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 1,530-2,544[2][1][3] 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 (also known as Westerlund 1 BKS A and Westerlund 1 BKS AS; sometimes abbreviated as W26 and W1-26; 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, an extreme red supergiant. It is approximately 1,530 solar radii (up to 2,544 by some estimates). It was discovered by the astronomer Bengt Westerlund in 1961.

The super star cluster Westerlund 1 in visible light.

Since its discovery, Westerlund 1-26 has been well known as a strong radio source. Its physical properties are not well understood, due to very high interstellar extinction between Earth and the cluster. Its strong radio emission leads to varying estimates of its size. But precise estimates give it the radius within 1,500 solar radii. However, such scales are given, with some taking it larger than 2,500 solar radii.

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.[4]

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 Wright, Nicholas J.; Roger Wesson; Drew, Janet E.; Geert Barentsen; Barlow, Michael J.; Walsh, Jeremy R.; Albert Zijlstra; Drake, Jeremy J. et al. (2013). "The Ionized Nebula surrounding the Red Supergiant W26 in Westerlund 1". arXiv:1309.4086v1 [astro-ph.SR].
  3. ^ Clark, J. S.; Ritchie, B. W.; Negueruela, I. (2010). "A serendipitous survey for variability amongst the massive stellar population of Westerlund 1". Astronomy and Astrophysics 514: A87. arXiv:1003.5107. Bibcode:2010A&A...514A..87C. doi:10.1051/0004-6361/200913820.  edit
  4. ^ "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