Sentence:  If the nebulosity were due to an <WSD>aggregate</WSD> 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 something other than a star cluster. 
Meaning of the word marked with <WSD> tags:  A mass, assemblage, or sum of particulars; something consisting of elements but considered as a whole.