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, typically between 0.5 and 6 solar masses.

A Red giant forms when after a few billion years, a main sequence star exhausts all of its hydrogen fuel in its core used for nuclear fusion, as the hydrogen located outside of the core 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.

Depending of the mass of the star, there are two different routes the red giant's evolution will take. For those stars that are less then 2.55 solar masses, the core will continue to contract until it becomes degenerate. At this point, the core will not contract any further, but will continue to heat up. As the temperature reaches 10⁸ K, it will become hot enough for the core to begin fusing helium to carbon and to a lesser extent, oxygen. As degenerate matter is an excellent conductor of heat, all the helium will be fused nearly simultaneously in a helium flash. For more massive stars, the 10⁸ K temperature required to fuse helium will happen as the core is still contracting, allowing for a much more fluid fusion of helium, avoiding the flash. http://hyperphysics.phy-astr.gsu.edu/Hbase/astro/redgia.html

Once the helium begins fusing, the star again contracts and leaves the red giant stage. For stars around the mass of our Sun, insufficient mass exists to fuse oxygen and carbon into heavier elements. Once the star lacks any more helium to fuse, the star will eject its expanded outer layers as a planetary nebula, exposing the core. The star thus becomes a white dwarf, slowly cooling off over time. More massive stars will continue to fuse the carbon and oxygen into heavier elements. Any star that are 10 solar masses or greater will instead become red supergiants before ending their lives in a supernova explosion. While low mass red dwarfs will not have sufficient mass to fuse helium and never enter a red giant phase, but instead will simply cool over time once all the hydrogen fuel is consumed.

Our sun's fate as a red giant

It is estimated that in about 5 billion years our sun will have burned through its hydrogen fuel needed for nuclear fusion and enter into the red giant phase itself. The sun is then estimated to expand to 200 times its current diameter but will loss significant mass, perhaps enough so that the orbit the Earth will widen enough so as not to be swallowed by the expanding red giant, due to its weakened gravity, although this is debated. If the Earth does survive intact the red giant phase of the sun, its atmosphere will be striped and the surface will become hot enough to be molten. However a few billion years before this stage, the Earth's biosphere will have already been destroyed by the sun as it grows ever more luminous over time. Eventually the sun will consume all of its helium fuel and puff away its outer layers, entering its final phase of life as a white dwarf.