CONSTITUTION OF THE STARS— EDDINGTON 143 



examination not to fulfill what was initially claimed for them — on 

 the strength of which they were recommended. And secondly it is 

 not unfair to describe them as agreeing with the physicist on a matter 

 as to which he knew nothing, at the expense of disagreeing with him 

 on matters as to which he claimed to know a great deal. 



All that has changed now that these subatomic processes have been 

 studied in the laboratory. They are found to require comparatively 

 low speeds of the particles, corresponding to comparatively low 

 temperatures, such as the stellar investigations had indicated. The 

 first criticism I heard, after the experiments on disintegration of 

 elements by protons had begun, was that 40 million degrees was too 

 high a temperature for the sun; and it could not be much ovej 15 

 million degrees without blowing up. Happily we had been before- 

 hand; and the revised astronomical calculations had already lowered 

 the temperature to a point which makes the sun safe for posterity. 



New experimental discoveries have helped us to come to an 

 important decision as to the nature of the subatomic energy released 

 in the stars. For 15 years we have been hesitating between two 

 alternative suggestions. The energy might be provided by electrons 

 and protons anniliilating one another, thus setting free the whole 

 energy of their constitution in the form of radiation. Or it might 

 be provided by transmutation of the elements. Even in this applica- 

 tion it remains true that we need distinguish only two kinds of stellar 

 matter, namely hj^^drogen and not-hydrogen; so the transmutation 

 can be more precisely defined as the transmutation of hydrogen into 

 not-hydrogen. The annihilation of a proton by an electron corre- 

 sponds to the complete disappearance of a hydrogen atom. The 

 energy released by the transmutation of a hydrogen atom into other 

 elements is only about K20 of the energy which would be released by its 

 complete disappearance. Thus the annihilation hypothesis provides 

 more than 100 times as much energy as the transmutation hypothe- 

 sis; and the possible lifetime of a star is correspondingly increased. 



Attempts to decide between the two alternatives by astronomical 

 evidence were inconclusive. But recent progress in physics seems 

 to point decidedly to the transmutation hypothesis; and the annihila- 

 tion hypothesis seems to have been generally abandoned. Perhaps 

 the most serious blow to it was the discovery of the positron by 

 Anderson at Pasadena. The positron, not the proton, is the true 

 opposite of an electron; and positrons and electrons do annihilate 

 one another. Our lust for slaughter being thus satisfied, it would be 

 incongruous to bring in the proton as an alternative agent; and we 

 look on the supposed annihilation of electrons by protons as a rather 

 misdirected anticipation of the real cancelling. 



Simultaneously the very long time-scale, which corresponds to the 

 annihilation hypothesis, has lost its attractiveness. The phenomenon 



