Stephenson 2-18

Stephenson 2-18 is a Red Hypergiant located 20,000 light-years away from Earth in the constellation Scutum. It is the largest star ever discovered. Not only that, but it would take light 9 hours to circle it's surface, compared to the Sun only 14.5 seconds. If placed at the centre of the solar system, it would engulf the orbit of Saturn! It lies near the open cluster Stephenson 2. The cluster stretches across about 6’ of the sky and contains a core group of 26 red supergiants, the largest known population in the Milky Way. Stephenson 2-18 has a radial velocity about 20 km/s below that of the other cluster stars, which indicates that it is not a foreground giant. It may be a cluster member and its observed radial velocity is offset by a thick expanding envelope. The same study stated that the star appeared to be on the verge of shedding its outer layers and evolving blueward into a luminous blue variable or a Wolf-Rayet star. The cluster is also known as RSGC2, one of several massive open clusters in Scutum, each containing multiple red supergiants. The designation St2-18 (short for Stephenson 2-18) is often used for the star.

## Discovery

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 brightest star in the cluster was given the designation 'Stephenson 2 Identifier 1' in an analysis of the star cluster. However, it wasn't considered a member of the cluster due to its position, unusually high brightness, and has a different proper motion from the other stars in the cluster.

However, some authors note *Identifier 1* as having a significant infrared excess, probably because of an extreme mass-losing outburst. This made them say that it is a red hypergiant.

Later, *Identifier 1* was assigned the number 18 and was considered a member of a group of stars named Stephenson 2 SW. The designation by Shuji Deguchi, Stephenson 2-18, is often used for the star. To avoid confusion from using the same number for different stars and different numbers for the same star, designations are typically given a prefix of DFK or D, for example, Stephenson 2 DFK 1.

## Physical Properties

Stephenson 2-18 shows the traits and properties of a highly luminous and extreme red supergiant, with a late spectral type of M6, which is unusual for a supergiant star. This places it in the top right corner of the Hertzsprung–Russell diagram.

A calculation for finding the bolometric luminosity by fitting the Spectral Energy Distribution (SED) gives the star a luminosity of nearly 440,000 sun luminosities, with an effective temperature of 3,200 K, which corresponds to a very large radius of 1,495,342,200km or 10.0AU, which would be considerably larger and more luminous than theoretical models of the largest, and most luminous red supergiants possible (roughly 1,043,262,000Km and 320,000 sun luminosities respectively). An alternate but older calculation from 2010, still assuming membership of the Stephenson 2 cluster at 17,930ly but based on 12 and 25 μm fluxes, gives a much lower and relatively modest luminosity of 90,000 *suns*. A newer calculation, based on SED integration and assuming a distance of 18908ly, gives a bolometric luminosity of 630,000 suns, although the authors are doubtful that the star is actually a member of the cluster and at that distance.

Stephenson 2-18 was probably born as a main-sequence star a few tens of times the mass of our Sun, and has likely lost about half its mass by now.