Using European Southern Observatory’s Very Large Telescope Interferometer (VLTI), Olivier Chesneau (Observatoire de la Côte d’Azur, Nice, France) and an international team of collaborators have found that the yellow hypergiant star HR 5171 A is 1300 times the diameter of the Sun and much bigger than was expected. It is in the constellation of Centaurus (visible in the Southern Hemisphere). This makes it the largest yellow star known. It is also in the top ten of the largest stars known — 50% larger than the famous red supergiant Betelgeuse — and about one million times brighter than the Sun. Yellow hypergiants are very rare, with only a dozen or so known in our galaxy — the best-known example being Rho Cassiopeiae. They are among the biggest and brightest stars known and are at a stage of their lives when they are unstable and changing rapidly. Due to this instability, yellow hypergiants also expel material outwards, forming a large, extended atmosphere around the star.
Despite its great distance of nearly 12 000 light-years from Earth, the object can just about be seen with the naked eye by the keen-sighted. HR 5171 A has been found to be getting bigger over the last 40 years, cooling as it grows. HR 5171A and its companion, HR 5171B, are so close that they touch, and the whole system resembles a gigantic peanut. HR 5171B is only slightly hotter than HR 5171 A’s surface temperature of 5,000 degrees Celsius.
13 March 2014