A STUDY of Schiaparelli's planispheres, which we gave in the last chapter, shows that some regions of the heavens are especially rich in lucid stars and others especially poor.
Neither telescope nor planisphere is necessary to show that many of those stars are collected in clusters. That the Pleiades form a group of stars by itself is clear from the consideration that six stars so bright would not fall so close together by accident. This conclusion is confirmed by their common proper motion, different from that of the stars around them. The singular collection of bright stars which form Orion, the most brilliant constellation in the heavens, and the little group called Coma Berenices — the Hair of Berenice — also suggest the problem of the possible connection of the stars which form them.
The question we now propose to consider is whether these clusters include within their limits an important number of the small stars seen in the same direction. If they and all the small stars which they contain were within their actual limits removed from the sky, would important gaps be left? The significance of this question will be readily seen. If important gaps would be left, it would follow that a large proportion of the stars which we see in the direction of the clusters really belong to the latter, and that, therefore, most of the stars would be contained within a limited region. The clusters which we shall especially study from this point of view are the Pleiades, Coma Berenices,. Praesepe and Orion.
The Pleiades. — In the case of this cluster the question was investigated by Professor Bailey, by means of a Harvard photograph 2° square, having Alcyone near its center. It was divided into 144 squares, each 10' on a side. The brighter stars of the cluster were included within 42 of these squares. The count of stars gave the results:
Within cluster: 1,012 stars, or 24 per square.
It, therefore, seems that the portion of the heavens covered by the cluster is actually poorer in stars than the region around it.
Two opposite conclusions might be drawn from this fact. Assuming that the difference is due to the presence of the cluster, we might suppose that the latter was formed of material that otherwise would have