Red giant

A Red giant is a very luminous, enormous 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 of less than 4 solar masses.

A Red giant forms when after a few billion years, a main sequence star exhausts all of its fuel in its core used for nuclear fusion. At this point the outer shell of the star, still made mainly up of hydrogen, begins to collapse under gravity when the hydrogen gas heats up. At this point the shell surrounding the core of the star will become hot enough to fuse the hydrogen into helium, providing a new source of energy. As the core star is now hotter than it was during the stars main sequence life, the outer parts of the star 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.

At this point the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. The core will then continue to heat until it reaches a temperature of roughly 10⁸K, hot enough to begin fusing helium to carbon. Once the core reaches this temperature, depending on the mass of the star, a helium fusion may occur. In more massive stars over 2.57 solar masses, the collapsing core will reach 10⁸K point before electron degeneracy can occur, therefore it will begin to fuse carbon without a helium flash.

Once the helium begins fusing, the star again contracts and leaves the red giant stage. A star who become a red giant lacks the necessary mass to generate enough energy to fuse carbon. Once the helium has been all converted to carbon, the star will eject its outer layers as a planetary nebula, exposing the core. The star thus becomes a white dwarf, slowly cooling off over time.

Our sun's fate as a red giant

It is estimated that in about 5 billion years our sun will have burned through it's hydrogen fuel needed for nuclear fusion and enter into the red giant phase itself. The sun is then estimated to expand to 200 times it's current diameter but will loss significant mass, perhaps enough so that the orbit the Earth will widen enough not to be swallowed by the expanding red giant, due to it's weakened gravity, although this is debated. If the Earth does survive intact the red giant phase of the sun, it's atmosphere will be striped and the surface will become hot enough to be molten.