NATURAL SCIENCES OF PHILADELPHIA. 159 



Additions to the Catalogue of STARS which have changed their colors.* 



BY JACOB ENNIS. 



17. Spica. This star was described by Humboldt in 1850, as being" decidedly 

 white.'' Cosmos, vol. iii. p. 181, Donati also arranges Spica among the white 

 stars. It is now conspicuously blue, and has been observed of this color by 

 myself and several friends since early in March. Inadvertently, in my commu- 

 nication of March in these Proceedings, I mentioned this star as Beta Librse, 

 which is in the near vicinity, but without speaking of a change of color. 



18. Altair. Humboldt, in 1850, classes this star along with three others 

 which he says " have a more or less decidedly yellow light." Donati also ar- 

 ranges it among the yellow stars. Altair is now, June, 1863, plainly blue. 



19. Deneb, or Alpha Cygni. " Decidedly white," are the words employed 

 by Humboldt to register this star in 1850. At present it is decidedly blue, and 

 it has been of this color since early in May. 



Vega. This star is placed in No. 9 of this Catalogue. Humboldt, in 1850, 

 described it as bluish ; Donati, in 1860, as white, on the authority of Schmidt. 

 During February and March of this year, I often compared Vega and Capella 

 together when they were at the same altitude in the Northern sky, and they 

 seemed of the same tint, " a delicate pale blue." An observer in this city, in 

 the middle of May, was very decided in saying to me that Vega was much 

 bluer than Capella. On a reobservation of Vega, and from my recollection 

 of Capella, I assented to this opinion freely. This star, I believed, had deepened 

 its blue. Immediately there recurred to my mind the sentence of Kearny, 

 about Capella, in a letter to Herschell, in 1859 : " By the way, the color of Ca- 

 pella seems less blue than it used to be." Thus both these stars had changed 

 their blue, though in opposite directions. On the 8th or 9th of June, I re- 

 quested a friend who is not at all a star observer, to tell me of what color he 

 regarded that star, pointing to Vega. After looking carefully a sufficient time 

 he said it was green. I again looked myself, and was surprised to see that it 

 appeared really green. Every night since then I have anxiously watched it3 

 appearance, and in very clear nights it seems green, but when the air is vapoury 

 or hazy, it seems blue. I have referred it to some half a dozen individuals, 

 and they, when the nights have been clear, have also pronounced it of a green 

 color. Last winter, in clear moonlight nights, I ofted remarked that the green 

 color of Sirius was obscured by the intermingling rays of the moon, reflected 

 from the atmosphere. Now also Vega scarcely appears green by moonlight. 

 There is reason to think that this change in the color of stars from blue to 

 green is not uncommon. Humboldt says, " when forced to compare together 

 the colors of double stars, as reported by several astronomers, it is particularly 

 striking to observe how frequently the companion of a red or orange-colored 

 star is reported by some observers as blue, and by others as green." Cosmos ; 

 vol. iii. p. 284, note. 



Donati, in the memoir referred to in my communication of March, gives 

 Humboldt in Cosmos, and Schmidt in Ast. Nach., as the authorities for his 

 classification of the colors of the stars. Therefore I was misled in the dates 

 for the color of his stars, for they cannot be as late as that of his memoir. 

 Hence, also, the supposed discrepancy between him and Kearny disappears. 

 Moreover, whenever he departs from the earlier of his authorities, Humboldt 

 in 1850, I suppose it must be in favor of the latter, Schmidt. If this supposi- 

 tion be correct, then it would indicate that between the observations of Hum- 

 boldt, and those of Schmidt, several changes of color among the stars had oc- 

 curred. Humboldt gives Procyon as a yellow star, Donati as white. Humboldt 



* See pages 26 and 96 of this volume. 



1863.] 



