<P> A red giant is a luminous giant star of low or intermediate mass (roughly 0.3--8 solar masses (M)) in a late phase of stellar evolution . The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,000 K (8,540 ° F) 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 . </P> <P> The most common red giants are stars on the red - giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert 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 a hydrogen burning shell just beyond that . </P> <P> Red giants are stars that have exhausted the supply of hydrogen in their cores and have begun thermonuclear fusion of hydrogen in a shell surrounding the core . They have radii tens to hundreds of times larger than that of the Sun . However, their outer envelope is lower in temperature, giving them a reddish - orange hue . Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size . Red - giant - branch stars have luminosities up to nearly three thousand times that of the Sun (L), spectral types of K or M, have surface temperatures of 3,000--4,000 K, and radii up to about 200 times the Sun (R). Stars on the horizontal branch are hotter, with only a small range of luminosities around 75 L. Asymptotic - giant - branch stars range from similar luminosities as the brighter stars of the red giant branch, up to several times more luminous at the end of the thermal pulsing phase . </P>

A low mass star that converts helium to carbon is a