The Resolution of Messier 32, NGC 205, and the Central Region of the Andromeda Nebula.

Abstract

Recent photographs on red-sensitive plates, taken with the 100-inch telescope, have for the first time resolved into stars the two companions of the Andromeda nebula-Messier 32 and NGC 205-and the central region of the Andromeda nebula itself. The brightest stars in all three systems have the photo- graphic magnitude 21.3 and the mean color index +1.3 mag. Since the revised distance-modulus of the group is m - M = 22.4, the absolute photographic magnitude of the brightest stars in these systems is Mpg = -1.1. The Hertzsprung-Russell diagram of the stars in the early-type nebulae is shown to be closely related to, if not identical with, that of the globular clusters. This leads to the further conclusion that the stellar populations of the galaxies fall into two distinct groups, one represented by the well-known H-R diagram of the stars in our solar neighborhood (the slow-moving stars), the other by that of the globular clusters. Characteristic of the first group (type I) are highly luminous 0- and B-type stars and open clusters; of the second (type II), short-period Cepheids and globular clusters. Early-type nebulae (E-Sa) seem to have populations of the pure type II. Both types seem to coexist in the intermediate and late-type nebulae. The two types of stellar populations had been recognized among the stars of our own galaxy by Oort as early as 1926