Red supergiant
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Red supergiants (RSGs) are supergiant stars (luminosity class I) of spectral type K-M. They are the largest stars in the universe in terms of volume, although they are not the most massive.
Stars with more than about 10 solar masses after burning their hydrogen become red supergiants during their helium-burning phase. These stars have very cool surface temperatures (3500-4500 K), and enormous radii. The four largest known red supergiants in the Galaxy are Mu Cephei, KW Sagitarii, V354 Cephei, and KY Cygni, which all have radii about 1500 times that of the sun (about 7 astronomical units, or 7 times as far as the Earth is from the sun). The radius of most red giants is between 200 and 800 times that of the sun, which is still enough to reach from the sun to Earth and beyond.
These massively large stars are little more than "hot vacuums", having no distinct photosphere and simply "tailing off" into interstellar space. They have a slow, dense, stellar wind and if their core's nuclear reactions slow for any reason (such as transitioning between shell fuels) they may shrink into a blue supergiant. A blue supergiant has a fast but sparse stellar wind and causes the material already expelled from the red supergiant phase to compress into an expanding shell.
The mass of many red supergiants allow them to eventually fuse elements up to iron. Near the end of their lifetimes, they will develop layers of heavier and heavier elements with the heaviest at the core.
The red supergiant phase is relatively short, lasting only a few hundred thousand to a million or so years. The most massive of the red supergiants are thought to evolve to Wolf-Rayet stars, while lower mass red supergiants will likely end their lives as a type II supernova.
Betelgeuse and Antares are the best known examples of red supergiants.
[edit] In fiction
The red sun around which the fictitious planet Krypton orbits in both Superman and Superman Returns is also a red supergiant (as opposed to that of a red dwarf star in the comics) that undergoes a supernova explosion, causing Krypton's destruction by means of the shockwaves emitted by the dying star (in the comic series, Krypton was destroyed by the planet's unstable core).
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