ON THE STRUCTURE OF THE UNIVERSE 



5 



F, G, K, M, N. The spectra of stars of class B are characterised 

 by strong helium lines, and those of class A by strong hydrogen 

 lines, other lines being comparatively weak. In classes F, G, K 

 the hydrogen lines become progressively weaker and the metallic 

 lines more and more prominent. Stars of class G have spectra 

 similar to our sun, which is itself a G star, and which contain 

 numerous fine metallic lines. In classes M, N flutings due to 

 carbon compounds make their appearance. The stars of early 

 type are supposed to be the hottest and gradually to cool as 

 they get older. 



The spectral types of an immense number of stars have been 

 determined at Harvard, and Prof. Pickering has tabulated the 

 numbers of different types, brighter than the visual limit 6*5 m., 

 in eight zones of galactic latitude, chosen so as all to have 

 practically an equal area. His results are given in Table II., 

 in which in each column after the first are given the number of 

 stars of the type at the head of the column, for the eight zones 

 whose average galactic latitude is given in the first column. 



Table II 



If the stars of each type were distributed uniformly over the 

 sky, the numbers in each vertical column should be approximately 

 equal. It will be seen that in no case is this so, and that whilst 

 the stars of type B exhibit a very strongly marked crowding 

 towards the galaxy, the distribution in the various zones becomes 

 more uniform with successive spectral types, until with class M 

 there is little or no indication that the density in the galaxy is 

 greater than that at the galactic poles. The most remarkable 

 concentration is, however, shown by a small class of stars known 

 as Wolf-Rayet stars, whose spectra apparently indicate a transi- 

 tion stage between nebulae and B-type stars, and which therefore 



