102 THE POPULAR SCIENCE MONTHLY. 



quite correct in assuming that the distance is not less than two hun- 

 dred billions of miles. This star is, indeed, ten times as far from us 

 as a Centauri, which is generally considered to he the sun's nearest 

 neighbor in our sidereal system. The proper motion and the distance 

 of 1830 Groombridge being both assumed, it is easy to calculate the 

 velocity with which that star must be moving. The velocity is indeed 

 stupendous and worthy of a majestic sun ; it is no less than 200 miles 

 a second. It would seem that the velocity may even be much larger 

 than this. The proper motion of the star which we see is merely the 

 true proper motion of the star foreshortened by projection on the sur- 

 face of the heavens. In adopting 200 miles a second as the velocity 

 of 1830 Groombridge, we therefore make a most moderate assumption, 

 which may and probably does fall considerably short of the truth. 

 But, even with this very moderate assumption, it will be easy to show 

 that 1830 Groombridge seems in all probability to be merely traveling 

 through our system, and not permanently attached thereto. 



The star sweeps along through our system with this stupendous 

 velocity. Now, there can be no doubt that if the star were perma- 

 nently to retain this velocity, it would in the course of time travel 

 right across our system, and, after leaving our system, would retreat 

 into the depths of infinite space. Is there any power adequate to re- 

 call this star from the voyage to infinity ? We know of none, unless 

 it be the attraction of the stars or other bodies of our sidereal system. 

 It therefore becomes a matter of calculation to determine whether the 

 attraction of all the material bodies of our sidereal system could be 

 adequate, even with universal gravitation, to recall a body which seems 

 bent on leaving that system with a velocity of 200 miles per second. 

 This interesting problem has been discussed by Professor Newcomb, 

 whose calculations we shall here follow. In the first place, we require 

 to make some estimate of the dimensions of the sidereal system, in 

 order to see whether it seems likely that this star can ever be recalled. 

 The number of stars may be taken at one hundred million, which is 

 probably double as many as the number we can see with our best tele- 

 scopes. The masses of the stars may be taken as on the average five 

 times as great as the mass of the sun. The distribution of the stars is 

 suggested by the constitution of the milky waj 7 . One hundred million 

 stars are presumed to be disposed in aflat, circular layer of such dimen- 

 sions that a ray of light would require thirty thousand years to traverse 

 one diameter. Assuming the ordinary law of gravitation, it is now 

 easy to compute the efficiency of such an arrangement in attempting 

 to recall a moving star. The whole question turns on a certain criti- 

 cal velocity of twenty-five miles a second. If a star darted through 

 the system we have just been considering with a velocity less than 

 twenty-five miles a second, then, after that star had moved for a cer- 

 tain distance, the attractive power of the system would gradually bend 

 the path of the star round, and force the star to return to the system. 



