NATURAL SCIENCES OP PHILADELPHIA. 27 



study. Catalogues have been made of some of these classes of stars, but no 

 catalogue has yet been made of stars which have changed their color. In- 

 deed, Humboldt, in writing about the red color ascribed to Sirius by the an- 

 cient Greeks, says, "Sirius, therefore, affords the only example of an histori- 

 cally proved change of color, for it has at present (1S50 ?) a perfectly white 

 light." And yet, in apparent contradiction to this, he, in other pages of the 

 third volume of his Cosmos, mentions other fixed stars whose colors in modern 

 times have been known to change. 



This change of color is one of the greatest physical events. Think of our 

 own intensely-lighted sun, 2,770,000 miles in circumference, as beiug deeply 

 red, then turning to be perfectly white, then changing to purple, aud then 

 again to green ! What mighty causes must be in operation to produce such 

 grand results. This should be made a distinct section of astronomical study, 

 and allied to that of the other classes of stars just mentioned ; and I therefore 

 offer the following as an enumeration of stars whose colors have changed. 



Catalogue. 



1. The temporary star described by Tycho Brahe in 1572. " For the first two 

 or more months it was white ; then it passed through yellow into red. At 

 last, when very small, it again became white, but of a dull whiteness. These 

 changes of color were attendant upon changes in amount of light. While 

 this star was so large as to be seen with keen eyes by day, and even through 

 the clouds by night, when all other stars were hidden, yet it scintillated more 

 strongly than stars of the first magnitude," thus indicating a constitutional as 

 well as an atmospheric cause for this scintillation. 



2. Eta of Argus. This star, so wonderfully variable in the amount of its 

 light, is also variable in its color. In 1843, Mr. Mackay, at Calcutta, observed 

 that it was similar in color to Arcturus, and was therefore reddish-yellow. 

 In Feb., 1850, Lieut. Gilliss, at Santiago, in Chili, writes of it as being " of a 

 darker color than Mars," and therefore deeply red. 



3. Beta of Ursa Minor. Heis, one of the most eminent German observers, 

 writing to Humboldt in May, 1850, says that " this star is not always equally 

 red ; at times it is more or less yellow, at others most decidedly red." 



4. Alpha Crucis. Humboldt, in Cosmos, vol. 3d, says, "My old friend, 

 Captain Berard, who is an admirable observer, wrote from Madagascar in 

 1847, that he had for some years seen this star growing red." 



5. Capella. In the tenth century this star was described by an Arabian 

 astronomer as red. In 1850, Humboldt, in the third volume of his Cosmos, 

 says, "it is now yellow, with scarce a tinge of red." In Sept., 1859, Rev. 

 J. B. Kearney, in a letter to Sir J. Herschell, printed in the 20th volume, 

 number one, of the Monthly Notices of the Royal Astronomical Society, says, 

 " By the way, the color of Capella seems much less blue than it used to be." 

 To myself, at present its color appears to be a delicate pale blue. 



6. Sirius. In the times of the old Greek astronomers Sirius was red. In 

 the Middle Ages the Arabian astronomers did not name Sirius among the red 

 stars, neither did the earlier astronomers of the west of Europe. Therefore, 

 it seems probable that its color changed from red to white between the times 

 of the Greek and those of the Arabian observers. Humboldt, as I have said, 

 writes in 1850 of this star as being " perfectly white." Two years ago, when 

 another change was observed by Dr. Wilcox, a friend of his, who was accus- 

 tomed to distinguish nice shades of color, pronounced the light of Sirius to be 

 purple, in which opinion he concurred. Four months ago, when he made the 

 announcement to me, I regarded it as blue with a decided tinge of green. At 

 present it seems to myself and some friends as the most deeply-colored star 

 in the sky, but as more green than blue.* 



* A "th star, Procyon, is to be added to the above catalogue, its change of color having been 

 ascertained a night or two after the presentation of the others. 



1863.] 



