Westerlund 1-26

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Westerlund 1-26
Surprise Cloud Around Vast Star-inset.png
Westerlund 1 with the inset showing W1-26 and the associated ionised hydrogen cloud
Credit: ESO
Observation data
Epoch J2000      Equinox J2000
Constellation Ara
Right ascension 16h 47m 05.403s[1]
Declination −45° 50′ 36.76″[1]
Apparent magnitude (V) 17.194[2]
Spectral type M2-M6Ia[3]
Apparent magnitude (B) 22.1[4]
Apparent magnitude (J) 4.31[1]
Apparent magnitude (H) 1.35[1]
Apparent magnitude (K) 1.9[5]
U−B color index −7.445[2]
B−V color index +5.000[2]
Distance 3.55k[6] pc
Radius 1,530[6] R
Luminosity 380,000[6] L
Age 3.5-5.0[4] Myr
Other designations
Westerlund 1 W26, 2MASS J16470540-4550367, Westerlund 1 BKS AS, Westerlund 1 BKS A
Database references

Westerlund 1-26 or Wd 1-26 is a red supergiant or hypergiant star within the outskirts of the Westerlund 1 super star cluster. It is one of the largest known stars discovered so far, although the radius is poorly known. It is approximately 1,530 solar radii, or a radius of 1,064,880,000 kilometres (661,685,755 miles; 7 AU). If placed at the center of the Solar System, its photosphere would engulf the orbit of Jupiter.


Westerlund 1 was discovered by Bengt Westerlund in 1961 during an infrared survey in the Zone of Avoidance of the sky, described as "a heavily reddened cluster in Ara". The spectral types of the component stars could not be determined at the time except for the brightest star which was tentatively considered type M.[7][8]

In 1969, Borgman, Kornneef, and Slingerland conducted a photometric survey of the cluster and assigned letters to the stars they measured. This star, identified as a strong radio source, was given the letter "A".[9] This leads to the designation Westerlund-1 BKS A used at Simbad, although the cluster was not known as Westerlund 1 at that time. At the time it was referred to as Ara A, with another strong radio source in the cluster called Ara C. Its brightness in the radio spectrum makes it one of the rare "radio stars". Westerlund made spectroscopic observations of the cluster, still not known as Westerlund 1, published in 1987 and numbered the stars, giving the number 26 and the spectral type M2I.[8]

Modern terminology stems from 1998 when the cluster was referred to as Westerlund 1 (Wd1), with a paper describing Ara A as star 26 and Ara C as star 9.[3]


The star is located 16,500[citation needed] light-years from Earth and is almost obscured at visible wavelengths by severe extinction due to interstellar dust, hence it has been studied extensively in the longer infrared to radio wavelengths. The star has the size between 1,530 to 2,544 solar radii.[citation needed] Its spectral type identifies it a red star with a high luminosity. At radio wavelengths it is 310,000 times brighter than the Sun,[citation needed] making its luminosity range somewhere around 380,000 times brighter than the Sun (mag -9.2).[6]

Westerlund 1-26 is classified as a luminous supergiant. With its surface temperature of about 3000 K, it occupies the upper right corner of the Hertzsprung-Russell diagram. The cool temperature means it emits most of its energy in the infrared spectrum. It also shows huge mass loss of atmospheric material, suggesting that it may further evolve into a Wolf-Rayet star.[citation needed]

Westerlund 1-26 has been observed to change its spectral class during several periods, but it has not been seen to change its luminosity. One possibility is that the dust extinction only passes a particular wavelength in the spectrum.[citation needed]

In October 2013, astronomers using the European Southern Observatory's Very Large Survey Telescope (VST) discovered that Westerlund 1-26 is surrounded by a glowing cloud of ionized hydrogen. This is the first ionized nebula to be have been discovered around a red supergiant star. The nebula extends 1.30 parsecs of the star and contains considerable material with a temperature of 800 K.[6]

See also[edit]


  1. ^ a b c d Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T.; Fowler, J.; Gizis, J.; Howard, E.; Huchra, J.; Jarrett, T.; Kopan, E. L.; Kirkpatrick, J. D.; Light, R. M.; Marsh, K. A.; McCallon, H.; Schneider, S.; Stiening, R.; Sykes, M.; Weinberg, M.; Wheaton, W. A.; Wheelock, S.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR On-line Data Catalog: II/246. Originally published in: 2003yCat.2246....0C 2246: 0. Bibcode:2003yCat.2246....0C. 
  2. ^ a b c . doi:10.1088/0004-6256/145/2/46.  Missing or empty |title= (help)
  3. ^ a b Clark, J. S.; Ritchie, B. W.; Negueruela, I.; Crowther, P. A.; Damineli, A.; Jablonski, F. J.; Langer, N. (2011). "A VLT/FLAMES survey for massive binaries in Westerlund 1". Astronomy & Astrophysics 531: A28. arXiv:1105.0776. Bibcode:2011A&A...531A..28C. doi:10.1051/0004-6361/201116990. 
  4. ^ a b Clark, J. S.; Negueruela, I.; Crowther, P. A.; Goodwin, S. P. (2005). "On the massive stellar population of the super star cluster Westerlund 1". Astronomy and Astrophysics 434 (3): 949. arXiv:astro-ph/0504342. Bibcode:2005A&A...434..949C. doi:10.1051/0004-6361:20042413. 
  5. ^ Piatti, A. E.; Bica, E.; Claria, J. J. (1998). "Fundamental parameters of the highly reddened young open clusters Westerlund 1 and 2". Astronomy and Astrophysics Supplement 127 (3): 423. Bibcode:1998A&AS..127..423P. doi:10.1051/aas:1998111. 
  6. ^ a b c d e Wright, N. J.; Wesson, R.; Drew, J. E.; Barentsen, G.; Barlow, M. J.; Walsh, J. R.; Zijlstra, A.; Drake, J. J.; Eisloffel, J.; Farnhill, H. J. (16 October 2013). "The ionized nebula surrounding the red supergiant W26 in Westerlund 1". Monthly Notices of the Royal Astronomical Society: Letters 437 (1): L1–L5. arXiv:1309.4086. Bibcode:2014MNRAS.437L...1W. doi:10.1093/mnrasl/slt127. 
  7. ^ Westerlund, Bengt (1961). "A Heavily Reddened Cluster in ARA". Publications of the Astronomical Society of the Pacific 73: 51. Bibcode:1961PASP...73...51W. doi:10.1086/127618. 
  8. ^ a b Westerlund, B. E. (1987). "Photometry and spectroscopy of stars in the region of a highly reddened cluster in ARA". Astronomy and Astrophysics Supplement Series (ISSN 0365-0138) 70: 311. Bibcode:1987A&AS...70..311W. 
  9. ^ Borgman, J.; Koornneef, J.; Slingerland, J. (1970). "Infra-red photometry of a heavily reddened cluster in Ara". Astronomy and Astrophysics 4: 248. Bibcode:1970A&A.....4..248B. 
Preceded by
NML Cygni
Largest known star
2013 — 2014
Succeeded by
UY Scuti