Westerlund 1-20 (abbreviated to Wd 1-20 or just W20) is a red supergiant (RSG) located in the Westerlund 1super star cluster. Its radius was calculated to be around 965 solar radii[1] (6.72 × 108km, 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 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.[8] 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.[9]
^ abcdeFok, Thomas K. T.; Nakashima, Jun-Ichi; Yung, Bosco H. K.; Hsia, Chih-Hao; Deguchi, Shuji (2012). "Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters". The Astrophysical Journal. 760 (1): 65. arXiv:1209.6427. Bibcode:2012ApJ...760...65F. doi:10.1088/0004-637X/760/1/65. S2CID53393926.
^Westerlund, B. E. (1987). "Photometry and spectroscopy of stars in the region of a highly reddened cluster in Ara". Astronomy and Astrophysics Supplement Series. 70: 311. Bibcode:1987A&AS...70..311W.
^Andrews, H.; Fenech, D.; Prinja, R. K.; Clark, J. S.; Hindson, L. (2018), "Asymmetric ejecta of cool supergiants and hypergiants in the massive cluster Westerlund 1", Monthly Notices of the Royal Astronomical Society: Letters, 477 (1): L55 –L59, arXiv:1803.07008, Bibcode:2018MNRAS.477L..55A, doi:10.1093/mnrasl/sly046