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The biggest star in the Galaxy and its mysterious nebula

posted Nov 7, 2013, 2:47 PM by Jeremy Drake   [ updated Nov 7, 2013, 2:47 PM ]
Prof. Janet Drew of the University of Hertfordshire in the UK was looking over some of the first images from a new survey she is leading of the plane of our Milky Way galaxy when she spotted something very interesting and unique. The survey is being carried out by a 2.6m telescope at the European Southern Observatory, named rather confusingly the Very Large Telescope Survey Telescope, or VST for short.  The aim is to map the Galactic plane in "H-alpha" - a specific red light emitted by excited hydrogen atoms that betrays the presence of interesting types of stars and nebulae.  Prof. Drew was looking at images of what is currently the most massive known "starburst" cluster in our Galaxy, Westerlund 1 pictured to the left, lying about 11,000 light years away and chanced upon a ring-like nebula. 
It was around a particular star, imaginatively known as "W26", which is a fairly rare red supergiant - a star of perhaps up to 30 times the mass of the Sun, in its last stage of evolution in a short life of only a few million years before a spectacular but catastrophic demise as a core-collapse ("Type II") supernova.  W26 and its surrounding nebula, together with an extended filament to the north, are shown in the figure to the lower right.

Royal Society Fellow Nick Wright, a recent predoc and postdoc alumnus of the Smithsonian and also now at the University of Hertfordshire, lead the data analysis.  He found W26 is probably the largest star currently known, at about 1500 times the size of the Sun.  Such stars are fairly unstable and throw off considerable mass during this last evolutionary phase.  It is this material that forms the nebula we see, but W26 is not hot enough to ionize the hydrogen that pervades the nebula gas. It is likely the gas is heated and ionized by the intense ultraviolet light from the many neighbouring massive, hot stars in the cluster, or perhaps by a hot, unseen binary companion.  Such mass loss episodes are only very rarely glimpsed but could be important for understanding the last years of the life of massive stars, as well as the enrichment of galaxies in elements like silicon, magnesium and oxygen that are the requisite ingredients for planets and life.  The paper, lead by Nick Wright, was published in the Monthly Notices of the Royal Astronomical Society on October 16, 2013, and a press release was made on October 18 2013.