| Westerlund 2 | |
|---|---|
 The cluster Westerlund 2 and its surroundings Credit:Hubble Space Telescope | |
| Observation data (J2000epoch) | |
| Right ascension | 10h 23m 58.1s[1] |
| Declination | −57° 45′ 49″[1] |
| Distance | 20000 ly (6000 pc) |
| Apparent magnitude (V) | 10.5[1] |
| Physical characteristics | |
| Contains some of the hottest, brightest, and most massive stars. | |
| Other designations | ESO 127-18, VDBH 95[1] |
| Associations | |
| Constellation | Carina |
| See also:Open cluster,List of open clusters | |
Westerlund 2 is an obscured compact young star cluster (perhaps even asuper star cluster[2]) in theMilky Way, 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 asGum 29, located 20,000 light-years away in the constellation Carina. It is half a degree from the naked eyeCepheid variableV399 Carinae.[3]
The cluster contains at least a dozenearly O stars, of which at least three areeclipsing binaries. All are hotter than 38,000 K and more luminous than 230,000 L☉.[4] There are around 20 further O class stars in the cluster, allmain sequence objects implying a very young age for the cluster.[5]
SeveralWolf–Rayet stars are found in the vicinity of Westerlund 2, although not in the central core.WR 20a, a binary consisting of two WR stars, and the single starsWR 20aa,WR 20b, andWR 20c 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 denserstellar winds so that they show the emission lines of a Wolf-Rayet star.WR21a, itself a massive binary, lies in the same direction but is unlikely to be a member of Westerlund 2.[6]
Westerlund 2 also contains a large number of pre-main sequence stars with masses below 2.5 M☉. These stars constrain the age of the cluster to near 2 Myr.[7]
As its name indicates, the Westerlund 2 cluster was discovered byBengt Westerlund in the 1960s[8] but its stellar content was assessed only in later years.[9]
On 23 April 2015 an image of the Westerlund 2 cluster was chosen to celebrate the 25th anniversary of theHubble Space Telescope.[3]
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