Westerlund 1
A massive young super star cluster in the Milky Way.
Westerlund 1 (Wd1, sometimes called the Ara Cluster) is a compact young super star cluster roughly 3.8 kpc (12,000 ly) from Earth. It is thought to be the most massive young star cluster in the Milky Way, discovered by Bengt Westerlund in 1961 but long understudied due to high interstellar absorption. The cluster hosts many rare, evolved high-mass stars and is expected to eventually evolve into a globular cluster.
Quick Facts
- Mass msol
- 63,000
- Age
- 3.50 Myr
- Constellation
- Ara
- Names
- Ara Cluster
- Westerlund 1
- ESO 277-12
- C 1644-457
- VDBH 197
Facts from the source article.
Observations
The brightest O7–8V main sequence stars in Wd1 have V-band magnitudes near 20.5, so the cluster visually is dominated by highly luminous post-main sequence stars (V magnitudes 14.5–18, absolute magnitudes −7 to −10) and less luminous post-main sequence stars of classes Ib and II (V magnitudes 18–20). Heavy interstellar reddening makes U- and B-band observation difficult, so most work is done in R- or I-bands or the infrared. Stars are named after Westerlund’s classification, though Wolf–Rayet stars often follow a separate convention. In X-rays, diffuse emission from interstellar gas and point sources from high-mass post-main sequence and low-mass pre-main sequence stars are seen; the magnetar is the brightest X-ray point source, with the sgB[e] star W9, the presumed binary W30a, and Wolf–Rayet stars WR A and WR B also strong. About 50 other X-ray sources have luminous optical counterparts. At radio wavelengths, W9 and red supergiants W20 and W26 are strong emitters, and many cool hypergiants, some OB supergiants, and Wolf–Rayet stars are detected.
Age and evolutionary state
The cluster’s age is estimated at 4–5 Myr by comparing its evolved stars with stellar evolution models. The simultaneous presence of Wolf–Rayet stars and red and yellow supergiants strongly constrains this age: theory holds that red supergiants appear only after about 4 Myr, while the Wolf–Rayet population declines after 5 Myr. Infrared observations of late-O main sequence stars are broadly consistent with this range, though a lower age of about 3.5 Myr has been suggested from lower-mass stars. If Wd1 formed with a typical initial mass function, it originally contained many very massive stars; current age estimates exceed those stars’ lifetimes, implying 50–150 supernovae have occurred, at a rate of roughly one per 10,000 years over the last million years. Only one definitive supernova remnant—the Westerlund 1 magnetar—has been detected, and the lack of other compact objects and high-mass X-ray binaries is puzzling. Proposed explanations include high supernova kick velocities disrupting binaries, formation of slowly accreting black holes, or binary systems where both objects are now compact, but the issue remains unresolved. Because the stars share the same age, composition, and distance, the cluster is ideal for studying massive star evolution, though models still fail to correctly predict the observed distribution of Wolf–Rayet subtypes.
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