304 FROM NEBULA TO NEBULA 



gases were confined and from under which they all the 

 more violently escape. There is, then, an arbitrary limit 

 imposed by Nature beyond which no star can grow, how- 

 soever favorably it may be situated for further growth, 

 and howsoever stout may be its armor against cometary 

 missiles from without. Here, in this simple explana- 

 tion, we have harmoniously coordinated the following 

 important considerations: (1) How the greater nebulae 

 are formed; (2) Why stars, though differing greatly 

 in mass, yet observe a certain order of size, like trees 

 and animals; (3) Why there is not and can never be a 

 great " central sun" ; (4) The dispersive force by which 

 the destructive centralizing tendency of gravitation is 

 counteracted and the balance of Nature automatically 

 maintained. Just how large a star may become is 

 another matter. Even this problem, however, is sus- 

 ceptible of mathematical determination within reasonable 

 approximation, though we cannot stop to deal with this 

 complex problem now. In my opinion Doctor Campbell 

 greatly overestimates the possibilities, when he says 

 (Stellar Motions, p. 158) : 



The parallax of Canopus can scarcely exceed .01 or .02 of 

 a second as a maximum. We can scarcely doubt that Canopus is 

 radiating certainly 1000 and perhaps 100,000 times as much light 

 as the Sun. If the effective radiating power of its surface equals 

 that of the Sun, the surface must be fully 1000 times as great as 

 the Sun's. Its corresponding volume would be 31,000 solar 

 volumes. Its mass must greatly exceed the Sun's mass, probably 

 between 1000 and 30,000 fold. 



When stars of very large size, and of precarious 

 structure because of their relatively thin shell, are sub- 

 ject to the stress of an especially powerful stellar re- 

 sultant of short stem (a condition which occurs when 

 many stars are in close proximity), they are forced to 

 rotate on their axes with extreme rapidity, enough so 

 at times as to flatten them out sufficiently thin to cause 

 their poles to coalesce, and so to bring into being two 

 stars of smaller size. This phenomenon is called star- 

 fission and, though rare, is well-authenticated by the 

 observers. In the majority of cases, it appears, this 



