CHAPTERS ON THE STARS. 5 



Then, the angle N P C is called the position-angle of the pair. It 

 is counted round the circle from 0° to 360°. The angle drawn in the 

 figure is nearly 120°. Were the companion C in the direction S the 

 position angle would be 180°; to the right of P it would he 270°; to 

 the right of N it would be between 270° and 360°. 



The distance is the angle P C, which is expressed in seconds of arc. 



We cannot set any well-defined limits to the range of distance. The 

 general rule is that the greater the distance beyond a few seconds the 

 less the interest that attaches to a double star, partly because the ob- 

 servation of distant pairs offers no difficulty, partly because of the in- 

 creasing possibility that the components have no physical connection 

 and so form only an optically double star. With every increase of tele- 

 scopic power so many closer and closer pairs are found that we cannot 

 set any limit to the number of stars that may have companions. It is 

 therefore to the closer pairs that the attention of astronomers is more 

 especially directed. 



The difficulty of seeing a star as double, or, in the familiar lan- 

 guage of observers, of 'separating' the components, arises from two 

 sources, the proximity of the companion to the principal star and the 

 difference in magnitude between the two. It was only in rare cases 

 that Struve could separate a pair of distance half a second. Now 

 Burnham finds pairs whose distance is one-quarter of a second or less; 

 possibly the limit of a tenth of a second is being approached. It goes 

 without saying that a very minute companion to a bright star may, 

 when the distance is small, be lost in the rays of its brighter neighbor. 

 For all these reasons no estimate can be made of the actual number 

 of double stars in the heavens. With every increase of telescopic 

 power and observing skill more difficult pairs are being found without 

 a sign of a limit. 



The great interest which attaches to double stars arises from the 

 proof which they afford that the law of gravitation extends to the 

 stars. Struve, by comparing his own observations with each other, or 

 with those of Herschel, found that many of the pairs which he meas- 

 ured were in relative motion; the position angle progressively chang- 

 ing from year to year, and sometimes the distance also. The lesser 

 star was therefore revolving round the greater, or, to speak with more 

 precision, both were revolving round their common center of gravity. 

 To such a pair the name binary system is now applied. 



There can be no reasonable doubt that the two components of all 

 physically connected double stars revolve round each other. If they 

 did not their mutual gravitation would bring them together and fuse 

 them into a single mass. We are therefore justified in considering all 

 double stars as binary systems, except those which are merely opti- 

 cally double. For reasons already set forth, the pairs of the latter 



