What happens when a main-sequence star exhausts its core hydrogen fuel supply?

Explanation:

when a main-sequence star exhausts its core hydrogen fuel supply the core starts to shrink ( lack of fusion reactions) and the rest of the star starts to expands. The fusion reaction leaves the main sequence and begin to fuse helium in a shell outside the core. This mass stars become red supergiant  and then evolve to become blue super giant.

 Their temperature decreases dramatically, but their luminosity increases only slightly.

Explanation:  This is exact from Plato

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