its culmination near the zenith. Sometimes
the work was continued through the afternoon, but the morn-
ing observations proved to be sufficient.
As soon as the curves had been measured and reduced,
and the pyrheliometer observations plotted, the full advan-
tages of the mountain station appeared. Not only was the
precision of the work much greater than before: even more
important was the fact that daily observations, continued for
many weeks, brought the exact nature of the phenomenon to
light. Through the latter part of the month of July, 11)05,
the value of the solar constant increased slightly from day
Does the Solar Heat Vary? 217
to day, until it reached a maximum. It then declined in the
same grradual manner. From these results Abbot concluded
that the solar heat had temporarily undergone actual change,
not to be ascribed to any modification of our own atmosphere.
Does this mean a greater outpouring of the solar radia-
tion, caused by an actual increase in the surface temperature
of the Sun? Or had the absorption of the solar atmosphere
decreased for a time, returning later to its normal value?
Much study will be required to answer this question, though
the uncertainties may be partially cleared up when the 1906
observations have been reduced. Increased solar activity,
represented by numerous sun-spots and flocculi, may prob-
ably be taken to indicate the existence of more numer-
ous and more violent convection currents, bringing larger
quantities of heat from the Sun's interior to the surface.
At times of great solar activity, therefore, we might expect
increased radiation. But this might soon be checked by
the diffusion through the solar atmosphere of materials
thrown upward by the violent eruptions, which characterize
such periods of activity. Indeed, the increased absorption,
persisting after the subsidence of unusual activity, might
result in a reduction of the radiation below its normal value.
Evidently a comparison must be made between observa-
tions of various kinds, carried on simultaneously. Spectro-
heliograph plates, bearing the record of the area covered by
the flocculi, afford an index to the solar activity. The
absorption of the solar atmosphere may also be measured by
allowing the solar image to drift slowly across a bolometer,
and photographing the galvanometer deflections upon a fall-
ing plate. During the summer of 1906 both of these classes
of work were carried on at Mount Wilson, simultaneously
with Abbots measurements of the solar constant. When all
the results are discussed together, new light may be thrown
on the subject.
218 Stellar Evolution
But the work is barely started, and must be continued for
many years under the best conditions. Simultaneous obser-
vations at several widely separated mountain stations are
greatly to be desired, to make certain that local changes in
our own atmosphere are in no wise concerned in the apparent
solar changes. Moreover, the work should go on without
the interruptions caused by the rainy season. If, for example,
a holographic outfit were established at the Solar Observatory
at Kodaikanal, in south India, at an elevation of 7,000 feet,
the dry season there would correspond with the rainy season
in southern California. An Australian station might also
accomplish very important results. It is to be hoped that
adequate provision may soon be made to carry out this im-
portant work.
But, it may be asked, must not such fluctuations of the
solar radiation, if real, be the cause of marked changes of
terrestrial temperature, easily detected and obvious in their
effects? Abbot believes that the thermometric records do
actually reflect these solar variations, but Newcomb holds the
contrary view. It is evident that complex meteorological
phenomena may be involved, and that their disentanglement
may require long-continued research. For this reason the
studies of the solar radiation undertaken by the Interna-
tional Union for Co-operation in Solar Research, the co-
operation in meteorological work set on foot by the Solar
Commission, and the labors of such an institution as the
observatory recently established on Mount Weather, Vir-
ginia, by the United States Weather Bureau, should prove
of value. In the exhaustive study of so important a problem
the cordial co-operation of many investigators is essential to
success.
CHAPTER XXIII
THE CONSTRUCTION OF A LARGE REFLECTING
TELESCOPE
The grinding and polishing of a 60-inch mirror involve
a variety of operations, described in detail in Ritchey's
memoir On the Modern Reflecting Telescope and the
Making and Testing of Optical Mirrors,^ the most authori-
tative treatise on the subject. A brief account of these
operations, taken in large part from the above source, may
be of interest here.
It is first necessary to obtain a suitable disk of glass.
The disk (of plate glass) made by the French Plate Glass
Works, of St. Gobain, France, for the reflecting telescoj^e of
the Solar Observatory is 60 inches in diameter, 8 inches
thick, and weighs a ton. It must be remembered that the
requirements for a large mirror are very different from those
for a lens through which light is to pass. The mirror disk
is merely a support for the thin silver film on its front sur-
face, from which the light is reflected without entering the
glass. For this reason the great perfection of a lens disk is
not necessary. Nevertheless, the glass must be free from
striae and other evidences of irregularity of structure. It
should contain no laro^e bubbles, thouorh a few small ones, if
they do not lie on the surface, are not objectionable. The
most important condition, however, is freedom from strain
caused by imperfect annealing. Evidences of strain are
^ detected by a test with polarized light. Such a test, how-
* ever, cannot be final, as an incident in the history of a great
telescope objective illustrates. The disk had been carefully
I Published by the Smithsonian Institution.
219
220 Stellar Evolution
annealed and was supposed to be suitable for its purpose.
During the process of grinding it flew to pieces, on account
of internal strain, the serious nature of which had not been
recognized in the test with polarized light.
It may not be obvious why the disk must be so thick,
when its sole purpose is to support the thin film of silver
on its accurately figured face. Great thickness, however,
is absolutely essential, to diminish the effects of bending
due to the weight of the glass and to temperature changes.
The thickness of a mirror should not be less than one-
eighth or one-seventh of the diameter. Even with such
thickness a special support system is necessary to prevent
flexure.
Glass is chosen in preference to other materials for tele-
scope mirrors because of its uniformity of structure, com-
parative ease of working, and capacity for a high polish.
Its lightness, when compared with such substances as specu-
lum metal (formerly employed for telescope mirrors), is
an important advantage. Furthermore, a surface of pure
silver, first used by Foucault, reflects a much larger propor-
tion of light than polished speculum metal.
The grinding-machine, designed and constructed by
Ritchey for his work on the 60-inch mirror, is shown in
Plate XCII. The glass disk rests on a heavy cast-iron turn-
table, carried by a vertical steel shaft. Between the lower
surface of the glass (ground flat) and the turn-table are two
thicknesses of Brussels carpet, which form an admirable
support during the grinding and polishing process. The
edge of the glass is ground true by means of a rapidly
rotating iron face-plate, held against the disk while the turn-
table is slowly rotated. The cutting material is powdered
carborundum, carried down between the glass and the face-
plate by a slow stream of water. After the edge-grinding
is completed, the two faces of the glass are ground plane
A Large Reflecting Telescope 221
and parallel, before the process of making one of these sur-
faces concave is undertaken.
The grinding-tools employed for this work are circular
plates of cast-iron, strongly ribbed on the back, and divided
into a series of small squares on the grinding surface, by
two sets of parallel grooves, planed at right angles to one
another. The tool rests on the surface of the glass, though
in Plate XCIII it is shown suspended from the lever arm,
employed to swing the heavy tools into or out of position.
During the grinding the disk is slowly rotated and the tool,
also kept in rotation, is moved over its surface in a series
of strokes from four to eight inches in length, by means
of the arm shown above the disk in Plate XCIII. On its
riorht-hand extremitv this arm terminates in a steel shaft,
which moves back and forth through a swiveled bearing
supported on an adjustable slide. In this way the position
of the grinding-tool on the disk can be changed laterally,
so as to bring the stroke across the center of the glass or
near the edge. If it is found, for example, that the center
is being cut away too rapidly, the tool is moved near the
edore and the crrinding: continued there until the error is
conected. The tool is not kept at any one position for a
great length of time, to avoid producing low zones in the
glass.
For the grinding process, various grades of carborundum
are prepared in the following way: The powdered carbo-
rundum is mixed with water and thoroughly stirred. After
settling for two minutes the coarse particles reach the bottom
of the bucket and the liquid, containing "two-minute" car-
borundum and the finer grades, is siphoned off into another
bucket. After the contents of the second bucket have been
allowed to stand four minutes, the liquid is poured off and
the "two-minute" carborundum at the bottom of the bucket is
set aside for fine grinding purposes. In the same way, carbo-
222 Stellar Evolution
rundum which has regained in suspension for periods up to
one hundred and twenty minutes, or even longer, is prepared,
These very fine grinding materials are used to give the
smooth and almost polished surface obtained after the grind-
ing with coarser carborundum is completed.
A perfectly true Brown & Sharpe steel straight-edge is
used to determine whether the surface of the glass is approx-
imately plane. When it is found to be sufficiently so for
the preliminary work, the fine grinding is commenced,
beginning with two-minute carborundum and continuing
with finer grades. In this work the iron grinding-tool is
counter-poised by placing weights on a lever arm con-
nected by a shaft with the tool. The pressure is reduced
from one-third pound to the square inch for the five- or ten-
minute carborundum, to about one-twelfth pound per square
inch for the one-hundred-and-twenty- and two-hundred-and-
forty-minute carborundum. Unless this precaution is taken
there is great danger of scratching the glass.
After being fine ground, the back of the mirror is
polished with rouge in the manner described later. No
great pains are taken with this surface, although it is made
very nearly plane, and is then polished so as to permit silver-
ing (Plate XCIV). It is desirable to silver the back of the
mirror, as well as the front, in order to prevent temperature
changes from affecting the two surfaces in unequal degree.
The front surface, after it has been given a plane figure,
is ready to be made concave. For this pur[)ose a convex
iron tool, of suitable curvature, is employed. In the case of
the 60-inch mirror the radius of curvature is 50 feet. The
curvature of the tool, and also of the glass, is tested from
time to time by a spherometer. This consists of a tripod,
with a micrometer screw at its center, which permits the
deviation of the surface from a plane to be accurately deter-
mined. After the desired curvature has been secured, the
A Large Reflecting Telescope 223
fine grinding is carried to a point where the surface is very
smooth and ready for polishing.
The [X)lishing and figuring are done by means of a tool
built up of narrow strips of wood, saturated with paraffine to
prevent change of figure. The face of this tool is covered
with squares of rosin, of a certain degree of hardness, which
can be determined only by experience. The rosin squares
are finally coated with a thin layer of beeswax, which forms
the polishing surface. The soft wax is very useful, since
small hard particles that may happen to be present in the
polishing material are likely to bed themselves in it, thus
reducing the danger of scratches. As a preliminary to
polishing, the tool is placed in contact with the glass disk
and pressed against it, by weights placed on the back, so
that it may acquire the same curvature as the surface.
After pressing for some hours, until the waxed squares
appear smooth and bright in all parts, the polishing may
begin. This is accomplished by moving the tool over the
rotating glass, by the main arm of the machine, as in the
case of the grinding process. The polishing material is
powdered jewelers' rouge, used commercially for polishing
plate glass. The tine rouge is separated from impurities
and coarser particles by a washing process similar to that
used for carborundum. The rouge, mixed with distilled
water, is applied to the surface of the glass by means of a
wide brush of cheese-cloth.
The greatest precautions must be taken throughout the
polishing process to avoid scratches. For this purjx)se the
room in which the work is done is fitted up in such a way as
to eliminate danger from dust. In the polishing-rooms of
the Solar Observatory optical shop (Plate XCV) the plas-
tered walls and ceilings are heavily varnished, and a canvas
screen is hung above the glass, to protect it from any falling
particles. The cement floor is painted, and kept wet when the
224 Stellar Evolution
polishing is in progress. The windows are double and care-
fully sealed, outer air being admitted to the room through a
cheese-cloth filter. The temperature is maintained constant,
within two or three degrees, by means of a hot- water furnace,
controlled by a thermostat. The motor, driving-shaft, and
apparatus for varying the speed of the grinding-machine,
are carefully inclosed, only the slow-moving belt coming out
into the room. No one is permitted to enter the room except
the optician, who wears a surgeon's gown and cap. By
observing such precautions the work may be continued for
months without producing even microscopic scratches in the
glass surface.
We may now assume that the glass has been polished,
after receiving an approximately spherical surface. It then
becomes necessary to apply a more accurate test than the
spherometer permits. For this purpose the glass is turned
into a nearly vertical position, where it is supported by a steel
edge-band (Plate XCV). An artificial star, consisting of
a hole about 3^^^ of an inch in diameter illuminated by an
acetylene lamp or other brilliant source of light, is placed at
the center of curvature, 50 feet from the glass surface. The
light from the articifial star then falls upon the disk and is
reflected back so as to form an image close beside the pin-
hole. If the surface is perfectly spherical, it will appear,
when examined by the eye placed at this point, to be bril-
liantly and uniformly illuminated. With an eye-piece, the
image of the pin-hole will then be perfectly sharp, showing
the most minute details or irregularities of the hole itself.
It is much more probable, however, that the surface will
have many zonal errors. To detect and interpret these, the
"knife-edge test," due to Foucault, is employed. If all the
zones come to a focus at the same point, and a knife edge is
moved across this point, while looking at the glass, the light
will be cut ofp instantly from all parts of the disk. If, how-
A Large Reflecting Telescope 225
ever, the curvature of certain zones is greater or less than
the average curvature, these zones will resemble projecting
or receding rings on an otherwise uniformlv bright surface.
The effect is as though the light were shining from one side,
producing an appearance of relief by lights and shadows.
The test is so sensitive that an error of ^ ^^ (^^^ ^ ^, part of an
inch can be detected. If, for example, the finger is placed
for a few moments on the glass, the heating of the surface
will cause a swelling easily to be detected by the knife-edge
test.
The process of figuring consists in removing the high
and low zones by means of the polishing tool, the stroke
and position of which must be modified in accordance with
the results of the knife-edge test. After a perfectly spheri-
cal form has been obtained in this way, the difficult process
of changing the spherical to a paraboloidal surface is begun.
As is well known, the parallel rays from a star, falling on a
spherical surface, will not be brought to a focus at a central
point, but in an irregular figure, called a "caustic." A
paraboloid, however, brings all parallel rays to a single
focus, and produces a perfect stellar image. In the case of
the 60-inch mirror, which has a focal length of 25 feet, the
paraboloid is deeper than the sphere at the center of the
disk by a quantity less than ywoJ} ^^ ^^ inch. Months of
figuiing are required, however, to produce this small differ-
ence, because of the necessity of giving each zone of the
paraboloid precisely the right curvature. In testing the
surface from the center of curvature, the measured radius of
each narrow zone of the mirror (the other parts being
covered by a cardboard screen) must corres}X)nd with the
calculated radius. The extreme difficulty of accomplishing
this may be appreciated when it is remembered that the
deviation of any zone from the surface of a perfect para-
boloid must not be greater than lool^^o ^^ ^^ inch, which
226 Stellar Evolution
would correspond to a change of y^^^^ of an inch in the
radius of curvature.
When parallel light is available, the difficulties of secur-
ing a perfectly satisfactory test of a paraboloidal mirror are
greatly reduced. In this case the mirror, when seen from
its focal plane (25 feet from the glass, or one-half the
radius of curvature) appears like a uniformly illuminated
plane surface when a perfectly paraboloidal form has been
obtained. This method of testing with parallel light has
been developed by Ritchey, and was used by him to secure
the last degree of perfection in the figure of the 60-inch
mirror.
As already explained, the problem of mounting a large
mirror is quite as serious as that of figuring it. It is neces-
sary, in the first place, to support the mirror in such a way
that it will retain its form, without bending, in any position
of the telescope. Furthermore, it must be held so that it
will not slip laterally, since the slightest change in the posi-
tion of the mirror with respect to the tube will cause a dis-
placement of the star images on the photographic plate.
The mirror, thus supported, must be carried at the lower
end of a tube, of skeleton construction, open at the top, and
so mounted that it can be pointed toward any part of the
heavens and made to follow the apparent motion of the
stars by rotation about an axis parallel to the axis of the
Earth, Strength and stability of the mounting, freedom
from flexure, perfection of optical and mechanical construc-
tion and adjustment, and the greatest precision of driving —
all these conditions must be met before a large reflector can
be expected to give satisfactory results, in the more exacting
departments of photographic work.
The difficulties thus presented have been most successfully
solved by Ritchey, whose design for the mounting of the
60-inch mirror is shown in Plate XCVI. The telescope tube
A Large Reflecting Telescope 227
is huuor between the arms of a massive cast-iron fork, which
is bolted to the upper end of the polar axis. This axis, a
hollow forging of nickel steel, is inclined at an angle corre-
sponding to the latitude of Mount Wilson (34° 13') and
thus rendered parallel to the axis of the Earth. Leveling
screws, by which the base of the mounting is supported on
its pier, permit this adjustment to be made with great pre-
cision. In order to relieve the great friction of this axis on
the upper and lower bearings in which it lies, a hollow steel
float. 10 feet in diameter, is bolted to its upper end, just
below the fork. This float dips into a tank filled with mer-
cury. Thus the entire instrument is floated by the mercury,
and in this way the friction on the bearings is reduced to a
minimum.
The 60-inch mirror rests at the lower end of the tube, on
a support system consisting of a large number of weighted
levers, which press against the back of the glass and dis-
tribute the load. A similar series of weighted levers around
the circumference of the mirror provide the edge support.
The path of the rays from the star may be as shown in
Plate XCVII, Figs. 1, 2. 3, or 4. In the first arrangement
(the Newtonian telescope), the parallel rays, after striking
the mirror, are reflected back and would come to a focus at a
point just beyond the end of the tube. They are intercepted,
however, by a plane mirror of silvered glass, which turns
them at right angles and forms the image on the photo-
graphic plate, which is mounted on the side of the tube near
the upper end. In this case the focal length of the instru-
ment is 25 feet, and the image is formed without secondary
magnification.
If. however, it is desired to secure, for certain classes of
work, the advantages of a greater focal length, a different
arrangement is adopted. The upper section of the tube,
bearing the plane mirror, is removed, and a shorter section
228 Stellar Evolution
substituted for it. This carries a hyperboloidal mirror,
which returns the rays toward the center of the large mirror
and causes them to converge less rapidly. They then meet
a small plane mirror, supported at the middle of the tube
near its lower end, which sends them to one of the following
instruments, mounted in the focal plane: (1) a double-slide
plate-holder, carrying a sensitive plate, for the photography
of the Moon, planets, bright nebulae, etc., with an equivalent
focal length of 100 feet (Fig. 3) ; (2) a spectrograph mounted
in place of this photographic plate, in which case a convex
mirror of different curvature is employed, and the equivalent
focal length is 80 feet (Fig. 4) ; or finally (3) a third convex
mirror may be used and the plane mirror inclined so as to
form the star image (after sending the light down through
the hollow polar axis) on the slit of a powerful spectrograph,
of 13 feet focal length, mounted on a pier in a constant-
temperature chamber (Fig. 2). In this case the equivalent
focal length is 150 feet.
The telescope is moved in right ascension or declination
by electric motors, controlled from the floor of the observing-
room. The driving-clock moves the telescope in right ascen-
sion by means of a worm-gear, 10 feet in diameter, carried
by the polar axis. The cutting of the teeth of this worm-
gear is a mechanical operation requiring the highest precision
of workmanship. Each tooth was spaced off by means of a
finely divided circle attached to the polar axis, and read with
a microscope. The rotating cutter was driven by an electric
motor. After all the teeth had been cut, the worm and worm-
gear were ground together for many hours, until all slight
residual errors had been eliminated. The operation was
completed with jewelers' rouge, which leaves a smooth and
highly polished surface.
All of the heavy parts of this mounting were made, after
Ritchey's designs, by the Union Iron Works Company, of
A Large Reflecting Telescope 229
San Francisco. They were then shipped to Pasadena, where
the mounting has been erected in the Solar Observatory shop
(Plate XCVIII). Here the worm-gear was cut, and all of
the smaller parts, including the driving-clock, setting-circles,
slow motions, motors, etc., are being fitted and adjusted.
All of these parts were made in the Observatory instrument
shop, which is equipped with the best machinery obtainable
for work of this kind (Plate XCIX).
As soon as this mounting has been completed, the 60-inch
mirror will be put in place and the telescope thoroughly
tested, by actual photography of the heavens. It will then
be necessary to transport the instrument to Mount Wilson—
an operation of considerable difficulty, as several of the cast-
ings are very large, and weigh about five tons each.
The building for the 60-inch reflector is of steel con-
struction throughout (Plate C). The thin inner walls will
be shielded from the Sun by outer walls, and air will be }>er-
mitted to circulate in the space between the two. The dome,
60 feet in diameter, will be rotated by an electric motor,
either rapidly, when passing from one part of the heavens
to another, or at a slow, uniform rate, of such a speed as
to keep the opening (15 feet wide) constantly opposite the
end of the telescope tube, when it is following a star. The
observer, when photographing in the principal focus, will
stand on a platform suspended from the dome and rotating
with it. The double-slide plate-carrier, with which stars
and nebulae will be photographed, is similar to that used
with the Yerkes telescope (Plate XVII).
CHAPTEE XXIV
SOME POSSIBILITIES OF NEW INSTRUMENTS
In looking toward the future and endeavoring to imagine
what appliances will be employed by the astronomer of the
next generation, the line of least resistance is to consider the
possibilities of improving existing telescopes and the auxil-
iary apparatus employed with them; for the prevision of
more radical departures is beyond our province. It is safe
to predict that the equatorial refractor, of which the Lick
and Yerkes telescopes are types, will hold an important
place in observatories for many years to come. The ease
with which such instruments can be pointed toward any part
of the heavens; the absence of reflecting surfaces; the per-
manence of object-glasses, as contrasted with the necessity
of silvering mirrors from time to time; the convenient posi-
tion of the observer at the lower end of the tube, rather than
at the upper end of a Newtonian reflector: these and other
considerations point to the long-continued use of the standard
refractor. In its most perfect form this instrument is still
capable of some improvements, the most important of which
will be the introduction of truly achromatic object-glasses,
capable of uniting the rays of all colors at the same focus.
It seems probable that the uses of the equatorial refractor
will be confined more and more to visual observations, and
to certain departments of photography, especially those
involving great precision of measurement or the inclusion
of large fields on a single plate. For the latter work the
refracting telescope, particularly in the portrait-lens form,
possesses great advantages, on account of the very limited
field of the reflector. It does not at present appear desir-
230
Some Possibilities of New Instruments 231
able to increase the aperture of refractors beyond the limit
of 40 inches reached in the Yerkes telescope. The resolving
power of such an aperture, when the atmospheric condi-
tions are good enough to permit its realization, is suffi-
ciently great for the most exacting demands of visual work.
Increased light-gathering power, which is much to be desired
for the investigation of faint objects, will be most easily and
effectively obtained through the use of large reflecting tele-
scopes. Increased focal length, on the other hand, which is
needed to give larger solar images, can best be secured
through the use of some form of fixed telescope. We may
now consider what types of telescopes are likely to prove
most serviceable in photographic and spectroscopic studies
of the Sun, stars, and nebulae.
Many important investigations require the use of a tele-
scope giving a sharply defined solar image, of large diameter,
at a fixed position within a laboratory. The focal length of
such a telescope must not change rapidly when the instru-
ment is exposed to the Sun. The image must not rotate,
and the laboratory conditions must permit the successful use
of the largest and most powerful spectrographs and spec-
troheliographs. The Snow telescope meets most of these
requirements in a very satisfactory manner. The one diffi-
culty with this instrument is the distortion of the image and
the change of focus when the mirrors are exposed for some
time to the Sun. When the precautions described in chap.
XV are taken, these obstacles are easily overcome in cur-
rent work with the 5-foot spectroheliograph and the Littrow
spectrograph. But with long exposures, such as are required
with a spectroheliograph of very high dispersion, the change
?of focus during the exjxjsure would be a serious obstacle. It
is probable that by substituting very thick mirrors for those
now used in the Snow telescope, and by reflecting sunlight
upon their rear surfaces, which should be silvered like the
232 Stellar Evolution
front surfaces, the tendency to distortion could be overcome.
For a very thick mirror would resist the bending which
results from the expansion of the front surface; and even if
the figure were changed, the compensating effect produced
by heating the rear surface should restore it. But the Snow
telescope is fully occupied with its present work, for which
it is well adapted. Accordingly, a new type of fixed tele-
scope has been devised for the purpose of supplementing the
Snow telescope, particularly in photographic work involving
long exposures.
In the new instrument the coelostat, provided with mir-
rors a foot thick, will be mounted at the summit of a steel
tower 65 feet in height (Fig. 7), From the second mirror
the sunlight will be sent vertically downward to a 12-inch
object-glass, mounted a short distance below it. This object-
glass, of 60 feet focal length, will form an image of the Sun
near the ground level. The new instrument will thus consist
essentially of a fixed refracting telescope, pointing directly
to the zenith and receiving light from a coelostat and second
mirror.
The spectroscopic laboratory at the base of the tower will
be excavated in the earth, to insure constancy of temperature
and great stability of the instruments it will contain. Of
these, the one shown on the left in Fig. 7' is a Littrow
spectrograph, similar to the one employed with the Snow
telescope, but of much greater power. This instrument will
have a focal length of 30 feet, and be provided with a large
plane grating. On the right is shown a spectroheliograph of
30 feet focal length, designed for extending the monochromatic
photography of the Sun to many of the finer lines of the
spectrum (p. 236). The atmospheric calm that prevails on
1 This is only a general diagram, omitting all details, such as the steel house, at
the base of the tower, which covers the upper ends of the spectroscope and spectro-
heliograph : tlie small electric elevator, to convey the observer from the bottom of
the underground laboratory to the summit of the tower, etc.
Some Possibilities of New Instruments 233
Mount Wilson during the best observing season may permit
the inner tower to be used merely as a skeleton, if firmly
stayed in position by strong steel guy-ro{)es. If, on experi-
ment, it is found that the wind produces too much vibration
of the structure, an outer tower, covered with canvas louvers,*
will be erected to shield the inner one, as indicated in Fio:. 7.
It remains to be seen whether this type of telescojje will
meet the rigorous conditions demanded in the case of a fixed
instrument for solar research. The vertical beam of ligrht
should be less affected by unequal temj^rature conditions
than a horizontal beam, and the considerable height of the
coelostat and object-glass above the ground may also prove
advantageous. Should it prove successful, a similar instru-
ment of larger aperture, and of about 150 feet focal length,
may ultimately be constructed, on account of the importance
of providing a very large image of the Sun for certain classes
of spectroscopic and spectroheliographic work. It is prob-
able enough that some other type of fixed telescope would be
better than this, but the results of our experience up to the
present time give reason for the belief that the present design
will prove satisfactory.
Since the principal difficulty to be overcome in the con-
struction of a fixed telescoj^e for solar work is the distortion
of the mirrors by the Sun's heat, it is to be hoped that homo-
geneous disks of fused quartz can ultimately be employed
for mirrors, in place of glass. The coefficient of expansion
of fused quartz is only about one-tenth that of glass, and
hence it is but slightly subject to change of figure by heat.
Many small quartz disks have been made in an electric fur-
nace at the Solar Observatory, but the presence of numerous
bubbles, which cannot be removed from the very viscous fluid
by stirring, have proved an insuperable obstacle to the use
lOr perhaps with flue wire netting, which should break the wind, and yet not
heat sufficiently in sunlight to produce convection currents.
234
Stellar Evolution
-ho
^^^
FIG. 7
Vertical Coelostat Telescope
Some Possibilities of New Instruments 235
of these disks for optical purposes. Day has met with better
success in the geophysical laboratory of the Carnegie Insti-
tution, where an electric furnace of special type permitted
quartz to be fused under pressure. His results are suffi-
ciently promising to lead to the hope that, if a large furnace,
of suitable design, were constructed, disks of 15 to 20 inches
in diameter might be made. In view of the expense of such
a furnace, it has seemed best to defer further experiments
in this direction until very thick glass mirrors can be
thoroughly tested.'
Another important need of the future is a machine capable
of rulincj grratinors of much laro^er dimensions than those of
Rowland. The best Rowland orratings, which have rendered
possible the great advances of the last quarter-century in
spectroscopy, have a ruled surface about 5^ inches long.
The resolving power of a grating depends upon the total
number of lines it contains, but there are many reasons
why it is not desirable that the number should exceed 20,000
per inch. If a good 15- or 20-inch grating could be ruled,
with lines from 10 to 15 inches in length, a great advance
in solar spectroscopy would be rendered possible. Such a
grating, if one of the spectra were very brilliant, would be
exactly what is required for a spectroheliograph caj^ble
of photographing the Sun through narrow dark lines.
If used in a spectrograph of from 40 to 50 feet focal
length, it would furnish a photographic map of the solar
sjiectrum much superior to Rowlands, and be of the greatest
service in the photography of sun-spot spectra, the study of
the solar rotation, and many other investigations. For this
purpose the sj^ectra of one of the higher orders (from second
to fourth) should be bright, and the precision of ruling
1 Since the above was written ttie "tower telescope" has been constructed and
tested. The thick mirrors are so little affected by sunlight that the focus wiU
remain constant during the long exposures required with the 30-foot spectrohelio-
graph (Plates CI acd CII).
236 Stellar Evolution
should, of course, be so high as to permit the theoretical
resolving power to be attained. In view of Michelson's
recent work in ruling 8-inch and 10-inch gratings, the
realization of his plan for the construction of a machine
capable of making gratings of much larger size is more
earnestly to be desired than that of any other project for
the development of spectroscopy.
Still another important need of the spectroscopist is
homogeneous glass, in large masses, for prisms. At the
present time it is almost impossible to obtain prisms of
large size that will give good definition. The repeated
failures of the best makers of optical glass indicate that the
problem is not an easy one, though it can probably be
solved. A careful study of this question, made with special
reference to the possibility of improving the present methods
of annealing, should yield valuable results. Large prisms
are urgently required for use in stellar spectrographs of large
aperture and high dispersion, such as the one which is to be
mounted in a constant-temperature chamber in conjunction
with the 60-inch reflector. However, if sufficiently large
and perfect gratings can be obtained, which concentrate
nearly all of the light in a single spectrum, they may be
better for this purpose than prisms.
In the further development of solar research, no instru-
ment seems to offer more possibilities than the spectrohelio-
graph. Recent experiments with a temporary spectrohelio-
graph of 30 feet focal length, used in conjunction with the
Snow telescope, have demonstrated the feasibility of photo-
graphing sun-spots with the lines that are strengthened or
weakened in their spectra. The resulting pictures show the
distribution of the corresponding vapors in and around the
spots, and should be capable of throwing much new light
on solar phenomena when taken daily and systematically
studied. It is expected that the 30-foot spectroheliograph
Some Possibilities of New Instruments 237
of the "tower telescope" will be employed in this way, but
even the great dispersion of this instrument will be inade-
quate for work with the finest lines. It is evident that if
the photograph is to represent the distribution of the gas
or vapor corresponding to the line employed, the line must
be as wide as the second slit, in order that light from the
adjoining continuous spectrum may not obliterate or confuse
the image produced by it. When it is remembered that the
solar spectrum contains more than 20,000 lines, and that
any one of these may be capable of furnishing a photograph
comparable in interest with the results already obtained with
hydrogen and calcium lines, it will be appreciated that no
effort should be spared to increase the dis|)ersion and optical
perfection of the spectroheliograph. The further applica-
tions of this instrument to the study of the level of the
tloccidi; the absorption of the solar atmosphere; the growth
of the flocculi and prominences, which can be shown, as if
in accelerated progress, by the aid of a series of pictures
taken in rapid succession and projected on a screen with a
kinematograph ; the use of stereoscopic methods in spectro-
heliographic work: these, and many other investigations,
leave no doubt that this field of solar research is but barely
opened, and still contains many untried possibilities.
Passing over other considerations that tend to confirm
one's optimistic belief in the future of solar research, we may
now inquire as to the type of telescope that appears most
promising for photographic and spectrographic studies of
stars and nebulae. In much of this work it is not essential,
as in the case of the Sun, that the image should be fixed in a
laboratory. For this reason, an equatorially mounted reflect-
ing telescope seems to meet the requirements admirably.
Even when a fixed image is required, it is possible, as illus-
trated in Fig. 2, Plate XCVII, to send the light from objects
lying within a certain zone of the heavens into a constant-
238 ' Stellar Evolution
temperature laboratory, for analysis by the most powerful
spectrographs. As already explained, such a telescope is
also adapted for many other classes of work, either in the
principal focus of the great mirror or with an enlarged
image given by a convex mirror, after the manner of Casse-
grain.
As an object-glass increases in size, the absorption, due
to its increased thickness, rapidly diminishes the percentage
of light it transmits. The loss is especially serious for the
blue and violet rays, since these are absorbed more completely
than the red and yellow. In the case of a mirror, the light
passes through no glass, but falls on a surface of pure silver,
from which it is reflected to the focal plane. Thus every
square inch added to the area of a telescope mirror means
a proportional increase in the light-gathering power. It
is evident that if the mechanical and optical difficulties
can be overcome, reflecting telescopes much more power-
ful than any now in existence can advantageously be con-
structed.
With this object in view, Mr. John D. Hooker has pre-
sented to the Carnegie Institution a sum sufficient to pur-
chase for the Solar Observatory a glass disk 100 inches in
diameter and 13 inches thick, and to meet other expenses
incident to the construction of a 100-inch mirror for a reflect-
ing telescope of 50 feet focal length. The construction of a
telescope so far surpassing all previous instruments in size
must, of course, be partly in the nature of an experiment.
The immense block of glass will weigh 4^ tons, four and one-
half times as much as the disk of the 60-inch mirror. The
difficulty of providing a mounting capable of carrying it with
the necessary precision is not slight. The glass is certain to
be more or less distorted by temperature changes, which
would ruin its performance if not obviated. The atmospheric
conditions, even on Mount Wilson, may not be sufficiently
Some Possibilities of New Instbumexts 239
good to permit so great an aperture to be used to full advantage.
Of these and other obstacles Mr. Hooker is fully informed,
and he does not underestimate their importance. But he
perceives and appreciates, with the understanding of one who
has himself invented and developed mechanical appliances,
that experiment is necessary to progress. He therefore does
not hesitate to provide the means for undertaking an optical
experiment on a large scale. Let us consider its probable
outcome.
In the first place, the question arises whether a sufficiently
homogeneous glass disk of the required dimensions can be
obtained. Our long experience with the Plate Glass Com-
pany of St. Gobain (France) leads us to believe that no
insuperable difficulty will be encountered. This old and reli-
able company has cast for us scores of disks, from which
Ritchey has made many plane and concave mirrors, from the
smallest sizes up to 60 inches. In all of these cases the
quality of the disks has left nothing to be desired. The
60-inch, 8 inches thick, and weighing a ton, is fully equal to
the smaller ones. We are therefore inclined to believe, since
the St. Gobain Company expresses its deliberate opinion that
a satisfactory disk, 100 inches in diameter and 13 inches
thick, can be produced, that they will be able to carry out
the order we have given them.
As for the work of grinding and figuring, no one who
has watched the progress of the 60-inch mirror would be
likely to doubt Ritchey's ability to accomplish this difficult
task. The method of parabolizing which he has perfected
will apply as well to a 100-inch mirror as to the 60-inch. It
eliminates the necessity of handwork, except for a few finish-
ing touches, and has yielded an essentially perfect parabo-
loidal figure in the case of the 60-inch mirror. I am con-
fident that he will find no difficulty in bringing the 100-inch
mirror to this highest order of perfection.
240 Stellar Evolution
The mounting should offer no great obstacles, especially
as it will not be built until the mounting of the 60-inch
has been thoroughly tested on Mount Wilson. In these
days of large and perfect machinery, the mechanical diffi-
culties are much less formidable than they would have
appeared twenty years ago. On this score, therefore, we
see no cause for fear.
The prevention of change of figure due to changing tem-
perature should not prove a very serious problem. During
the fine nights of the best observing season on Mount Wilson
the temperature remains almost perfectly constant after 9 p. m.
It will therefore only be necessary to maintain the mirror
(or possibly the entire telescope) at approximately this tem-
perature throughout the day, by means of suitable refrigerat-
ing machinery. In the long periods of cloudless weather the
change of temperature from night to night is extremely
small, so that little difficulty should be encountered on this
score. If the slowly falling temperature during the early
evening should prove to give trouble, the observational work
may be deferred until after nine o'clock. The dome and
building, like those for the 60-inch reflector, will be so con-
structed that no air can enter during the day ; they will also
be shielded from the heat of the Sun. The problem is, of
course, altogether different from that encountered in the
case of the Snow telescope, where the mirrors are required
to give good images in spite of their exposure to direct
sunlight.
Assuming that these various difficulties can be success-
fully overcome, it still remains a question whether the atmos-
pheric conditions on Mount Wilson will be sufficiently good
to permit the telescope to give satisfactory images. This
cannot be definitely determined until after the 60-inch reflector
has been used for some time. Even if it should prove, how-
ever, that only a very few nights in the course of a year can
Some Possibilities of New Instruments 241
be utilized to the fullest advantage, the construction of such
a telescope would nevertheless be desirable. For under the
average summer conditions, which are much finer than those
in the eastern part of the United States, results of great value
can undoubtedly be obtained in many classes of work, such
as the photography of stellar spectra, the measurement of the
heat radiation of the stars, etc. The immense amount of
light which this mirror will collect should render it particu-
larly suitable for spectroscopic work of all kinds.
It need hardly be said that the 100-inch mirror, when
suitably mounted, will play a most important part in the
scheme of research of the Solar Observatory. The investiga-
tion of stellar evolution frequently calls for adequate spectro-
scopic study of stars beyond the reach of existing instru-
ments. With the 40-inch Yerkes telescope, for example, it
was impossible to obtain satisfactory evidence, positive or
negative, as to the transition from solar stars to those of the
fourth type. The large number of stars within the reach of
a 100-inch reflector (which will give imaofes about ten times
as bright as the 40-inch) should greatly increase the chances
of finding possible intermediate types, so important in their
bearing upon the relationship of solar and red stars. This is
onlv a single instance, but it forciblv sugforests itself when con-
sidering our programme of research. In other fields the large
reflector should be equally valuable, especially for the pho-
tography of the numerous small spiral nebulae, the details of
which should be brouo^ht out to good advantage with a focal
length of 50 feet ; minute investigation of the larger nebulae,
in the hope of detecting changes in their form; the study,
with very high disjjei-sion, of the spectra of bright stars, etc.
The remarkable calm of the summer nights on Mount Wilson
should assist materially in all of this work, since vibration of
the tube, caused by the wind, would undoubtedly be a
serious drawback under less favorable conditions.
242 Stellar Evolution
It is impossible to predict the dimensions that reflectors
will ultimately attain. Atmospheric disturbances, rather
than mechanical or optical difficulties, seem most likely to
stand in the way. But perhaps even these, by some process
now unknown, may at last be swept aside. If so, the astron-
omer will secure results far surpassing his present expecta-
tions.
CHAPTER XXV
OPPORTUNITIES FOR AMATEUR OBSERVERS
I SHALL never forget my delight, when as a boy, I first
learned of the spectroscope. Its extraordinary achievements,
and the endless jx)ssibilities. vaguely imagined, of its further
applications in astronomical research, filled me with enthusi-
asm, and kindled a strong desire for immediate work. The
visual study of flames, with a simple one-prism spectroscoj^e,
aroused an ambition to photograph spectra. This was soon
accomplished, by substituting an ordinary camera for the
Dbserving telescope. But the scale of the photographs was
too small, so I built a longer camera of wood. Later, when
Rowland was making his earliest gratings, one of the small-
est size was secured, and substituted for the prism. The
marvelous increase in resolving power, and the greatly aug-
mented beauty of the solar spectrum, led to observations of
the solar prominences, and subsequently to more serious
research. But none of the pleasures of later years, during
which I have enjoyed the privilege of using larger and more
[X)werful instruments, has surpassed the delight of the initial
work, much of which was done with simple and inexpensive
apparatus of my own construction.
These remarks are called forth by certain criticisms I have
heard of great modern observatories. Some amateurs, I
am told, believe that their efforts are rendered futile by
the more powerful equipment and better atmospheric advan-
tages of other investigators. If this feeling were well-
grounded, it might fairly be asked whether the great observa-
tories are worth their cost. For the history of astronomy
teaches that much of the pioneer work has been done by
243
244 Stellar Evolution
amateurs, usually with modest means and in unfavorable cli-
mates. To discourage this class of workers, unfettered as
they are by the traditions of institutions, and driven by their
own initiative into unexplored fields, would be a serious error,
hardly to be atoned for by any services the larger observa-
tories can render.
We may therefore inquire whether useful work, of such j
a nature as to contribute in important degree to the progress '
of science, can still be done with simple and inexpensive j
instruments. This question may at once be answered in the
affirmative. The results of amateur observations may not
only be useful — they may equal, or even surpass, the best ;
products of the largest institutions. Great care must be exer- '
cised in choosing the subject of research, in constructing the
instruments, in making the observations by the best methods
and at the most favorable hours, and in the reduction and
discussion of the results. If such precautions are observed, |
discouragement will soon give way to confidence and success. ]
Take, for example, the direct photography of the Sun, A
2-inch objective, of 40-feet focal length, will give beautiful
solar photographs, over 4 inches in diameter, perfectly
adapted for the study of the solar rotation, the proper motions :
of the spots, and other important purposes. Details sepa- j
rated by less than two seconds of arc will not be resolved on j
these photographs, but in many classes of work little gain j
would result from increased resolving power. Such an ob-
jective should be mounted so as to send the beam horizontally
(better vertically) across shaded ground, or within a building, >
to the photographic plate. If no coelostat is available, a small
mirror, with optically plane reflecting surface, will serve the ;
needs of direct photography. It is only necessary to mount it '
on a wooden support, so that it can be held at the angle required
to reflect sunlight through the objective. The exposures —
made by the rapid motion of a wooden shutter, pierced by a
Opportunities for Amateur Observers 24:5
narrow slit with brass edges, mounted just in front of the
plate — are very short, and the slight drift of the solar image
during this time can be overcome, when desired, by a very
simple driving mechanism. Between exposures the small
mirror should be shielded from the Sun, The apparatus used
by the American parties to photograph the last transit of
Venus across the Sun was of this type, except that a J:-inch
objective and larger mirror were used.
It will probably be found that the best solar definition
occurs in the early morning, before the ground is greatly
heated. A careful study should be made of the local con-
ditions before selecting the hours of work.
Solar photographs, made in this way at intervals of from
one to several hours, may be combined in the stereoscope
with striking results. More important, however, would be
a long series of photographs, made at short intervals, and
examined with a kinetoscope. These should show the Sun
rotating under one's eyes, the spots near the equator moving
more rapidly than those in higher latitudes. The effect of
proper motion, in causing some spots to overtake others in
the same latitude, should also be very finely brought out.
Even more interesting, however, would be the changing
forms of spots, and the manner of their growth and decay,
which have never yet been observed by this method.
The same horizontal telescope, with some modifications,
would give an admirable image for spectroscopic work. The
objective should, if jxjssible, be of from -t to 6 inches aperture,
and from 40 to 60 feet focal length. The mirror should also
be increased in the same ratio, and mounted as a coelostat,
with its plane parallel to the Earth's axis. If the mirror is
very thick — 3 inches or more — its form will be changed but
little by sunlight. A second mirror will be needed to send
the beam to the spectrograph, as in the Snow telescope
(Plate LVIII). If this arrangement appears formidable, it
246 Stellar Evolution
should be remembered that almost all the parts can be made
of hard wood, thoroughly soaked in melted paraffine, to pre-
vent warping. The bearings are practically the only parts
that need be of metal. A cheap clock movement, with heavy
spring, will serve for a driving-clock, or a small electric
motor may be used. With moderate ingenuity, any amateur
accustomed to the use of tools can build such an instrument
for a very small sum.
The spectrograph is even more simple. It should be of
the Littrow form (p. 153), and the aperture of the single
plano-convex lens that serves for both collimator and camera
should be from 1 to 1^ inches. Its focal length will be
determined by the diameter and focal length of the objective
used to form the solar image on the slit. If these are 4
inches and 60 feet, respectively, the ratio will be 1:180.
Hence the focal length of the spectrograph lens should be 180
times its aperture, or from 15 feet to 22 feet 6 inches. The
grating should be a 2-inch Rowland, or, if this is too expen-
sive, a good replica by Ives, Wallace, or Thorpe. The repli-
cas have the disadvantage of being made on transparent films,
for use with transmitted light; but they can perhaps be cour
verted into reflecting gratings by silvering.
The collimator-camera lens should be mounted on a ver-
tical wooden bracket, arranged to slide 3 or 4 inches for
focusing. The grating may also have a wooden support,
consisting of a bracket, which can be tipped forward or
back, mounted on a circular wooden table, permitting rota-
tion about a vertical axis in the plane of the grating. Such
rotation is necessary in order to bring different spectra upon
the photographic plate, or to pass from one region to another
in the same spectrum. The height of the spectrum on the
plate can be adjusted by tipping the grating forward or back.
It is also necessary to make the lines of the grating parallel
to the slit; this can easily be done by hanging the bracket
Opportunities fob Amateur Observers 2^1
from above, and defining its position by two side screws,
passing through wooden blocks attached to the circular table.
Plate cm shows a wooden lens and grating support in reg-
ular use as part of a Littrow spectrograph of 18 feet focal
length in the laboratory of the Solar Observatory.
The extreme simplicity of the slit end of the same instru-
ment is illustrated by Plate CIV. A short slit, with one jaw
movable by a screw, is supported by a tube fitting tightly in
a hole bored through a wooden bracket. Below is the plate-
holder, held in a frame that slides up and down, permitting
many narrow spectra to be photographed on the same plate.
In another similar instrument the slit and plate-holder sup-
port stands on a pier, and fits into a partition, so as to exclude
all light from the room except that which enters through the
slit.' In this case no tube is necessary between the plate and
lens. The latter is mounted, with the grating, on a pier at a
distance from the slit equal to the focal length of the lens.
In spite of the simplicity and cheapness of such a spectro-
graph, no better instrument could be asked. Its one draw-
back— the reflections of the slit from the surfaces of the lens
— is easily removed by placing a bar across the lens (as shown
in Plate CIV) . Wooden spectrographs are in constant use
at the Solar Observatory, and give results which are very
satisfactory.
Any of the solar spectroscopic work described in this book
can be done with such an instrument. The resolving power,
even with only an inch aperture, will be sufficient for the sepa-
ration of very close solar lines. The spectra of sun-spots, the
solar rotation, the remarkable differences between the spectra
of the center and limb of the Sun, and many other phenom-
ena can be studied by its aid with the greatest precision and
success. The exposures, it is true, must be longer than with
• This room is part of a long hall, for testini? optical mirrors, in the Pasadena
shop of the Solar Observatory. By opening large light-tight doors, the hall can be
used for the transmission of light in the knife-edge tests.
248 Stellar Evolution
a spectrograph of larger aperture, but this is not a serious
obstacle. Indeed, it may be said that at the present time
only two or three observatories in the world are using equip-
ment as powerful as this for the classes of solar work just
enumerated.
I might go on to describe a wooden spectroheliograph,
fitted up with spare lenses and prisms, which gave excellent
results with the Snow telescope before the 5-foot spectro-
heliograph was completed. Indeed, the photographs were
quite equal to those taken with the latter instrument, except
that they did not include the entire solar image, which is
unnecessary for many kinds of work. The small coelostat
telescope described above would give as good results as the
Snow telescope with such a spectroheliograph, except that the
exposures would be longer. The entire apparatus is easily
within the reach of any intelligent amateur of limited means.
Those who desire to undertake solar work would do well
to procure the Transactions of the International Union for
Co-operation in Solar Research.^ The aim of the Union is
to encourage co-operation among observers, in the various
fields where this is desirable. For example, it is impossible,-
in visual observations of sun-spot spectra, for one person to
make a thorough study of more than a limited region. By
mutual agreement, the spectrum is therefore divided up
among many observers, who record their results on a common
plan, Spectroheliographs, distributed from India across
Europe to California, are also operated in harmony, and co-
operation is practiced in other fields as well. Apart from
such routine, however, every observer is encouraged to act
on his own initiative, for the Solar Union recognizes that
the greatest advances will come from individual effort, which
no amount of co-operation can replace.
' Vol. I was published by the University Press, of Manchester, England, in 1906.
Vol. II will soon appear.
INDEX
Abbot: tes-ts of Mount Wilson atmos-
phere, 129; solar radiation, 214 ; pyrhe-
liometer, 215.
Absorption: spectrum, 52; in solar
atmosphere. 53, 68 ; in hydrogen flocculi,
96 ; in stellar atmospheres, 173.
Adams: metallic and spot spectra, 159;
titanium oxide in spots. 162 ; spectrum
of Arcturus. 168; si>ectrum of a Orioni".
170 ; Trapezium stars, 1S9 ; ''Orion'' type
stars. 190.
Altitudes : advantages of high, 111-20.
.\mateues: opportunities for, 27, 243-19.
Andromeda nebula. 41, 44.
Anomalous dispersion and solar phe-
nomena. 148.
Antares, 195.
Arcturus: spectrum, 16S; heat radiation,
172, 17.3.
Astrophysics : relat ion to astronomy and
physics, 6.
.Atmosphere: absorption in Earth's, 63.
114, 128; unsteadiness, 111, 112. 127.
Barnard: photography of Milky Way.
30-3.3, 128; micrometric observations,
103 ; comparative photographs at Mount
Wilson and Lake Geneva. 128, 129; tests
of Mount Wilson definition, 129.
Barnard and Ritchey- photography
of corona. 76.
Betelgeuze: spectrum, 170.
Binaries: spectroscopic, 105.
Bolometer, 215.
Boys: stellar heat, 171.
Brlte spectrograph, 104, 167.
Bruce telescope, 29-33.
Burnham: discoveries with small tele-
scope, 27; observations with Verkes
refractor, 103.
Calcium : lines, H and K, 84, 91 ; flocculi.
8>>-93, 143, 147 ; vapor, radial motion of,
92.
Calcium hydhide : m sun-spots, 163.
Calvert: corona, 73.
Camera lens ; stellar photography with.
28-32.
Campbell : stellar motions, 105.
Canes Venatici: spiral nebula in, 38, 39.
Caebox: in chromosphere, 80; in red
stars, 195.
Carnegie. Andrew: establishment of
Carnegie Institution, 109.
Carnegie Institution : purpose of. 109.
Chamberlin : criticisms of nebular
hypothesis. 182-86; planetesimal hy-
pothesis, 208-10.
Chromosphere, 15, 84; si)ectrum, 78;
"flash" spectrum, 80.
Coelostat, 75, 109, 245 ; advantages, 131 ;
Snow telescope, 133, 134; "tower" tele-
scope, 231.
Co-operation in research, 98, 218, 249.
CORNU: telluric lines. 63, 64.
Corona, 16, 73-75 ; spectrum of, 74.
Crossley reflector, 42, 45.
Cygnus: nebula in, 44.
Darwin, Charles : Origin of Species, 1 ;
correlation in research, 97.
Daewin, Sir George : tidal friction, 183;
meteoroidal swarm, 183, 204.
Deslandees: level of calcium flocculi,
90; spectra of flocculi. 96; spectn -helio-
graph, 96; Foucault siderostat, 1.31.
Draper, 41, 54.
Echelon. 65.
Eclipse: solar, 73; apparatus. 75.
Elleeman : work with Kenwood spectro-
heliograph, 86; work with Rumford
spectroheliograph, 89; work with .5-foot
spectroheliograph, 138; photography of
spot spectra. 152, 16.3.
Eveeshed : spectroheliograph. 96.
Evolution : early views, 1 ; general
problem, 3.
Faculae. 15, 71. 72, 85, 86, 90, 146.
Flagstaff. 1'23.
"Flash" spectrum, 80.
Flocculi : calcium, 85; daily motion. 87,
142, 146; minute, 89; levels, 90; eruptive,
92; hydrogen, 94; iron. 96; h- liographic
I>osit ions. 144 ; proi)er motions, 146 ; level
of calcium and hydrogen, 147; levels,
150; areas. 150. 217.
Foucault: siderostat, 131.
Fowleb: magnesium hydride in spots,
163; titanium oxide in red stars, 195.
Fox : measures of Kenwood plates, 144.
Fbaunhofeb: dark lines in solar spec-
trum, 47; objective prism, 189; stellar
spectra, 189.
Frost: stellar spectroscopy. 104. 105;
heat radiation of sun-smits, 149 : Trape-
zium stars, 189; "OriotC' type stars, 190.
Furnace : electric, 160.
249
250
Stellar Evolution
Gale: metallic spectra, 159.
Galileo: early discoveries, 9.
Globe measuring machine, 144.
Gratings: Rowland, 56-59, 2.35, 236;
Michelson, 65, 66, 2.36; Jewell, 68.
Greenwich Obskrvatory: spot posi-
tions, 144.
Harvard Observ.\tory : refractor, 41;
objective prism, 189; stellar spectra,
201.
Heliomicrometer, 144.
Helium: in Snn, 78, 79; terrestri:i], 78;
in '■'Orion"' stars, 79; in nebulae, 190.
Herschel, Sir John : clusters and
nebulae, 46.
Herschel, Sir William: clusters and
nebulae, 46 ; condensation of nebulae,
187.
HiGGS: map of solar spectrum, 62.
Hooker, J. D. : gift of 100-inch mirror,
238.
Hooker expedition, 30, 128.
Hooker telescope, 238-42.
HuGGiNS: stellar spectra, i)3; promi
ueiices, .54, 76; spectrum of nebulae, .54;
helium, 79; stellar motions, 105; s-tellar
hear, 171; stellar evolution, 199; tem-
perature of nebulae, 207.
Hydrogen: spectrum, 78; in stars, 79,
170, 191, 193, 195, 199, 200, 206; in promi-
nences, 83; in nebulae, 190; in meteor-
ites, 20">.
Hydrogen flocculi, 93-95; level, com-
pared with calcium, 147.
Interferometer, 65.
Janssen: prominences, 54, 76; solar
photography, 70.
Jewell: telluric lines, 63; gratings, 66.
Julius: anomalous dispersion thi ory,
148.
Kapteyn: structure of universe, 202.
Keeler: spiral nebulae, 3, 45; pho-
tography with Crossley reflector. 42;
Saturn's rings, 182 ; chief nebular line,
205.
Kenwood Observatory, 83; spectro-
heliograpli, 84 ; spot spectra, 1.52.
Kirchhoff: explanation of solar spec-
trum, 51-.53.
Kirk wood: nebular hypothesis, 185.
KODAIKANAL OBSERVATORY, 119.
Laboratory: Yerkes Observatory, 107;
Solnr Observatory, 1.56.
Lane's law, 191.
Langley: sun-spots, 69; photospheric
grannies, 69-71 ; color of Sun, 193; solar
radiation, 214; bolometer, 215.
Laplace: nebular hypothesis, 2, 17.5-X6.
Lick Observatory, 42, 119, 120, 205.
Lick telescope, 26, 41.
Littrow spectrograph; laboratory,
1.56; of Snow telescope, 134, 153; of
"tower" telescope, 232 ; wooden, 246-48.
Lockyer: prominf^nces. .54, 76; helium,
78; sun spot spectra, 151; dissociation
in sun spots, 151; temperature of sun-
spots, 1.52; temperature of stars. 173;
enhanced lines, 194; stellar classifici-
tion, 194, 207; meteoritic hypothesis,
204-8.
Magnesium hydride: in sun-spots, 163.
Magnifying power, 22.
Mars: period of inner satellite, 183.
M.aunder: band lines in spots, 1.52.
Maxwell : Saturn's rings, 182.
Meteorites : spectra, 205.
Michelson: interferometer, 65; stand-
ard wave-lengths, 65, echelon, 65;
gratings, 65, 66, 236.
Milky Way: photographs of, .30-.33.
Mills spectrograph, 167.
Mirror : 60-inch, figuring. 219-26 : method
of testing, 222, 224-26; 100-inc 1.2.38-41.
Mirrors: distortion, 1.37, i:«, 231-35, 240.
Momentum: moment of, 185.
Moon : photography of, Si.
Mont Blanc, 119.
MouLTON : criticisms of nebular hypoth-
esis, 182-86.
MouLTON AND Chamberltn: planetesi-
mal hypothesis. 208-10.
Mount Etna: expedition to, 116-19.
Mount Hamilton, 119. 120.
Mount Wilson, 123-30.
Mount Wilson Solar Observatory:
origin, 110; plan of research, 121; site,
123-30; Snow tel(>scope, 13i-:W; work
with spectroheliograph, 139-.50; sun-
S(iot spectra, 1.5.3-64 ; laboratory, 1.5.5-.58,
160; stellarspectroscopy.l6i-71 ; 60-inch
reflector, 219-29; "tower" telescop ,
232-35 ; 100-inch reflector, 238-42.
Mountains : as observatory sites, 113-.30.
Nebula : spiral in Canes Venatici, 38, 39;
in Andromeda, 41, 44,45; in Cygnus, 44;
in Orion, 44, 188-90; in Draco, .54; La-
place's, 177.
Nebulae: spiral, 7, .38, 39. 41, 44, 45, 188,
203; relationship to star-, .3, 32. .5.5, 177,
187-90, 198-201, 206, 207, 209, 210; and
clusters, 18, 46, 47, .54; in Milky Way, 31,
32; in Pleiades, 44, 129, 198, 202; spec-
trum of, .54, 190, 206; condensation,
17>-81, 183, 187, 200.201,212; planetary,
187; temperature of, 207.
Nebular hypothesis, 2, 17.5-86.
Nebulum, 204.
Neptune: orbits of satellites, 183.
Index
251
Newton: analysisof ^uiilieht, 47: advan-
tatres of high altitudes. 111.
Nichols: stellar heat, 172.
Olmsted: calcium hydride in spots. 163.
Orifiin of Species, 1.
Orion nebula, 44, 188-90; Trapezium
stars, 188. 190.
"OWo/l" TYPE STABS. 79, 190.
Photography: advantaees. 28: star
trails, 29: with camera, 29-.3.3: clusters.
a3. 44; Moon, 3-3, M; with Yerkes tel-
escope. 3.3-.%): with reflectors, 4(»-45; of
nebulae. 41-4t, 203; solar spectrum, tO-
64, 247. 24X; Sun. 70-72. 244. 24.5; eclipse.
74-76; with spectroheliograph, 81-96,
1.39-42, 149, I.tO, 2.36. 2:^7. 248; stellar
spectra. 101, 105. 16.=)-71. 189-97. 203 ; coro-
na, without eclipse, ll.V 18; Milky Way,
128, 129; with Snow telescope, 137, 1.38;
sun-spot spectra, 1.52-54 : metallic spec-
tra, 1.58-62.
Photosphere : strnctnre, 69-72.
Physics : fundamental importance of, 5.
Pic du Midi. 119.
Pickering, E. C. : objective prism, 189;
stellar spectra, 201.
Pike's Peak: expedition to. 11-5. 116.
Planetesimal hypothesis, 208-10.
Pleiades, 18. 44, 129, 177, 198, 202.
Potsdam Astrophysical Observatory,
100. 167.
Prism: formation of sjjectrum, 48; ob-
jective, Fraunliofer, 189; objective,
Pickering. 1>9: glass. 236.
Prominences, 15; nature of, 76; without
eclipse, 77; sp'ctrum, 78; seen with
open slit. 81; quie-cent and ruptive.
81. 9;}; photography of, 82; H and K
in, 84.
Pyrheliometer, 213.
Ql'artz: fused, for telescope mirrors,
2:i.3-35.
Radiometer. 172.
Radilm. 212. 213.
Kamsay : discovery of helium. 78.
Reversing layer, 14:1.
Rit(HEy: photograp' s of Moon. :». 34;
photography with Yerkes telescope, 3.3,
36; 24-inch reflector. 43; phot-'grapsy
with reflector. 44; telescope construc-
tion. 43, ::19-:10; 100- inch reflector, :::»-40.
Ritchey and Barnard: photography
of corona. 76.
Roberts: Jnc/romcfo nebula, 41.
Rosse: 6-foot reflector, 38, .39.
Rotation; solar, by spots. 143; by facu-
lae. 143; by flocculi, 143, 146.
Rowland: gratings. .56-.59; composition
of Sun, 62: map of solar spectrum, 62;
solar spectrum wave-lengths, 62.
RUMFORD spectroheliograph. 88.
Rcnge; helium. 79.
RiTHERFURD: Stellar spectra, 2, 53;
gratings, 57.
Satcbx : ring system, 179; constitution
of rings, 182; n-volution of rings, 184,
Schuster : stellar evolution, 199.
Secchi : St liar spectra, -53; prominences,
76: classification of stellar si>ectra, 170.
Siritis: spectrum, 191.
Smith, Piazzi: Teneriffe expedition, 119.
Smithsonian Observatory, 214.
Snow telescope, 1:^2-38.
Solar Union, 218, 248.
Spectra; stellar. 2, 18, 104. ia5. 164-71,
189-91, Ift3-2a3, 207, 208; nebulae. 18, .54,
203-7; solar, 47, 51-.5.3, 60-64. 215; con-
tinuous, 49, 51 ; bright-line. 49-.5;l 60. 61,
107, 141, 157-63. iOj ; dark-line. .51-54, 9.3,
94; prominences, .54, 76-81 ; gratine. •5'^-
60: chromosph' re, 78-81: "Hasli.'' 80;
flocculi. 85. 86, 90. 91,9.3-96; faculae, 90;
sun-spots. 108, 151-64 : arc, 1.59-62; elec-
tric furnace, 160. 161 ; .Saturn's ring. 182 ;
Sun.c nter and limb, 192; '•enhanced"
lines. 194.
Spectrograph : Bruce, 104, 167 : Littrow,
1.34. 1.53; laboratory, 1.56; Mills, 167;
Potsdam, 167: grating, for stars, 167,
22^; of "tower" telescope, 232; woi'den,
246-48.
Spectroheliograph; principle of. 82;
Kenwood. 84: Rumford, 88; use of dark
lines, 93; .5-foot, 139; operation. 141;
30-foot, 2.33 ; future development, 236.
Spectroscope: Kirchhoff's, 51; plane
grating. .58, 77; concave grating, .59. 60;
objective prism, 80, 189.
Spectroscopic binaries, 105.
Spencer : nebulae. 47.
Spurious disk, 23.
Stars : clusters, 18, 46 ; colors, ix. 170, 173,
191. 193, 19.5. 199; size of image. 23;
clouds. 31; spectra. .53, 104, 105, 16-71,
17.3, 174. 189-203. 207, 208, 241 ; tw nkling,
111 ; heat radiation, 171 : red, 17:i, 195-
97, 208; terajjerature, 173; helium. 19i);
■'Orion'' type, 190; white, 191 ; dark, 197.
Stereocomparator. 147.
Sun; visual apijearance. 15, 68; composi-
tion, 16: activity, 16, 217; as a star, 17;
line absorption, 53; absorption in at-
mosphere, 68; direct photography, 70,
245; inclination of axis, 143; rotation,
143, 146; contraction. 191; spectra of
center and limb, 192; radiation, 212-16.
Sux-SPOTS, 15, 69; periodicity, 16; level,
71, 148; heliograpiiic positions, 143- 6;
dissociation in, 1.51; darkness. 151, 163;
spectrum, 151-64; temperature, 163.
Telescope; magnifying power. 22;
brightness of image, 22: resolving
p< iwer, 'ii ; large and small, 24-27 ; fixed,
131.232; "tower," 232.
252 Stellar Evolution
Telescope: reflecting, Rosse, 38; devel- Vega: heat radiation, 172, 173.
opmentof, 38-4.-) ; advantages. 42 ; < 'ross- Vogel : stellar motions, 105.
ley, 42, 4.); 24-inch, 43; Snow, 132-38:
60-inch, 219-30; lOU-inch, 23«-42.
Telescope: refracting, 21, 2.30; Yerkes. Wadsavokth: reflector mounting, 43.
2.5, 26, 33-.36, 43, X8, 101-4; Lick, 26, 41; Wilson: sun-spots as cavities, 71.
Burnham's 27; camera, 28-:<3; Bruce,
29,30; Kenwood, 33, 84; development Yerkes Obseevatoky: policy. 98; origin.
**^' *^- 99; plan of building, 100; instrument
Telluric LINES, 63, 64. and optical sliops, 106; spectroscopic
Teneriffe expedition 119. laboratory, 107; site, 113; coelostat
m • ^ inn room, 172.
Titanium oxide: in sun-spots, 162. ,, , ^ i „., o.
,,_ ,, „.,„ Y erkes refractor: photography, .3.?-.Jb,
"Tower' telescope, 232. 88; mounting, 34, 101 ; compared witli
TrapeziMwi STARS, 188,190. reflector, 44; objective, 101 ; operation
Turner: coelostat, 132. ^^■^ ^^-■
Twinkling of STARS, 111. Young: discovery of "flash" spectrum,
80; prominences. 81 ; H and K in ijrom-
inences, 84; calcium flocculi, 8.i; pho-
Uranus: orbits of satellites, 183. tography of spot spectra, 1.52.
Direct Photograph. Showing the Six as it Appears to the Eye
The Solak Chromospheke and Pkomixexces
PLATE IV
fUltf I
Fig. 1
Chabactekistic Spectba of (a) White, (b) Yeu>ow, axd (c) Red Stabs
(Huggins)
Fig. 2
The Solab Coboxa
'hotographed by Yerkes Observatory Eclipse Expedition. May 28. 1900 (Barnard and Ritchey)
PLATE VI
Stab Tkalls Photographed with 2} o -inch Poktkait Lexs
(Ritchey)
PLATE VII
The Bbuce Telescope of the Yerkes Obsebvatoby
PLATE VIII
Stab Cli •stek Me^-'ier 11 and the SiRRorNDiNci Milky AVay
Small-scale photograph takea with lantern lens (Barnard)
f
PLATE IX
Stab Clusteb Messier 11 and the Sl-bboundcg Mtlky \Vay
Larger-scale photograph taken with 10-inch Bruce telescope (Barnard)
PLATE X
The Milky Way xeak p Ophiuchi
Photographed with 10-inch Brnce telescope (Barnard)
I
I
PLATE XI
Stab Cluster Messier 11
Large-scale photograph taken with -lO-inch Yerkes telescope (Ritchey)
i
PLATE XII
Thb Moon
Photo^aphed with the 12-mch Kenwood refractor (Ritchey)
PLATE XIII
Lunar Cbateb Theophilus axd SuKRorNDiNG Region
Photographed with the 40-inch Yerkes refractor (Ritchey)
PLATE XIV
The 40-inch Refractor of the Yemkes Observatory
— tf
I —
90-FOOT Dome of the Yerkes; Ob^jekvatoky
PLATE XVII
Eye-Esd of Yebkes Telescope
Showing donble-slide plate-holder
PLATE XVIII
y /.
Thb 24-inch Reflsctob of xhi: Yebkes Obsebvatoby
PLATE XIX
Stak Cluster Messier 13
Photographed with the 24-inch reflector of the Yerkes Observatory (Ritchey)
PLATE XX
Star Cluster Messier 13
Photographed with the 40-inch Yerkes refractor (Ritchey)
PLATE XXI
The Great Xebixa in Orion
Photographed with the 24-inch reflector (Ritchey)
f
\
frr -—
I ^
I
1
PLATE XXIII
f
SlK WiLUAM HUGGINS
1^
<
P-i
a-{^ =
•i -f!
i- ^
MOISTAIR DRYAIR
PLATE XXVI
Lax(;ley"s Dkawing of the Typical Srx-SpuT of DEtEMHEK. 1?<7:^
-:afc.':^-.'.^afy^#>".--?^»>-ri.-... v^i
<
a
PLATE XXXII
(a)
(6)
Bright H and K Lixes on the Disk la. h. and <■>. in the
Chromosphere ib). and in a Prominence o '
PLATE XXXIV
Spectkoheuogkai'H Attached to 1:2-lnch Kenwood Keikaitok
PLATE XXXV
iBLTTIVE PkOMIXEXCE PHOTOGRAPHED WITH THE KeXWOOD SpECTBOHELIOGKAPH
March i"). ISftj. lOt 40m. Height of prominence. 162.000 miles
PLATE XXXVI
Ibitttve Promcjesce Showx rs Plate XXXV Fhgtcghaphed 1? Mixutes Lateb
Height of prominence. 2»1,OCO miles
PLATE XXXVII
Rf MFOBD SpEerBOHEUOGRAPB ATTACHED TO 40-IXCH YkRKSS RkFRACTOB
PLATE XXXVIII
The Sl n. Showing the Calcicm Flocculi
August 12. 1903, Sh 52i«
5- S
= i
z r
:i ft
QQ
PLATE XL
■ */?SU'.
.^^^m £ . ^b
''^'^f.
Fig. 1.— 3h40m. Second slit set ou Hi
Fig. 2.— 3h 31m. Second slit set on H2 Same region of the Sun
as that shown in Fig. 1
Mejute Stblctuke of the Calcium Floccuu
PLATE XLI
Fig. 1
Pkism Tkaix of the Rumfokd Spectkoheliogbaph
Fig 2
H ASD K Lines of Caixh'm ix the Elbctkic Arc
o o
z 5
PLATE XLIII
Fig. 1.— 3h 57m. Calcinm floccali iKj)
Fig. Z.— lli^ i»"'. Ilvviro^eu floccnii {Hy) \bri«;.ii r.i.,.i... .„-,.>. „.. .i.c;>t of .-pot/
Hydboges axd Caixitm Flocxxtj. JrLY 7. 190S
^
o
PLATE XLVI
WM
The BbICE SpECTHOGKAPH of the YeBKES OsgEKVATOKY
Mounted on its carriage, with constant temperatare case removed
X
r
\
PLATE LVI
Fig. 1.— At tlie Yerkes Observatory. Exposure 40"
Fig. 2— At Mount Wilson. Exposure 41"!
Star Cluster Messier 35
Photographed with the Bruce telescope (Barnard)
mm <^
r
PLATE LXIII
DiKECT Photogkaph of the Sln
August 25. 1906, 6l» 09ni a. m.
PLATE LXIV
The Sfx, Photographed with the 5-foot Spectroheuogkaph
AugTist 25, 1906. 6h 22n> a. m.
Camera slit set on H| line of calciam
PLATE LXV
The Sin. Photographed vrrxH the 5-foot Spectbohelioghaph
August i"). 1906. eh 18m A. M.
Camera slit set on Hj line of catcinm
PLATE LXVI
The Six. Phutookaphed with the 5-foot SpECXKOHELiixiKAPH
August 25. 1906. 6h :i6"' x. M.
Camera slit set cm Hs line of hydrogen
PLATE LXVII
The Srx. Photographed with the 5-foot Spectbohkuogkaph
Auf^ust 25. 1906, eh 2gm a. m.
Camera slit set on the iron line A 4046
PLATE LXVIII
Fig. 1.— Th 46>'i. Hydrogen Flocculi
Fig. 2.— 7h .>4in. Iron Flocculi
(A 4046)
;ex and Ikox FLocnLi Photookaphed with the 5-foot Spectkoheliogkaph,
November l'^. 1907
PLATE LXX
The Heuomichometkb
- M
'Z s
3 .2
X =
3
3 »
i z
a '2
O 1=
Si » "^
o -
IL
1
>
X
X
m
5 O
.
III!
O te
1 1
00. Z ^ z.
I S X x
qMRi
p=-^
PLATE LXXXIII
,
5300
i
1
5400
i 1
5500
1
1
seoo
i
5700
, i
^^K
P
m
r
1
f r
1
T1
iR^
]
J
1
Carljon
Arc
132 Schj.
(IV)
a Ifrinnis
(III)
Fig. 1
Region of Yellow Cakbon Flutinu in Electbic Abc. Fol-bth
Type Stab (182 Schji'llertip\. and Thibd
Type Stab (a Ononis)
t
L
Fig. 2
Spectba of Focb Foubth Tyte Stabs
[Photograp'ie J with the 40-iDch Yerkes refractor, showing how the dark carbon
band becomes stronger as the star cools
< X
PLATE LXXXVI
The Pleiades
Photoj^raphed with tlie 24-iiicli reflector of the Vt rkc- Ohservatory (Ritchey)
PLATE LXXXVII
Xeblla in Cyfjnus, X. G. C. 6Hir2
Photographed with the 24-inch reflector (Ritchey)
PLATE LXXXVIII
Spiral Nebula Messier 51 Canum Venaticorum
Photographed with the 24-inch reflector iRitcheyi
PLATE LXXXIX
Spibai^ Nebula Messier 101
Photographed with the 24-inch reflector (Ritchey)
PLATE XC
Spiral Nebula Messier :^ Trianguli
Photograplied with the 24-inch reflector (Ritchey)
7. ■_
PLATE XCIV
♦50-INCH Disk after Both Sukfaces had bees Fixe-Gkoixd and Poushed
i
PLATE XCVII
^^''^
^^
.^<^ -''
\ ^ '^ ^^
^ ^^ _,
\ ^ "'1^^
^^<^--
.'^
^A>'^^
i'- "
-'■' ''^
\ ->-c-
;^^^?
y_A
^ [^
>>
1
\
Fig. 1
Fig. 2
Fi... :-; Fig. 4
Vabious Mibrok Combinations ix GO-ixch Rkflectixg Telescope
PLATE XCVIII
MorXTrXG of ♦JO-IXCH REKUClTLNti TeI.E^J OPE
Under construction in Pasadena instrument shop of the Solar Observatory
* ?
h o
■a.
Live Music Archive
Librivox Free Audio
Metropolitan Museum
Cleveland Museum of Art
Internet Arcade
Console Living Room
Books to Borrow
Open Library
TV News
Understanding 9/11