18 PROCEEDINGS OF THE AMERICAN ACADEMY 



maximum and one minimum in each period, the other two maxima and 

 two minima in the same time. As examples, /3 Lyres and S Cephei 

 may be noted, f^ifthly, we have a class of stars which during the 

 greater part of the time remain unchanged in brightness, but at regu- 

 lar intervals lose in the course of a few hours a large part of their 

 Tight, and regain it with equal rapidity. These changes appear to be 

 repeated with the greatest regularity, so that the interval can be com- 

 puted in some cases within a fraction of a second. Algol, or i3 Persei, 

 is the most striking example of this class to which 8 Cancri and 

 8 LihrcE also belong. 



Various theories have been advanced to account for these phenomena. 

 Probably different causes act in the case of the different classes. One 

 theory would assume that by a collision, or by the liberation and ignition 

 of a vast amount of hydrogen, the star was suddenly heated to incan- 

 descence, and gradually lost its light by cooling. This explanation 

 would apply only to stars of the first class; it is strengthened in the 

 case of the new star in Cygnus by the observations with the spectro- 

 scope. The spectrum gave at first the lines of incandescent hydrogen 

 which disappeared as the light faded. It has been urged that, to 

 account for the rapid cooling, the star must have been small, perhaps 

 only a few miles in diameter, and consequently not very distant. This 

 view is contradicted by the absence of perceptible parallax. If we con- 

 sider how quickly a meteorite becomes heated, and again gives up its 

 heat, this argument loses its force. The star may be large and dis- 

 tant, the surface only being heated, and soon losing its heat by radia- 

 tion and conduction. This explanation appears more probable than 

 that the light is cut off by clouds of smoke or steam, as has been sug- 

 gested by some astronomers. 



Stars constituted like our Sun, but in which the variations in size of 

 the spots would be far greater, might undergo considerable changes in 

 light. While it is difficult to account for the great changes in class 

 two in this way, those in class three may be thus explained. A popu- 

 lar theory for the variation of stars of short period is that it is due to the 

 revolution of the star upon its axis, when the different portions are of 

 unequal biightnoss. The variation in light of lapetus, the outer satel- 

 lite of Saturn, is commonly explained in tliis way. A similar effect 

 would be produced if the star was not spherical, and in revolving ex- 

 posed a disk of varying area. A great variation could not thus be 

 produced without the revolving body assuming a condition of unstable 

 equilibrium. For the application of these principles to lapetus, see 

 Annals of Harvard College Observatory, xi. 204. This theory may ex 



