Star Cluster

A

number of the naked-eye stars in the constellation Orion also form a large loosely organized cluster, compobA almost wholly of the very hot stars of spectral class B. There are similar large loosely defined groups of B stars in Vela, Centaurus, Scorpio and Perseus. In fact, J. C. Kapteyn's researches and the subse quent investigations by C. V. L. Charlier and Harlow Shapley have shown that these various groups of B stars all together form a larger system of which the total diameter is probably in excess of a thousand parsecs (a parsec is 3 X kilometers, approxi mately). This system of loose clusters, named the "local system" by Shapley, is known to contain a great number of stars besides those in the localized B type clusters; according to recent re searches by F. H. Seares, it also includes the vast majority of all stars surrounding the sun. The local system is, in fact, rather in the nature of a star cloud, like those in the Milky Way, or like the Magellanic Clouds, and as such it need receive no further consideration in this discussion of star clusters.

Systems more definitely circumscribed than the constellations just named are the Hyades, the Pleiades, Coma Berenices, Prae sepe, and the Double Cluster in Perseus. All these clusters are

visible with the unaided eye, and for all of them the proper mo tions (angular speeds) have been studied in much detail. An in vestigation of the proper motions of the individual stars of a cluster leads to the discovery, first made for the Hyades by Lewis Boss, that the paths converge towards a common point (or diverge from one). The convergence is obviously a matter of perspective. When the convergent point is accurately deter mined, and the motion of one or more of the stars of the cluster in the line of sight has been determined spectroscopically, then we can determine with high accuracy the linear distance to the cluster and the actual speed of the stars. If V is the radial velocity of the star in kilometers per second, its proper motion in seconds of arc per year, and 0 the angular distance from the star to the convergent point, then the distance in parsecs, R, is given by