Page:Popular Science Monthly Volume 58.djvu/429

Rh II.—Southern part, from Gould.

To derive the best conclusions from these numbers we must compare them with the mean star-density for the sky in general, and for the regions near the galactic plane. Heis has 3,903 stars north of the equator; Gould, 6,755 south of it. The area of each hemisphere is 20,626 square degrees. It will be convenient to express the various star densities in terms of 100 square degrees as the unit of area. Thus we have the following star-densities according to the two authorities:

The first two pairs of numbers lead to the remarkable and unexpected conclusion that the darker regions of the Milky Way are but slightly richer in lucid stars than the average of the whole sky; certainly no richer than is due to the general tendency of all the stars to crowd toward the galactic plane. On the other hand, the bright areas are 60 per cent, richer according to Heis, and more than 100 per cent, richer according to Gould, than the darker areas seen among and around them. The conclusion is that an important fraction of the lucid stars which we see in the same areas with the agglomerations of the Milky Way is really in those agglomerations and form part of them.

A study quite similar to this has been made by Easton for the portions of the Milky Way between Cygnus and Aquila, where the diversities of brightness are greatest. His count of the stars in the bright and dark regions differs from that made above, principally by including all the stars of the Durchmusterung, which we may suppose to extend to about the ninth magnitude.