THE STELLAR UNIVERSE. 



373 



is discovered. In most of those in which it has been discovered, its amount, 

 even after the lapse of years, is still but small. In one or two it is suf- 

 ficiently great to be detected by very ordinary means of astronomical obser- 

 vation. The greatest proper motion which has hitherto been observed in 

 any single star is found in the star p in the constellation of Cassiopeia. The 

 annual displacement of this star amounts to 3 /7 '74, so that in 500 years it 

 will be removed from the place it now occupies by a space equal to the ap- 

 parent diameter of the moon. The annual proper motion of Arcturus is 

 about half that of p Cassiopeia. In the following table is collected the proper 

 motions as they affect the declination and right ascension of some of the 

 stars in which this phenomenon is most conspicuous. The sign -j- prefixed 

 to the annual variation, shows that it is to be added, and that it is to be 

 subtracted, to find the true place of the object at any time : 



But it is among the double stars that we find the most remarkable examples 

 of proper motion. These systems, while their component stars revolve one 

 round the other, or rather round their common centre of gravity, seem to 

 be carried forward in some determinate direction with a motion in which 

 they both participate. Thus the individuals which compose the double star 

 61 Cygni (of which Professor Bessel has discovered the annual parallax), 

 have remained constantly at nearly the same distance from each other for 

 sixty years last past, but have at the same time been continually shifting their 

 position on the firmament, and are now about five minutes from the place 

 they occupied sixty years ago. In 350 years this double star will move over 

 a space on the firmament equal to the diameter of the moon. 



The only conceivable explanation of the phenomena of the proper motions 

 of the stars, is the supposition that these bodies actually have real motions 

 through space, such as to produce the apparent changes of position which we 

 observe. If the distance of any star having a proper motion be known, the 

 rate at which it moves may be easily calculated. Thus, if we assume the 

 distance of 61 Cygni, as determined by Bessel's observations on the parallax, 

 to be 60,000,000,000,000 miles, their motion must be at the rate of one hun- 

 dred and seventy-seven thousand miles an hour, in order to produce the ap- 

 parent annual displacement which has been observed. This velocity would 

 be double that of the orbitual motion of the earth. 



Among the proper motions of the stars, there is no apparent relation noth- 

 ing to lead to the conjecture that these phenomena are ascribable to any 

 common physical cause affecting at once all these bodies. We must then 

 infer that they are independent motions affecting these distant systems inde 

 pendent at least, so far as our present knowledge extends. 



