Red supergiants (RSGs) are stars with a supergiant luminosity class of spectral type 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.
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) late in their lives.
A circumstellar envelope (CSE) is a part of a star that has a roughly spherical shape and is not gravitationally bound to the star core. Usually circumstellar envelopes are formed from the dense stellar wind, or they are present before the formation of the star. Circumstellar envelopes of old stars eventually evolve into protoplanetary nebulae, and circumstellar envelopes of young stellar objects evolve into circumstellar discs.
WOH G64 is an unusual red supergiant (RSG) star in the Large Magellanic Cloud (LMC) satellite galaxy in the southern constellation of Dorado. It is one of the largest known stars, being described as possibly being the largest star known. It is also one of the most luminous and massive red supergiants, with a radius calculated to be around 1,540 times that of the Sun (R☉) and a luminosity around 282,000 times the solar luminosity (L☉).
VY Canis Majoris is an extreme oxygen-rich (O-rich) red hypergiant (RHG) or red supergiant (RSG) and pulsating variable star 1.2 kiloparsecs from the Solar System in the slightly southern constellation of Canis Major. It is one of the largest known stars, one of the most luminous and massive red supergiants, and one of the most luminous stars in the Milky Way.
VX Sagittarii is an asymptotic giant branch star located more than 1.5 kiloparsec away from the Sun in the constellation of Sagittarius. It is a pulsating variable star with an unusually large magnitude range. It is one of the largest stars discovered, with a radius varying between 1,350 and 1,940 solar radii (940,000,000 and 1.35×109 km; 6.3 and 9.0 au). It is the most luminous known AGB star, at bolometric magnitude –8.6, which is brighter than the theoretical limit at –8.0.
W Hydrae is a Mira-type variable star in the constellation Hydra. The star is nearly located within the Solar neighborhood, between 75 and 120 parsecs, likely at 320 light years from the Sun. It has a visual apparent magnitude range of 5.6 to 10. In the near-infrared J band it has a magnitude of -1.7, is the 7th brightest star in the night sky, and is even brighter than Sirius.
IRC+10420, also known as V1302 Aql, is a yellow hypergiant star located in the constellation of Aquila at a distance of 4-6 kiloparsecs of the Sun.
NML Cygni or V1489 Cygni is a red hypergiant or red supergiant (RSG) in the constellation Cygnus. It is one of the largest stars currently known by radius, and is also one of the most luminous and massive cool hypergiants, as well as one of the most luminous stars in the Milky Way.
U Equulei is a variable star in the Equuleus constellation with an apparent magnitude of +14.50 in the B band. It lies at an estimated distance of 5,000 light-years from the Solar System.
PZ Cassiopeiae is a red supergiant star located in the Cassiopeia constellation, and a semi-regular variable star.
HD 179821 or V1427 Aquilae is either a post-red supergiant yellow hypergiant or a post-AGB yellow supergiant star in the constellation of Aquila, surrounded by a detached dust shell. It is a semi-regular variable nearing the end of its life.
AH Scorpii is a red supergiant variable star located in the constellation Scorpius. It is one of the largest stars known by radius and is also one of the most luminous red supergiant stars in the Milky Way.
IRC −10414 is a red supergiant and runaway star in the constellation Scutum, a rare case of a red supergiant with a bow shock.
XX Persei is a semiregular variable red supergiant star in the constellation Perseus, between the Double Cluster and the border with Andromeda.
IK Tauri or NML Tauri is a Mira variable star located about 280 parsecs (910 ly) from the Sun in the zodiac constellation of Taurus.
Stephenson 2 DFK 49 or St2-11 is a putative post red supergiant star in the constellation Scutum. It is located in the massive open cluster Stephenson 2. It is possibly one of the largest known stars with radius estimates ranging from 884 solar radii to 1,300 solar radii. If the upper estimate is correct, then Stephenson 2 DFK 49 has a volume 2.2 billion times that of the Sun. If it was placed at the center of the Solar System, its photosphere will potentially approach or engulf Jupiter's orbit. It loses mass at a very high rate, resulting in large amounts of Infrared excess.
CIT 6 is a carbon star in the constellation Leo Minor. It is a semiregular variable star, with a period of about 628 days, and has been given the variable star designation RW Leonis Minoris. It is perhaps the second most studied carbon star, after CW Leonis. CIT 6 was discovered in 1966 by a group at the California Institute of Technology who found it using the same 62-inch infrared telescope on Mount Wilson that was used to produce the Two-Micron Sky Survey. It is the second brightest carbon star in the near-infrared, after CW Leonis.
RT Virginis is a variable star in the equatorial constellation of Virgo, abbreviated RT Vir. It ranges in brightness from an apparent visual magnitude of 7.7 down to 9.7, which is too faint to be visible to the naked eye. Based on parallax measurements made with the VLBI, the distance to this star is approximately 740 light years. It is receding from the Sun with a radial velocity of 17 km/s.
R Crateris is a star about 700 light years from the Earth in the constellation Crater. It is a semiregular variable star, ranging in brightness from magnitude 8.1 to 9.5 over a period of about 160 days. It is not visible to the naked-eye, but can be seen with a small telescope, or binoculars. R Crateris is a double star; the variable star and its magnitude 9.9 F8V companion are separated by 65.4 arcseconds.
