Page:Popular Science Monthly Volume 58.djvu/427

Rh the course of the galaxy from that constellation in the opposite direction from that which we have been following. South of Cassiopeia we see, near the central line, the well-known cluster forming the sword handle of Perseus. Farther south the belt grows narrower and fainter; although the irregularities of structure continue, they are far less striking than on the other side. On a moonlight evening it will scarcely be visible at all. If the moon is absent and the air clear we shall see that it grows slightly brighter toward the southern horizon, near which will be the narrowest part of its entire course. Below is the broad and diffused region in Argo.

One conclusion from the inequalities of structure which we have described will be quite obvious. The Milky Way is something more than the result of the general tendency of the stars to increase in number as we approach its central line. There must be large local aggregations of stars, because, as we have already pointed out, there cannot be such diversity of structure shown in a view of a very widely stretched stratum of stars. When, instead of a naked eye view of the belt, we study the photographs of the Milky Way, we find this evidence of clustering to grow still stronger. It is shown very strikingly in the photograph by Barnard, showing the singular rifts in the Milky Way in the constellation Ophiuchus. Yet more singular are three-minute openings in the constellation Aquila, the positions of which are:

The fundamental question which we meet in our farther study of this subject is: At what magnitude do these agglomerations of stars begin? Admitting, as we must, that they are local, are they composed altogether of stars so distant as to be faint, or do they include stars of considerable brightness? We consider this question in a way quite similar to that in which we discussed the clustering of the stars in the last chapter. We mark out on a map of the Milky Way the brightest regions—that is, those which include the densest agglomeration of very faint stars. We also mark out the darkest regions, including the coal sack. For this purpose I have taken the maps found in Heis's Atlas Cælestis for the northern portion of the Milky Way and the Atlas of Gould's Uranometria Argentina for the southern portion. In order to enable any one to repeat and verify the work I give the position of the central part of each patch or region studied. This serves simply for the purpose of identification. The outlines can be drawn by any one when the patch is identified. The third column of the table is given, approximately, the number of square degrees in the patch as outlined. Then follows the number of stars as found on the map. Here are included stars somewhat fainter than those regarded as lucid.