Red supergiants (RSGs) are stars with a supergiant luminosity class and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.
Betelgeuse pulsating and showing spectral line profile changes (HST UV images)
A red supergiant ends its life as a type II supernova (bottom left) in a spiral arm of M74
RSGC1, the first of several massive clusters found to contain multiple red supergiants.
The Orion region showing the red supergiant Betelgeuse
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.
The four brightest stars in NGC 4755 are blue supergiant stars, with a red supergiant star at the centre. (ESO VLT)
The disc and atmosphere of Betelgeuse (ESO)
RS Puppis is a supergiant and Classical Cepheid variable.