RECENT ADVANCES IN SCIENCE 551 



Distances of Spiral Nebula. — The recent discovery of a 

 " nova " or new star in the nebula N.G.C. 6946 started a 

 search at the Mt. Wilson and Lick Observatories to ascertain 

 whether other novae in spiral nebulae could not be found by 

 comparing old negatives of the nebulae with others of more 

 recent date. The result has been that several other instances 

 of novae occurring in spiral nebulae have been found, whilst 

 previous to the discovery of the nova on N.G.C. 6946, only two 

 instances were known. There seems to be little doubt that 

 these novae are actually associated with the spirals in whose 

 photographs they have been detected, and that they are not 

 seen simply projected upon the nebulae, for the novae un- 

 connected with spirals only occur close to the galaxy, whereas 

 spiral nebulae are most frequent away from the galaxy. This 

 fact has been used by H. Shapley {Pub. Ast. Soc. Pac. 39, 213, 

 191 7) and H. D. Curtis {Lick Obs. Bull. No. 300) to derive 

 estimates as to the distances and dimensions of the nebulae. 

 A comparison of the mean apparent magnitudes of the novae 

 associated with nebulae and those associated with our galactic 

 system shows that the former are about io m fainter ; assum- 

 ing that both classes of novae are similar objects, and therefore 

 equally bright on the average, the spiral nebulae in which novae 

 have been observed (probably the nearer spirals) are about 

 100 times as distant as the galactic novae. The latter are 

 probably several times as distant as B-type stars of the same 

 magnitude, whence it follows that the distance of the spirals 

 is of the order of not less than 100,000 light-years. This 

 great distance is confirmed by other reasoning : the galactic 

 novae probably increase in brightness about i6 m , so that 

 the novae in spirals would be about 30 m before their outburst. 

 The stars of i$ m in our galaxy are about 20,000 light-years 

 away, and to appear of 30 111 would have to be removed to 

 a distance of about 20 million light-years. Assuming the 

 two classes of stars to be of the same type, this will be the 

 distance of the novae . Moreover, since even the most powerful 

 telescopes do not resolve the spirals into discreet stars, all the 

 stars in them must be fainter than 2i m , so that, as a lower 

 limit to their distance, we again deduce several million light- 

 years . 



This evidence seems conclusive that such nebulae are other 

 universes, quite separate from our own stellar universe. A 



