Page:Popular Science Monthly Volume 58.djvu/430

422 He divides the regions studied into six degrees of brightness. For our present purpose it is only necessary to consider three regions, the brightest, the faintest and those intermediate between the two. Besides the count from the Durchmusterung he made a count of the same sort from Dr. Wolf's photographs and from Herschel's gauges of the heavens. In the following table I have reduced all his results, so as to express the number of stars in a square degree in the three separate regions. At the top of each column is given the authority, whether Argelander, Wolf or Herschel. Wolf had two sets of photographs, one supposed to include all the stars to the eleventh, the other to the twelfth magnitude. The magnitudes included are given in the second line. That Herschel's count extends to the fifteenth magnitude is by no means certain; but we can judge from the great number of his stars that it goes considerably beyond Wolf's in the faintness of the stars included. Below this we give, in the regions A, B and C, which are, respectively, those of least, of medium and of greatest brightness, the number of stars per square degree according to each of the authorities:

The vastly greater number of individual stars per square degree in the brighter regions is what we should expect from the studies we have made of the lucid stars. But what is of most interest in the table is the continual increase in the proportion of faint stars in the separate regions. We notice that, when we consider only the stars of the ninth magnitude, there are twice as many in the brightest as in the darkest portions. When we go to the eleventh magnitude, as shown by Wolf's photograph A, we find the number of stars in the brighter regions to be threefold. When the twelfth magnitude is included we find that there are between five and six times as many stars in the bright regions as in the dark ones. Finally, when we come to stars from Herschel's gauges there are fourteen times as many stars per square degree in the brighter regions as in the dark.

At first sight this result seems to show a great difference between the clusters of stars described in the last chapter, and the collections of the Milky Way, in that the former include few or no faint stars, while the latter include a greater and greater number as we ascend in the scale of magnitude. This difference is important as showing a vastly greater range of actual brightness among the galactic stars than among those which form the scattered clusters. Allowing for this difference,