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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 a giant star, i.e., with radii on the order of an astronomical unit, much larger than that of a main sequence star. Their luminosity is like that of an early main sequence star, such as an A-type star or B-type star, but being less massive, the sphere formed from gravitational attraction is less compact. The stellar core's produced energy is diluted as it reaches the larger surface area, such that a lower-temperature photosphere is sufficient to emit enough EMR to maintain a balance. This lower temperature results in a red color, i.e., K-type star, M-type star, or S-type star.
Stars at various stages in the post-main-sequence generate more luminosity from fusion in a larger volume and using additional kinds of fuel, resulting giant stars such as red giants, including the red-giant branch (RGB), horizontal branch (HB), and asymptotic giant branch (AGB). However some giant stars are blue (blue giant) or yellow (yellow giant). Example red giants are Arcturus and Betelgeuse.