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.^[8]

Westerlund 1 W237

Westerlund 1 super star cluster (north is towards top left). The location of Wd 1-237 is circled. Credit: ESO
Observation data Epoch J2000      Equinox J2000
Constellation	Ara[1]
Right ascension	16h 47m 03.1075s[2]
Declination	−45° 52′ 18.957″[2]
Apparent magnitude (V)	19.008[3]
Characteristics
Evolutionary stage	either a red supergiant or a foreground giant[4]
Spectral type	M3Ia[5] or M6+ III[6]
Apparent magnitude (B)	22.8[citation needed]
Apparent magnitude (R)	13.634[3]
Apparent magnitude (G)	11.259[2]
Apparent magnitude (J)	5.075[7]
Apparent magnitude (H)	3.01[7]
Apparent magnitude (K)	2.18[7]
Astrometry
Proper motion (μ)	RA: -3.09±0.15[2] mas/yr Dec.: -4.12±0.12[2] mas/yr
Parallax (π)	0.3370±0.1235 mas[2]
Distance	3,000[8] or 623[9] pc
Absolute magnitude (MV)	–6.96[10]
Details
if a supergiant
Radius	1,241±70[10] R☉
Luminosity	219,000±26,000[10] L☉
Temperature	3,550[10] - 3,605[9] K
Age	7.9[10] Myr
if a foreground giant
Radius	216[9] R☉
Luminosity	1,000[4] or 7,178 – 7,379[9] L☉
Temperature	3,605[9] K
Other designations
Westerlund 1 W237, Westerlund 1 BKS B, 2MASS J16470309-4552189, Gaia DR3 5940105904023386752
Database references
SIMBAD	data

Physical characteristics

Westerlund 1 W237 compared to the other 3 RSGs (Red Supergiants) in the Westerlund 1 star cluster.

Westerlund 1 W237 is classified as a luminous cool supergiant emitting most of its energy in the infrared spectrum.^[11] 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 0.07 M_☉ 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 2×10⁻⁵ M_☉ per year.^[12]

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 219,000 L_☉, the radius of Westerlund 1 W237 would be 1,241 times the solar radius (R_☉), 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 28 M_☉ or 32 M_☉ for a non-rotating star.^[10]

Distance

The distance of Westerlund 1 W237 is assumed to be around 8,500+2,000
−1,300 light years or 2,600+600
−400 parsecs^[13] 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).^[12] Another but older source suggests a similar distance of 3,000±500 parsecs.^[8]

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 1,000 L_☉.^[4] Gaia Data Release 2 gives a parallax of 1.64±0.2608 mas for W237, implying a distance of 623+139
−96 pc and a luminosity of 7,178–7,379 L_☉ with a corresponding radius of 216 R_☉.^[14][9] In 2020, the parallax was revised to the much smaller value of 0.3370±0.1235 mas, corresponding to a distance of 2967±1087 pc (9670±3540 ly).^[2]

See also

• Westerlund 1-75