CONSTITUTION OF THE STARS— EDDINGTON 141 



thej^ put US in a favorable position to learn something about the laws 

 of subatomic energy. Many well-known lines of argument have 

 convinced us that the sun and stars have a lifetime to be reckoned in 

 thousands of millions of years — which means that evolutionary 

 changes are extremely slow, and that the heat radiated by a star into 

 space is almost exactly balanced by the heat liberated from subatomic 

 sources in the interior. So when we measure the radiation of a star, 

 we measure the generation of subatomic energy. You see then that 

 the measurement of subatomic energy is just a common everyday 

 astronomical measurement. 



To the engineer the release of subatomic energy on a practical 

 scale is, and seems likely to remain, a Utopian dream. To the phys- 

 icist it was, until 3 years ago, a field of uncontrolled theoretical 

 speculation. To the astronomer it has long been an everyday 

 phenomenon which it would be absurd to close his eyes to. 



Having then measured the rate of release of subatomic energy in 

 all types of stars, we can correlate it to the temperatures and densities 

 which we have found in the interior. This more or less direct inves- 

 tigation of the conditions of release can be supplemented by a theo- 

 retical examination of the conditions of stability of stars containing 

 such a source — a lino of attack initiated by Prof. H. N. Russell. 



If the star contracts, the liberation of subatomic energy must be 

 stimulated; otherwise the star is unstable. We cannot deduce 

 astronomically whether the stimulus comes from the increased 

 temperature or the increased density; but for simplicity we shall 

 suppose it to be mainly the temperature. Then each star contracts 

 until its internal temperature reaches the value at which the hberation 

 of subatomic energy is equal to the heat radiated, and there it 

 sticks — not quite indefinitely, but for a very long period until the 

 sources of subatomic energy show signs of exhaustion. The stars 

 on the main series appear to be those which have reached this balance 

 and stuck. Now it is one of the results of our previous investigations 

 that the stars of the main series, from the most massive to the lightest, 

 have practically the same internal temperature. We used to give 

 the central temperature as 40 million degrees, but the figure has come 

 down — partly by the recognition of the abundance of hydrogen and 

 partly by the substitution of a less condensed model, and the present 

 estimate is about 15 million degrees. But whatever it is, it is nearly 

 the same for all. It appears, therefore, that on the main series a 

 small star which requires a small amount of energy per gram to 

 maintain its radiation and a massive star requiring 1,000 times as 

 much energy per gram both have to rise to 15 million degrees to 

 liberate it. Or to put it another way the liberation must increase 

 1,000 fold in a rise of temperature scarcely large enough for us to 

 notice in our rather rough calculations. 



