WEIGHING THE WORLDS 



volve about each other in a two-hundred year period. 

 As Professor Moulton has suggested, if there are 

 planets in this system, the phenomena of night and 

 day must be curiously complicated. 



The second magnitude star Algol, which lies about 

 half way between the W in Cassiopeia and the 

 familiar cluster of the Pleiades, is a double star of 

 peculiar interest because one of the components is 

 a dark body that chances to revolve in such a direc- 

 tion that it partly eclipses its bright companion. As 

 it makes a complete revolution in less than three 

 days, the fading out and rejuvenescence of Algol 

 takes place in a period of a few hours during which 

 the star sinks from second magnitude almost to in- 

 visibility and returns again to second magnitude. 

 These changes may readily be observed with the 

 naked eye. 



On the opposite side of the pole star from Algol 

 lies the very interesting constellation Lyra. The 

 very bright star Vega enables one readily to locate 

 this constellation. This star, as we have seen, chances 

 to lie within a few degrees of the direction in which 

 our solar system is drifting. 



Near Vega is one of the most famous of double 

 stars, known as Beta Lyrae, which changes its bright- 

 ness by more than a magnitude in a period of a little 

 less than thirteen days. It has been estimated to be 

 a double with components 10,000,000 miles in diam- 

 eter, and having respectively ten and twenty-one 

 times the mass of our sun. These two colossal bodies 

 revolve about a common orbit so small that the two 

 stars, which are necessarily elongated by their mutual 



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