x Supplement to “ Nature,” May 12, 1923 

giant stars in the state of a perfect gas, and they are 
therefore useless for the present discussion. But within 
the last year we have been fortunate enough to obtain 
complete and very accurate information for one of the 
giant stars, Capella. This is another of the benefits 
which astronomy has derived from Prof. Michelson’s 
interferometer method of observation. The brighter 
component of Capella (which is a double star) has a 
mass 4:2 times that of the sun and a luminosity 160 
times greater. We can use these facts to calculate the 
opacity of Capella in the way I have described ; it 
turns out to be 150 in C.G.S. units. To illustrate the 
meaning of this, let us enter Capella and find a region 
where the density is that of the terrestrial atmosphere 
we are accustomed to ; a slab of this gas only 6 inches 
thick would form an almost opaque screen. Only 51; 
of the radiant energy falling on one side would get 
through to the other, the rest being absorbed by the 
gas. > 
ABSORPTION OF X-RAYS IN STARS. 
It seems at first surprising that 6 inches of gas could 
stop the zther-waves so effectively ; but we might 
have anticipated something like this from general 
physical knowledge. We give different names to 
zther-waves according to their wave-length. The 
longest are the Hertzian waves used in wireless tele- 
graphy ; then come the invisible heat-waves; then 
light-waves ; then photographic or ultra-violet waves. 
Beyond these we have X-rays, and finally—the shortest 
of all—the y-rays which are emitted by radioactive 
substances. 
Where in this series are we to place the ather-waves 
in the interior of a star? It is solely a question of 
temperature, and the zther-waves at stellar tempera- 
tures are those which we call X-rays—more precisely, 
they are very “soft” X-rays. Now X-rays, and soft 
X-rays especially, are strongly absorbed by all sub- 
stances. The opacity which we have found in Capella 
is of the same order of magnitude as the opacity of 
terrestrial substances to X-rays measured in the 
laboratory. The following table shows a few of the 
laboratory results compared with the astronomical 
value for Capella : 

Absorption-coefficient (opacity) in 
Wave-length 
(A). 
Aluminium. Capella. 
We have been performing an investigation of the 


absorption of X-rays in a star, parallel to investigations 
on the same subject made in the laboratory. In one 
respect the physicist has a big advantage because he 
can vary the material experinfented on, whereas we 
have to be content with the material, whatever it is, 
composing the stars. But, as you see from the table, 
the physicist is also interested in finding how the 
absorption changes for different wave-lengths. We can 
follow him in this, and even do better than him, because — 
he is restricted by certain practical difficulties to a 
narrow range of wave-length, whereas we can explore 
a range of wave-length covering a ratio of at least 
ro to 1, by using stars of different temperatures. It 
is true that our results are not yet very accurate ; we 
have only one star, Capella, for which a really good 
determination is possible, but for other stars rough 
values can be found. The terrestrial results indicate 
an extremely rapid change of absorption for slight 
alterations of wave-length (as is seen from the table) ; 
the astronomical results, on the contrary, give a nearly 
steady absorption-coefficient. We cannot yet detect 
certainly whether it increases or decreases with wave- 
length; at any rate there is nothing like the rapid 
change shown in the foregoing table. This profound 
discrepancy between astronomical and laboratory re- 
sults leads us to inquire more deeply into the theory 
of absorption in a star. 
a good reason for it. 
We have been taking advantage over our cloud- 
bound physicist by having a preliminary peep at an 
actual star. We are not going to allow him to do that. 
He must not use astronomical observations to deter- 
mine the opacity, but must be able to predict the 
astronomical value either from pure theory or from 
terrestrial experiments. This study is of special in- 
terest because it plunges us at once among those 
problems which are most exercising practical physicists 
at the present time. We started to explore the interior 
of a star; we shall presently find ourselves in the 
interior of an atom. 
It is now generally agreed that when zther-waves 
fall on an atom they are not absorbed continuously. 
The atom lies quiet waiting its chance and then 
suddenly swallows a whole mouthful at once. The 
waves are done up in bundles called quanta and the 
atom has no option but to swallow the whole bundle 
or leave it alone. Generally the mouthful is too big 
for the atom’s digestion, but the atom does not stop 
to consider that ; it falls a victim to its own greed— 
in short, it bursts. One of its satellite electrons shoots 
away at high speed, carrying off the surplus energy 
which the atom was unable to hold. The bursting 
could not continue indefinitely unless there were some 
counter-process of repair. The ejected electrons travel 
It will be found that there is — 
ee eee 


