Page:Popular Science Monthly Volume 58.djvu/21

Rh an observable inequality. Such an inequality can arise only through the action of a neighborhood of a mass at least comparable with that of our sun. A new field of astronomical research is thus opened, the exploration of which must occupy many years. The ultimate result may be to make as great an addition to our knowledge of the heavens as has been made during the last century by the telescope.

A star-cluster is a bunch or collection of stars separated from the great mass of stars which stud the heavens. The Pleiades, or 'seven stars,' as they are familiarly called, form a cluster, of which six of the components are easily seen by the naked eye, while five others may be distinguished by a good eye.

About 1780 Michell, of England, raised the question whether, supposingthe stars visible to the naked eye to be scattered over the sky at random, there would be a reasonable possibility that those of the Pleiades would all fall within so small a space as that filled by the constellation. His correct conclusion was in the negative. It follows that this cluster does not consist of disconnected stars at various distances, which happen to be nearly in a line from our system, but is really a collection of stars by itself. Besides the stars visible to the naked eye, the Pleiades comprise a great number of telescopic stars, of which about sixty have been catalogued and their relative positions determined. The principal star of the cluster is Alcyone or η Tauri, which is of the third magnitude. The five which come next in the order of brightness are not very unequal, being all between the fourth and fifth magnitudes. Six are near the sixth magnitude. The remainder, so far as catalogued, range from the seventh to the ninth.

In this case there is a fairly good method of distinguishing between a star which belongs to the cluster and one which probably lies beyond it. This test is afforded by the proper motion. All the stars of the group have a common proper motion in the same direction of about seven seconds per century. The first accurate measures made on the relative positions of the stars of the cluster were those of Bessel, about 1830. In recent years several observers have made yet more accurate determinations. The most thorough recent discussion is by Elkin. One result of his work is that there is as yet no certain evidence of any relative motion among the stars of the group. They all move on together with their common motion of seven seconds per century, as if they were a single mass.

A closer cluster, which is plainly visible to the naked eye and looks like a cloudy patch of light, is Præsepe in Cancer. It is very well seen in the early evenings of winter and spring. Although there is nothing in the naked-eye view to suggest a star, it is found on telescopic