Constitution and Evolution of the Stars 101
and brightness, of the stars are determined directly by the
fundamental properties of the very atoms of which they
are composed. It may be shown, for example, from
Eddington’s equation, that a mass of gas will shine as a
giant star when, and only when, the ratio of the diameter
of the star to the average distance between the atoms which
compose it is about twenty times the ratio between the
charge of an electron and the average mass of an atom
(provided that this mass is measured, not in the ordinary
way, but, as in the electrical case, by its power of attract-
ing a similar mass at a given distance). The latter ratio
is very large, about 4×1017, so that the number of atoms
in the star is enormous, and the star itself a very large
mass.
One of the most impressive consequences of the whole
theory is that the masses of the stars are determined by
the interplay of the two forces, gravitation and radiation
pressure, which, among all those in nature, are so feeble,
under the conditions of ordinary experiment, that it taxes
the skill of the experimenter to build an apparatus delicate
enough to measure the effects of either one. Were we
confined to experiments in enclosed laboratories, isolated in
space, without the Earth’s attraction to prove to us the
existence of gravitation, it would probably have been long
before the very existence of either of these forces would
have been suspected; yet these forces, and these alone, when
working on the grand scale, are powerful enough to shape
the stars.
One question still remains. How long a time is required
for this sequence of evolutionary changes? What is the
life of a star? Here, again, the answer which we would
now give depends upon knowledge which has come within
the last decade or two. We have, even now, no direct