a perfect screen. If we are thinking of light-waves this seems an astonishing opacity for material as
tenuous as air; but we have to remember that it is an opacity to X-rays, and the practical physicist knows
well the difficulty of getting the softer kinds of X-rays to pass through even a few millimetres of air.
There is a gratifying accordance in general order of magnitude between the opacity inside the star, determined
from astronomical observation of leakage, and the opacity of terrestrial substances to X-rays of more or
less the same wave-length. This gives us some assurance that our theory is on the right track. But a
careful comparison shows us that there is some important difference between the stellar and terrestrial
opacity.
In the laboratory we find that the opacity increases very rapidly with the wave-length of the X-rays that
are used. We do not find anything like the same difference in the stars although the X-rays in the cooler
stars must be of considerably greater wave-length than those in the hotter stars. Also, taking care to
make the comparison at the same wave-length for both, we find that the stellar opacity is less than the
terrestrial opacity. We must follow up this divergence.
There is more than one way in which an atom can obstruct ether-waves, but there seems to be no doubt
that for X-rays both in the stars and in the laboratory the main part of the opacity depends on the process
of ionization. The ether-wave falls on an atom and its energy is sucked up by one of the planet electrons
which uses it to escape from the atom and travel away at high speed. The point to notice is that in the
very act of absorption the absorbing mechanism is broken, and it cannot be used again until it has been
repaired. To repair it the atom must capture one of the free electrons wandering about, inducing it to
take the place of the lost electron.
In the laboratory we can only produce thin streams of X-rays so that each wave-trap is only called upon
to act occasionally. There is plenty of time to repair it before the next time it has a chance of catching
anything; and there is practically no loss of efficiency through the traps being out of order. But in the
stars the stream of X-rays is exceedingly intense. It is like an army of mice marching through your larder
springing the mouse-traps as fast as you can set them. Here it is the time wasted in resetting the traps --
by capturing electrons -- which counts, and the amount of the catch depends almost entirely on this.
We have seen that the stellar atoms have lost most of their electrons; that means that at any moment a
large proportion of the absorption traps are awaiting repair, For this reason we find a smaller opacity in
the stars than in terrestrial material. The lowered opacity is simply the result of overworking the absorbing
mechanisms -- they have too much radiation to deal with. We can also see why the laws of stellar and
terrestrial opacity are somewhat different. The rate of repair, which is the main consideration in stellar
opacity, is increased by compressing the material, because then the atom will not have to wait so long
to meet and capture a free electron. Consequently the stellar opacity will increase with the density. In
terrestrial conditions there is no advantage in accelerating the repair which will in any case be completed
in sufficient time; thus terrestrial opacity is independent of the density.
The theory of stellar opacity thus reduces mainly to the theory of the capture of electrons by ionized
atoms; not that this process is attended by absorption of X-rays -- it is actually attended by emission --
but it is the necessary preliminary to absorption. The physical theory of electron-capture is not yet fully
definitive; but it is sufficiently advanced for us to make use of it provisionally in our calculations of the
hindering factor in the leakage of radiation from the stars.
The Relation of Brightness to Mass
We do not want to tackle too difficult a problem at first, and so we shall deal with stars composed of
perfect gas. If you do not like the technical phrase 'perfect gas' you can call it simply 'gas', because
all the terrestrial gases that you are likely to think of are without sensible imperfection. It is only under
high compression that terrestrial gases become imperfect. I should mention that there are plenty of
examples of gaseous [Note:Unless otherwise indicated 'gaseous' is intended to mean 'composed of
perfect gas'] stars. In many stars the material is so inflated that it is more tenuous than the air around