VARIABLE STARS. 227 



A great difference in the maximum of brightness is exhibited 

 by Mira Ceti. In the year 1779, for instance (on the 6th of 

 November), Mira was only a little dimmer than Aldebaran, and 

 indeed not unfrequently brighter than stars of the 2nd mag- 

 nitude ; whereas at other times this variable star scarcely 

 attained to the intensity of the light of 8 Ceti, which is of the 

 4th magnitude. Its mean brightness is equal to that of y Ceti 

 (3rd magnitude). If we designate by the brightness of the 

 faintest star visible to the naked eye, and that of Aldeba- 

 ran by 50, then Mira has varied in its maximum from 20 to 

 47. Its probable brightness may be expressed by 30 : it is 

 oftener below than above this limit. The measure of its 

 excess, however, when it does occur, is in proportion more 

 considerable. No certain period of these oscillations has as 

 yet been discovered. There are however indications of a 

 period of 40 years, and another of 160. 



The periods of variation in different stars vary as 1 : 250. 

 The shortest period is unquestionably that exhibited by 

 Persei, being 68 hours and 49 minutes; so long at least as 

 that of the polar star is not established at less than two 

 days. Next to Persei come 8 Cephei (5d. 8h. 49m.), 

 77 Aquilse (7d. 4h. 14m.), and Geminorum (lOd. 3h. 35m.). 

 The longest periods are those of 30 Hydrse Hevelii, 495 days; 

 X Cygni, 406 days; Variabilis Aquarii, 388 days; Serpentis 

 S, 367 days ; and Mira Ceti r 332 days. In several of the vari- 

 able stars it is well established that they increase in brilliancy 

 more rapidly than they diminish. This phenomenon is the 

 most remarkable in 8 Cephei. Others, as for instance /3 

 Lyrae, have an equal period of augmentation and diminution 

 of light. Occasionally, indeed, a difference is observed in this 

 respect in the same stars, though at different epochs in their 

 process of light. Generally Mira Ceti (as also 8 Cephei) is 

 more rapid in its augmentation than in its diminution ; but 

 in the former the contrary has also been observed. 

 -" Q2 



