The Milky Way has several star clusters; collections of stars pulling each other into a tight group. But now astronomers have located a super star cluster, containing hundreds of thousands of stars in a region only 6 light-years across. It's called Westerlund 1, and nobody discovered it before now because it's hidden behind thick clouds of dust.
Super star clusters are groups of hundreds of thousands of very young stars packed into an unbelievably small volume. They represent the most extreme environments in which stars and planets can form. Stars are generally born in small groups, mostly in so-called "open clusters" that typically contain a few hundred stars. In some active ("starburst") galaxies, scientists have observed violent episodes of star formation (see, for example, ESO Press Photo 31/04), leading to the development of super star clusters, each containing several million stars. Until now, super star clusters were only known to exist very far away, mostly in pairs or groups of interacting galaxies.
The open cluster Westerlund 1 is located in the Southern constellation Ara (the Altar). It was discovered in 1961 from Australia by Swedish astronomer Bengt Westerlund, who later moved from there to become ESO Director in Chile (1970 - 74). This cluster is behind a huge interstellar cloud of gas and dust, which blocks most of its visible light. The dimming factor is more than 100,000 - and this is why it has taken so long to uncover the true nature of this particular cluster.
Westerlund 1 contains hundreds of very massive stars, some shining with a brilliance of almost one million suns and some two-thousand times larger than the Sun (as large as the orbit of Saturn)! From their observations, the astronomers conclude that this extreme cluster most probably contains no less than 100,000 times the mass of the Sun, and all of its stars are located within a region less than 6 light-years across. Westerlund 1 thus appears to be the most massive compact young cluster yet identified in the Milky Way Galaxy.
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