644 POPULAR SCIENCE MONTHLY. 



It is found that several other stars vary in the same way as Algol; 

 that is to say, they are invariable in brightness during the greater part 

 of the time, but fade away for a few days at regular intervals. This is 

 a kind of variation which it is most difficult to discover, because it will 

 be overlooked unless the observer happens to notice the star during the 

 time when an eclipse is in progress, and is thoroughly aware of its 

 previous brightness. One might observe a star of this kind very ac- 

 curately a score of times, without hitting upon a moment when the 

 partial eclipse was in progress. On the principle that like effects are 

 due to like causes, we are justified in concluding that in the cases of all 

 stars of this type, the eclipses are caused by the revolution of a dark 

 body, now called 'Algol variables,' round the principal star. 



A feature of all the Algol variables is the shortness of the periods. 

 The longest period is less than five days, while three are less than 

 one day. This is a result that we might expect from the nature 

 of the ease. The nearer a dark planet is to the star, the more likely 

 it will be to hide its light from an observer at a great distance. If, for 

 example, the planet Jupiter were nearly as large as the sun, the chances 

 would be hundreds to one against the plane of the orbit being so nearly 

 in the line of a distant observer that the latter would ever see an 

 eclipse of the sun by the planet. But if the planet were close to the 

 sun, the chances might increase to one in ten, and yet farther to almost 

 any extent, according to the nearness of the two bodies. 



Still, we cannot set any definite limit to the period of stars of this 

 type; all we can say is that, as the period we seek for increases, the 

 number of stars varying in that period must diminish. This fol- 

 lows not only from the reason just given, but from the fact that the 

 longer the interval that separates the partial eclipses of a star of the 

 Algol type, the less likely they are to be detected. 



STARS OF THE BETA LYRiE TYPE. 



The star Beta Lyrae shows variations quite different in their nature 

 from those of Algol, yet having a certain analogy to them. Anyone 

 who looks at the constellation Lyrae a few nights in succession and 

 compares Beta with Gamma, a star of nearly the same brightness in its 

 neighborhood, will see that while on some evenings the stars are of 

 equal brightness, on others Beta will be fainter by perhaps an entire 

 magnitude. 



A careful examination of these variations shows us a very remark- 

 able feature. On a preliminary study, the period will seem to be six 

 and one-half days. But, comparing the alternate minima, we shall find 

 them unequal. Hence the actual period is thirteen days. In this 

 period there are two unequal minima, separated by equal maxima. 



