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A red giant is a star that has exhausted the primary supply of hydrogen fuel at its core. An average-sized star like our Sun will spend the final 10 percent of its life as a red giant. In this phase, a star's surface temperature drops to between 3,140 and 6,741°F (1,727 and 3,727°C) and its diameter expands to 10 to 1,000 times that of the Sun. The star takes on a reddish color, which is what gives it its name.

With no hydrogen left at the core of a star to fuel the nuclear reaction that keeps it burning, the core begins to contract. The core's contracting releases gravitational energy into the surrounding regions of the star, causing it to expand. Consequently, the outer layers cool down and the color of the star (which is a function of temperature) becomes red. The star may slowly shrink and expand more than once as it evolves into a red giant.