Westerlund 2 is an obscured compact young star cluster (perhaps even a

super star cluster A super star cluster (SSC) is a very massive young open cluster that is thought to be the precursor of a globular cluster. These clusters called "super" because they are relatively more luminous and contain more mass than other young star clusters. ...

) in the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

, with an estimated age of about one or two million years. It contains some of the hottest, brightest, and most massive stars known. The cluster resides inside a stellar breeding ground known as Gum 29, located 20,000 light-years away in the constellation Carina. It is half a degree from the naked eye

Cepheid variable A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid varia ...

V399 Carinae.

Cluster members

The cluster contains at least a dozen early O stars, of which at least three are

eclipsing binaries A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

. All are hotter than 38,000 K and more luminous than . There are around 20 further O class stars in the cluster, all

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

objects implying a very young age for the cluster. Several

Wolf–Rayet star Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface ...

s are found in the vicinity of Westerlund 2, although not in the central core.

WR 20a WR 20a is an eclipsing binary star belonging to or recently (0.5 millions years before present) ejected from the young, massive cluster Westerlund 2. It was discovered in 2004 to be one of the most massive binary systems known, for which ...

, a binary consisting of two WR stars, and the single stars

WR 20aa WR may refer to: Arts and media * '' W.R.: Mysteries of the Organism'', a Serbian film, * Wikipedia Review, an Internet forum for the discussion of Wikimedia projects, particularly the English Wikipedia. *Willie Revillame, a Philippine TV host * ...

WR 20b WR may refer to: Arts and media * '' W.R.: Mysteries of the Organism'', a Serbian film, * Wikipedia Review, an Internet forum for the discussion of Wikimedia projects, particularly the English Wikipedia. *Willie Revillame, a Philippine TV host * ...

, and

WR 20c WR may refer to: Arts and media * '' W.R.: Mysteries of the Organism'', a Serbian film, * Wikipedia Review, an Internet forum for the discussion of Wikimedia projects, particularly the English Wikipedia. *Willie Revillame, a Philippine TV host * ...

are all thought to be members of the cluster, although possibly now runaway members. All five Wolf Rayets are extremely young massive objects with OIf^*/WN spectral types, amongst the most luminous stars in the galaxy. This composite spectral type indicates young very massive hydrogen-burning stars that are just starting to convect nitrogen and helium to the surface and develop denser

stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. D ...

s so that they show the emission lines of a Wolf-Rayet star.

WR21a WR 21a is an eclipsing binary star in the constellation Carina. It includes one of the most massive known stars and is one of the most massive binaries. WR 21a lies near the Westerlund 2 open cluster and likely to be an ejected member. ...

, itself a massive binary, lies in the same direction but is unlikely to be a member of Westerlund 2. Westerlund 2 also contains a large number of pre-

stars with masses below . These stars constrain the age of the cluster to near 2 Myr.

Discovery

As its name indicates, the Westerlund 2 cluster 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 the 1960s but its stellar content was assessed only in later years.

Hubble 25th Anniversary Image

On 23 April 2015 an image of the Westerlund 2 cluster was chosen to celebrate the 25th anniversary of the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

References

External links

Simbad

*

Astronomy Picture of the Day Astronomy Picture of the Day (APOD) is a website provided by NASA and Michigan Technological University (MTU). According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written ...

Young Star Cluster Westerlund 2
26 June 2010 * {{DEFAULTSORT:Westerlund 2 Open clusters Carina (constellation) Star-forming regions

Cluster members

Discovery

Hubble 25th Anniversary Image

See also

References

External links