$ 264] Doubh Stars 343 



members of Herschel's newly discovered double stars were 

 " beyond arithmetic." 



264. Twenty years after the publication of his first 

 catalogue Herschel was of Michell's opinion, but was 

 now able to support it by evidence of an entirely novel 

 and much more direct character. A series of observations 

 of Castor, presented in two papers published in the Philo- 

 sophical Transactions in 1803 and 1804, which were fortu- 

 nately supplemented by an observation of Bradley's in 

 1759, had shewn a progressive alteration in the direction 

 of the line joining its two components, of such a character 

 as to leave no doubt that the two stars were revolving 

 round one another; and there were five other cases in 

 which a similar motion was observed. In these six cases 

 it was thus shewn that the double star was really formed by 

 a connected pair of stars near enough to influence one 

 another's motion. A double star of this kind is called a 

 binary star or a physical double star, as distinguished from 

 a merely optical double star, the two members of which have 

 no connection with one another. In three cases, including 

 Castor, the observations were enough to enable the period 

 of a complete revolution of one star round another, assumed 

 to go on at a uniform rate, to be at any rate roughly 

 estimated, the results given by Herschel being 342 years 

 for Castor,* 375 and 1,200 years for the other two. It was 

 an obvious inference that the motion of revolution observed 

 in a binary star was due to the mutual gravitation of its 

 members, though Herschel's data were not enough to 

 determine with any precision the law of the motion, and 

 it was not till five years after his death that the first attempt 

 was made to shew that the orbit of a binary star was such 

 as would follow from, or at any rate would be consistent 

 with, the mutual gravitation of its members (chapter xin., 

 309 : cf. also fig. 101). This may be regarded as the first 

 direct evidence of the extension of the law of gravitation to 

 regions outside the solar system. 



Although only a few double stars were thus definitely 

 shewn to be binary, there was no reason why many others 



* The motion of Castor has become slower since Herschel's time, 

 and the present estimate of the period is about 1,000 years, but it 

 is by no means certain. 



