|Annu. Rev. Astron. Astrophys. 1989. 27:
Copyright © 1989 by . 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.