ORIGIN OF THE SOLAR SYSTEM JEANS 147 



Probably we ought not to regard the two possibilities just men- 

 tioned as sharply cut alternatives. It is more likely that they repre- 

 sent the two extreme ends of a continuous chain of possible histories 

 for the family of stars born out of a single nebula. It seems quite 

 possible that what we describe as "the main mass of the stars" may 

 be nothing more than a collection of clusters of stars, each cluster 

 having originated out of a single nebula. The clusters are by now so 

 intermingled that it is difficult to look on them as distinct groups 

 of stars, although we can still find some evidence that this may be 

 the proper way of regarding them. In 1905 Kapteyn showed that 

 the stars in the neighborhood of the sun formed what he described 

 as two " star streams," each stream moving with its own velocity in 

 space. Except that it begs the question as to the extent of these 

 streams in space, it would have been equally accurate to describe 

 them as forming two intermingled moving clusters. Shortly after. 

 Eddington and Halm, independently, found a third stream or mov- 

 ing cluster, constituted of the very hot stars which the astronomer 

 classifies as stars of types B and O. In this case we know the extent 

 of the cluster in space and also its approximate shape. According 

 to Charlier, it is shaped like a round biscuit lying parallel t© the 

 Milky Way, its diameter being about 2.8 times its thiclaiess. Any 

 cluster of stars having a common origin, whatever shape it may as- 

 sume at first, will be rapidly knocked out of shape when it begins to 

 intermingle with other stars. Dynamical theory shows that after 

 it has been knocked about ad infinitum in our universe of stars, such 

 a cluster ought to assume the shape of a round biscuit parallel to 

 the Milky Way, the ratio of its diameter to its thickness being about 

 2.5. This agrees sufficiently well with what is observed to suggest 

 that all the stars in this stream have a common origin, and the same 

 is true of many of the smaller known moving clusters, such as the 

 Ursa Major cluster already mentioned. Thus, although we can not 

 claim that anything is definitely proved, there is every justification 

 for thinking of the main mass of the stars as a jumble of inter- 

 mingled moving clusters, each cluster owing its existence to a sepa- 

 rate nebula. This possibility has no very direct bearing on the ques- 

 tion of the origin of our solar system ; it has been mentioned merely 

 as rounding off our knowledge of what appears to be the main evolu- 

 tionary process of the stars. 



In all its essentials except one, this evolutionary process is similar 

 to, and in its earlier stages almost identical with, that which Laplace, 

 in his famous nebular hypothesis, imagined as the origin of the 

 solar system. We have seen before our eyes the rotating and shrink- 

 ing nebula finally shedding matter from its equator; we have 

 watched the condensation of this matter into separate masses, and 



