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Astronomers have taken the first close-up image of an individual dying supergiant star in our neighbouring galaxy, the Large Magellanic Cloud. By combining pairs of 8.2-metre telescopes from the European Southern Observatory’s (ESO) Very Large Telescope Array in Chile as an interferometer, a team of researchers achieved the super-sharp resolving power of a 60-m telescope, and discovered that the dying supergiant star WOH G64 is developing a thick dust torus around its middle, rather than a spherical shell which is normally assumed. This means that the star is less hidden than had been thought, and therefore about half as massive. They estimated that the star had an initial mass of about 25 times the mass of our Sun, but that it is shedding material so rapidly that it has already lost 10-40 percent of its initial mass into the dusty torus, by means of a violent stellar wind. "The characteristics of the star mean that it may be experiencing a very unstable phase accompanied by heavy mass loss," says Markus Wittkowski from ESO. "We estimate that the belt of gas and dust that surrounds it contains between three and nine solar masses, which means that the star has already lost between one tenth and a third of its initial mass."
Infrared images of the Large Magellanic Cloud, our nearest galactic neighbour at a distance of 160,000 light years. The location of WOH G64 is indicated in the right hand image. Image: NASA/Spitzer/SAGE Team. "For the first time we could take a close-up view of an individual star outside our Galaxy, and this is an important first step to understand how dying stars in other galaxies differ from their counterparts in our Milky Way,” says Keiichi Ohnaka at the Max Planck Institute for Radio Astronomy. When a star becomes older, it ejects a huge amount of material, which becomes embedded in a thick envelope around the star. For decades, researchers have been trying to understand how such aging stars lose a considerable amount of their mass before their death, but now, by taking a close look at the envelope surrounding such a star, astronomers are able to begin to answer some of these outstanding questions.
Artist impression of the dust torus around star WOH G64. The diameter of the supergiant star is as large as the orbit of Saturn in our Solar System, the inner edge of the torus is at 120 AU. Image: ESO. Although WOH G64 is speeding toward its final fate as a supernova, it is unlikely to explode for another few thousand years. But when it does, future astronomers will observe a bright supernova, visible to the naked eye in the Southern Hemisphere, similar to the SN1987A supernova explosion, also in the Large Magellanic Cloud. The explosion will rip away most of the remaining mass of WOH G64, which will then be recycled as the building blocks for stars of the next generation.
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