Stephenson 2-18

Stephenson 2-18 (St2-18, also known as Stephenson 2-18 and RSGC2-18)^[3] is a red supergiant (RSG) or red hypergiant (RHG) star. It is close to, and a possible member of, the open cluster Stephenson 2. It is about 6,000 parsecs (20,000 ly) away from Earth in the constellation of Scutum and is one of the largest known stars.

Stephenson 2-18

Dense starfield around the red supergiant star St2-18 (brightest star in the image) as seen from the 2MASS astronomical survey. The picture was published in 2003. Credit: Université de Strasbourg/CNRS
Observation data Epoch J2000      Equinox J2000
Constellation	Scutum
Right ascension	18h 39m 02.3709s
Declination	−06° 05′ 10.5357″
Characteristics
Evolutionary stage	Red supergiant
Spectral type	~M6[1]
Apparent magnitude (G)	15.2631±0.0092
Apparent magnitude (J)	7.150
Apparent magnitude (H)	4.698
Apparent magnitude (K)	2.9
Astrometry
Proper motion (μ)	RA: −3.045±0.511 mas/yr Dec.: −5.950±0.480 mas/yr
Parallax (π)	−0.0081 ± 0.3120 mas
Distance	19,560 ly (6,000 pc)
Details
Radius	2,150 R☉
Luminosity	90,000–440,000; 630,000[2] L☉
Temperature	3,200 K
Other designations
Stephenson 2-18, Stephenson 2 DFK 1, RSGC2-18, 2MASS J18390238-0605106, IRAS 18363-0607
Database references
SIMBAD	data

It is also possibly one of the most luminous of its type, if it is a member of the cluster. It was estimated to have an estimated radius of 2,150 times that of the Sun,^[4] and has a volume 10 billion times that of the Sun, however these parameters are unreliable and it is likely not a member of Stephenson 2.^[4][5] This radius is bigger than the largest possible red supergiant size and so is probably smaller.

This star has a spectral type of M6,^[1] which is pretty unusual for a supergiant star (below the Hayashi limit) and corresponds to its cool temperature of 3,200 Kelvin.^[4]

The clear single image of stephenson 2-18 was first taken in the year 2007 using 2MASS All-Sky Survey camera from Cerro Tololo Inter-American observatory in Chile.^[6]