338 A Short History of Astronomy [Cn. xn 



These facts suggested obviously the inference that the 

 difference between nebulae and star clusters was merely a 

 question of the power of the telescope employed, and accord- 

 ingly Herschel's next sentence is : 



" This tends to confirm the hypothesis that all are composed 

 of stars more or less remote." 



The idea was not new, having at any rate been suggested, 

 rather on speculative than on scientific grounds, in 1755 

 by Kant, who had further suggested that a single nebula 

 or star cluster is an assemblage of stars comparable in 

 magnitude and structure with the whole of those which 

 constitute the Milky Way and the other separate stars which 

 we see. From this point of view the sun is one star in a 

 cluster, and every nebula which we see is a system of the 

 same order. This " island universe " theory of nebulae, as 

 it has been called, was also at first accepted by Herschel, 

 so that he was able once to tell Miss Burney that he had 

 discovered 1,500 new universes. 



Herschel, however, was one of those investigators who 

 hold theories lightly, and as early as 1791 further observa- 

 tion had convinced him that these views were untenable, 

 and that some nebulae at least were essentially distinct from 

 star clusters. The particular object which he quotes in 

 support of his change of view was a certain nebulous star 

 that is, a body resembling an ordinary star but surrounded 

 by a circular halo gradually diminishing in brightness. 



" Cast your eye," he says, " on this cloudy star, and the 

 result will be no less decisive. . . . Your judgement, I may 

 venture to say, will be, that the nebulosity about the star is not 

 of a starry nature" 



If the nebulosity were due to an aggregate of stars so 

 far off as to be separately indistinguishable, then the central 

 body would have to be a star of almost incomparably greater 

 dimensions than an ordinary star; if, on the other hand, 

 the central body were of dimensions comparable with those 

 of an ordinary star, the nebulosity must be due to some- 

 thing other than a star cluster. In either case the object 

 presented features markedly different from those of a star 

 cluster of the recognised kind ; and of the two alternative 



