Giant star degenerate helium fusion begins explosively, but most of the energy goes into lifting the degeneracy and the core becomes convective. The energy generated by helium fusion reduces the pressure in the surrounding hydrogen-burning shell, which reduces its energy-generation rate. The overall luminosity of the star decreases, its outer envelope contracts again, and the star moves from the red-giant branch to the horizontal branch. When the core helium is exhausted, a star with up to about has a carbon–oxygen core that becomes degenerate and starts helium burning in a shell. As with the earlier collapse of the helium core, this starts
Red giant stars develop into supergiants that follow an evolutionary track that takes them back and forth horizontally over the HR diagram, at the right end constituting red supergiants. These usually end their life as a type II supernova. The most massive stars can become Wolf–Rayet stars without becoming giants or supergiants at all. Red giants with known planets: the M-type HD 208527, HD 220074 and, as of February 2014, a few tens of known K-giants including Pollux, Gamma Cephei and Iota Draconis. Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary