THE BINARY STARS. 317 



separates them from us. Of course, it is certain that there are 

 many stars which appear double when viewed with a telescope, in 

 which the close proximity of the pair is merely apparent and not 

 real, the two stars not being in any way connected with one 

 another, one being immensely more distant than the other, and 

 their apparent nearness to each other being merely due to perspec- 

 tive, both happening to lie very nearly in the same line of sight. 



On the other hand, there are large numbers of double stars in 

 which careful and persistent observation has brought to light the 

 fact that both the components are moving in concert, and are in 

 fact two suns performing revolutions round each other, or rather 

 round the common centre of gravity of the pair, and it is these 

 movements of revolution which have been made to yield such 

 important results. Perhaps it will be best to divide the double 

 stars into four classes : — 



First, the binary stars proper, in which movements of revolu- 

 tion have been detected, such as Castor, 61 Cygni, and Sirius. 



Second, pairs in which no such movement has been discovered 

 (owing, perhaps, to its being too slow to be noticed), but in which 

 a common proper motion of both components through space 

 along parallel lines and in the same direction is found, rendering 

 it more than probable that some physical connection exists 

 between the pair — Mizar and its companion, together with Alcor 

 in Ursa Major, may be taken as examples of this class. 



Third, those doubles which are distinguished by quite opposite 

 characteristics, and in which no movement of revolution can be 

 detected, and where the proper motion of one is not participated 

 in by the other. Vega is an instance of this class of star. 



Our fourth class consists of those stars which — although we 

 cannot see that they are double, and which no telescope in the 

 world will show as more than a single point of light — we yet know 

 by a process of reasoning (which is as certain in its results as 

 ocular demonstration) to be really double, and to be composed of 

 two suns revolving round one another, but placed so close together 

 in comparison to the immense distance at which they are situated 

 from us that the spectroscope alone is able to divide them, and to 

 afford us proof of their double character. The primary star of the 

 Mizar system (which, when taken together, has supplied an 



