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Red giants are extremely large stars approaching the end of their current life cycle that glow red because of their current low surface temperature.
All stars pass through this
stage. When a star nears a phase transition, its rising core temperature causes
it to expand. Previously all the helium from converted hydrogen descended to the
heartcore of the star where it was encircled by a hydrogen casing.
Since there was no longer enough hydrogen to generate the necessary energy to hold up the star's exterior layers, the star focused its energy inward until the temperature and pressure were high enough to generate helium burning through carbon fusion. The star then used the radiant energy from the helium burning to expand outward again, becoming a Red Giant.
At this stage, the star
enlarged significantly to many times its original size, but, still generated
about the same amount of energy which caused the surface temperature to
decrease, changing the star's color from a hot white or blue to a cool red.
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Astronomical
Cosmological Extraterrestrial 
