260-262] The Process of Fission 251 



262. The most plausible conjecture that can be made is perhaps that in 

 the far interior of a star the atoms must be almost completely broken up into 

 their constituent electrons*. In this case, the effective molecular weight 

 would approximate, as Eddington has pointed outf , to 2. This limit would 

 be almost independent of the chemical structure of the matter, since the 

 number of electrons in all atoms except hydrogen is nearly equal to half 

 the atomic weight. 



This reduction of the effective molecular weight will greatly modify our 

 numerical estimates. Assuming the star to be made of air (molecular weight 

 about 30) we found in 259 a critical density p = ^. If other factors remained 

 unaltered a reduction of molecular weight from 30 to 2 would reduce this 

 critical density from J to -fa. Other factors do not, however, remain un- 

 altered. On our present tentative view of the interior structure of a star, 

 the effective molecular weight m must have its minimum value, nearly equal 

 to 2, at the centre of the star, and must gradually increase as we pass out- 

 wards towards the surface. Our critical value 7 = 2'2 was determined on the 

 assumption that m had a uniform value throughout the star. It was found 

 that a decrease of m on passing outwards would increase the critical value of 

 7 ; in the same way a decrease of m on passing inwards must decrease the 

 critical value of 7. This might possibly result in a still further decrease of 

 the critical density p. 



Beyond this there are general physical considerations which require a 

 still further adjustment of the critical density. Eddingtonj has pointed out 

 the importance of radiation-pressure in the internal mechanism of a star. 

 When there is extreme ionisation in a star's interior, Eddington's conclusions 

 will need modification, and the disturbing effect of radiation-pressure will be 

 less than that originally estimated but it will still be appreciable . Finally 

 the departure of our ordinary gas laws from the laws of a perfect gas, on 

 which the relation between 7 and the density depends, arises from the "size" 

 of the molecules and the extension of the field of force surrounding them. 

 When the gas is highly ionised, we have to deal rather with the extension of 

 the field of force round individual electrons and positive nuclei, and it is 

 almost impossible even to guess at the density at which the departure from 

 the gas laws becomes appreciable. 



All these considerations suggest that our preliminary theoretical con- 

 clusions must be viewed at least with suspicion, so that there is certainly 

 no ground for surprise that they have not proved to be confirmed by 

 observation. 



* Eddington, Monthly Notices R.A.S. 77 (1917), p. 596. 



"t The Observatory, 40 (1917), p. 44. 



+ Monthly Notices E.A.S. 77 (1917), p. 16. 



Eddington, Monthly Notices RA 8. 77 (1917), p. 603. 



