140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



ever, has treated it seriously and worked out the degree of ionization 

 in various conditions, including comparatively small masses in which 

 the pressure is relatively low and the ionization is not very high. He 

 has thus obtained formulae which he is able to apply to the planets. 



I turn now to the subject of subatomic energy wliich we believe to 

 be the source which maintains a star's heat. This is a matter on 

 which, until about 3 years ago, terrestrial experiment gave us no help 

 at all. Conditions have now changed, and physical laboratories 

 throughout the world have given themselves up to an orgy of atom- 

 splitting. It is of immense importance for the future of astronomy 

 that a new laboratory technique enables us to experiment directly on 

 the processes of liberation of energy by transmutation of atomic 

 nuclei, since these are almost certainly the processes which keep the 

 stars alight. But at present it is too early to expect results this way. 

 The theory of stellar constitution, which I have been describing, was 

 built up without any laboratory Imowledge of subatomic energy. 

 Tliis was possible because the problem of the source of maintenance 

 of a star's heat could be segregated almost completely from the rest 

 of the problem. By Lane's method we could determine the tempera- 

 ture — how much heat there was in the star — without speculating as 

 to how it came to be there; and we could show that a star so endowed 

 must radiate at the moment a calculable amount of light and heat — 

 without inquiring how it managed to go on radiating it for thousands 

 of millions of years. In short, the structural problem could be segre- 

 gated from the evolutionary problem. 



The only point at which the segregation is not complete is this: The 

 concentration of density toward the center of a star depends to some 

 extent on how the source maintaining the heat is distributed. It 

 seems clear from present-day experiment, as well as from astronomical 

 evidence, that the liberation of subatomic energy increases rapidly 

 with temperature; so that we may expect it to occur mainly in the 

 hottest central part of the star. This has the effect of diminishing the 

 concentration of density to the center — making it less than in the 

 standard model which has generally been emplo3^ed. This effect is, 

 however, limited ; because, if the star overdoes it, convection currents 

 are set up, which bring about compensation. To describe our present 

 conclusion I must use technical terms: The density distribution near 

 the outside has a poly tropic index 3 which gradually diminishes to 1.5 

 at the center, where there is a convective core. I am speaking of 

 ordinary stars such as the sun ; but curiously enough this specification 

 of the density distri})ution applies also to white dwarfs — for which it 

 has long been the recognized model — though in the white dwarfs it 

 comes about in quite a different way. 



Apart from this refinement, the researches which I have hitherto 

 described are not affected by theories of subatomic energy. But 



