PROGRESS OF MODERN ASTRONOMY 



stars are found thus closely connected into gravitation 

 systems; indeed, there are all gradations between bi- 

 nary systems and great clusters containing hundreds or 

 even thousands of members. It is known, for example, 

 that the familiar cluster of the Pleiades is not merely 

 an optical grouping, as was formerly supposed, but an 

 actual federation of associated stars, some two thou- 

 sand five hundred in number, only a few of which are 

 visible to the unaided eye. And the more carefully 

 the motions of the stars are studied, the more evident 

 it becomes that widely separated stars are linked to- 

 gether into infinitely complex systems, as yet but lit- 

 tle understood. At the same time, all instrumental ad- 

 vances tend to resolve more and more seemingly single 

 stars into close pairs and minor clusters. The two 

 Herschels between them discovered some thousands 

 of these close multiple systems; Struve and others in- 

 creased the list to above ten thousand ; and Mr. S. W. 

 Burnham, of late years the most enthusiastic and suc- 

 cessful of double-star pursuers, added a thousand new 

 discoveries while he was still an amateur in astronomy, 

 and by profession the stenographer of a Chicago court. 

 Clearly the actual number of multiple stars is beyond 

 all present estimate. 



The elder Herschel's early studies of double stars 

 were undertaken in the hope that these objects might 

 aid him in ascertaining the actual distance of a star, 

 through measurement of its annual parallax that is to 

 say, of the angle which the diameter of the earth's 

 orbit would subtend as seen from the star. The ex- 

 pectation was not fulfilled. The apparent shift of 

 position of a star as viewed from opposite sides of the 



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