RELATIONS BETWEEN THE SPECTRA AND OTHER 

 CHARACTERISTICS OF THE STARS. 



By henry NORRIS RUSSELL. 

 {Read April 20, 19 12.) 



To the student of the stars, who attempts to arrange our exist- 

 ing knowledge in such a manner that some hght may be thrown 

 upon the problems connected with stellar evolution, the spectral 

 classification developed at Harvard is of vital importance. 



In such investigations, we must deal, if possible, not with single 

 instances, but with representative averages for groups of stars. 

 But really, representative averages are often much harder to obtain 

 than might be supposed. Consider, for example, the actual bright- 

 ness of the stars. We can find this only when we know the dis- 

 tance of the star — and out of the hundreds of thousands of stars 

 which have been catalogued, we know the distance of barely five 

 hundred. But even if we knew the exact distances of the 6,000 or 

 more stars which are visible to the naked eye, we would not have 

 a fair sample of the general run of stars. To explain how this may 

 happen, let us suppose that there were only two kinds of stars, 

 one equal to the sun in brightness, and the other 100 times as 

 bright as the sun, and that these were distributed uniformly 

 through space, in the proportion of 100 stars of the fainter kind for 

 every one of the brighter. To be visible to the naked eye, a star 

 of the fainter sort must lie within about 55 light-years from the 

 sun ; but all the stars of the brighter kind which lay within 550 

 light-years would be visible. We would therefore be searching for 

 these stars throughout a region of space whose volume was 1,000 

 times greater than that to which our method of selection limited us 

 in picking out the fainter ones, and our list of naked-eye stars 

 would consequently contain ten stars of the brighter kind to every 

 one of the fainter — though if we could select instead the stars 



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