Wide Field Imager image of Westerlund 1 (annotated)

This image of the young star cluster Westerlund 1 was taken with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. This remarkable cluster contains hundreds of very massive stars, some shining with a brilliance of almost one million suns. Although most stars in the cluster are hot blue supergiants, they appear reddish in this image as they are seen through interstellar dust and gas. European astronomers have for the first time demonstrated that the magnetar — an unusual type of neutron star with an extremely strong magnetic field — that lies in the cluster was formed from a star with at least 40 times as much mass as the Sun. The result presents great challenges to current theories of how stars evolve, as a star as massive as this was expected to become a black hole, not a magnetar.