SECT, xxxvn.] VARIABLE STARS. 423 



burst forth in the constellation of Serpentarius, which con- 

 tinued visible for a year ; and a more recent case occurred 

 in the year 1670, when a new star was discovered in the 

 head of the swan, which, after becoming invisible, re- 

 appeared, and, having undergone many variations in light, 

 vanished after two years, and has never since been seen. 

 In 1572 a star was discovered in Cassiopeia, which rapidly 

 increased in brightness till it even surpassed that of Jupiter ; 

 it then gradually diminished in splendour, and, having ex- 

 hibited all the variety of tints that indicate the changes of 

 combustion, vanished sixteen months after its discovery, 

 without altering its position. It is impossible to imagine 

 anything more tremendous than a conflagration that could 

 be visible at such a distance. It is, however, suspected that 

 this star may be periodical, and identical with the stars 

 which appeared in the years 945 and 1264. There are, 

 probably, many stars which alternately vanish and re- 

 appear among the innumerable multitudes that spangle the 

 heavens. The periods of several have already been pretty 

 well ascertained. Of these the most remarkable is the star 

 Omicron, in the constellation Cetus : it appears about 

 twelve times in eleven years, and is of variable brightness, 

 sometimes appearing like a star of the second magnitude ; 

 but it does not always attain the same lustre, nor does it 

 increase or diminish by the same degrees. According to 

 Hevelius, it did not appear at all for four years, y Hydras 

 also vanishes, and reappears every 494 days. 



Variable stars are very common ; about 20 are decidedly 

 go, and 50 others are supposed to be either periodical or 

 irregular in their changes. Sir John Herschel has given an 

 instance of periodicity in the star Algol, or /3 Persei, which 

 is described as retaining the size of a star of the second 

 magnitude for two days and fourteen hours ; it then sud- 

 denly begins to diminish in splendour, and in about three 

 hours and a half is reduced to the size of a star of the 

 fourth magnitude ; it then begins again to increase, and in 



