COLOR INDICES AND THE DISTANCES OF GALACTIC CLUSTERS

Abstract

Although it is true, as Dr. Trumpler has remarked in a recent paper,1 that the data on color indices in the galactic clusters are relatively incomplete and inhomogeneo us, the writer thought it might be appropriate to call attention to the importance of further work in this field, since the color indices, as well as the spectral types, afford a complete solution to the problem of determining the distances of these objects. Furthermore, the colors appear to have several advantages over the spectral types, which make it seem likely that for the smaller clusters, the color-index method will be the only one possible. Ability to reach stars three to four magnitudes fainter than those for which spectral types can be determined and good resolving power are of importance for studying these objects. With the aid of colors, distances for most of the galactic clusters may be obtained from main-sequence stars, whose dispersion in absolute magnitude is smaller than that for the brightest stars, and from the stars in the central regions of the clusters, where a high percentage of the stars are cluster members. Illustrated in Figure 1 are the relations between color and apparent magnitude which one may expect to observe in the galactic clusters. Since the cluster stars are all at very nearly equal distances from us, the apparent magnitudes represent the absolute magnitudes, and the resulting relation closely resembles the conventional Russell-Hertzsprung diagram. Figure la represents an unobscured cluster, distant r0 parsecs. Figure 'b shows the same relation for a similar cluster, situated at a distance ri behind an obscuring cloud which has absorbed A magnitudes and introduced a color excess E. The distance of the unknown cluster (Figure 'b) may readily be obtained. The color excess, E, may be estimated from the situation of the color-magnitude relation, assumed to be intrinsically similar in form to that of the known cluster, with respect to the zero of color index.