Westerlund 1-20

Westerlund 1-20 (abbreviated to Wd 1-20 or just W20) is a red supergiant (RSG) located in the Westerlund 1 super star cluster. Its radius was calculated to be around 965 solar radii^[2] (6.72 × 10⁸ km, 4.48 au), making it one of the largest stars discovered so far. This corresponds to a volume 899 million times bigger than the Sun. If placed at the center of the Solar System, the photosphere of Westerlund 1-20 would almost reach the orbit of Jupiter.

Westerlund 1-20 compared to the other 3 RSGs in the Westerlund 1 star cluster.

Westerlund 1-20

Westerlund 1 super star cluster. The location of Wd 1-20 is circled. Credit: ESO
Observation data Epoch J2000      Equinox J2000
Constellation	Ara[1]
Right ascension	16h 47m 04.69s[2]
Declination	−45° 51′ 23.9″[2]
Characteristics
Evolutionary stage	Red supergiant
Spectral type	M3[3] - M6I[4]
Apparent magnitude (J)	6.378
Apparent magnitude (H)	4.381
Apparent magnitude (K)	2.61
Astrometry
Proper motion (μ)	RA: −2.478[5] mas/yr Dec.: −4.041[5] mas/yr
Parallax (π)	0.1690±0.1073 mas[5]
Distance	2,600+600 −400 (assumed)[6] pc
Details
Radius	858±48[3] R☉
Luminosity	101,000±13,000[3] L☉
Temperature	3,284 - 3,550[3] K
Other designations
Westerlund 1 W20, Westerlund 1 BKS D, 2MASS J16470468-4551238, Gaia EDR3 5940106007092341120
Database references
SIMBAD	data

The star is classified as a luminous cool supergiant emitting most of its energy in the infrared spectrum.^[7] W20 occupies the upper right corner of the Hertzsprung-Russell diagram. Using the effective temperature of 3,500 K,^[2] the bolometric luminosity of 126,000 L_☉^[2] and the solar effective temperature of 5,772 K,^[8] its radius can be calculated using the Stefan-Boltzmann law.^[a]

Westerlund 1-20 was observed to have an extended, cometary shaped nebula, similar to the other red supergiant Westerlund 1 W26. It is therefore likely that its morphology was affected by either the intracluster medium or the cluster wind of Westerlund 1.^[9] The nebulae of both Westerlund 1-20 and Westerlund 1 W26 are extended outward from the cluster core and most bright at inward direction, indicating the outward cluster wind.^[10]

See also

• Westerlund 1-237
• Westerlund 1-75
• List of largest known stars

Notes

1. ${\displaystyle {\sqrt {\left({\frac {5772}{3500}}\right)^{4}\cdot 125893}}\approx 964.98\ R_{\odot }}$