Page "Giant star" Paragraph 3

If a star is more massive than 0. 25 solar masses, then when it consumes all of the hydrogen in its core available for fusion, the core will begin to contract and hydrogen will begin to fuse to helium in a shell around the helium-rich core, and the portion of the star outside the shell expands and cools.

During this portion of its evolution, labeled the subgiant branch on the Hertzsprung – Russell diagram, the luminosity of the star remains approximately constant and its surface temperature decreases.

Eventually the star will start to ascend the red giant branch on the Hertzsprung – Russell diagram.

At this point, the surface temperature of the star, now typically a red giant, will remain approximately constant as its luminosity and radius increase drastically.

The core will continue to contract, raising its temperature .< sup >, § 5. 9 .</ sup >