646 POPULAR SCIENCE MONTHLY. 



our solar system, and which we should never have suspected, had it not 

 been for observations of this star. 



The gap between the variable stars of the Algol type and those of 

 the Beta Lyrse type is, at the present time, being filled by new discov- 

 eries in such a way as to make a sharp distinction of the two classes dif- 

 ficult. It is characteristic of the Algol type proper that the partial 

 eclipses are due to the interposition of a dark planet revolving round 

 the bright star. But suppose that we have two nearly equal stars, A 

 and B, revolving round their common center of gravity in a plane 

 passing near our system. Then, A will eclipse B, and, half a revolution 

 later, B will eclipse A } and so on in alternation. But, when the stars 

 are equal, we may have no way of deciding which is being eclipsed, and 

 thus we shall have a star of the Algol type, so far as the law of varia- 

 tion is concerned, yet, as a matter of fact, belonging rather to the Beta 

 Lyras type. If the velocity in the line of sight could be measured, the 

 question would be settled at once. But only the brightest stars can, so 

 far, be thus measured, so that the spectroscope cannot help us in the ma- 

 jority of cases. 



The most interesting case of this kind yet brought to light is that 

 of Tau Cygni. The variability of this star, ordinarily of the fourth mag- 

 nitude, was discovered by Chandler in December, 1886. The minima 

 occurred at intervals of three days. But in the following summer he 

 found an apparent period of 1 d. 12 h., the alternate minima being 

 invisible because they occurred during daylight, or when the star was 

 below the horizon. With this period the times of minima during the 

 summer of 1888 were predicted. 



It was then found that the times of the alternate minima, which, 

 as we have just said, were the only ones visible during any one season, 

 did not correspond to the prediction. The period seemed to have 

 greatly changed. Afterward, it seemed to return to its old value. 

 After puzzling changes of this sort, the tangle was at length unraveled 

 by Duner, of Lund, who showed that the alternate periods were un- 

 equal. The intervals between minima were one day nine hours, one 

 day fifteen hours, one day nine hours, one day fifteen hours, and so on, 

 indefinitely. This law once established, the cause of the anomaly be- 

 came evident. Two bright stars revolve round their common center 

 of gravity in a period of nearly three days. Each eclipses the other 

 in alternation. The orbit is eccentric, and, in consequence, one-half 

 of it is described in a less time than the other half. If we could dis- 

 tinguish the two stars by telescopic vision, and note their relative posi- 

 tions at the four cardinal points of their orbit, we should see the pair 

 alternately single and double, as shown in the following diagrams: 



