also known as Stephenson 2 DFK 1 or RSGC2-18, is possibly a red supergiant or red hypergiant star in the constellation of Scutum, although some sources consider it to be a foreground object. It lies near the open cluster Stephenson 2, which is located about 6,000 parsecs (20,000 ly) away from Earth in the Scutum–Centaurus Arm of the Milky Way galaxy, and is assumed to be one of a group of stars at a similar distance. It is possibly among the largest known stars, and one of the most luminous red supergiants, with a possible radius around 2,150 times that of the Sun (R☉), which would correspond to a volume nearly 10 billion times that of the Sun. If placed at the center of earth's Solar System, its photosphere could potentially engulf the orbit of Saturn.
The open cluster Stephenson 2 was discovered by American astronomer Charles Bruce Stephenson in 1990 in the data obtained by a deep infrared survey. The cluster is also known as RSGC2, one of several massive open clusters in Scutum, each containing multiple red supergiants.
The brightest star in the region of the cluster was given the identifier 1 in the first analysis of cluster member properties. However, it was not considered to be a member of Stephenson 2 due to its outlying position, abnormally high brightness, and slightly atypical proper motion. The authors however note Stephenson 2-18 as having a significant infrared excess, possibly because of an extreme mass-losing episode. As a result, they state that the star may be a red hypergiant.
In a later study, the same star was given the number 18 and assigned to an outlying group of stars called Stephenson 2 SW, assumed to be at a similar distance to the core cluster. The designation St2-18 (short for Stephenson 2-18) is often used for the star, following the numbering from Deguchi (2010). To avoid confusion from using the same number for different stars and different numbers for the same star, designations from Davis (2007) are often given a prefix of DFK or D, for example Stephenson 2 DFK 1.
St 2-18 shows the traits and properties of a highly luminous red supergiant, with a spectral type of M6, which is unusual for a supergiant star. This places it at the top right corner of the Hertzsprung–Russell diagram, a region characterized for exceptionally large and luminous low-temperature stars.
One calculation for finding the bolometric luminosity by fitting the Spectral Energy Distribution (SED) gives the star a luminosity of nearly 440,000 ☉, with an effective temperature of 3,200 K, which corresponds to a very large radius of 2,150 R☉ (1.50×109 km; 10.0 au; 930,000,000 mi),[a] which would be considerably larger and more luminous than theoretical models of the largest, and most luminous red supergiants possible (roughly 1,500 ☉ and 105.5 ☉ respectively). An alternate but older calculation from 2010, still assuming membership of the Stephenson 2 cluster at 5.5 kpc but based on 12 and 25 μm fluxes, gives a much lower and relatively modest luminosity of 90,000 ☉.A newer calculation, based on SED integration and assuming a distance of 5.8 kpc, gives a bolometric luminosity of 630,000 ☉. However, it has been noted that its SED is somewhat peculiar, with the fluxes that couldn't fit with the accepted range of appropriate temperatures for an RSG. This would suggest a higher extinction, which would make it be even more luminous. Because of this unusually high luminosity, the star's membership to the Stephenson 2 cluster has been considered doubtful, though it is also stated that it cannot necessarily be ruled out yet.[b]LRLLL
In 2013, an article describing the red supergiants in Stephenson 2 stated that Stephenson 2-18 (referred to as D1) and D2 (another member of Stephenson 2) have maser emissions, indicating that they have the highest mass loss in the cluster.
It has been debated for a while if this star is actually part of its supposed cluster. Due to its radial velocity being below the other cluster stars, some sources state that the star is unlikely to be a foreground giant; however, more recent papers considered the star an unlikely member due to its extreme properties.
The distance of Stephenson 2-18 has been stated to have an uncertainty greater than 50%, and the radius of 2,150 ☉ is very likely an overestimation because it greatly exceeds the theoretical limit of 1,500 ☉. Another estimate of the luminosity gave a value of 90,000 ☉.
Epoch J2000 Equinox J2000
Right ascension 18h 39m 02.3709s
Declination −06° 05′ 10.5357″
Evolutionary stage Red supergiant, possible red hypergiant
Spectral type ~M6
Apparent magnitude (G) 15.2631±0.0092
Apparent magnitude (J) 7.150
Apparent magnitude (H) 4.698
Apparent magnitude (K) 2.9
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 18,900 ly
Radius ~2,150[a] R☉
Luminosity 437,000 (90,000–630,000[b]) L☉
Temperature 3,200 K