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CHAPTER XIII 
THE GREAT NEBULAE 
297. It has already been noticed how the “ great nebulae ” form what 
Herschel described as a system of “ island-universes,” distinct and detached 
both from one another and from the galactic system of stars. Hubble has found 
that these nebulae are all of comparable size, being, as fig. 2 (p. 15) has shewn, 
of size comparable with, although smaller than, the galactic system. 
This of itself would encourage the conjecture that the great nebulae may 
be star-clouds, of the same general nature as the cloud of stars surrounding 
the sun. This view of the nature of the great nebulae has been very prevalent 
since the time of the Herschels, and various items of recently gained know 
ledge appear to give it support rather than the reverse. 
Viewed from a fairly remote nebula, our galactic system of stars would 
appear as a cloud of faint light, which telescopes of terrestrial power would 
be unable to resolve into separate stars. Since the average light from these 
stars gives a spectrum of F or 0 type, the composite spectrum of this cloud of 
stars would closely resemble a stellar spectrum of F or G type, and this is 
precisely the type of spectrum shewn by the great nebulae, their spectra 
even being crossed by dark lines of the same general character as the 
Fraunhofer lines in the solar spectrum. 
Viewed from a near nebula, through a telescope of terrestrial power, the 
galactic system would be fairly easily resolved into separate stars, so that if 
the near nebulae are clouds of stars similar to the galactic system, they ought 
to admit of resolution in our telescopes. In actual fact some of them have 
been so resolved, at least in their outermost regions. Plate VII shews a small 
area of the outer region of the Andromeda nebula M 31, photographed with 
the 100-inch telescope, and the resolution into distinct stars can be easily 
seen *. In M 33 (Plate XI) the resolution into separate stars is even easier. 
By resolving such regions into distinct stars, and detecting Cepheid variables 
in them, Hubble has been able to estimate the distances of these nebulae. 
He has further found the nearer nebulae to be of the same general size and 
luminosity as the two Magellanic clouds, and as these latter are quite obviously 
and unmistakably clouds of stars, it would appear reasonable to conjecture that 
the nebulae also may be. Finally, those who maintain that the nebulae are 
merely remote clouds of stars, island universes like our own, can point to the 
fact that they are of the same general shape and build as the galactic system, 
namely, flattened discs with high central condensation. 
Against this, all spiral nebulae, so far as can be j udged from those we see 
* I am indebted to Dr Hubble for preparing and sending me this and many other photographs.