A luminous giant star in a late phase of stellar evolution, characterized by a cool surface temperature (giving it a red color) and an enormously expanded outer envelope. These stars have exhausted hydrogen in their cores and are burning helium or heavier elements.
The term emerged in early 20th-century astronomy, combining 'red' (describing the star's cool surface temperature around 3000-4000K) with 'giant' (referring to its enormous size, often 10-100 times the Sun's radius). The classification became standard with the development of stellar evolution theory.
Red giants are stellar senior citizens that have literally lost their grip - their outer layers are so loosely bound that they're slowly puffing away into space! When our Sun becomes a red giant, it will be so large it might engulf Earth's orbit.
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