150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



We now come to the crux of the whole question. Nowhere on this 

 course have we found our solar system or anything in the least degree 

 resembling it. If our sun had been unattended by planets we should 

 have had no difficulty in guessing its origin. It might reasonably 

 be supposed to have been born out of a nebula in the normal way, 

 but to have emerged with insufficient rotation to have carried it on 

 to the later stages of fission into a binary or a multiple system. It 

 might, in fact, be supposed to have had the same evolutionary career 

 as half of the stars in the sky. In support of the conjecture that 

 our sun had been born out of a nebula in the ordinary way, we could 

 note that its mass is about equal to what we calculate ought to be the 

 mass of a star born out of a nebula, and that it is, apart from its 

 planets, similar in every way to millions of other stars to which we 

 may ascribe a nebular origin. In support of the conjecture that it 

 had stopped short on its evolutionary course from want of adequate 

 rotation to carry it on further, we should merely have to note the 

 slowness of its present rotation. A simple calculation shows that 

 the sun has only a small fraction of the amount of angular mo- 

 mentum requisite for fission. Even if we add the angular mo- 

 mentum of all the planets, as we ought if we suppose that these at 

 one time formed part of the sun, the result is the same — the whole 

 system can never have had more than a fraction of the angular mo- 

 mentum necessary for a rotational break-up into a binary star. 



Thus the sun is a quite intelligible structure. The difficulty of 

 our problem is not the origin of the sun but the origin of the planets 

 and of their satellites. 



Certain special types of astronomical structure have already been 

 mentioned as not falling into place on the main line of evolutionary 

 development. The particular examples chosen Avere the planetary 

 nebulae, the Cepheid variables, and the long-period variables. The 

 question now arises as to whether we must add the solar system to the 

 list. The circumstance that certain structures do not find a place in 

 the evolutionary main line suggests that off this main line may be 

 branch lines on to which the development of a system may in cer- 

 tain circumstances be turned. This, indeed, is only what might be 

 anticipated. We should no more expect two stars to have precisely 

 the same experiences in their careers than we should expect it of two 

 humans. Our normal star has been supposed to develop in a uni- 

 verse of its own, where its angular momentum remained constant 

 and where it was in every way unmolested by its neighbors. The 

 mathematician finds it convenient to allot a whole infinite universe to 

 each star, but nature does not. Nevertheless, the conditions postu- 

 lated by the mathematician are nearer to the truth than is often the 

 case in his idealized problems. On the scale we have already used, on 

 which the sun was represented by a microscopic particle ys^d?^ i^ch 



