A low or intermediate mass star (0.4–8 M) in the late phase in its evolution becomes a red giant. After exhaustion of the central hydrogen fuel, the stellar core contracts, and the envelope expands (to tens or hundreds of times its size on the main sequence) and cools. Hydrogen fuses to helium in a shell surrounding the core, releasing more power (energy per unit time) than on the main sequence, and the point representing the star ascends the red giant branch of the Hertzsprung-Russell diagram. The ascent stops when core temperature allows helium fusion, which takes place in the red clump (for metal-rich stars) or in the horizontal branch (for metal-poor stars). More massive stars become red supergiants, which are substantially more luminous, while red dwarfs are fully convective and never evolve to red giants (no differentiation occurs between core and envelope).