THE LIGHT OF THE STARS. 55 



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 explanation, but this hypothesis 

 does not agree with the actual figures. An examination of the num- 

 ber 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 thou- 

 sand double stars in the ' Mensuras Micrometrica?/ the number of stars 

 optically double, or of those which happen to be in line, according to 

 the theory of probabilities, is only about forty. This fact should be 

 recognized in any conclusions regarding 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 furnish 

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

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

 guishing 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 mark- 

 edly 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 C 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 spec- 

 trum is intermediate between classes A and C, 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 actu- 

 ally fewer of these stars in the Milky Way than outside of it. One 

 class of stars, the fifth type, class O, has a very remarkable spectrum 

 and distribution. A large part of the light is monochromatic. Of 

 the ninety-six stars of this type so far discovered, twenty-one are in the 

 Large Magellanic Cloud, one in the Small Magellanic Cloud, and the 

 remainder follow the central line of the Milky Way so closely that the 

 average distance from it is only two degrees. All of these stars, with 

 the exception of sixteen, have been found by means of the Henry 

 Draper Memorial. 



It will be seen from the above discussion that stellar photometry 

 in its broadest sense furnishes the means of attacking, and perhaps of 

 solving, the greatest problem presented to the mind of man — the struc- 

 ture and constitution of the stellar universe, of which the solar sys- 

 tem itself is but a minute and insignificant molecule. 



