THE STELLAR UNIVERSE. 



two, during the first three hours and a half 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 splendour at midnight 

 on the first day of the month, its changes would be as follows : 



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



o o o to 2 14 o It appears of second magnitude. 



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



2, 1 7 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 frequently and conveniently observed. It is 

 situate near the foot of the constellation Andromeda, and lies a 

 few degrees north-east of three stars of the fourth magnitude, 

 which form a triangle. 



Goodricke, who discovered th e periodic phenomena o f A Igolvo. 1782, 

 explained 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. 

 The more recent observations on this star indicate a decrease 

 of its period, which proceeds with accelerated rapidity. Sir J. 

 Herschel thinks that this decrease will attain a limit, and will be 

 followed by an increase, so that the variation of the period will 

 prove itself to be periodic. 



The stars 5 in Cepheus and in Lyra are remarkable for the 

 regular periodicity of their lustre. The former passes from its 

 least to its greatest lustre in thirty-eight hours, and from its 

 greatest to its least in ninety-one hours. The changes of lustre 

 of the latter, according to the recent observations of M. Argelander, 

 are very complicated and curious. Its entire period is 12 days 

 21 hrs. 53 min. 10 sec., and in that time it first increases in lustre, 

 then decreases, then increases again, and then decreases, so that 

 it has two maxima and two minima. At the two maxima its 

 lustre is that of a star of the 3 '4 magnitude, and at one of the 

 minima its lustre is that of a star of the 4*3, and at the other that 

 of a star of the 4-o magnitude. In this case also the period of the 

 star is found to be periodically variable. 



43. In the following Table the stars periodically variable, dis- 

 covered up to 1848, are given, with their periods and extremes of 

 lustre. This Table has been collected from various astronomical 

 records by Sir J. Herschel. 



198 



