Westerlund 1-26

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Westerlund 1-26
Surprise Cloud Around Vast Star.jpg
False-colour image of Westerlund 1 in H-alpha (green), I-band infra-red (red), and visual (blue). Wd 1-26 is the bright yellow star just below and right of the intense green spot (the triangular nebula) towards the top of the central cluster of stars. It is one of the brightest stars in the cluster at this wavelength. Credit: ESO
Observation data
Epoch J2000      Equinox J2000
Constellation Ara
Right ascension 16h 47m 05.403s
Declination −45° 50′ 36.76″
Apparent magnitude (V) 16.79
Spectral type M2-M6Ia[1]
Apparent magnitude (K) 1.9
B−V color index 5.31
Distance 3.55k[2] pc
Radius 1,530[2] R
Other designations
Cl* Westerlund 1 BKS AS, Cl* Westerlund 1 W26, Cl* Westerlund 1 BKS A, 2MASS J16470540-4550367
Database references

Westerlund 1-26 (pronounced "Westerlund 1, star 26"; standard abbreviation 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,[2] 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 (as "a heavily reddened cluster in Ara") by Bengt Westerlund in 1961 during an on-course infrared survey in the Zone of Avoidance of the sky. 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.[3][4] 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".[5] 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.[4] 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.[1]


The star is located 16,500 light-years from Earth and is polluted by severe dust extinction by interstellar dust, hence it was studied extensively in the longer infrared to radio wavelengths. The star has the size ranging from 1-530 to 2,544 solar radii, giving the average size of 1,530 solar radii, making it the 5th largest known star, although the parameters of the size estimate were unknown. Its spectral type makes it an extremely red star with a high luminosity. At radio wavelengths it is 310,000 times brighter than the Sun, making its luminosity range somewhere around 380,000 times brighter than the Sun (mag -9.2).

Westerlund 1-26 is classified as a luminous supergiant, occupying the upper right corner of the Hertzsprung-Russell diagram. With its surface temperature of about 3000 K, it is a very cool supergiant, emitting mostly its energy in the infrared spectrum. It also shows huge mass loss of considerable material, suggesting that it may further evolve into a Wolf-Rayet star.

Westerlund 1-26 has been seen as a star that changes its spectral class during several periods, but it has not been seen to change its luminosity, unlike other stars. No one knows why. One possibility is that the dust extinction only passes a particular wavelength in the spectrum, allowing only the color to be seen while the inert brightness is blocked. But if it doesn't change its luminosity, it will be the first discovered variable star of its type.

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 giant star. The nebula extends 1.30 parsecs of the star and contains considerable material with a temperature of 800 K.[2] Oddly enough, the nebula was very similar to the one at Sanduleak -69° 202 before it exploded as SN 1987A.[6]

See also[edit]


  1. ^ 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.  edit
  2. ^ a b c d 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. doi:10.1093/mnrasl/slt127. 
  3. ^ Westerlund, B. (1961). "A Heavily Reddened Cluster in ARA". Publications of the Astronomical Society of the Pacific 73: 51. doi:10.1086/127618.  edit
  4. ^ a b Bibcode1987A&AS...70..311W
  5. ^ Bibcode1970A&A.....4..248B
  6. ^ "Surprise Cloud Around Vast Star". ESO Picture of the Week. Retrieved 15 Oct 2013. 
Preceded by
NML Cygni
Largest known star
2013 — 2014
Succeeded by
UY Scuti