CHAPTER X 



THE EVOLUTION OF STAR-CLUSTERS 



218. From a purely theoretical discussion of the evolution of a mass of 

 rotating gas we have been led to the hypothesis that the spiral nebulae are 

 merely masses of rotating gas which have reached a stage of disintegration, 

 the rotation having become so great through shrinkage that configurations of 

 equilibrium are no longer possible. It would be of the utmost interest to 

 follow out dynamically the different processes of this disintegration but un- 

 fortunately the mathematical difficulties have so far proved to be too great. 



We have, however, found that the masses of these spirals must be supposed 

 to be enormously greater than that of our sun, and the general nature of the 

 disintegration has been seen to consist of the formation in the nebular arms 

 of condensing nuclei each of mass just about comparable with that of our 

 sun. Thus the hypothesis which has already been adopted seems to lead 

 irresistibly to the conclusion that the final result of the process of disintegra- 

 tion which we see going on in the spiral nebulae must be the formation of 

 star-clusters. 



As to the features to be expected in these final star-clusters our dynamical 

 analysis has so far told us almost nothing. It seems not unreasonable to expect 

 that the star-clusters will be of the type we have described as " globular " 

 thus we may conjecture that the observed spiral nebulae are forming star- 

 clusters similar to observed globular star-clusters and that the observed 

 globular clusters have originated out of spiral nebulae. 



For the present we shall regard this conjecture merely as a hypothesis 

 whose truth is to be tested. The hypothesis commits us to what is com- 

 monly called the "island universe" theory just as the spiral nebulae are 

 distinct independent objects in space, so the star-clusters formed from them 

 may be expected to be distinct independent objects in space. The " island 

 universe " theory need not, however, be accepted in any extreme or categorical 

 form. Twin nebulae are known to exist in the sky, and these may easily be 

 imagined to form overlapping or intermingling star-clusters. Thus the " island 

 universes " may overlap or intermingle one " island " may be entirely enclosed 

 inside another and larger " island." These, however, will be exceptional phe- 

 nomena ; normally we regard the stars as falling into detached clusters or 

 separate " island universes." The particular cluster of which our sun is a 

 member will be spoken of as the galactic universe. 



