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

1.1 The Two-Point Diagram

Baade's original criteria for distinguishing Population I from Population II included four parameters: location, color, association with interstellar material, and kinematics. However, he found that these features are interrelated in such a way that all stars seemed to be classifiable by only two parameters. Figure 1 illustrates this division into types, represented by two points in a plane.

Figure 1. The traditional two-point population diagram.