Ill
THE CONSTITUTION AND EVOLUTION OF
THE STARS
THE preceding lectures have, I hope, presented to
you a picture—drawn rather in outline, owing to the
limitations of time—of the stars as they are known at
present, their dimensions, masses, densities, surface bright-
ness, and the like. It remains to speak of what has been
done to correlate these facts into a theory of the constitu-
tion of the stars, and their probable evolution and age.
What makes this problem tractable, in spite of the
limitations imposed by the remoteness of the stars in space,
and our ephemeral duration in time, is that we have to
deal only with the simpler and more general properties
of matter. The vast variety of the forms of rock and
mountain depends upon the solidity of their materials;
the still greater diversity of the forms of organic life is
based on the presence of chemical compounds of great
complexity—and neither of these conditions can exist at
all in bodies as hot as even the coolest stars. In the stars
all matter must be gaseous, and the laws of gases are
among the simplest known to physics. Add to them the
still more general laws that govern gravitation, radiation,
and the structure of atoms, and we have the controlling
factors in the evolution of the stars.
Considering a star, then, as a mass of gas, isolated in
space, we notice first that it must be in internal equilibrium
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