MOTION OF THE STARS. 197 



question naturally arises. Is the world of the fixed stars com- 

 oosed merely of a number of neighboring partial systems di- 

 vided into groups, or must we assume the existence of a uni- 

 versal relation, a rotation of all self-luminous celestial bodies 

 (suns) around one common center of gravity which is, either 

 filled with matter or void J We here, however, enter the 

 domain of mere conjecture, to which, indeed, it is not im- 

 possible to give a scientific form, but which, owing to the 

 incompleteness of the materials of observation and analogy 

 which are at present before us, can by no means lead to the 

 degree of evidence attained by the other parts of astronomy 

 The fact that we are ignorant of the proper motion of an in- 

 finite number of very small stars from the tenth to the four- 

 teenth magnitude, which appear to be scattered among the 

 brighter ones, especially in the important part of the starry 

 stratum to which we belong, the annuli of the Milky Way, 

 is extremely prejudicial to the profound mathematical treat- 

 ment of problems so difficult of solution. The contempla- 

 tion of our own planetary sphere, whence we ascend, from 

 the small partial systems of the moons of Jupiter, Saturn, 

 and Uranus, to the higher and general solar system, has 

 naturally led to the belief that the fixed stars might in a 

 similar manner be divided into several individual groups, 

 and separated by immense intervals of space, which again 

 (in a higher relation of these systems one to another) may 

 be subject to the overwhelming attractive force of a great 

 central body (one sole sun of the whole universe).* The in- 

 ference here advanced, and founded on the analogy of our 

 own solar system, is, however, refuted by the facts hitherto 

 observed. In the multiple stars, two or more self-luminous 

 stars (suns) revolve, not round one another, but round an 

 external and distant center of gravity. No doubt something 

 similar takes place in our own planetary system, inasmuch 

 as the planets do not properly move round the center of the 

 solar body, but around the common center of gravity of all 

 the masses in the system. But this common center of grav- 

 ity falls, according to the relative positions of the great plan- 

 ets Jupiter and Saturn, sometimes within the circumference 

 of the sun's body, but oftener out of it.t The center of 

 gravity, which in the case of the double stars is a void ia 



* Toe value or worthlessness of such views has been discussed by 

 Argelanderin his essay, "Ueber die eigene Bewegung des Sonnensystemt 

 hergeleilet aus der eigenen Bewegung der Sterne, 1837, s. 39. 



t See Cosmo*, vol. i., p. 145. (Madler, Astr., p. 400.) 



