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Astronomy with an online telescope

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Figure 3 The post main sequence evolution of a star like the Sun. The right-hand diagram shows a sample of stars from a globular cluster such as M13. Because globular clusters are old, the more massive stars have already left the main sequence to become red giants. Only the lower portion of the main sequence, containing stars of similar mass to the Sun and smaller, is still intact. These less massive stars have longer lifetimes so have not yet left the main sequence.

 1.2 Expansion and the red giant branch