THE BOUNDARIES OF ASTRONOMY. 103 



If therefore, the velocities of the stars were under no circumstances 

 more than twenty-five miles a second, then, supposing the system to 

 have the character we have described, that system might be always 

 the same. The stars might be in incessant motion, but they must 

 always remain in the vicinity of our present system, and our whole 

 sidereal system might be an isolated object in space, just as our solar 

 system is an isolated object in the extent of the sidereal system. We 

 have, however, seen that for one star at all events the velocity is no 

 less than 200 miles a second. If this star dash through the system, 

 then the attractions of all the bodies in the system will unite in one 

 grand effort to recall the wanderer. This attraction must, to some 

 extent, be acknowledged ; the speed of the wanderer must gradually 

 diminish as he recedes into space ; but that speed will never be lessened 

 sufficiently to bring the star back again. As the star retreats farther 

 and farther, the potency of the attraction will decrease ; but, owing 

 to the velocity of the star being over twenty-five miles a second, the 

 attraction can never overcome the velocity ; so that the star seems 

 destined to escape. This calculation is of course founded on our as- 

 sumption as to the total mass of the stars and other bodies which form 

 our sidereal system. That estimate was founded on a liberal, indeed, 

 a very liberal interpretation of the evidence which our telescopes have 

 afforded. But it may still fall short of the truth. There may be more 

 than a hundred million stars in our system : their average weight may 

 be more than five times the weight of our sun. But, unless the as- 

 sumption we have made is enormously short of the truth, our inference 

 can not be challenged. If the stars are sixty-four times as numerous, 

 or if the whole mass of the system be sixty-four times as great as we 

 have supposed, then the critical velocity would be 200 miles a second 

 instead of twenty-five miles a second. Our estimate of the system 

 would therefore have to be enlarged sixty-four-fold, if the attraction 

 of that system is to be adequate to recall 1830 Groombridge. It should 

 also be recollected that our assumption of the velocity of the star is 

 very moderate, so that it is not at all unlikely that a system at least one 

 hundred times as massive as the system we have supposed would be 

 required if this star was to be recalled. The result of this inquiry is 

 really only to be stated as an alternative : either our sidereal system 

 is not an entirely isolated object, or its bodies must be vastly more 

 numerous or more massive than even our most liberal interpretation of 

 observations would seem to warrant. It seems more reasonable to 

 adopt the first branch of the alternative. If this be so, then we see 

 that 1830 Groombridge, having traveled from an indefinitely great 

 distance on one side of the heavens, is now passing through our sys- 

 tem for the first and the only time. After leaving our system this star 

 will retreat again into the depths of space, to a distance which, for 

 anything we can tell, may be practically regarded as infinite. Although 

 we have only used this one star as an illustration, yet it is not to be 



