The constitution and evolution of the stars

104 Recent Advances in Stellar Astronomy

We may, therefore, say, with considerable confidence, that
the life of the Sun, and doubtless also of the stars in
general, must extend over many billions of years.

But here we meet with a serious difficulty. We know
the rate at which the Sun is radiating energy to the earth,
and, from consideration of the way in which the Earth in
turn radiates this energy into space, we can be sure that
the Sun is also sending out an equal amount of heat into
space in every direction. The total output is so great that
ɪt would exhaust the whole huge fund of energy, which
would be made available by the Sun’s contraction from an
indefinitely extended size, in about twenty million years, as
Lord Kelvin showed long ago. When we allow for the
fact that some of this heat is still stored in the Sun’s in-
terior, and that it was probably much brighter in its earlier
stages of evolution than at present, we see that, if gravita-
tional energy alone was available as the source of its radia-
tion, the Sun’s past life as a star must have occupied but a
very few million years. In view of the geological and
radio-active evidence, there seems to be no escape from the
conclusion that the Sun must have some other, and far
greater, store of internal energy upon which to draw.

Further evidence in favor of this view has been found
by Eddington in the behaviour of the star Delta Cephei.
This is a typical giant star, about eight hundred times as
bright as the Sun. Eddington has given good reason to
believe that the cause of its variation in light is a peri-
odic expansion and contraction of the whole star by
about ten per cent, on each side of the mean. The period
of this change would depend on the density of the star,
and diminish if this increased. Hence, if the mean dia-
meter was gradually contracting, the period should
shorten. Eddington calculates that, if the radiation is

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