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stars or the so-called fifth type stars (Vogel II/>), it is obvious that 

 we must place these stars at the head of the series, as it has also 

 been done by Miss CannoiN in her investigation of the southern 

 spectra (H. C. 0. Ann. Bd. 28) '). 



Some of these stars show a relative intensity of the metallic 

 lines ditFerent from that of the ordinary stellar spectra ; Vogel and 

 ScHEiNER have found this before in a Cygni and a Persei (Public. 

 Potsdam Bd. 7, part 2). Maury found representatives of this group 

 in almost all the classes from III to XIII, and classed them in a 

 parallel series designated by IIIc — XIII c, in contradistinction to 

 which the great majority are called a stars. 



According to the most widely spread oi)inion a star goes succes- 

 sively through all these progressive stages of development. It com- 

 mences as an extremely tenuous mass of gas which grows hotter by 

 contraction, and after having reached a maximurh temperature de- 

 creases in temperature while the contraction goes on. Before the 

 maximum temperature is reached, there is a maximum emission of 

 light; past the maximum temperature the brightness rapidly decreases 

 owing to the joint causes: fall of temperature and decrease in volume. 

 That the first type stars are hotter than the stars of the second type 

 may be taken for certain on the strength of their white colour ; 

 wheth-^r the maximum temperature occurs here or in the Orion 

 stars is however uncertain. 



This development of a tenuous mass of gas into a dense and cold 

 body, of which the temperature first increases and then decreases is 

 in harmony with the laws of physics. In how far, however, the 

 different spectral types correspond to the phases of this evolution is 

 a mere hypothesis, a more or less probable conjecture; for an actual 

 transition of a star from one type into the other has not yet been 



1) According to Campbell's results (Astronomy and Astrophysics XIII, p. 448), 

 the characteristic lines of the Wolf-Rayet stars must be distinguished in two groups 

 and according to the relative intensity of the two groups these stars must 

 be arranged in a progressive series. One group consists of the first secondary 

 series and the first line of the principal series of hydrogen : H/3' 5414, Hy' 4542, 

 Uy .4201, principal line 4G8G) ; it is that group wliich in Maury's classes I— III 

 occurs as dark lines and vanishes and which in the classes towards the other 

 side (class Oe — Ob Gannon) is together with the ordinary H lines more and more 

 reversed into emission lines. The other group, which as compared with the 

 hydrogen lines becomes gradually stronger from this point, consists of broad 

 bands of unknown origin of which the middle portions according to Cannon's 

 measurements of yVelorum have the wavelengths 5807, 5G92, 5594, 5470, 4654, 

 4443. The brightest hand is 4654; its relative intensity as compared willi the 

 H Hne 4689 gradually increases in the series: 4, 47, 5, 48, 42 (Campbell's 

 star numbers). 



