What is Westerlund 1?
[[Image:Surprise Cloud Around Vast Star.jpg|thumb|right|300px|Westerlund 1: the hot OB supergiants usually emit blue light – however they appear as red stars in the visible light image. This is because the blue light from the stars has been absorbed by gas, causing the reddening.]] '''Westerlund 1''' ('''Wd1''', also called '''Ara Cluster'''<ref>[http://simbad.u-strasbg.fr/simbad/sim-id?Ident=ARA+CLUSTER&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id Simbad]</ref>) is a compact young [[super star cluster]]. It is in the [[Milky Way galaxy]], about 3.5–5 kilo[[parsec]]s (12000–16000 [[light years]]) away from Earth. It is the most massive compact young star cluster known in the entire [[Local Group|Local Group of galaxies]]. It was discovered by Bengt Westerlund in 1961.<ref>{{Cite journal |last=Westerlund |first=B. |year=1961 |title=A heavily reddened cluster in Ara |journal=Astronomical Journal |volume=70 |issue= |page=57 |doi=10.1086/108585 |bibcode=1961AJ.....66T..57W|s2cid=119636689 }}</ref> It remained largely unstudied for many years due to high [[interstellar extinction]] (absorption) in its direction. In the future, it will probably evolve into a [[globular cluster]].<ref name=gallagher02>{{cite journal| author = Gallagher & Grebel | year = 2002| title = Extragalactic star clusters: speculations on the future| url = https://archive.org/details/arxiv-astro-ph0109052 | journal = Extragalactic Star Clusters, IAU Symposium| volume = 207| pages = 207| bibcode = 2002IAUS..207..745G |arxiv = astro-ph/0109052 }}</ref> The cluster contains a large number of rare, evolved, high-mass stars, including: six [[yellow hypergiant]]s, four [[red supergiant]]s, 24 [[Wolf-Rayet star]]s, a [[luminous blue variable]], many [[OB star|OB supergiants]], and an unusual supergiant [[Be star|sgB[e] star]] which may be the remnant of a recent [[Stellar collision|stellar merger]].<ref name=clark>{{Cite journal |last=Clark |first=J. S. | display-authors = etal |year=2005 |title=On the massive stellar population of the super