Red giant causing helium in a shell to begin fusing. At the same time hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch, a second red-giant phase. The helium fusion results in the build up of a carbon–oxygen core. A star below about will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf. The ejection of the outer mass
Giant star Stars still more luminous than giants are referred to as supergiants and hypergiants. A hot, luminous main-sequence star may also be referred to as a giant, but any main-sequence star is properly called a dwarf no matter how large and luminous it is. A star becomes a giant after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence. The behaviour of a post-main-sequence star depends largely on its mass. For a star with a mass above about 0.25 solar masses (), once the core is depleted of hydrogen it