Page:Popular Science Monthly Volume 87.djvu/239

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not follow that the more distant and fainter stars will show the same preferential motions as the brighter and nearer ones which led Kapteyn to his hypothesis, though it should be said that a fairly extensive study of stars fainter than the Kapteyn stars made by Comstock led to results in good agreement with Kapteyn's. May it not be possible that the preferential motions observed are in some way connected with rotational phenomena within our stellar system, especially as the line of preferential motions lies approximately in the plane of the Milky Way, or are local to what we may call our region of the system, and not be true of the system as a whole?[1]

An alternative hypothesis of prevailing stellar motions, proposed by Schwarzschild, seems to have advantages from the point of view of probability, but it appears not to accord so well with the facts of observation. Schwarzschild suggests that if from a given point we draw vectors whose directions and lengths represent the directions and speeds of existing stellar motions, then the outer extremities of these vectors will define the surface of an ellipsoid (of preferential motions) having three unequal axes.

(To be continued)

  1. Turner has proposed the following explanation of the two star streams: The whole mass of the stellar system exerts a gravitational influence on the motion of each star in the system, and the individual stars revolve around the center of mass of the system in their elongated orbits. One star stream comprises all those stars moving away from the center, and the other stream all those stars moving toward the center. We can not doubt that the motions of the individual stars are influenced by the gravitational attractions of the stellar system and of the group of stars nearest to them; but observational data on stellar motions must be vastly more extensive than at present in order to test Turner's hypothesis.

    Halm, of the Cape of Good Hope, has given evidence of the existence of a third star stream, much less extensive than Kapteyn's two streams.