Red-giant branch Red-giant branch The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature. Red giants were identified early in the 20th century when the use of the Hertzsprung–Russell diagram made it clear that there
Red supergiant star than a hotter supergiant. For example, Alpha Herculis is classified as a giant star with a radius of while Epsilon Pegasi is a K2 supergiant of only . Although red supergiants are much cooler than the Sun, they are so much larger that they are highly luminous, typically . There is an upper limit to the luminosity of a red supergiant at around . Stars above this luminosity would be too unstable and simply don't form. Red supergiants have masses between about and . Main-sequence stars more massive than about do not expand and cool to become red supergiants. Red