DOUn..E STAllS. 209 



stances in which a briUiant white star (1527 Lconis, 1768 

 Can. ven.) is accompanied by a small blue star ; others, where 

 in a double star (6 Serp.) both the principal and its companion 

 are blue.=^ In order to determine whether the contrast of 

 colors is merely subjective, he proposes (when the distance 

 allows) to cover the principal star in the telescope by a thread 

 or diaphragm. Commonly it is only the smaller star that 

 is blue : this, however, is not the case in the double star 23 

 Orionis (GOG in Struve's Catalogue, p. Ixxx.), where the prin- 

 cipal star is bluish, and the companion pure white. If, in 

 the multiple stars, the difierently colored suns are frequently 

 surrounded by planets invisible to us, the latter, being difTer- 

 ently illuminated, must have their white, blue, red, and green 

 days.f 



As the 'periodical vai'iahilityX of the stars is, as we have 

 already pointed out, by no means necessarily connected with 

 their red or reddish color, so also coloring in general, or a 

 contrasting difference of the tones of color between the prin- 

 cipal star and its companion, is far from being peculiar to 

 the multiple stars. Circumstances which we find to be fre- 

 quent are not, on that account, necessary conditions of the 

 phenomena, whether relating to a periodical change of light, 

 or to the revolution in partial systems round a common cen- 

 ter of gravity. A careful examination of the bright double 

 stars (aiid color can be determined even in those of the ninth 

 magnitude) teaches that, besides white, all the colors of the 

 solar spectrum are to be found in the double stars, but that 

 the principal star, whenever it is not white, approximates in 

 general to the red extreme (that of the least refrangible rays), 

 but the companion to the violet extreme (the limit of the 

 most refrangible rays). The reddish stars are twice as fre- 

 quent as the blue and bluish ; the white are about 2\ times 

 as numerous as the red and reddish. It is moreover remark- 

 able that a great difference of color is usually associated with 



oped by the action of the accompanying star, which is generally much 

 the more brilliant of the two." (Arago, in the Annuaire pour 1834, p. 

 295-301.) 



* Struve, Ueber Doppelsterne nach Dorpater Bcobachtungen, 1837, s. 

 33-36, and Mensurce Microm., p. Ixxxiii., enumerates sixty-thi-ee double 

 stars in which both the principal and companion are blue or bluish, and 

 in which, therefore, the colors can not be the effect of contrast. When 

 we are forced to compare together the colors of double stars, as report- 

 ed by several astronomers, it is particularly striking to observe how fre- 

 quently the companion of a red or orange-colored star is reported by 

 some observers as blue, and by others as green. 



t Arago, Annuaire pour 1834, p. 302. t Vide supra, p. 130-136. 



