Stephenson 2-18


Stephenson 2-18, also known as Stephenson 2-DFK 1, or RSGC2-18, is a red supergiant star within the open cluster Stephenson 2 located around 6,000 parsecs away from Earth in the constellation of Scutum. It is among the largest known stars and one of most luminous red supergiants, with an estimated radius around 2,150 times that of the Sun, which corresponds to a volume around 10 billion times bigger than the Sun. If placed at the center of the Solar System, its photosphere would engulf the orbit of Saturn.

Observation history

The open cluster Stephenson 2 was discovered by American astronomer Stephenson in 1990 in the data obtained by a deep infrared survey.
The brightest star in the region of the cluster was given the number 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 the same distance as the core cluster.

Physical properties

St 2-18 shows traits of an extreme star, being an extremely luminous, large, and cool red supergiant. This implies its position on the Hertzsprung–Russell diagram, occupying the top left corner. St 2-18 is near the limit of the luminosity that can be obtained by a red supergiant.
A calculation of the bolometric luminosity by fitting the Spectral Energy Distribution gives the star a luminosity of nearly, with an effective temperature of, which corresponds to a very large but extreme radius of, which would be considerably larger, cooler and more luminous than theoretical models of the largest, coolest, and most luminous possible red supergiants. An alternate but older calculation from 2010 gives a much lower and relatively modest luminosity of.

Stage in life

Based on current stellar models and theories on stellar evolution, St 2-18 has the potential to evolve into a yellow hypergiant, and then into a luminous blue variable or Wolf–Rayet star, at which stage its outer layers will be stripped off by strong stellar winds. Eventually, St 2-18 will begin to fuse iron, when it will undergo type IIb, IIn, or type Ib/Ic supernova, and judging by its extreme parameters, leave behind a black hole, not a neutron star.