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 its evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, from 5,000 K and lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars. Because these stars have expanded so much, they are rarely found with more than one usable planet-if any at all.

The most common red giants are stars nearing the end of the so-called red-giant-branch (RGB) but are still fusing hydrogen into helium in a shell surrounding a degenerate helium core. Other red giants are: the red clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and sometimes with a hydrogen burning shell just beyond that.

The nearest red giant to Terra is Gamma Crucis, 88 light years away, but the orange giant Arcturus is described by some as a red giant and it is 36 light years away.