204 THE NEW QUANTUM THEORY 



the material breaks up (ionises) the outer electron sys- 

 tems of the atoms, so that the fragments can be packed 

 much more closely together. At ordinary temperatures 

 the minute nucleus of the atom is guarded by outposts 

 of sentinel electrons which ward off other atoms from 

 close approach even under the highest pressures; but at 

 stellar temperatures the agitation is so great that the 

 electrons leave their posts and run all over the place. 

 Exceedingly tight packing then becomes possible under 

 high enough pressure. R. H. Fowler has found that in 

 the white dwarf stars the density is so great that classi- 

 cal methods are inadequate and the new statistical 

 mechanics must be used. In particular he has in this 

 way relieved an anxiety which had been felt as to their 

 ultimate fate; under classical laws they seemed to be 

 heading towards an intolerable situation — the star could 

 not stop losing heat, but it would have insufficient energy 

 to be able to cool down!* 



Transition to a New Theory. By 1925 the machinery 

 of current theory had developed another flaw and was 

 urgently calling for reconstruction; Bohr's model of the 

 atom had quite definitely broken down. This is the 

 model, now very familiar, which pictures the atom as 

 a kind of solar system with a central positively charged 

 nucleus and a number of elecrons describing orbits about 

 it like planets, the important feature being that the 

 possible orbits are limited by the rules referred to on 

 p. 190. Since each line in the spectrum of the atom is 

 emitted by the jump of an electron between two par- 



* The energy is required because on cooling down the matter must 

 regain a more normal density and this involves a great expansion of 

 volume of the star. In the expansion work has to be done against the 

 force of gravity. 



