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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 0.8Mo . 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. The diagram on the right focuses on the turn-off of the main sequence and the red giant branch. The diagram on the left indicates a number of different objects. HB indicates the horizontal branch, RGB is the red giant branch, SGB is the subgiant branch, and BSS indicate the blue stragglers. Blue stragglers will be discussed later in this review and the interested reader can consult [46Jump To The Next Citation Point] for a discription of the other objects. Figure taken from Ferraro et al. [46].
That there is a 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 [49]. Thus globular clusters are made up of old, population II stars.

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