EVIDENCE --- Image of young stellar association(s).
The question remains, do we have evidence to support this hypothesis? After all, it was developed using purely qualitative arguments based on basic physical principles. Figure XX shows a Hertzsprung-Russell Diagram for three young stellar populations: the Orion Nebula Cluster, the Upper Scorpius subgroup of the Scorpius-Centaurus OB Association, and the Pleiades open cluster. [maybe only one example?] Notice that the hotter and presumably more massive stars lie closer to theoretical predictions for the ZAMS than do the cooler stars, which tend to lie well above the ZAMS. However, the degree to which the cooler stars lie above the ZAMS is not constant and depends on which stellar population we consider, but hotter star remain near the ZAMS in each case, lending support to our intuitive reasoning.