The constitution and evolution of the stars

Constitution and Evolution of the Stars 93
miles in its surface, and less light given out per square
mile, so that its light will fall off rapidly, and it will grow
fainter and redder until at last it disappears.

During its history, therefore, it will pass through any
surface temperature lower than the maximum twice—once
when of large diameter, low density, great luminosity and
rising temperature, and again when its diameter is small,
density high, luminosity low, and temperature falling. It
is obvious that these contrasted groups of characteristics
are exactly those which differentiate the giant and dwarf
stars. The theoretical and observed pictures, indeed, agree
not merely in their general outlines, but in every detail.
For example, the lower the temperature selected for
study, the greater will be the theoretical difference between
the groups of stars of rising and falling temperature, and
the greater is the actual difference between the giant and
dwarf stars. The approximate equality in brightness among
the giant stars of the various spectral classes, and the
great differences among the dwarfs, find also a complete
explanation.

Stars of large mass, as can easily be shown, should
attain a greater maximum temperature than those whose
mass is smaller, and should be more luminous than the lat-
ter, for the same surface temperature, especially in the giant
stages. The great masses and luminosities of the B-stars
are thus accounted for. The are not massive be-
cause they are hot, but hot because they are massive.
Lesser masses never attain the B stage of temperature, but
stop at A; and still smaller ones may not get beyond Class
F, or even G. As we go down the spectral series, there-
fore, we are continually adding to our list stars of mass
too small to get into any of our earlier groups at all—so it
is no wonder that the average mass decreases for the red-

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