A relatively old star whose diameter is about 100 times bigger than it was originally, and had become cooler (the surface temperature is under 6,500 K). They are frequently orange-red in color. Betelgeuse is a red supergiant.
Red supergiants (RSGs) are supergiant stars (luminosity class I) of spectral type K-M and a luminosity class of I. They are the largest stars in the universe in terms of physical size (volume), although they are not the most massive.