THE LIGHT OF THE STARS 383 



and that the faint stars are not necessarily the most distant. If the 

 number of stars were infinite, and distributed according to the laws of 

 chance throughout infinite and empty space, the background of the 

 sky would be as bright as the surface of the sun. This is far from 

 being the case. While we can thus draw general conclusions, but little 

 definite information can be obtained, without accurate quantitative 

 measures, and this is one of the greatest objects of stellar photometry. 

 If we consider two spheres, with the sun as the common centre, and 

 one having ten times the radius of the other, the volume of the first 

 will be one thousand times as great as that of the second. It will, 

 therefore, contain a thousand times as many stars. But the most dis- 

 tant stars in the first sphere would be ten times as far off as those 

 in the second sphere, and accordingly if equally bright would appear 

 to have only one one-hundredth part of the apparent brightness. Ex- 

 pressed in stellar magnitudes, they would be five magnitudes fainter. 

 In reality, the total number of stars of the fifth magnitude and 

 brighter is about 1500, of the tenth magnitude, 373,000, instead of 

 1 ,500,000, as we should expect. An absorbing medium in space, which 

 would dim the light of the more distant stars, is a possible explana- 

 tion, but this hypothesis does not agree with the actual figures. An 

 examination of the number of adjacent stars shows that it is far in 

 excess of what would be expected if the stars were distributed by 

 chance. Of the three thousand double stars in the Mensurae Micro- 

 metricse, the number of stars optically double, or of those which hap- 

 pen to be in line, according to the theory of probabilities, is only 

 about forty. This fact should be recognized in any conclusions re- 

 garding the motions of the fixed stars, based upon measures of their 

 position with regard to adjacent bright stars. 



We have here neglected all conclusions based upon the difference 

 in composition of different stars. Photographs of their spectra fur- 

 nish the material for studying this problem in detail. About half of 

 the stars have spectra in which the broad hydrogen lines are the dis- 

 tinguishing feature. They are of the first type, and belong to Class A 

 of the classification of the Henry Draper Memorial. The Milky Way 

 consists so completely of such stars, that if they were removed, it 

 would not be visible. The Orion stars, forming Class B, a subdivision 

 of the first type in which the lines of helium are present, are still more 

 markedly concentrated in the Milky Way. A large part of the other 

 stars, forming one third of the whole, have spectra closely resembling 

 that of the sun. They are of the second type, and form classes G and 

 K. These stars are distributed nearly uniformly in all parts of the 

 sky. Class M, the third type, follows the same law. Class F, whose 

 spectrum is intermediate between classes A and G, follows the same 

 law of distribution as classes G and K, but differs from them, if at all, 

 in the opposite direction from Class A. There therefore seem to be 



