( 144 ) 



they ought to be placed in tlie series of evolution remains a riddle. 

 There is a regular continuity in the series F — G—K—M; and accord- 

 ing as we suppose the development to take place in one direction 

 or in the other we find in the transition Gr — K either cooling accom- 

 panied with expansion, or heating accompanied with contraction. The 

 puzzling side of this hypothesis can also be expressed in the follow- 

 ing way : while in the natural development of the celestial bodies, 

 as we conceive it, the temperature has a maximum but the density 

 continuously increases, the values obtained here would according 

 to this interpretation point at a maximum density in the spectral 

 classes F and G. 



In Vol. XI of Astronomy and Astrophysics Maunder has drawn 

 attention to several circumstances, which indicate that the spectral 

 type rather marks a difference in constitution than difference in the 

 stage of development. "There seems to me but one way of recon- 

 ciling all these different circumstances, viz. : to suppose that spectrum 

 type does not primarily or usually denote epoch of stellar life, but 

 rather a fundamental difference of chemical constitution" '). One of 

 the most important of these facts is that the various stars of the 

 Pleiades, which widely differ in brightness and, as they are lying at 

 the same distance from the sun, also in actual volume show yet 

 the same spectrum. The result found here confirms his supposition. 



One might feel inclined to look for a certain relation between 

 these K and M stars and the c stars, which, according to Hertzsprung, 

 have also a much greater luminosity, hence either less density or 

 greater mass than the similar a stars ; and the more so as these c stars 

 reach no further than class XIII. Yet to us this seems improbable; 

 the K stars are numerous, they constitute 20 7o of all the stars, 

 while the c stars are rare. Moreover the spectra of all the K stars 

 are with regard to the relative intensity of the metallic lines perfectly 

 identical with the a stars of preceding classes such as the sun and 

 Capella. Therefore it as yet remains undecided to which other 

 spectra we have to look for other phases in the K star lives and 

 to which spectra for those in the c star lives. The c stars, except a 

 few, are all situated in or near the Milky Way : this characteristic feature 

 they have in common with the Wolf-Rayet stars and also with the 

 4^'' type of SECcm (Vogel's 1116), although these spectra have no lines 

 in common which would suggest any relation between them. 



§ 7. The constitution found here for the Arcturian stars among 

 the third type stars may perhaps be tested by means of other 



1) Stars of the first and second types of spectrum, p. 150. 



