No. 3.

Temporary Stars: Possibly Cases of Partial Impact.

From the very considerable velocity of the proper motion of the so-called fixed stars, and the want of any definite order in their motion it is almost certain that they must frequently page 5 come within the influence of each other's effective attraction and sometimes into collision. When collision occurs, as I have shown in my last letter, partial impact will be the general case and complete impact the exception. I shall also attempt to shew that partial impact offers an explanation of all cases of temporary stars. The companion to Sirius, the existance of variable stare, and the high temperature of radiation into space, all point to dark or feebly luminous bodies in space. The following reasonings apply equally to the collision of these bodies or to ordinary luminous stars. If two bodies come into complete collision with a definite velocity the destruction of the molar motion would develope a high temperature. If, with the same velocity, a partial collision were to occur, a piece of each would be struck off and would coalesce. The motion destroyed will be identically the same in this case, and consequently the temperature will the same in the coalesced body, no matter how small it may be. In these two cases we have two masses, one largo and one small, and both at the same temperature. It is possible to show that in complete collision, produced by attraction from infinite distance, the heat developed is far smaller than that necessary to project the whole of the matter into space, away from the influence of the bodies' attraction. Let us suppose that each of the bodies struck off one-tenth of their mass, the temperature is the same as with complete coalescence, but the attraction is a very small fraction, clearly the molecular velocity due to the temperature may be great enough to carry every molecule independently into space If one-tenth is not sufficient take one-hundredth, the temperature is still the same, but the attractive force almost nothing. Here is a definite "vera causa," which exactly agrees with the great initial brilliancy and rapid disappearance of these stars. The star of Tycho Brabe is supposed to be a recur rant temporary star, may it not be that partial impact occurred, that the two retreating masses returned by mutual attraction and, struck off a star at each journey, such a return is highly probable; but if a second impact is not very likely, so there would be a very striking difference in the time of return. The extreme brilliancy of Tycho's star supposes a large piece struck off, but the rapidity of its disappearance proves it to have been only partial impact. It is only necessary to vary the conditions of impact, and I have not read of any case of temporary stars which cannot be explained on this hypothesis.

Thus, partial impact appears to be at once, not only very much the most frequent case of collision, but it also possesses capabilities of producing a variety of these phenomena to which no other single hypothesis approaches.

Note—In complete coalescence the velocity destroyed will always be somewhat greater than a similar case of partial impact, as the centres of the two bodies will not be so close, and consequently the attraction will be less and the velocity acquired also leas, neither will there be such complete destruction of motion. Any physicist will readily see that this is not enough to affect the reasoning in the slightest degree, for it is only necessary to assume a smaller fraction at impact to obtain the same result.