-v at approximately the same time. As a cluster ages, later and later spectral types evolve away from the main sequence. This means the turn-off point shifts to later spectral types as the cluster ages. We can tell the relative age of two clusters by comparing their turn-off points. If we know how long different spectral type stars actually stay on the main sequence, we can tell the absolute age of a cluster from its turn-off point.

We note that there are some galactic clusters, not shown in Fig. 13.9, that are missing the lower (cooler) end of the main sequence. We think that these clusters are very young. The lower mass stars are still in the process of collapse, and have not yet reached the main sequence. (We think that lower mass stars take longer to collapse than higher mass stars.)

An HR diagram for a composite of globular clusters is shown in Fig. 13.11. These appear to be different from the HR diagrams for galactic

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