VARIABLE STARS.* 



By Prof. C. A. Young. 



For the most part the stars maiiitaiu their places aud brightness 

 apparently unchanged; the heavens of today are substantially the 

 same as those of Job and Homer. The comparison of a modern star 

 catalogue with that of Hipparchus as preserved by Ptolemy shows that 

 with few and generally slight exceptions the principal stars still bold 

 the same relative positions and the same rank in brightness which they 

 did two thousand years ago. And yet we know that all the time they 

 have been swiftly moving in all directions at rates sometimes exceed- 

 ing 100 miles a second — some approaching us and some receding. 

 Then, too, they have all been growing old together, some advancing 

 from stellar infancy toward adult vigor, and some descending toward 

 extinction. The apparent i:)ermaneuce is apparent only, — due simply to 

 the fact that the scale of human life and all our terrestrial surround- 

 ings is practically infinitesimal as compared with that of the stellar 

 universe. Ev^eu the nearest star is so remote that if it were rushing 

 directly toward us with the speed of 100 miles a second it would gain 

 in brightness only about 2i iier cent in a century, an amount barely 

 observable by our best ])botometers. As to changes due merely to 

 advancing age, a century bears only some such ratio to the lifetime of 

 a star as a single minute to that of an aged man. 



But while all this is true there are several hundred stars T>hich are 

 known as " variables," and undergo considerable changes of bright- 

 ness, sometimes quite rapidly. They form the subject of a most inter- 

 esting and important chapter in astronomy, — a chapter which may be 

 regarded as opened by Tycho's observations of the marvellous star of 

 1572, but even yet only begun ; in fact, nearly all the systematic work 

 that has been done in this line has been accomplished within the last 

 sixty years. 



These "variables" may be roughly classified as follows: First, we 

 have a few stars that seem to be gradually, and more or less steadily, 

 growing brighter or fainter, judging from their present rank as com- 

 pared with that which they used to hold; Alpha Geminorum is a case 



* From The Popular Science News, Boston, Dec, 1893; vol. xxvii, pi). 177, 178. 



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