Variable Stars. 75 



for more thin seven months, when it flashes up into a star of the 

 second or third magnitude, reaching its maximum in about one 

 month, and then more slowly fades away, dropping to its minimum 

 in less than three months. Its ordinary light curve may be repre- 

 sented thus : 



• id mag. 



A 



" / I mo. V 12th mag 



But this is by^ no means its invariable career. Sometimes it 

 scarcely reaches naked eye visibility at its maximum, and its period 

 is subject to strange caprices, having a variation in length often to 

 twenty-five days. Even its maximum brightness is apparently sub- 

 ject to periodical changes, and its greatest brightness is believed to 

 be at every eleventh maximum. 



A large proportion of the known variable stars are of the Mira 

 type. 



There is now a very faint star in the southern constellation Argo 

 which has an interesting history. We can not positively assign it 

 either to the first or to the second-class of variables, being on the 

 border line between them. Its position is so near the South Pole 

 that it can not be seen in this latitude. In 1677, the astronomer 

 Halley saw it at the Island of St. Helena, as a star of the third 

 magnitude. In 1751 Lacaille found that it had increased to the 

 second magnitude. In 1837 its light had augmented till it fairly 

 matched the bright star .Alpha Centauri. 



It then diminished slightly in brightness for four or five years, but 

 in 1842 and 1843 it blazed up brighter than ever and nearly equaled 

 Sirius in splendor. Then for twenty-five years it slowly and steadily 

 diminished, and in 1867 was barely visible to the naked eye, and 

 the year following vanished from the unassisted view. Though it 

 has not yet begun to increase in brightness, it is suspected to be a 

 variable of a long and irregular period. 



The most striking variable in the whole heavens, and one which 

 is at tht same time a model specimen of the third type — those having 

 short and exact periods — is the bright star Algol in the northern 

 constellation Perseus. It swings from the second to the fourth 

 magnitude and back again in a period a little less than three days. 

 This period is as sharply defined as that of a planet in its orbit, or 

 the rotation of a planet on its axis. It is precisely 2 days, 20 hours, 



