red giant - Earth's Sun, Red giants in fiction

A cool red star, 10–100 times the radius of the Sun, and hundreds of thousands of times as luminous, but of similar mass. It develops in a late stage of stellar evolution, after the main sequence, when hydrogen in the core is exhausted and the outer layers expand.

According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M;

They are stars of 0.4 - 10 times the mass of the Sun which have exhausted their supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core. In stars massive enough to ignite helium fusion, an analogous process occurs when central helium is exhausted and the star switches to fusing helium in a shell, although with the additional complication that in many cases hydrogen fusion will continue in a shell at lesser depth — this puts stars onto the asymptotic giant branch. ,, The decrease in surface temperature shifts the star's visible light output to the red — hence red giant. Stars of spectral types O through K are believed to become red giants (or supergiants in the case of O and B stars).

Very low mass stars are thought to be fully convective , and thus may not accumulate an inert core of helium, and thus may exhaust all of their fuel without ever becoming red giants.

If the star is less than 2.57 solar masses, the addition of helium to the core by shell hydrogen fusing will cause a helium flash—a rapid burst of helium fusing in the core, after which the star will commence a brief period of helium fusing before beginning another ascent of the red giant branch. Stars more massive than 2.5 solar masses enter the helium fusing phase of their lives much more smoothly. The core helium fusing phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the Hertzsprung-Russell diagram of many star clusters. Metal-rich helium-fusing stars do not lie on a horizontal branch, but instead lie in a clump (the red clump) in the Hertzsprung-Russell diagram.

Earth's Sun

As Earth's Sun is of one solar mass, it is expected to become a red giant in about five billion years.

Red giants in fiction

In the Superman movie series, the doomed planet Krypton orbited a red giant called Rao. It explodes in a supernova explosion, causing shockwaves that in turn destroys Krypton itself (the comic book series has a red dwarf star and Krypton exploding due to an unstable core). The star in Charn is described as being red, cold, and much larger than the Sun, just like the red giants in our own universe.