Westerlund 1-26 or Wd 1-26 is a

red supergiant Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Ant ...

within the outskirts of the

Westerlund 1 Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster) is a compact young super star cluster about 3.8 kpc (12,000 ly) away from Earth. It is thought to be the most massive young star cluster in the Milky Way, and was discovered by Ben ...

super star cluster. It is one of the largest known stars discovered so far although its radius is uncertain but is calculated to be , with a possible but very unlikely upper estimate of , corresponding to a volume between 1.58 and 16 billion times bigger than the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. Assuming the upper estimate is correct, if placed at the center of the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

, its photosphere would engulf the orbit of

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...

Discovery

Westerlund 1 was discovered by

Bengt Westerlund Bengt Westerlund (January 17, 1921 - June 4, 2008) was a Swedish astronomer who specialised in observational astronomy. He received his PhD from Uppsala University in 1954. In 1957 he was appointed astronomer at the Uppsala Southern Station at M ...

in 1961 during an infrared survey in the

Zone of Avoidance The Zone of Avoidance (ZOA, ZoA), or Zone of Galactic Obscuration (ZGO), is the area of the sky that is obscured by the Milky Way. The Zone of Avoidance was originally called the Zone of Few Nebulae in an 1878 paper by English astronomer Richard ...

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. 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". 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. Westerlund also discovered another notable red supergiant,

WOH G64 WOH G64 (IRAS 04553-6825) is an unusual red supergiant (RSG) star in the Large Magellanic Cloud (LMC) satellite galaxy in the southern constellation of Dorado. It is one of the largest known stars, being described as possibly being the ...

, found in the Large Magellanic Cloud in the constellation

Dorado Dorado () is a constellation in the southern sky. It was named in the late 16th century and is now one of the 88 modern constellations. Its name refers to the dolphinfish (''Coryphaena hippurus''), which is known as ''dorado'' in Spanish, altho ...

. 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.

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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

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 In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

(and thus its temperature) during several periods, but it has not been seen to change its luminosity. The star is almost obscured at visible wavelengths by

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

of around 13 magnitudes due to

interstellar dust Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 micrometers). Larger particles are c ...

, 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 Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a ...

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 (), 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 () based on effective temperatures of 3,450 and 3,660 K for spectral types M5 and M2 respectively. 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 VY Canis Majoris (abbreviated to VY CMa) is an extreme oxygen-rich (O-rich) red hypergiant (RHG) or red supergiant (RSG) and pulsating variable star from the Solar System in the slightly southern constellation of Canis Major. It is o ...

. An earlier calculation of the luminosity and the temperature by fitting the

spectral energy distribution A spectral energy distribution (SED) is a plot of energy versus frequency or wavelength of light (not to be confused with a 'spectrum' of flux density vs frequency or wavelength). It is used in many branches of astronomy to characterize astron ...

and based on the spectrum by using DUSTY model gave a far higher luminosity near , considerably more luminous than expected for a red supergiant. The model also gave a photospheric temperature of , corresponding to a radius of . 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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

. 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. The nebula extends 1.30

parsecs The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...

from the star. 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. 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 , with an uncertainty of ± .

Discovery

Physical characteristics

See also

Notes

References

Further reading