It will be only when the two bodies, coming from contrary directions, collide with equal momentum that the entire motion will be stopped. But in the case of stellar masses moving, as it were, at random in every direction this is a condition which will but rarely occur. Accordingly, in most cases the resulting stars will have more or less motion. In short, the stars should, according to the theory, be moving in all directions and with all varieties of velocity. Further, it follows that these motions ought to be in perfectly straight lines, and not in definite orbits of any kind. So far as observation has yet determined, all these conditions seem to be fulfilled. Sometimes it will happen that the two bodies will strike each other obliquely. In this case the resulting star, both as to the direction and velocity of its motion, will, to a large extent, be the resultant of the two concurrent forces.
III. Motion of the Stars not due to their Mutual Attractions.
According to the theory the absolute motion of the stars is due, not to the influence of gravity, but to motions which originally belonged to the two component masses out of which the star arose; motion regarding the origin of which science can no more inform us than it can regarding the origin of the masses themselves. There is strong presumptive evidence that the motion of the stars is due to this cause. We know that there are stars which have a far greater velocity than can result from gravitation, such, for example, as the star 1830 Groombridge, which has a velocity of 200 miles per second. Suppose, with Professor Newcomb, that the number of stars belonging to the universe amounts to 100,000,000, and that these have, on the average, five times the mass of the sun, and are spread out in a layer across which light requires 30,000 years to pass. Then computation shows that, unless the attractive power of the whole were sixty-four times greater than it really is, it could not have conferred on Groombridge the motion which it possesses, or arrest it in its onward course.[[7]] We are therefore forced, as Professor Newcomb remarks, to one of two alternatives, viz.: “Either the bodies which compose our universe are vastly more massive and numerous than telescopic examination seems to indicate, or 1830 Groombridge is a runaway star, flying on a boundless course through infinite space, with such momentum that the attraction of all the bodies of the universe can never stop it.”
As regards the theory we are discussing, it is the same which alternative is taken, for both are equally favourable. If the former, then, according to the theory that stellar heat had its origin in collision, it is presumptive evidence that space is occupied by dark bodies far more numerous and massive than the luminous ones which the telescope reveals. If the latter, viz. that the star has a velocity which never could have been produced by attraction, “then,” as says Professor Newcomb, “it must have been flying forward through space from the beginning, and, having come from an infinite distance, must now be passing through our system for the first and only time.” The probability is, however, that the star derived its motion from the source from which it derived its light and heat; namely, from the collision of the two masses out of which it arose. If the star is ever to be arrested in its onward course, it must be by collision; but such an event would be its final end.
There are other stars, such as 61 Cygni, ε Indi, Lalande 21258, Lalande 21185, μ Cassiopeiæ, and Arcturus, possessed of motions which could not have been derived from gravity. And there are probably many more of which, owing to their enormous distances, the proper motions have not been detected. α Centauri, the nearest star in the heavens, by less than one-half, is distant twenty-one millions of millions of miles; and there are, doubtless, many visible stars a thousand times more remote. A star at this distance, though moving transversely to the observer at the enormous rate of 100 miles per second, would take upwards of thirty years to change its position so much as one second, and consequently 1,800 years to change its position one minute. In fact, we should have to watch the star for a generation or two before we could be certain whether it was moving or not.
IV. Probable Origin of Comets.
Great difficulty has been experienced in accounting for the origin of comets upon the nebular hypothesis. They approach the sun from all directions, and their motions, in relation to the planets, are as often retrograde as direct. Not only are their orbits excessively elliptical, but they are also inclined to the ecliptic at all angles from 0° to 90°. It is evidently impossible to account satisfactorily for the origin of comets if we assume them all to have been evolved out of the solar nebula, although this has been attempted by M. Faye and others. Comets are evidently, as Laplace and Professor A. Winchell both conclude, strangers to our system, and have come from distant regions of space. If they belonged to the solar system they could not, says Professor Winchell, have parabolic and hyperbolic paths. “Only a small portion of the comets,” he remarks, “are known to move in elliptic orbits.”[[8]] This assumption that they are foreigners will account for all the peculiarities of their motions; but how are we to account for their coming into our system? How did they manage to leave that system in which they had their origin? If a comet have come from one of the fixed stars trillions of miles distant, the motion by which it traversed the intervenient space could not, possibly, have been derived from gravity. We are therefore obliged to assume that the motion was a projected motion. Comets, in all probability, have the same origin as meteorites. The materials composing them, like those of the meteorites, were probably projected from nebulæ by the expulsive force of the heat of concussion which produced the nebulæ. Some of them, especially those with elliptic orbits, may have possibly been projected from the solar nebula.
V. Nebulæ.
It is a curious circumstance that the theory here advanced seems to afford a rational explanation of almost every peculiarity of nebulæ, as I have, on former occasions, endeavoured, at some length, to prove.[[9]]
1. Origin of nebulæ.—We have already seen that the theory affords a rational account of the origin of nebulæ.