Studies in Spectral Classification. II. The HR Diagram of NGC 6530.

New, more accurate, spectral types on the MK system have been obtained for twenty-five of the brightest members of the cluster NGC 6530, which is located in the central region of the emission nebula M8. A discussion of these types, together with the UB V photometry of Johnson and of Walker, indicates modification of Walker's conclusion that a normal main sequence from OS to about A0 is present; the well-defined clustering located in the central region of the emission nebula consists-for the brightest a group of stars of luminosity classes IV and V; these do not define a "normal" main sequence, in the sense of such clusterings as that surrounding the star' Orionis. The mean of the directly determined color excesses is +0.36 mag. The values of (U - B),, computed from the individual values of E( v), suggest that there is an ultraviolet excess-or that there are systematic errors in the photometry in the ultraviolet. The early B stars of the cluster are mostly of class Be, and tend to have abnormally strong absorption lines of hydrogen; in this respect, they show similarity to the stars of the Orion Nebula cluster. In addition, star No. 66 of Walker's catal6gue i a "helium star," with abnormally strong absorption lines of He I. A true distance modulus of 11 0 is found for NGC 6530, on the assumption of a ratio of 3 to 1 for total to selective absorption.