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2.1 Globular cluster stars

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Figure 2: Hubble Space Telescope photograph of the dense globular cluster M80 (NGC 6093).

Because the clusters are of great age, most of the stars above about 0.8M ⊙ have already evolved off the main sequence. Thus, a large number of red giants are readily visible in most pictures of globular clusters (see Figure 2View Image). When viewing the color–magnitude diagram (CMD) for a globular cluster, one can clearly see the red giant branch lifting up away from the main sequence. The horizontal branch of evolved stars is also seen in the CMD for M80 shown in Figure 3View Image.

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Figure 3: Color–magnitude diagram for M80. Figure taken from the catalog of 52 globular clusters [203]. The entire catalog is available at the Padova Globular Cluster Group website [231].

The clearly visible turn-off point in the CMD for globular clusters is evidence for the roughly coeval nature of the stars in the cluster. During the early stages of the evolution of a globular cluster, most of the gas and dust has been swept away. Subsequent replenishment of the intercluster gas by stellar winds from evolved stars is removed during periodic passages of the cluster through the plane of the galaxy. The remaining gas is generally too hot for any star formation to take place [67]. Thus globular clusters are made up of old, population II stars.


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