358 THE STELLAR UNIVERSE. 



PERIODIC STARS. 



The stars in general, as they are stationary in their apparent positions, are 

 equally invariable in their apparent magnitudes and brightness. To this, 

 however, there are several remarkable exceptions. Stars have been observed, 

 sufficiently numerous to be regarded as a distinct class, which exhibit periodi- 

 cal changes of appearance. Some undergo gradual and alternate increase and 

 diminution of magnitude, varying between determinate limits, and presenting 

 these variations in equal intervals of time. Some are observed to attain a cer- 

 tain maximum magnitude, from which they gradually and regularly decline 

 until they altogether disappear. After remaining for a certain time invisible, 

 they reappear and gradually increase till they attain their maximum splendor, 

 and this succession of changes is regularly and periodically repeated. 



Such objects have been denominated periodic stars. The most remarkable 

 of this class is the star called Omikron, in the neck of the Whale, which was 

 first observed by David Fabricius, on the 13th August, 1596. This star re- 

 tains its greatest brightness for about fourteen days ; being then equal to a large 

 star of the second magnitude. It then decreases continually for three months 

 until it becomes invisible. It remains invisible for five months, when it again 

 reappears, and increases gradually for three months until it recovers its maxi 

 mum splendor. This is the general succession of its phases. Its entire pe- 

 riod is about 334 days. This period is always the same, but the gradations 

 of brightness through which it passes are said to be subject to variation. 

 Hevelius states that in the interval between 1672 and 1676 it did not appear 

 at all. 



The star called Algol, in the head of Medusa, in the constellation of Perseus, 

 affords a striking example of the rapidity with which these periodical changes 

 sometimes succeed each other. This star generally appears as one of the 

 second magnitude ; but an interval of seven hours occurs at the expiration of 

 every sixty-two, during the first three and a half ho;irs of which it gradually 

 diminishes in brightness till it is reduced to a star of the fourth magnitude, 

 and during the remainder of the interval it again gradually increases until it 

 recovers its original magnitude. Thus, if we suppose it to have attained its 

 maximum splendor at midnight on the first day of the month, its changes would 

 be as follows : 



D. H. M. D. H. M. 



to 2 14 It appears of second magnitude. 



2 14 to 2 17 24 It decreases gradually to fourth magnitude. 



2 17 24 to 2 20 48 It increases gradually to second magnitude. 



2 20 48 to 5 10 48 It appears of second magnitude. 



5 10 48 to 5 14 12 It decreases to fourth magnitude. 



5 14 12 to 5 17 36 It increases to second magnitude. 



&c. &c. &c. 



This star presents an interesting example of its class, as it is constantly 

 visible, and its period is so short that its succession of phases may be fre- 

 quently and conveniently observed. It is situate near the foot of the constel- 

 lation Andromeda and lies a few degrees northeast of three stars of the fourth 

 magnitude which form a triangle. It passes the meridian of New York in 

 December, at about four o'clock in the afternoon, and may therefore be seen 

 toward the west during the early hours of the night. 



Goodricke, who discovered the periodic phenomena of Algol in 1782, ex- 

 plained these appearances by the supposition that some opaque body revolves 

 round it, being thus periodically interposed between the earth and the star, so 

 as to intercept a large portion of its light. Whatever be their cause, these 

 phenomena indicate an extraordinary system of rapid motions and changes in 



