Annu. Rev. Astron. Astrophys. 1989. 27: 139-59
Copyright © 1989 by . All rights reserved

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The concept of stellar populations, developed primarily by W. Baade in the 1950s, was inspired by the apparent dichotomy of galaxy content. Some galaxies (spirals and irregulars) seemed to be made up largely of bright, blue stars, with an admixture of interstellar material, whereas others (ellipticals) seemed to be made up exclusively of faint red stars (13). Baade called the two kinds of stars Population I and Population II, respectively. One of Baade's important insights was the relation of these two types of galaxy content with the properties of the stars in the Milky Way Galaxy (hereafter MWG), which provided him with kinematical information that eventually led to an astrophysical understanding of the differences in terms of the evolution of the MWG.

Much of Baade's original inspiration was derived from his detailed study of Local Group galaxies, and this particular sample of examples has continued to play an important part in the further development of our understanding of stellar populations. The history of this development has been the subject of a number of papers, reviews, and books, most recently that edited by Norman et al. (94). This review, therefore, limits itself primarily to a discussion of the most recent events, with a demonstration of both the failings and the strengths of Baade's arguments, especially in terms of the Local Group galaxies.