CONSTITUTION OF THE STARS — HUSSELL 155 



influence of a passing star, the sun itself must have been already 

 formed at that remote epoch. 



But we may go further. Life already existed on the earth in 

 Cambrian times, and it is a moderate estimate to say that the process 

 of organic evolution has lasted for a billion years. During all this 

 time the sun can never have been one stellar magnitude brighter 

 or fainter than it is now; for in the first case, its heat would have 

 raised the oceans to the boiling point, and, in the second, they would 

 have frozen solid — and either of these catastrophes would have put 

 an end to evolution and to all terrestrial life. Now the sun is a 

 typical dwarf star, and there is good reason to believe that it is now 

 well advanced in cooling and was once much brighter and hotter 

 than it is now — of class F, at least, though perhaps not of class A. 

 At such a time it must have been at least two magnitudes brighter 

 than at present. Yet in the whole of geological time it has probably 

 decreased half a magnitude or less. We may, therefore, say, with 

 considerable confidence, that the life of the sun, and doubtless also 

 of the stars in general, must extend over many billions of years. 



But here we meet with a serious difficulty. We know the rate 

 at which the sun is radiating energy to the earth, and, from con- 

 sideration of the way in which the earth in turn radiates this energy 

 into space, we can be sure that the sun is also sending out an equal 

 amount of heat into space in every direction. The total output is 

 so great that it would exhaust the whole huge fund of energy which 

 would be made available by the sun's contraction from an indefinitely 

 extended size, in about twenty million years, as Lord Kelvin showed 

 long ago. When we allow for the fact that some of this heat is 

 still stored in the sun's interior, and that it was probably much 

 brighter in its earlier stages of evolution than at present, we see 

 that, if gravitational energy alone was available as the source of its 

 radiation, the sun's past life as a star must have occupied but a very 

 few million years. In view of the geological and radioactive evi- 

 dence, there seems to be no escape from the conclusion that the sun 

 must have some other, and far greater, store of internal energy upon 

 which to draw. 



Further evidence in favor of this view has been found by Edding- 

 ton in the behavior of the star Delta Cephei. This is a typical giant 

 star, about eight hundred times as bright as the sun. Eddington has 

 given good reason to believe that the cause of its variation in light 

 is a periodic expansion and contraction of the whole star by about 

 10 per cent on each side of the mean. The period of this change 

 would depend on the density of the star, and diminish if this in- 

 creased. Hence, if the mean diameter was gradually contracting, 

 the period should shorten. Eddington calculates that, if the radia- 

 tion is supplied by gravitational contraction alone, the period should 



