IOO Recent Advances in Stellar Astronomy
first mass of 10 grams, the next 100 grams, the third 1,000
grams, and so on. Then, by means of his equation, we
find that the proportion which the radiation pressure bears
to the whole will be quite negligible in all the spheres up
to number 32, will increase rapidly for numbers 33 and
34; while for sphere 35 and all those beyond it the radia-
tion pressure will be the dominant partner, leaving little
for the gas-pressure to do.
Upon this long line of spheres, therefore, we find a
small region in which a certain natural factor changes
from an insignificant to a controlling role. On general
physical principles, therefore, as Eddington puts it, we
would expect “something to happen” in this critical inter-
val, and “what happens is the stars.” It is only when the
radiation pressure and the gas pressure share the gravita-
tional food that we get anything that can fairly be called
a star. Smaller masses do not give out light enough to
make them visible at interstellar distances, while the great
ones, in which the radiation pressure is almost sufficient to
counteract gravitation, would be in an almost unstable
condition, so that a small disturbance, such as might be
produced by a moderate rotation, would cause them to
break up into parts. Hence smaller masses do not shine,
and bigger ones break up, and only those in the critical
intervening range of mass remain as luminous stars. We
have seen that this should occur for masses comparable
with those of spheres 33 and 34 of the series. Now the
first of these is of half the mass of the Sun, and the second
has five times the Sun’s mass, so that the actual masses of
the stars fall very exactly into the range indicated by the
theory. Since the constants of this theory are derived from
those which are the most fundamental in modern physics,
we may truthfully say that the masses, and hence the sizes