Westerlund 1 W237

Westerlund 1 W237, also known as Wd 1-237 or Westerlund 1 BKS B, is a possible red supergiant (RSG) in the constellation of Ara. It is one out of four known red supergiants in the Westerlund 1 super star cluster. As a red supergiant, it would be one of the largest known stars and one of the most luminous of its type.

Physical characteristics
Westerlund 1 W237 is classified as a luminous cool supergiant emitting most of its energy in the infrared spectrum. It is surrounded by a radio nebula which is similar in mass to those of Westerlund 1-20 and Westerlund 1 W26, and moreover directly comparable to that of VY Canis Majoris. The elliptical structure of this nebula however indicates that it has been less affected by the cluster wind of Westerlund 1 (W20 and W26 have pronounced cometary shaped nebulae). The outflow velocity for the RSG wind is assumed to be around 30 km/s. The nebula itself seems to have a mass of and a radius of about 0.11 parsecs. This results in a kinematic age around 3,600 years and a time averaged mass loss rate of $$ per year.

The star occupies the upper right corner of the Hertzsprung-Russell diagram. With an effective temperature of 3,550 K and a bolometric luminosity of, the radius of Westerlund 1 W237 would be 1,241 times the solar radius , making it larger than the orbit of Jupiter. The initial mass of W237 has been calculated from its position relative to theoretical stellar evolutionary tracks to be around or  for a non-rotating star.

Distance
The distance of Westerlund 1 W237 is assumed to be around $$ or $219,000$ based on it being commonly thought of as a member of the Westerlund 1 star cluster (the elliptical shape of its nebula indicates that it might not be near the center of W1, while other RSGs like W20 and W26 are). Another but older source suggests a similar distance of $1,241$.

Westerlund's 1987 analysis assigned a spectral type of M6+ III to W1-237 and considered it to be a foreground giant with a luminosity only around. Gaia Data Release 2 gives a parallax of $2$ for W237, implying a distance of $8,500 light years$ and a luminosity of with a corresponding radius of. In 2020, the parallax was revised to the much smaller value of 0.3370$2,600 parsecs$ mas, corresponding to a distance of $3,000 parsecs$ ($1.64 mas$).