366 



THE STELLAR UNIVERSE. 



may be as distant from each other as any other stars in the firmament, notwith- 

 standing their apparent juxtaposition. But the individuals of a binary star are 

 at the same distance from the eye in the same sense in which the planet 

 Uranus and its attendant satellites are said to be at the same distance. 



More recent observation has fully confirmed these remarkable discoveries. 

 The catalogue of binary stars first given by Sir William Herschel, consisting 

 of from fifty to sixty, comprises nearly all the most considerable objects of that 

 class that have yet been detected. These stars require the best telescopes for 

 their observation, being generally so close as to render the use of very high 

 magnifying powers indispensable. 



The moment the revolution of one star round another was ascertained, the 

 idea of the possible extension of the great principle of gravitation to these re- 

 mote regions of the universe naturally suggested itself. Newton has proved 

 in his Principia, that if a body revolve in an ellipse by an attractive force 

 directed to the focus, that force will vary according to the law which charac- 

 terizes gravitation. Thus an elliptical orbit became a test of the presence and 

 sway of the law of gravitation. If, then, it could be ascertained that the 

 orbits of the double stars were ellipses, we should at once arrive at the fact 

 that the law of which the discovery conferred such celebrity on the name of 

 Newton is not confined to the solar system, but prevails throughout the uni- 

 verse. 



The first distinct system of calculation by which the true elliptic elements 

 of the orbit of a binary star were ascertained, was supplied in 1830, by M. 

 Savary, who showed that the motion of one of the most remarkable of these 

 stars ( £ Ursa Majoris), indicated an elliptic orbit described in 58J years. Pro- 

 fessor Encke, by another process, arrived at the fact that the star 70 Ophiuchi, 

 moved in an ellipse with a period of 74 years. Several other orbits were as- 

 certained and computed by Sir John Herschel. In the following table, given 

 by that astronomer, are exhibited the principal discoveries in this branch of 

 astronomy : — f 



Eccentricity. 



0-83350 



0-61125 

 8-75820 

 0-46670 

 0-41640 



The most remarkable of these, says Sir John Herschel, is y Virginis ; not 

 only on account of the length of its period, but by reason also of the great 

 diminution of apparent distance, and rapid increase of angular motion about 

 each other, of the individuals composing it. It is a bright star of the fourth 

 magnitude, and its component stars are almost exactly equal. It has been 

 known to consist of two stars since the beginning of the eighteenth century, 

 their distance being then between six and seven seconds ; so that any tolera- 

 bly good telescope would resolve it. Since that time they have been con- 

 stantly approaching, and are at present hardly more than a single second asun- 

 der ; so that no telescope that is not of very superior quality, is competent to 

 show them otherwise than as a single star somewhat lengthened in one direc- 

 tion. It fortunately happens, that Bradley, in 1718, noticed and recorded in 



