*THE POPULAR SCIENCE MONTHLY.*

quite correct in assuming that the distance is not less than two hundred billions of miles. This star is, indeed, ten times as far from us as *α* 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 surface 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 permanently 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 recall 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 telescopes. 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 way. One hundred million stars are presumed to be disposed in a flat, circular layer of such dimensions 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 critical 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 certain 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.