May 25, 1917] 



SCIENCE 



541 



Fig. 30. The Spectrum of Alcyone, photographed by Wright with the Mills Spectrograph of the Lick 



Observatory. 

 [The star spectrum occupies the central horizontal section. The bright hydrogen Alplia line is 

 one fourth inch fi-om the right end, and the dark hydrogen Beta line is one half inch from the left 

 end.] 



seems eiitirelj^ too great to let us conclude 

 that the blue stars in general could have 

 been formed from planetary nebulas such as 

 the 150 planetaries now known to us. 



For the hypothesis that the stars in gen- 

 eral have evolved from irregular nebulas a 

 vastly stronger case can be made. 



In a priceless possession of astronomy, 

 the Draper catalogue of stellar spectra. 

 Harvard College Observatorj' has classified 

 the spectra of a great many thousands of the 

 brighter stars. They have been arranged in 

 a sequence, running from the so-called ex- 

 tremely blue stars through the yellow stars 

 to the red stars, which can be readily de- 

 scribed. The main divisions are illustrated 

 in Fig. 29. Each main class has ten subdi- 

 visions, but we need not dwell upon details. 

 The dark lines in these spectra indicate the 

 presence of certain vapors and gases of the 

 chemical elements in the outer strata of the 

 stars. In the Class B stars the helium lines 

 rise to their maximum strength near the 

 middle of the B subgroups and sink to in- 

 significance in the later B or earlier A sub- 

 divisions, and the helium lines are not 

 found at all in Class F and the later types. 

 The hydrogen lines are fairly prominent in 

 the Class B stars, but they increase to maxi- 

 mum intensity in Class A, and then de- 



crease continuously throughout the remain- 

 ing groups. The hydrogen lines are very 

 inconspicuous in Class M, or red stars. 

 The magnesium lines go up to a maximum 

 in Class A and down to disappearance in 

 the F's and G's. Some of the metallic 

 lines, such as titanium and iron, usually 

 begin to show in the later subdivisions of 

 Class A, other metallic lines first enter upon 

 the scene in Class F, and thej^ increase in 

 numbers and prominence up to a maximum 

 in the red stars. The calcium lines are 

 weak or entirely wanting in the Class B's, 

 and they increase in intensity as we pass 

 down through the series, until they become 

 the most prominent features in the spectra. 

 We can not change the arrangement of 

 these spectral classes without throwing the 

 sequence of development of the spectral 

 lines into hopeless disorder. This sequence 

 is thought- to represent the order of stellar 

 evolution. A Class B star, according to 

 that hypothesis, is a comparatively young- 

 star. It should develop, in the course of 

 long ages, into an F, a G, a K, an 31, and so 

 on, to its final destination of darkness and 

 invisibility. 



Some of the Class B stars contain bright 

 lines of hydrogen. Alcyone (Fig. 30) and 

 Pleione, in the Pleiades, contain both bright 



Fig. 31. The Spectrum of Eta Carinje, photographed by Moore at the D. 0. Mills Oljserva.tory Santi- 

 ago, Chile. 

 [The series of bright lines above and below the star spectrum are the reference spectrum of iron.] 



