DOUBLE STARS 157 



others again in which the components are nearly in 

 contact. The theory will be further strengthened if in 

 such cases there appears other and independent evidence 

 of an early stage of stellar life. We have now to see 

 how such systems have been discovered, and by what 

 methods their dimensions have been made a matter of 

 scientific investigation. 



The first evidence of the existence of very close 

 doubles was suggested by the behaviour of a certain class 

 of variable stars. That a certain number of the stars 

 vary in brightness from time to time has been known 

 from very early times, and their number has been 

 enormously increased by recent observations. In some 

 cases, the variation is of an extremely irregular nature, 

 the behaviour of the star in the past supplying no indica- 

 tion of the future course of its light changes. In the 

 majority of cases, however, the variation displays at least 

 an approximation to a definite periodicity, and, in a few 

 instances, the light rises and falls in a definite period, and 

 in a manner that is repeated with almost mathematical 

 exactness. Of these regular variables, the oldest known, 

 as well as the easiest to observe, is situated in the 

 constellation of Perseus, and is known as Algol 1 or 

 Persei. 



Algol appears normally as one of the very brilliant 

 stars, its brightness being slightly below that of the 

 second magnitude. After shining steadily for about 

 fifty-nine hours, however, its light begins to fade, and 

 in the course of four-and-a-half hours it has descended to 

 thirty-eight per cent, of its original brilliancy. It then, 

 and apparently without a pause, begins a recovery which 

 is completely effected in a slightly longer time than that 



1 The name Algol is derived from the Arabic Al-Ghoul, the demon ; it 

 was probably so named from its changing appearance. 



