The Sun and Its Energy 7

from the central regions of the Sun will expand without
limit if the outside pressure is sufficiently reduced.

The high compressibility of the gaseous matter in the
Sun brings about a rapid increase of density as we go from
the surface toward the centre, and it has been calculated
that the central density of the Sun must exceed its mean
density by a factor of 50 (that is, the core of the Sun is 50
times denser than is the Sun taken as a whole). Since the
mean density of the Sun, calculated by dividing its known
mass by its dimensions (mass 2 X lo33 grammes; volume
1.4 X io33 cubic centimetres), amounts to 141 times the
density of water, we conclude that the gas filling the solar
interior is compressed to a density six times that of mercury.
On the other hand the outer layers of the Sun are quite
rarefied, and the pressure in the chromosphere, where the
absorption lines of the solar spectrum are formed, is only
one-thousandth the pressure of atmospheric air.

Although all our direct observational evidence concern-
ing the physics and chemistry of the Sun is limited to the
phenomena taking place in this rarefied solar atmosphere,
it is possible, if we start with these surface conditions and
make use of our general knowledge concerning the prop-
erties of matter, to learn about the conditions existing in
the solar interior almost with the same certainty as if we
could see it with our own eyes. Our mathematical analysis
of the solar interior is mainly owing to the work of the
British astronomer Sir Arthur Eddington; and Figure i
gives us the schematic picture of the internal structure of
the Sun as obtained from his calculations. In this picture
the values of T> P, and p give the temperatures, pressures,
and densities respectively at different depths under the
surface of the Sun.