Red-giant branch

The red-giant branch is a region of the Hertzsprung–Russell diagram traced by stars that have exhausted core hydrogen and expanded as hydrogen burns in a surrounding shell.

Main sequence: The long-lived phase in which stars fuse hydrogen into helium in their cores. Stars enter the red-giant branch after core hydrogen fusion ends.
Stellar evolution: The changes in a star’s structure, composition, and observable properties over its lifetime. The branch is one stage in the evolution of stars.
Red-giant branch tip: The brightest point reached by a low-mass star before helium ignites in its core. Its predictable luminosity makes the branch endpoint useful in stellar studies.
Red clump: A concentration of core-helium-burning stars on the cool side of the Hertzsprung–Russell diagram. It follows the red-giant branch after helium ignition in many low-mass stars.
Hydrogen-shell burning: Fusion of hydrogen in a layer surrounding a star’s inert helium core. This shell supplies energy as the star climbs the branch.
Stellar luminosity: The total energy a star emits per unit time across all wavelengths. Shell fusion raises luminosity as stars ascend the branch.
Tip of the red-giant branch distance indicator: A distance-measurement method using the nearly uniform peak luminosity of red-giant stars in the I band. The branch tip provides a standard candle for nearby galaxies.
Asymptotic giant branch: A late evolutionary phase of cool giants with inert carbon–oxygen cores and alternating hydrogen- and helium-burning shells. It is a later giant branch, reached after core helium burning ends.
Helium core: The central region of a star composed primarily of helium produced by hydrogen fusion. It grows and contracts while hydrogen burns around it.
Effective temperature: The temperature of a blackbody emitting the same energy per unit area as a star’s surface. Branch stars are cool at the surface despite their high luminosities.

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