52 F. W. Very — A Cosmic Cycle. 



in the spectrum of the solar star is very much darker on 

 account of the multitude and intensity of the Fraunhofer lines, 

 yet the narrow breadths of unabsorbed spectrum between the 

 lines are brighter in Capella than in Vega. In the extreme 

 ultra-violet the energy in the spectrum of the Sirian star every- 

 where prevails, but here it is probable that the general absorp- 

 tion of the short waves by the atmosphere of the solar star has 

 increased at a more rapid, rate, so that even the intervals 

 between the absorption lines are obscure. The comparison in 

 the ultra-violet, to be of any value, must be made beyond the 

 broad band of diffuse absorption in the spectrum of the Sirian 

 stars (0'35/>{ to 0*37//), and since the general absorption of the 

 earth's atmosphere is here both large and variable, the result 

 might be of doubtful interpretation. 



As the general absorption becomes small in the yellow, we 

 may anticipate that the interlinear supremacy of the solar star 

 will be found still greater in this part of the spectrum, and the 

 test is worth trying with iso-chromatic plates. Of course it 

 will be understood that the average brilliancy of the spectrum 

 of a solar star through a considerable range, without regard to 

 minor fluctuations, is relatively greater towards the red than 

 in the violet, because of the smaller number and intensity of 

 red absorption lines, and this seems to be even more the case 

 in the infra-red. 



Professor E. F. Nichols has determined the ratio of the total 

 radiation from Arcturus and Vega as 2*2.* There is no such 

 difference of brightness in these two stars. Their visual inten- 

 sities scarcely differ by 0*1 magnitude, while in the ultra-violet 

 Vega is the more intense. Consequently the total radiant 

 superiority of Arcturus means that it has not only absolutely, 

 but also relatively, a greater intensity than Vega in the infra- 

 red, and that the maximum energy in the spectrum of Arctu- 

 rus is of a greater wave-length than Vegans maximum. This 

 proves that the effective surface-temperature of Vega is the 

 greater. The meaning of this may be open to various inter- 

 pretations. It is possible that all photospheric surfaces are at 

 substantially the same temperature, the position of the photo- 

 sphere being determined by the level at which this temperature 

 is attained, and that variations of intrinsic brightness or of 

 distribution of energy in the spectrum are entirely due to 

 absorption by the stellar atmospheres. In this view the photo- 

 sphere of a solar star is situated at a greater depth below the 

 surface than in a Sirian star, and the preponderance of selec- 

 tive (line) absorption for the violet and ultra-violet rays by the 

 solar atmosphere, as well as the greater general obstruction to 



* Astrophysical Journal, voL xiii, p. 135, 1901. 



