Red giant red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that. Red giants are stars that have exhausted the supply of hydrogen in their cores and have begun thermonuclear fusion of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them a
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