1907-1908.] Nebulce and Nebular Hypotheses. 7 5 



ance of round discs, like planets on a gigantic scale. In 

 reality the discs are generally elliptical, and they give one 

 the impression that they represent objects consisting of several 

 concentric shells, — as if they had been formed by successive 

 outpourings of matter from a centre. Such nebulae are green 

 in colour and gaseous in constitution, so that they belong to 

 the same category as the irregular class. 



When nebulae were first discovered, it was imagined that 

 they might be external universes — systems of stars like our 

 own, but lying far beyond the confines of our own system, 

 and having no connection with it. But, as a matter of fact, 

 the stars and the nebulae form complementary parts of what 

 is essentially one system, and nothing shows this more clearly 

 than their relative distribution on the sky. Like all sidereal 

 objects, nebulae are dominated as regards their general distri- 

 bution by the influence of the Milky Way. Green nebulae, 

 as a rule, lie in the Galaxy ; white nebulae avoid it, and con- 

 gregate at its poles. Locally, too, nebulae are generally found 

 where stars are few in number, — often, indeed, a nebula 

 appears to have cleared a space for itself among the surround- 

 ing stars. And, strongest proof of all, nebulae and stars lie at 

 similar distances from us, since in very many cases photo- 

 graphs show the stars to be actually entangled in the nebul- 

 osity. Such photographs, too, seem to show the literal growth 

 of stars from nebulae. The argument is that no nebula can 

 be perfectly homogeneous. Aggregations of matter must occur 

 here and there throughout the mass ; and these, once formed, 

 will persist and will act as centres of attraction, slowly but 

 surely incorporating the surrounding nebulous matter, and 

 emerging at last as finished stars. 



Now many stars are clearly nebulous, and many nebulae are 

 obviously flocculent. They contain knots or nuclei more or 

 less advanced in consolidation ; and the inference is that year 

 by year those nuclei will grow at the expense of the argent 

 haze in which they are now embedded, until at last they 

 decisively predominate, and the object becomes more stellar 

 than nebular, developing finally into a star cluster. What 

 was considered to be an exquisite example of this stage is 

 afforded by the cluster of stars known technically as the 

 Pleiades, and popularly as the Seven Sisters. To the unaided 



