red giants

A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. The appearance of the red giant is from yellow-white to reddish-orange, including the spectral types K and M, sometimes G, but also class S stars and most carbon stars. Red giants vary in the way by which they generate energy: • most common red giants are stars on the red-giant branch that are still fusing hydrogen into helium in a shell surrounding an inert helium core • red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process • asymptotic-giant-branch stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen-burning shell just beyond that. Many of the well-known bright stars are red giants because they are luminous and moderately common.

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