156 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



decrease by about 40 seconds per year. The observations, which 

 cover more than a century, show indeed a decrease of period, but 

 at the rate of about a second in 12 years — five hundred times slower 

 than the previous theory would demand. Here again we have evi- 

 dence that the rate of stellar evolution — in a giant star this time — is 

 many hundreds of times slower than it would be if there was not 

 some internal store of energy to draw upon. 



It is certain that no corresponding evolution of heat from any 

 source occurs within the earth, and we must therefore suppose that 

 energy from the " unknown source " becomes available only at 

 exceedingly high temperatures, such as prevail inside the stars. But 

 if this is the case, and a star, in contracting, gets hot enough inside 

 to start this process going, why does this not make the interior 

 still hotter, and so cause a still more rapid transformation of the 

 unknown energy into heat, till the process ends in an explosion on 

 a colossal scale? I mulled over this idea for a couple of years 

 before I saw the simple answer. If heat energy is supplied to the 

 interior of a giant star, the star will have to expand, and if it ex- 

 pands, it must grow cooler. The process is the exact reverse of 

 that by which contraction makes the star hotter, and at the same 

 time compels the escape of heat from the surface into space. Hence, 

 if too much heat is supplied from the unknown source, the star 

 will expand and cool, shutting off further supplies. It is easy to 

 see that we have here a self-regulating process, which, in the long 

 run, will automatically adjust the supply of heat in the interior 

 so that it just makes up the loss due to leakage toward the surface 

 and radiation into space. In the short run, we might find alternate 

 overproduction, leading to expansion of the star and cooling, and 

 underproduction, permitting contraction and heating; and oscilla- 

 tions of just this sort appear to happen in the Cepheid variables. 

 Though the star may thus be kept shining for a very long time, it 

 can not go on forever, for the store of internal energy, however 

 vast, must be finite, and will gradually be used up. As this hap- 

 pens, the star will contract, although very slowly, and ultimately pass 

 through the various giant and dwarf stages, in substantially the man- 

 ner which was described earlier. 



Such a store of available energy will account for the facts; but 

 how shall we attempt to account for the store of energy itself ? One 

 thing is clear at the start. The only places small enough to contain 

 so huge an accumulation are the nuclei of the atoms. I say " small " 

 advisedly, for it is only when the constituent parts of which the 

 atoms are built come exceedingly close together that the forces be- 

 tween them can become great enough to account for their possession 

 of such an amount of energy. Radioactive energy, which comes 



