CHAPTERS ON THE .STARS. 647 



A B 



Position ( 1 ) , stars at pericenter * 



Interval, 16 hours. 

 Position (2) , A eclipses B 



Interval, 20 hours. 



B A 



Postion ( 3 ) , stars at apocenter 4 



Interval, 20 hours. 

 Position (4) , B eclipses A 



Interval, 16 hours. 

 Position ( 1 ) is repeated * 



U Pegasi is a star which proved as perplexing as Tan Cygni. It was 

 first supposed to be of the Algol type, with a period of about two days. 

 Then it was found that a number of minima occurred during this 

 period, and that the actual interval between them was only a few hours. 

 The great difficulty in the case arises from the minuteness of the 

 variation, which is but little more than half a magnitude between the 

 extremes. The observations of Wendell, at the Harvard Observatory, 

 with the polarizing photometer, enabled Pickering to reach a con- 



Q A X A 2 A 3 A ^A S A qA. 7 A. qA 9A 



Fig. 3. Light Curve or U Pegasi, of the Beta Lyr« Type, from Observa- 

 tions by Wendell at the Harvard Observatory. Magnitude at 

 Maximum, 9.32; at Principal Minimum, 9.90; at Secondary Minimum, 

 9.76. Period, 9 hours. 



elusion which, though it may still be open to some doubt, seems to be 

 the most likely yet attainable. The star is of the Beta Lyras type; its 

 complete period is 8 hours 59 minutes 41 seconds, or 19 seconds less 

 than nine hours; during this period it passes through two equal maxima, 

 each of magnitude 9.3, and two unequal minima 9.76 and 9.9, alter- 

 nately. 



The difference of these minima, Om. 14, is less than the errors which 

 really ordinarily affect measures of a star's magnitude with the best 

 photometers. Some skepticism has, therefore, been felt as to the 

 reality of the difference which, if it does not exist, would reduce the 

 periodic time below four and one-half hours, the shortest yet known. 



