638 POPULAR SCIENCE MONTHLY. 



CHAPTERS ON THE STARS. 



By Prof. SIMON NEWCOMB, U. S. N. 

 VARIABLE STAES. 



IT is a curious fact that the ancient astronomers, notwithstanding 

 the care with which they observed the heavens, never noticed that 

 any of the stars changed in brightness. The earliest record of such an 

 observation dates from 1596, when the periodical disappearance of 

 Omicron Ceti was noticed. After this, nearly two centuries elapsed 

 before another case of variability in a star was recorded. During the 

 first half of the nineteenth century Argelander so systematized the 

 study of variable stars as to make it a new branch of astronomy. In 

 recent years it has become of capital interest and importance through 

 the application of the spectroscope. 



Students who are interested in the subject will find the most com- 

 plete information attainable in the catalogues of variable stars, pub- 

 lished from time to time by Chandler in the 'Astronomical Journal.' 

 His third catalogue, which appeared in 1896, comprises more than 300 

 stars whose variability has been well established, while there is always 

 a long list of 'suspected variables' — whose cases are still to be tried. 

 The number to be included in the established list is continually in- 

 creasing at such a rate that it is impossible to state it with any ap- 

 proximation to exactness. The possibility of such a statement has been 

 yet further curtailed by the recent discovery at the Harvard Ob- 

 servatory that certain clusters of stars contain an extraordinary pro- 

 portion of variables. Altogether at the time of the latest publication, 

 509 such stars were found in twenty-three clusters. The total number 

 of these objects in clusters, therefore, exceeds the number known in the 

 rest of the sky. They will be described more fully in a subsequent 

 chapter. For the present we are obliged to leave this rich field out 

 of consideration and confine our study to the isolated variable stars 

 which are found in every region of the heavens. 



Variable stars are of several classes, which, however, run into each 

 other by gradations so slight that a sharp separation cannot always 

 be made between them. Yet there are distinguishing features, each 

 of which marks so considerable a number of these stars as to show some 

 radical difference in the causes on which the variations depend. 



We have first to distinguish the two great classes of irregular and 

 periodic stars. The irregular ones increase and diminish in so fitful a 

 way that no law of their change can be laid down. To this class belong 



