Absorption and Space Reddening in the Galaxy as Shown by the Colors of Globular Clusters

Abstract

The globular clusters, because of their great distances and intrinsic luminosities as compared with individual stars, furnish an excellent means for testing the transparency of space in the neighborhood of our galactic system. The photoelectric cell, when applied to the photometry of such objects, has the advantage of integrating the light effect from a luminous surface or a group of stars, and thus the total light or color of a cluster may be compared with that of a standard star. The present report is based upon observations made at Mount Wilson during the past year with a photoelectric cell attached to the 100-inch reflector. The photometer used was that which has been in operation for a number of years at the Washburn Observatory. The gas-filled potassium hydride cell by Kunz is of the type we have applied to the photometry of stars, and its construction in a quartz bulb eliminates the dark current, which is troublesome when currents of 1014 or 10-1l amperes are to be measured. The rate-ofcharge method with a Lindemann electrometer was used throughout the investigation. For measures of color the instrument is furnished with filters, which may be conveniently interposed in front of the cell. With a cell having a maximum sensitivity at 4500 A, the blue and yellow filters give effective maxima at about 4300 A and 4800 A, respectively. Between stars of spectrum AO and KO the combination gives a difference in color-index of 0.44 mag. This amount is not a large leverage for the determination of color-indices, but it maintains the possibility of reaching faint objects. At the Newtonian focus of the 100-inch reflector the ordinary limit for satisfactory measures of the total light of a star, nebula or cluster is about photographic magnitude 13.0, and with the loss in the filters the limit for colors is about 12.5. These limits can be extended somewhat, but for fainter objects the elimination of the effect of the sky background becomes increasingly difficult. The faintest object actually observed for total light was of magnitude 14.6, and satisfactory color-indices have been found for clusters and nebula ranging from 12.5 to 13.0. Since the recognized globular clusters are brighter than the thirteenth magnitude, it is possible to observe all of them that can be reached from Mount,Wilson, that is, all those north of declination -30?.