The illustration to the right is a Hertzprung-Russell diagram of two star clusters, and is used to infer ages for these clusters. How is that done? Stellar clusters are gravitationally bound and so the stars seem to be of the same general age and composition, also they are at about the same distance from us. So, on the physics of collapsing Hydrogen-rich gas clouds (in turn based on relativity, atomic physics, spectroscopy etc), star formation, and the resulting life cycle, in particular the model timeline for main sequence turnoffs to the giant band, we can estimate the age of the cluster. In this case, M67 is estimated at ~ 4BY, and NGC 188 at ~ 5 BY. (SOURCE: Wiki CCA, … Continue reading He said it: Toronto of TSZ etc on abductive inference to best explanation in science