Variable Stars. 73 



VARIABLE STARS. 

 By Wm. H. Knight. 



Read before the Cincinnati Society of Natural History, July 2, 1889. 



Among the stars which have received considerable attention 

 from astronomers and physicists in recent years, none possess a 

 deeper interest, or present more difficult problems for solution, than 

 those technically known as variable stars. 



As their name implies, they shine with fluctuating brilliancy. In 

 some of these stars the variation of light is marked and rapid, and 

 extends through several magnitudes. In other cases it is very slow 

 and gradual, and its range may be confined to less than a single 

 magnitude, so th it its variable character can only be detected by a 

 photometer, or light gauge. 



Prof. S. C. Chandler, of the Harvard College Observatory, a 

 distinguished specialist in this department of astronomy, catalogued 

 225 of these strange heavenly bodies in 18S8, and their range and 

 period of variation, spectra, and other peculiarities, have been the 

 subject of careful investigation by Chandler, Sawyer, Pickering, 

 Young, Huggins, Lockyer, and other eminent astronomers on both 

 sides of the Atlantic. 



Variable stars have been divided into many classes by different 

 writers on that subject, but for the purposes of this paper they will 

 be considered under three principal types : 



1. Those which exhibit fluctuations of light without any indica- 

 tions of a regtdar period. 



2. Those of long but not exact periods, though the terms of 

 their maximum and minimum brightness may be approximately 

 predicted. 



3. Those of short periods, in which the minimum is of brief dura- 

 tion and recurs with the precision of clock-work. 



Of the first class — those not having any recognized period — are 

 temporary stars, and stars once bright but now dim. The most 

 noted of the temporary stars was that known as Tycho's, which 

 suddenly blazed out in the constellation Cassiopeia, in November, 

 1572. For a few days it was as bright as Venus, and could be seen 



