STUDY OF THE VARIABLE STABS 



181 



Reference will be made in what follows only to classes II., IV. 

 and V. New stars may well be considered as a class apart. There is, 

 possibly, no sufficient reason for including them among variable stars, 

 technically so-called. The stars of class III. are few, doubtful, not 

 well understood, and relatively unimportant. 



II. Variable stars of long period and large variation in light are 

 perhaps the easiest to observe and the most difficult to interpret of all. 

 Many of them are bright enough to be observed, near maximum at 

 least, by the naked eye. and the variations are so great that observa- 

 tions of the highest precision are not essential for the determination of 

 the light-curves. The length of period ranges in general from 100 



\ Cuoui- . 7?U- 



G(aWluk 



E 



aHL UttUtl 



■ I 



Fig. 7. Stars of Type III. and Variable Stars of Type III. with 

 Bright Hydrogen Lines. 



days to 400 clays. Some of these stars have been observed by different 

 astronomers during the last two centuries, and elaborate investigations 

 concerning them have been made by Argelander, Schonfeld, Chandler, 

 Pickering, and others. Omicron Ceti, or Mini, The Wonderful, has 

 been studied more carefully than any other. Even here, however, 

 much remains to be learned. The light-curve of Mira is shown in 

 Fig. 1, and is fairly typical of the group. The variations in brightness 

 are irregular and a single light-curve can only represent mean results. 

 Irregularity characterizes all the phases; the exact time of any return 

 of maximum is uncertain, and the brightness at different maxima, and 

 at different minima, varies greatly. 



The spectrum of stars of this class is in general of Seechrs third 

 type, with heavy banded lines and flutings. A short time before maxi- 

 mum the bright lines of hydrogen appear, and persist till the star has 

 grown somewhat faint again. At least, this is true of Mira, and of 



