Page:Popular Science Monthly Volume 87.djvu/237

Rh great dimensions. This is the particular reason why a few astronomers suggest that the spirals may be distant systems of stars. They say that our own stellar system, if viewed from a great distance, might be seen to have a spiral structure: that it would be fairly circular in general outline if viewed from the poles of the Milky Way, or greatly elongated or spindle-shaped if the observer were in the plane of the Milky Way. We illustrate this point by means of well known spirals viewed broadside, and obliquely, and of the spindle-shaped nebulæ, which we do not doubt are spirals seen edgewise. Easton, the principal modern student of Milky Way structure, has even gone through the laborious task of assigning the stars, as seen from our viewpoint near the supposed center of the stellar system, to their assumed places in a spiral structure. But I need scarcely say that the subject is too vast for solution now, or in the near future. If our stellar system is one of a hundred thousand or more spiral nebulæ, we have at once the problem of determining the place of our stellar system in the larger universe of systems, necessarily beginning with the motion of our system as a whole with reference to the great numbers of surrounding systems.

It was supposed, until ten years ago, that the stars are moving approximately at random, both as to direction and as to speed. In 1904 Kapteyn announced, on the contrary, that the stars have decided preferences for motions toward two opposite points in the sky; one point in the northern edge of Orion, in the Milky Way; and the other point exactly opposite to this. Investigations by many others have in all cases confirmed Kapteyn's discovery. Kapteyn did not mean to say that the individual stars are moving parallel to a straight line joining these two