224 CURTIS: MODERN THEORIES OF SPIRAI. NEBUlyAE 



removed from the galactic plane, the presumption that these 

 spirals are themselves galaxies composed of hundreds of millions 

 of stars is a very probable one. 



If, moreover, we make the reasonable assumption that the 

 new stars in the spirals and the new stars in our own galaxy 

 average about the same in size, mass, and absolute brightness, 

 we can form a very good estimate of the probable distance of 

 the spiral nebulae, regarded as island universes. Our galactic 

 novae have averaged about the fifth magnitude. The new 

 stars which have appeared in the spiral nebulae have averaged 

 about the fifteenth magnitude, but it would appear probable 

 that we must inevitably miss the fainter novae in such distant 

 galaxies, and it is perhaps reasonable to assume that the average 

 magnitude of the novae in spirals may be about the eighteenth, 

 or thirteen magnitudes fainter than those in our own galaxy. 

 They would thus be about 160,000 times fainter than our galactic 

 novae, and on the assumption that both types of novae average 

 the same in mass, absolute luminosity, etc., the novae in spirals 

 should be four hundred times further away. We do not know 

 the average distance of the new stars which have appeared in 

 our own galaxy, but 100,000 light-years is perhaps a reasonable 

 estimate. This would indicate a distance of the order of 4,000,000 

 Ught-years for the spiral nebulae. This is an enormous distance, 

 but, if these objects are galaxies like our own stellar system, 

 such a distance accords well with their apparent dimensions. 

 Our own galaxy, at a distance of 10,000,000 Ught-years, would 

 be about 10 minutes of arc in diameter, or the size of the larger 

 spiral nebulae. 



On such a theory, a spiral structure for our own galax}^ would 

 be probable. Its proportions accord well with the degree of 

 flattening observed in the majority of the spirals. We have 

 very little actual evidence as to a spiral structure for our galaxy ; 

 the position of our sun relatively close to the center of figure of the 

 galaxy, and our ignorance of the distances of the remoter stars, 

 renders such evidence very difficult to obtain. A careful study 

 of the configurations and star densities in the Milky Way has 

 led Professor Easton, of Amsterdam, to postulate a spiral struc- 

 ture for our galaxy. 



