Characteristics
Red giants are stars that have exhausted the supply of hydrogen in their cores and switched to 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 large size. Red-giant-branch stars have luminosities about a hundred to several hundred times the Sun (L☉), spectral types of K or M, temperatures of 3,000–4,000K, and diameters about 20–100 times the Sun (R☉). Stars on the horizontal branch are hotter, while AGB stars are around ten times more luminous, but both types are less common than normal red giants.
Among the AGB stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements from helium burning are dredged up to the surface. The first dredge up occurs during hydrogen shell burning on the red-giant branch, but does not produce dominant carbon at the surface. The second, and sometimes third, dredge up occurs during helium shell burning on the AGB and convects carbon to the surface in sufficiently massive stars.
The stellar limb of a red giant is not sharply-defined, as depicted in many illustrations. Instead, due to the very low mass density of the envelope, such stars lack a well-defined photosphere. The body of the star gradually transitions into a 'corona' with increasing radii. The coolest red giants have complex spectra, with molecular lines, masers, and sometimes emission.
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