The image above is of the Westerlund 1 cluster, a group of stars estimated to be between 11,000 and 15,000 light-years from Earth. What distinguishes this group from many other pretty collections of stars is the red hypergiant star Westerlund 1-26.
Westerlund 1-26 is so large that if its center was where the Sun is, its outer edge would extend past the orbit of Jupiter, giving it a radius more than 1,500 times that of our own star. By comparison, Betelgeuse, the largest star by volume in our corner of the galaxy, is usually estimated at a little over half that radius, although measurements vary.
Like most clusters, the stars of Westerlund-1 are thought to have been born at the same time, approximately 3 million years ago. However, cluster members vary in mass. Since the larger the mass, the faster a star goes through its lifecycle, Westerlund 1-26 has aged rapidly, passing through stellar youth and middle age to reach the red giant stage – although in its case, it's so large it is termed a hypergiant.
Westerlund-1, in the constellation Ara, lies in the direction of the more densely packed parts of our galaxy and is heavily obscured by dust at visible wavelengths. Most of what we know about the cluster comes from radio and infrared observations, since the dust blocks less of these. Consequently, Hubble's image, shown above, is a rare example of a clear visible light shot.
Stars don't stay in the red giant phase long, at least by astronomical standards, and at some point in the not too distant future, Westerlund 1-26 will turn into a supernova and give us a spectacular show. Before that, however, it will become a Wolf-Rayet star, a stage in the evolution of very large stars marked by ionized emission lines, lack of hydrogen, and extraordinarily strong stellar winds.
Even without its prime attraction, Westerland 1 is one of the largest star clusters in the galaxy, with an estimated 63,000 solar masses. If you're having trouble picking Westerland 1-26 out of the photo above, that is because there are three other red supergiants in the cluster, along with many other enormous stars of types that are very rare elsewhere in the universe. One of the stars is thought to be the result of two large stars colliding and merging, and at least one remnant of a previous supernova has been detected.
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