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
Hydrogen-deficient star Hydrogen-deficient star A hydrogen-deficient star is a type of star that has little or no hydrogen in its atmosphere. Hydrogen deficiency is unusual in a star, as hydrogen is typically the most common element in a stellar atmosphere. Despite being rare, there are a variety of star types that display a hydrogen deficiency. Hydrogen-deficient stars had been noted prior to the discovery of their hydrogen deficiency. In 1797, Edward Pigott noted the profound variation in stellar magnitude of R Coronae Borealis (R CrB). In 1867, Charles Wolf and Georges Rayet discovered unusual emission line structure in Wolf-Rayet stars. Hydrogen deficiency