The Sun among the Stars 127

lines. It reads: "Oh, Be A Fine Girl, Kiss Me Right
Now . . ." As to whether S, the last letter, stands for
"Sweetheart" or for "Smack," there is a long-standing, still
unconcluded dispute between the Harvard and the Yerkes
astronomers.*

If the spectrum of a given star falls, according to its
properties, somewhere in between two of the above classes,
a decimal notation is used, e.g., As = two-tenths the dis-
tance from A to F, or KS = five-tenths the distance from K
to M (see Plate VII). In this Harvard classification our Sun
belongs to the class G (6000 degrees), Sirius to the class
A (i 1,200 degrees), and the faint star Krueger 60 B to the
"cold" class M (3300 degrees).

Knowing the star's surface temperature as given by its
spectral class, we can now estimate also its geometrical
dimensions by comparing the surface brightness that
should correspond to this temperature with the star's
absolute luminosity. We find in this way that the diameters
of Sirius and Y Cygni are respectively 1.8 and 5.9 times
larger than our Sun's, whereas the faint star Krueger 60 B
has a diameter half as long.

THE RUSSELL DIAGRAM

When we compare these four stars (including the Sun)
we may easily notice a very interesting regularity in the
fact that stars of higher luminosity generally possess higher
surface temperatures and larger radii. A more detailed
study of this relationship has led to a remarkable classifica-
tion of stars, which represents at the present time the most
important basis for theories of stellar properties and
evolution.

* Plate VII does not show the spectra for classes O and S.