A red giant is a luminous giant star of low or intermediate mass in a late stage of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature cool, between 5,000 K and 3,000 K. A red giant’s radius is typically between 10 and 100 times that of the Sun. Its luminosity can be up to 1,000 times that of the Sun. However, the final fate of stars like our Sun is to become white dwarf stars. Stars in this class are called “asymptotic-giant branch” (AGB) stars after they leave the main sequence (MS) turnoff point on the Hertzsprung–Russell diagram.
Red giants are evolved from main-sequence stars with masses in the range 0.3–8 solar masses (). When a star exhausts all its hydrogen fuel at its core it starts to fuse helium into carbon via the triple-alpha process where three helium nuclei combine to form carbon. For most mass ranges above about 0.5 solar masses (), this eventually causes thermal pulses during which energy production fluctuates strongly for periods of 10 million years or more. As such pulsing begins, a star swells out dramatically into an enormous bright red object known as a Mira variable; when these events subside however it becomes what is known as an Asymptotic Giant Branch star instead. Thereafter different types of fusion continue in layers outside the core: first hydrogen shell burning around a degenerate helium core; then helium shell burning around an inert carbon–oxygen core, before finally either carbon–oxygen or neon–oxygen fusion occurs at extremely high temperatures near the center of the star. If sufficient mass loss takes place during any one of these phases then some material may be ejected from outside even before it has reached nuclear ignition temperatures; if not enough mass loss takes place however then no further ejecta will occur until after degeneracy has lifted—at which point much more violent supernova explosions can take place instead.”