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red giant

(very bright red star)

A red giant is a star that is bright and red, with a sufficiently cool photosphere for the red color, achieving their brightness by being giant stars, with radii much larger than that of a main sequence star, on the order of an astronomical unit. Their luminosity is like that of an early main sequence star, such as A-type star or B-type star, but being less massive and opaque, the outward-moving energy (basically, radiation pressure) inflates them up larger than would a more massive star, and the energy spread out over the larger surface area leads to the lower-temperature photosphere, and a red color, i.e., K-type star, M-type star, or S-type star.

Stars at various stages in the post main sequence can be red giants, including the red-giant branch, horizontal branch, and asymptotic giant branch, but some giant stars are blue (blue giant) or yellow (yellow giant). Example red giants:

(star type,stellar evolution)

Referenced by:
asymptotic giant branch (AGB)
binary star
Canis Major Dwarf Galaxy
cosmic dust
evolutionary track
giant star
horizontal branch (HB)
H-R diagram (HRD)
low mass star
M-type star (M)
red-giant branch (RGB)
Schönberg-Chandrasekhar limit
symbiotic binary
Thorne-Żytkow object (TZO)
Wilson-Bappu effect