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Diagram showing Helium fusion and expansion in a post-main sequence star. On the left, a cross section of a star is shown with an orange core labelled as containing Helium, surrounded by yellow outer layers containing Hydrogen. Six inward-pointing arrows around the core indicate inward collapse. On the right is a larger start with red outer layers, labelled 'Radiation pressure causes outer envelope to expand' (the red colour indicates that these outer layers cool during the expansion). Outward facing arrows depict this expansion. In the centre is a hot core labelled 'Helium fusion 3 – alpha' surrounded by a thin circular yellow zone labelled 'Hydrogen fusion shell'.

 1.2 Expansion and the red giant branch