xii Supplement to ‘ Nature,” May 12, 1923 

actually does give a value of the absorption-coefficient 
agreeing with astronomical observation. Thus for 
Capella the calculated value is 110 as compared with 
the observed value 150. There are certain doubtful 
factors which permit of the result being varied by a 
factor 2 or possibly 3; and we lay no stress on the 
precise accordance. But it appears to be possible 
to predict on this hypothesis the brightness of a star 
of known mass like Capella to within a magnitude, 
which amply solves the problem proposed to our 
physicist on the cloudy planet. It may be added that 
the theory also explains why the absorption in giant 
stars is nearly independent of the wave-length; but 
that is a more elementary result which becomes 
apparent as soon as we realise that the problem is 
concerned with the rate of repair of the atoms ; many 
alternative theories of the conditions of repair would 
lead to the same conclusion. 
Source or STELLAR ENERGY. 
The store of etherial heat and the store of material 
heat in the star may be compared to the accumulators 
of a power station, We have not yet discovered the 
dynamos. The accumulators would run down in a 
few thousand years if they were not replenished. 
What is the source of the energy maintaining (and 
during the ascent of temperature increasing) this 
internal store? We believe now that the source is 
sub-atomic energy. One theory is that inside the 
star the simpler elements are gradually being built 
up into more complex elements, and energy is liberated 
in the process ; a more drastic view is that matter is 
being entirely annihilated, setting free the whole of 
its energy of constitution. Taking the first theory, 
the most conspicuous known case is in the formation 
of helium from hydrogen. We do not know how to 
make helium from hydrogen, but we know that it is 
so made ; we know also that o°8 per cent. of the mass 
disappears in the process, and this must be the mass 
of the energy —ether-waves—liberated when the 
change occurs. Aither-waves weigh very light, and 
the energy available from this source is colossal. If 
5 per cent. of the star consists of hydrogen which turns 
into helium as a first step in the formation of the 
higher elements, that would provide energy sufficient 
for all reasonable demands. 
We might perhaps expect that the earliest stars 
would consist almost entirely of hydrogen, the evolu- 
tion of the higher elements having little chance of 
beginning until the interior became hot enough to 
stimulate the process. But a difficulty arises here. 
For astronomical reasons it seems impossible to admit 

that even the earliest stars contain more than a very 
moderate proportion of hydrogen. I have referred 
to the fact that our calculations have been practically 
independent of the chemical canstitution of the star ; 
but one reservation ought to have been made— 
provided it is not made of hydrogen. Hydrogen gives 
results differing widely from all the other or 
elements. Z 
To assume hydrogen as the material would in most 
cases destroy the general accordance of theory and 
observation ; indeed it isa way of realising the goodness 
of this general accordance to note how it disappears 
when hydrogen is substituted instead of a normal 
element. I think, therefore, that the process of 
element-building from protons and electrons must 
have begun before the stellar stage is reached. This 
is a curious detached piece of knowledge to have 
come across in exploring the interior of a star—to be 
able to deny that it is mainly composed of hydrogen 
though any of the other 91 elements may be present 
to any extent ; and it is still more curious that hydrogen 
should be the element which we. were tempted to 
build the stars with, so that this apparently random 
denial hits the mark, 
Admixture of hydrogen diminishes the proportion 
of ztherial energy and etherial pressure, and so permits 
gravitation to aggregate larger masses. The occasional 
formation of stars of exceptionally large mass (20 
to 80 times the sun’s) may be due to-the accidental 
prevalence of hydrogen in the region where they 
originated—that is to say, the material was in a more 
primitive state as regards evolution of the elements. 
We need not be greatly concerned as to whether 
these rude attempts to explore the interior of a star 
have brought us to anything like the final truth. 
We have, I think, been able to recognise some of the 
leading factors participating in the problem and to 
learn how many varied interests are involved. The 
partial results already attained correspond well enough 
with what is observed to encourage us to think we 
have begun at the right end in disentangling the 
difficulties, and we do not anywhere come against 
difficulties which appear likely to be insuperable. 
The fact is that gaseous matter at very high tempera- 
ture is the simplest kind of substance for a mathe- 
matical physicist to treat. To understand all that 
is going on in the material of a desk, for example, is 
a really difficult problem almost beyond the aspira- 
tions of present-day science ; but it does not seem too 
sanguine to hope that in a not too distant future we 
shall be able to understand fully so simple a thing as 
a star. 
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