Giant Stars

Giant stars occupy the section above the main sequence on an HR-Diagram. In astronomy, stars that are even brighter and more giant than giants are supergiants and hypergiants. However, for this website, those types will also be in discussion on this page. The image above is of Betelguese, a well-known supergiant star

Giant stars are caused by the complete depletion of hydrogen fuel in their cores. Once that fuel has successfully burned out, the star leaves the main sequence. What happens from there is hugely dependent on mass. Stars with masses less than 0.25 solar masses can sustainably burn hydrogen for quite a while, around 10^12 years, much longer than the universe's current age. Around that time, they will then become helium white dwarfs once they run out of fuel.

For stars that are about 0.25 solar masses, they become slightly brighter and become subgiants. As subgiants, they'll live on for some more time before likely becoming degenerate, meaning that they exert outward pressure no longer through thermal effects but through quantum mechanical effects.

Stars with masses of about 0.40 solar masses don't reach the temperatures necessary to fuse helium. This means that it remains as a hydrogen-burning star until it runs out of fuel, making it a helium white dwarf. However, this process is theoretically predicted to last longer than the current age of the universe so it wouldn't have been observed as of yet, just like with stars of masses lower than 0.25 solar masses.

Stars with masses above 12 solar masses become blue giants and start core-helium burning. Once their cores become degenerate, they turn into red giants and don't see a big change in luminosity. After a point, they become far too unstable and explode into supernovae