CURTIS: MODERN THEORIES OF SPIRAL NEBULAE 223 



that this pecuUar grouping is only apparent, and due to some 

 phenomenon in our own galaxy. This point will be reverted to 

 later. 



It has been shown that the factors of space- velocity and space- 

 distribution separate the spirals very clearly from the stars of 

 our galaxy; from these facts alone, and from the evidence of the 

 spectroscope, the island universe theory is given a certain meas- 

 ure of credibility. 



Another Une of evidence has been developed within the past 

 two years, which adds further support to the island-universe 

 theory of the spiral nebulae. 



NEW STARS 



Within historical times some twenty-seven new stars have 

 suddenly flashed out in the heavens. Some have been of 

 interest only to the astronomer; others, like that of last June, 

 have rivaled Sirius in brilliancy. All have shown the same 

 general history, suddenly increasing in light ten thousand-fold 

 or more, and then gradually, but still relatively rapidly, sinking 

 into obscurity again. They are a very interesting class, nor 

 has astronomy as yet been able to give any universally accepted 

 explanation of these anomalous objects. Two of these novae 

 had appeared in spiral nebulae, but this fact had not been weighed 

 at its true value. Within the past two years over a dozen novae 

 have been found in spiral nebulae, all of them very faint, ranging 

 from about the fourteenth to the nineteenth magnitudes at 

 maximum. Their life history, so far as we can tell from such 

 faint objects, appears to be identical with that of the brighter 

 novae. Now the brighter novae of the past, that is, those which 

 have not appeared in spirals, have almost invariably been a 

 galactic phenomenon, located in or close to our Milky Way, 

 and they have very evidently been a part of our own stellar 

 system. The cogency of the argument will, I think, be ap- 

 parent to all, although the strong analogy is by no means a 

 rigid proof. If twenty-seven novae have appeared in our own 

 galaxy within the past three hundred years, and if about half 

 that number are found within a few years in spiral nebulae far 



