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

1.3 The Population Plane

Figure 3 represents a more morphological approach, including as it does possibilities of old metal-rich stars and young metal-poor stars. In the 40 years since the introduction of the two populations, increasing evidence has suggested that such stars do exist in certain environments, especially in other galaxies, leading to a further breakdown in the simple two-population classification scheme.

Figure 3. Morphology of the evolutionary plane.

Figure 4 illustrates how this situation arises, using the MWG and the Small Magellanic Cloud (SMC) as examples. Probably because of its lower mass, and thus of a consequent different star formation rate history, the SMC has experienced a slower enrichment of heavy elements. Therefore, it contains some stars that are relatively young but still metal poor. We do not, on the other hand, find or expect to find well-mixed galaxies or segments of galaxies that have old metal-rich stars together with young metal-poor ones, as enrichment, of course, only proceeds in one direction. Thus, the entire area of Figure 3 is not inhabitable by a given star group, though a composite galaxy, such as the MWG, can have a spread in these parameters.

Figure 4. Schematic enrichment lines for the Milky Way Galaxy (MWG) and the Small Magellanic Cloud (SMC).