Supplement to ‘ Nature,” May 12, 1923 Vv 

The Interior of a Star.1 
By Prof. A. S. Epprncton, F.R.S. 
N December 13, 1920, the angular diameter of a 
star was measured for the first time in history 
with an apparatus devised by Prof. A. A. Michelson. 
Hitherto every star had appeared as a mere point of 
light, and no test had been able to differentiate it 
from a geometrical point. But on that eventful 
evening a 20-foot interferometer constructed at the 
Mt. Wilson Observatory was turned on the star 
Betelgeuse, and the measurement revealed that this 
star had a disc ,\, of a second of arc in diameter— 
about the size of a halfpenny 50 miles away. The 
distance of Betelgeuse is known roughly (unfortunately 
it cannot be found so accurately as the distances of 
many stars), so that we can convert this apparent 
size into approximate actual size. Betelgeuse is not 
less than 200 million miles in diameter. The orbit 
of the earth could be placed entirely inside it. 
The stars are thus not limited to objects of com- 
paratively small bulk like the sun; there are among 
them individuals truly gigantic in comparison. We 
can add another step to the astronomical, multiplica- 
tion table—a million earths make one sun; ten 
million suns make one Betelgeuse. This is a com- 
parison of volume, not of amount of material. It 
leaves open the question whether, in order to obtain 
one of these giants, we should take the material of 
ten million suns rolled into one, or whether we should 
take the material of the sun and inflate it to ten 
million times its present size. There is no doubt that 
the latter answer is nearer the truth. Betelgeuse, I 
admit, contains more matter than the sun (perhaps 
50 times as much) ; but in the main its vast bulk is 
due to the diffuseness with which this material is 
spread out. It isa great balloon of low density, much 
more tenuous than air, whereas in the sun the material 
is compressed to a density greater than water. 
Whether the star is one of these balloon-like bodies 
or whether it is dense like the sun depends on the 
stage of its life at which we catch it. It is natural to 
think that the stars gradually condense out of diffuse 
material, so that they become denser and denser as 
their life-history proceeds. We can now see in the 
heavens samples of every stage in the development of 
a star. The majority of those seen with the naked 
eye are in the early diffuse state ; that is not because 
these young stars are really more numerous, but 
because their great bulk renders them brighter and 
more conspicuous. What I shall have to say about 
the inside of a star refers chiefly to the young diffuse 
stars—the giant stars as they are called. The reason 
1 Discourse delivered at the Royal Institution on February 23. 
is that we understand much more about the properties 
of matter when it is in the condition of a perfect gas 
than when it is condensed ; although the difficulties 
of treating a dense star like the sun are not insuperable, 
we have naturally made the most progress with the 
easier problem of giant stars. 
INTERNAL TEMPERATURES. 
We only observe the physical conditions at the 
surface of a star, and at first it might seem impossible 
to learn anything about the conditions in the interior. 
Consider, for example, the question of temperature. 
The nature of the light received from Betelgeuse 
teaches us that the temperature is 3000° C.—not an 
extravagantly high temperature judged even by 
terrestrial standards. But this refers, of course, to 
the layer near the surface from which the observed 
light is coming; it is just the marginal temperature 
of the furnace affording no idea of the terrific heat 
within. I shall not attempt to explain in detail how 
we manage to calculate the inside temperatures ; but 
I can perhaps show that there is a clue which can be 
followed up by appropriate mathematical methods. 
Elasticity is a well-known property of a gas, familiar 
to everybody through its practical application in the 
pneumatic tyre. What gives the gas. its elasticity or 
expansive force is its heat, that is to say, the energy 
of motion of its molecules hastening in all directions 
and continually tending to spread apart. The greater 
the heat the greater the expansive force. Now at any 
point inside the star a certain condition of balance 
must be reached ; on one hand we have the weight 
of all the layers above pressing down and trying to 
squeeze closer the gas inside; on the other hand we 
have the elasticity of this inside gas trying to expand 
and force the upper layers outwards. Since neither 
one thing nor the other happens and the star remains 
practically unchanged for hundreds of years, we must 
conclude that these two tendencies just balance. 
At each point the elasticity and therefore the heat 
has to be of the exact amount needed to bear the 
weight of the layers above. That is the principal 
clue by which we determine how much heat there 
must be at various depths inside the star. 
The internal temperature depends on the particular 
star considered, but it is generally from 2 to 20 million 
degrees at the centre. Do not imagine that this is a 
degree of heat so vast that ordinary conceptions of 
temperature have broken down. These temperatures 
are to be taken quite literally. Temperature is a mode 
of describing the speed of motion of the ultimate 
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