Other Parts of the Northern Sky 199 



because it seems the most reasonable to account for the 

 gradual obscuration and regular recovery of brightness, 

 but also because of information about the motion of 

 Algol afforded by that marvelous instrument, the spec- 

 troscope. An examination of the light by this instrument 

 enables us to tell whether a star is approaching or receding, 

 and how fast. By this means we know that 17 hours 

 before Algol becomes faintest, it is receding at the rate 

 of nearly 27 miles a second, and 34 hours later it is ap- 

 proaching us at the same rate. Algol and its dark 

 companion are revolving around their common center 

 of gravity. When the dark companion is coming toward 

 us, Algol is receding, and vice versa. The period of 

 revolution, less than 3 days, is very short. Astronomers 

 believe that Algol is about 1,160,000 miles in diameter; 

 its dark companion about 840,000 miles ; and the dis- 

 tance between their centers only about 3,250,000 miles. 

 Algol is only one of a number of variable stars. 



A new star. The constellation Perseus is notable also 

 because of a new star which appeared in it in February, 

 1901, and attracted much attention. In 3 days from 

 the time it was first noticed it became one of the brightest 

 stars in the heavens. It faded rapidly and by the end 

 of the year could no longer be seen with the naked eye. 

 It is still visible with the best telescopes. Like other 

 temporary stars, it turned red as it faded. The records 

 tell us of a few other new stars. 



Auriga, the Charioteer. This constellation is on the 

 opposite side of the pole from Lyra. Its brightest star, 

 Capella, is of the same brightness as Vega and 7 degrees 

 nearer the pole. When you have learned to recognize 



