Constitution and Evolution of the Stars 97
in the gas counts as much as any other—would be much
diminished. Secondly, the gas at this temperature would
emit a tremendous flood of radiation, most of it of such
short wave-length that it would resemble X-rays rather
than ordinary light. This radiation would not go very far
before it was scattered in all directions by the electrons, or
absorbed in detaching some fast-knit electron from the
remnant of an atom, only to be re-emitted when recombina-
tion took place. In either case the energy would be re-
layed back and forth from atom to atom, now in this
direction, again in that, until in the lapse of ages it leaked
gradually outward to the cooler parts of the star, on its
way to the surface.
Jeans, was, I believe, the first to call attention to this
extraordinary state of things, and Schwarzschild to point
out the fundamental importance of the exchange of radia-
tion in determining the conditions of equilibrium within a
star; but the general solution of the problem came later,
from Eddington, who was the first to appreciate one of
its most fundamental features.
The flood of entrapped radiation, in its attempts to
escape, exerts a pressure outward in all directions, just as
a compressed gas would do. The existence of this radia-
tion pressure was pointed out long ago by Maxwell’s
theory of light. With any light obtainable on Earth, even
full sunlight, it is so minute that apparatus of the most
delicate sort is required to indicate its existence; but at
the temperatures which prevail inside the stars, it may
amount (as Eddington pointed out) to hundreds of tons
per square inch, and be an important factor in preventing
the collapse of the star’s interior under the weight of the
outer parts. Indeed, under some conditions, it may do
more than the gas-pressure due to the motions of the