GENERAL MOVEMENT OF THE STARS. 229 



their brightness, and he thinks that the difference in their light may 

 often be referred either to a real difference in their diameters, or 

 possibly in their specific luminous intensity. He cites as a striking 

 example of this kind, the two stars of the constellation Cygnus, Alpha 

 and 61 ; the first having a parallax and proper movement almost imper- 

 ceptible, while the value of these elements in each is respectively 

 0".348 and 5". 22 ; whence he concludes that the latter must be at 

 least 30 times nearer to us than the former, and its absolute brilliancy 

 20,000 times less. 



It is to the attentive observation of the proper movements, com- 

 bined with the parallaxes directly obtained, that, according to our 

 author, we should attach ourselves for the determination of the dis- 

 tances and special physical relations which exist between the systems 

 of stars. As an essay towards absolute determinations in this way, 

 he compares among themselves the proper movements and parallaxes 

 for the stars to the number of 7, where this last element is approxi- 

 mately known, after the researches of Bessel, Maclear, and Peters. 

 These respective values are as follows: 



Proper movement c. Annual parallax jr. 



a of the Centaur 3".674 0°.912 



61st of Cygnus 5 .221 .348 



a of the Lyre .349 .103 



1830 of the catalogue of Groombridge 7 .020 .226 



Polar star .038 .067 



2 of the Greater Bear .535 .133 



Arcturus 2 .258 .127 



With the exception of the Polar star, in regard to which the small- 

 ness of the two elements renders their values vef y uncertain, we see 

 that the first much surpasses the second in magnitude, their mean 

 relation being that of 10 to 1. Macleare thinks the parallax of Sirius 

 less than the fourth of a second, while its proper movement is one 

 second and a fourth. 



M. Masdler, after having endeavored to determine anew, with all 

 possible care, the precise value of the annual proper movement of 61 

 of Cygnus, finds it to be 4". 282. Now, that of the Pleiades being, 

 in the mean, 0".0582, is consequently 73 times smaller. We may 

 then admit that in the rectilinear triangle PSC, which joins .the 

 Pleiades, the Sun, and 61 of Cygnus, the side SC is much the smallest. 

 The angle at the Sun S being 83°. 4, the angle at C should be near 

 90°, without by possibility exceeding 96°. The author admits, then, 

 the equality of the sides PS and PC to be very close, and conse- 

 quently that also of the real proper movements of the Sun and 61 

 Cygnus. Thus the annual proper movement of our Sun ought to be 

 seen very nearly under an angle of 4". 284 from a point in the celestial 

 space where the radius of the earth's orbit would appear under a 

 parallactic angle of 0".3483; whence it follows that the proper move- 

 ment of the Sun corresponds very nearly to 12,295 radii of the ter- 

 restrial orbit. 



This result is much greater than that of 1,623 obtained by W. 

 Struve, at the end of his Etudes Stdlaires, the grounds of which are 



