Westerlund 1-26 or Wd 1-26 is a red supergiant within the outskirts of the Westerlund 1 super star cluster. It is one of the largest known stars discovered so far although its radius is uncertain but is calculated to be 1,165–1,221 solar radii (810,000,000–849,000,000 km; 5.42–5.68 au), with a possible but very unlikely upper estimate of 2,519 solar radii (1.752×10⁹ km; 11.71 au), corresponding to a volume between 1.58 and 16 billion times bigger than the Sun. Assuming the upper estimate is correct, if placed at the center of the Solar System, its photosphere would engulf the orbit of Jupiter.

Westerlund 1-26

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]
Characteristics
Spectral type	M0.5–M6[3] (M2-M6Ia[4])
Apparent magnitude (B)	22.1[5]
Apparent magnitude (J)	4.31[1]
Apparent magnitude (H)	1.35[1]
Apparent magnitude (K)	1.9[6]
B−V color index	+4.906[2][5]
Astrometry
Radial velocity (Rv)	-49.42 km/s
Proper motion (μ)	RA: -1.841[7] mas/yr Dec.: -3.909[7] mas/yr
Parallax (π)	0.0534 ± 0.0917 mas[7]
Distance	11,500 ly (3,550[8] pc)
Details
Radius	1,165 ± 58 or 1,221 ± 120[3] R☉
Luminosity	256,000 ± 40,000 or 312,000 ± 37,000[3] L☉
Temperature	3,720 ± 183 or 4,000 ± 100[3] K
Age	7.9[3] Myr
Other designations
Westerlund 1 W26, 2MASS J16470540-4550367, Westerlund 1 BKS AS, Westerlund 1 BKS A, Gaia EDR3 5940106041452150272
Database references
SIMBAD	data

Discovery

Westerlund 1 was discovered by Bengt Westerlund in 1961 during an infrared survey in the Zone of Avoidance of the sky, and 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.[9][10]

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".[11] This leads to the designation Westerlund-1 BKS A as used by 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.[10] Westerlund also discovered another notable red supergiant, WOH G64, found in the Large Magellanic Cloud in the constellation Dorado.

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.[12]

Physical characteristics

Westerlund 1-26 is classified as a luminous cool supergiant. It occupies the upper right corner of the Hertzsprung–Russell diagram. The cool temperature means it emits a significant part of its energy in the infrared spectrum. It also shows huge mass loss of atmospheric material, suggesting that it may further evolve into a hotter supergiant. Westerlund 1-26 has been observed to change its spectral class (and thus its temperature) during several periods, but it has not been seen to change its luminosity.[8]

The star is almost obscured at visible wavelengths by extinction of around 13 magnitudes due to interstellar dust,[2] hence it has been studied extensively in the longer infrared to radio wavelengths, which made it easier to study. Its spectral type identifies it a red star with a high luminosity. The bolometric luminosity of Westerlund 1-26 has been calculated from its K-band infrared brightness to be between 320,000 and 380,000 times higher than the sun's (L_☉), depending on the spectral type between M2 and M5. These luminosities imply a radius between 1,530 and 1,580 times the Sun's radius (R_☉)[8] based on effective temperatures of 3,450 and 3,660 K for spectral types M5 and M2 respectively.[13][8] These parameters make Westerlund 1-26 one of most luminous red supergiants and are similar to those estimated for another notable red supergiant star, VY Canis Majoris.[8] An earlier calculation of the luminosity and the temperature by fitting the spectral energy distribution and based on the spectrum by using DUSTY model gave a far higher luminosity near 1,100,000 L_☉, considerably more luminous than expected for a red supergiant. The model also gave a photospheric temperature of 3,700 K, corresponding to a radius of 2,519 R_☉.[14][lower-alpha 1]

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 have been discovered around a red supergiant star through its optical emission lines, and follows the discovery of an ionized nebula around NML Cyg in 1982.[15][16] The nebula extends 1.30 parsecs from the star.^{[citation needed]} The nebulae of both Westerlund 1-20 and Westerlund 1-26 are extended outward from the cluster core and most bright at inward direction, indicating the outward cluster wind.[17] A later study analyzed the ejecta surrounding some of Westerlund 1's stars; the study determined the mass of Westerlund 1-26's ejecta to be 403×10⁻³ M_☉, with an uncertainty of ± 94×10⁻³ M_☉.[18]

See also

• Westerlund 1-237
• Westerlund 1-20
• Westerlund 1-75

Notes

References

Further reading

• MacKey, Jonathan; Castro, Norberto; Fossati, Luca; Langer, Norbert (2015). "Cold gas in hot star clusters: The wind from the red supergiant W26 in Westerlund 1". Astronomy & Astrophysics. 582: A24. arXiv:1508.07003. Bibcode:2015A&A...582A..24M. doi:10.1051/0004-6361/201526159. S2CID 54683876.

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