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
Hydrogen as dark matter and dark energy.) This element is found in great abundance in stars and gas giant planets. Molecular clouds of H are associated with star formation. Hydrogen plays a vital role in powering stars through the proton-proton reaction and the CNO cycle of nuclear fusion. Throughout the universe, hydrogen is mostly found in the atomic and plasma states, with properties quite different from those of molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged