(Description) 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.

Week 7: Variable stars

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Two Hertzsprung – Russell diagrams showing the evolution of a star of similar mass to the Sun after leaving the Main Sequence. The left hand diagram is a schematic with a diagonal orange line from top left to bottom right depicting the Main Sequence. From the lower right of the Main Sequence is a red line leading at first upwards and to the right. This part of the track is labelled with letters A, B, C, and D, and 'red giant branch'. The track then heads to the left, labelled D – E – F and 'horizontal branch, before heading upward and to the right again, labelled 'asymptotic giant branch'. The track ends at the top right of the diagram in the red giant area. On the right is the HR diagram of a globular cluster, with individual stars indicated by dots. Only the lower right section of the Main Sequence contains stars. The pattern of stars then extends upwards and to the right, following the track of the red path indicated on the left hand plot. The upper parts of the plot are labelled HB (Horizontal Branch), RGB (Red Giant Branch) and AGB (Asymptotic Giant Branch) corresponding to the labels on the left hand diagram.