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Stephenson 2-18 is a Red hypergiant Star that is in the constellation of Scutum within the Star cluster of Stephenson 2. It is 19,570 light-years away from the Earth, near the supermassive black hole named Sagittarius A*. It is the largest Star in the Galaxy. This Star is also known as St2-18, Stephenson 2 DFK 1, or RSGC2-18. This star’s volume is more than 10 billion times bigger than the Sun.

Observation history

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. 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 reduce 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.

Physical properties

St 2-18 shows the traits and properties of a highly luminous and extreme 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.

One calculation for finding the bolometric luminosity by fitting the Spectral Energy Distribution (SED) gives the star a luminosity of 437,000 L, with an effective temperature of 3,200 K, which corresponds to a very large radius of 2,158 R (1.50×109 km; 10.0 au; 930,000,000 mi), which would be considerably larger and more luminous than theoretical models of the largest, and most luminous red supergiants possible (roughly 1,500 R and 320,000 L 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 L. A newer calculation, based on SED integration and assuming a distance of 5.8 kpc, gives a bolometric luminosity of 630,000 L although the authors are doubtful that the star is actually a member of the cluster and at that distance.