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Red supergiant star

Red supergiants are supergiant stars of spectral type K-M and a luminosity class of I.

They are the largest stars in the universe in terms of physical size, 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.

Note:   The above text is excerpted from the Wikipedia article "Red supergiant star", which has been released under the GNU Free Documentation License.
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