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

1.2 The Population Line

As a better understanding of stellar evolution was developed, and as ideas about galaxy evolution progressed, these criteria were supplanted by two more fundamental ones: stellar ages and stellar chemical abundances. It appeared to be the case, at least for the MWG, that all of the other observed differences could be understood in terms of these two variables, and therefore that stellar populations could be represented by two segments of a line that described the chemical enrichment of a stellar group due to stellar evolution (Figure 2). As implied by this representation, there can be a variety of different ages among Population I or II stars, with a corresponding range in heavy element abundances. Intermediate populations were introduced, especially by those astronomers trying to understand the kinematics and spatial distribution of stars in the MWG, and the simple two-point system broke down, to be supplanted by a linear progression of types.

Figure 2. The two populations as evolutionary line segments.