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A '''red giant''' is an enormous, highly luminous star, typically tens to hundreds the times the diameter of our own [[sun]], that represents the late stages of the [[stellar evolution]] of a main sequence [[star]]. It is typically Typically for stars between 0.5 and 6 solar masses.. <ref>The Cambridge Atlas of Astronomy (2nd ed.). Cambridge University Press. 1988. pp. 255. ISBN 0-521-36360-8.</ref> The greater the mass of the star, the more quickly it will enter the red giant phase of its evolution. For a star like the Sun, this will take approximately 10 billion years.<ref>Zeilik, Michael A.; Gregory, Stephan A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. pp. 321–322. ISBN 0030062284.</ref>

A The red giant forms stage begins when a main sequence star exhausts all of the [[hydrogen]] fuel used for nuclear fusion in its core. The hydrogen located just outside of the core , however, will not fuse due to the lower temperature of the region. The star will then begin to contract under [[gravity]], and the outer layers of the star will start to collapse. As the shells of hydrogen gas contract towards the core, they heat up, eventually reaching the temperature required to fuse the hydrogen into [[helium]], providing a new source of energy and halting the collapse through a renewal of stellar fusion. As the star has now grown hotter than it was during the star's life during the main sequence, the outer parts of the star begin to swell. Because of the expansion of the outer layers of the star, the energy produced in the core of the star is spread over a vastly larger surface area. This results in a lower surface temperature, which shifts the star's visible output towards the red side of the visual spectrum.