The new VST image of the star cluster Westerlund 1. The stars in the cluster appear red due to foreground dust blocking out their blue light. The blue stars are foreground objects and are not related to the cluster. The star W26 is in the upper left of the cluster and is surrounded by a green glow. (Credit: ESO/VPHAS+ Survey/N. Wright) Enlarge

Oct. 16, 2013 — An international team of astronomers has observed part of the final death throes of the largest known star in the Universe as it throws off its outer layers. The discovery, by a collaboration of scientists from the UK, Chile, Germany and the USA, is a vital step in understanding how massive stars return enriched material to the interstellar medium – the space between stars – which is necessary for forming planetary systems.

The researchers publish their results in the Oxford University Press journalMonthly Notices of the Royal Astronomical Society.

Stars with masses tens of times larger than that of the Sun live very short and dramatic lives compared to their less massive siblings. Some of the most massive stars have lifetimes of less than a few million years before they exhaust their nuclear fuel and explode as supernovae. At the very ends of their lives these stars become highly unstable and eject a considerable amount of material from their outer envelopes. This material has been enriched by nuclear reactions deep within the star and includes many of the elements necessary for forming rocky planets like our Earth, such as silicon and magnesium, and which are also the basis for life. How this material is ejected and how this affects the evolution of the star is however still a mystery.

Using the Very Large Telescope Survey Telescope (VST) at the European Southern Observatory's Paranal Observatory in Chile an international team of astronomers has been surveying our Galaxy using a special filter to detect nebulae of ionized hydrogen. The VST Photometric H-Alpha Survey (VPHAS) has been searching our galaxy for ejected material from evolved stars and when the team observed the super star cluster Westerlund 1 they made a remarkable discovery.

Westerlund 1 is the most massive cluster of stars in our galaxy, home to several hundreds of thousands of stars, and is the closest analogue to some of the truly massive star clusters seen in distant galaxies. The cluster is about 16,000 light years from Earth in the southern constellation of Ara (The Altar), but our view of the cluster is hampered by gas and dust that makes it appear comparatively dim in visible light.

When the astronomers studied the images of Westerlund 1 they spotted something truly unique. Around one of the stars, known as W26, they saw a huge cloud of glowing hydrogen gas, shown as green in this new image. Such glowing clouds are ionised, meaning that the electrons have been stripped away from the atoms of hydrogen gas.

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