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 in part because as the Sun burns hydrogen, it loses mass causing Earth (and all planets) to orbit farther away. There are also significant uncertainties in calculating the orbits of the planets over the next 5–6.5 billion years, so the fate of Earth is not well understood. At its brightest, the red-giant Sun will be several thousand times more luminous than today but its surface will be at about half the temperature. Red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The