Red-giant branch Red-giant branch The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature. Red giants were identified early in the 20th century when the use of the Hertzsprung–Russell diagram made it clear that there
Red giant to the planet when the giant expands out to the orbital distance of the planet. Many of the well known bright stars are red giants, because they are luminous and moderately common. The red giant branch variable star Gamma Crucis is the nearest M class giant star at 88 light years. The K0 red giant branch star Arcturus is 36 light years away. In about 5 to 6 billion years, the Sun will have depleted the hydrogen fuel in its core. It will shrink, with the hydrogen outside the core able to compress enough for hydrogen there to fuse, and