CONSTITUTION OF THE STARS— EDDINGTON 137 



than water for which it was not intended. This was a complete sur- 

 prise. But the explanation was not difficult to find. We had been 

 taking it for granted that stellar matter would cease to behave as a 

 perfect gas when the density approached that of ordinary liquids or 

 solids. Ordinary terrestrial atoms then begin to jam together and 

 the material becomes almost incompressible. But in the stars the 

 temperature of 10 million degrees causes most of the satellite elec- 

 trons to be torn away from the atom, and what is left of the atom is 

 a tiny structure. The atoms or ions are so reduced in size that they 

 will not jam until densities 100,000 times greater are reached. For 

 this reason, the perfect gas state continues up to much higher den- 

 sities in the stars. The sun and other dense stars insisted on obey- 

 ing the theory worked out for a perfect gas, as they had every right 

 to do, since their material was perfect gas. 



There was, therefore, nothing to prevent stellar matter from becom- 

 ing compressed to exceedingly high density; and it suggested itself 

 that the densities which had been calculated from observation for 

 certain stars called white dwarfs, which had seejned impossibly high, 

 might be genuine after all. 



In reacliing this conclusion I was not without a certain misgiving. 

 I was uneasy as to what would ultimately happen to these superdense 

 stars. The star seemed to have got itself into an awkward fix. Ulti- 

 mately its store of subatomic energy would give out and the star 

 would then want to cool down. But could it? The enormous density 

 was made possible by the high temperature which shattered the 

 atoms. If the material cooled it would presumably revert to terres- 

 trial density. But that meant that the star must expand to say 

 5,000 times its present bulk. But the expansion requires energy — 

 doing work against gravity; and the star appeared to have no store 

 of energy available. What on eartli was the star to do if it was con- 

 tinually losing heat, but had not enough energy to get cold! 



The high density of the companion of Sirius was duly confirmed 

 by Professor Adams— but this puzzle remained. Shortly afterward 

 Prof. R. H. Fowler came to the rescue in a famous paper, in which 

 he applied a new result in wave mechanics which had just been dis- 

 covered. It is a remarkable coincidence that just at the time when 

 matter of transcendently great density was discovered in astronomy, 

 the mathematical physicists were quite independently turning atten- 

 tion to the same subject. I suppose that up to 1924 no one had 

 given a serious thought to abnormally dense matter; but just when 

 it cropped up in astronomy it cropped up in physics as well. Fowler 

 showed that the newly discovered Fermi-Dirac statistics saved the 

 star from the unfortunate fate which I had feared. 



I will say a word or two about Professor Fowler's explanation. My 

 colleague Fowler was in his youth a pure mathematician, and I am 



