CHAPTERS ON TEE STARS. 



649 



Beta Lyrse is always of the same brightness at the same hour of its 

 period, and Algol has always the same magnitude at minimum. It is 

 true that the length of the period varies slowly in the case of these 

 stars. But this may arise from the action of other invisible bodies 

 revolving around the visible stars. This general uniformity is in 

 accord with the theory which attributes the apparent variations to the 

 various aspects in which we see one and the same system of revolving 

 stars. 



Another variable star showing some unique features is Eta Aquilse. 

 What gives it special interest is that spectroscopic observations of its 

 radial motion show it to have a dark body revolving round it in a very 

 eccentric orbit, and in the same time as the period of variation. It 

 might therefore be supposed that we have here a star of the Algol or 

 Beta Lyrae type. But such is not the case. There is nothing in the law 

 of variation to suggest an eclipsing of the bright star, nor does it seem 

 that the variations can readily be represented by the varying aspects 

 of any revolving system. 



The orbit of this star has been exhaustively investigated by Wright 

 from Campbell's observations of the radial motion. The laws of change 

 in the system are shown by the curves below, which are reproduced, in 

 great part, from Wright's paper in the 'Astrophysical Journal.' 



5' t> d T* 



Fig. 4. Light-Curve and Radial Velocity of Eta Aqdilm. 



The lower curve is the light-curve of the star during a period of 

 7.167 days. Starting from a maximum of 3.5 mag., it sinks, in the 

 course of 5 days, to a minimum of 4.7m. It was found by Schwab 

 that the diminution is not progressive, but that a secondary maximum 

 of 3.8m. is reached at the end of the second day. After reaching the 

 principal minimum it rises rapidly to the principal maximum in %\ days. 



The upper curve shows the radial velocity of the star during the 



