HR 5171A: Astronomers Discover Largest Yellow Hypergiant Star

An international team of scientists using the Very Large Telescope Interferometer has discovered the largest known yellow hypergiant star in our Galaxy.

• Yellow hypergiants are very rare, with only a dozen or so known in Milky Way Galaxy. The best-known example is Rho Cassiopeiae.
• These stars 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, these objects also expel material outwards, forming a large, extended atmosphere around the star.
• The newly discovered hypergiant star, named HR 5171A, lies about 12,000 light-years away from Earth and measures more than 1,300 times the diameter of the Sun.
• HR 5171A is one of the 10 largest stars found so far. It is 50 per cent larger than huge Betelgeuse and about 1,000,000 times brighter than the Sun.
• The hypergiant is part of a double star system, with the second component so close that it is in contact with the main star.
• HR 5171A and its companion, HR 5171B, are so close that they touch and the whole system resembles a gigantic peanut.

HR 5171, the brightest star just below the center of this wide-field image, is a yellow hypergiant, a very rare type of stars with only a dozen known in our galaxy.

• Dr Chesneau and his colleagues made use of a technique called interferometry to combine the light collected from multiple individual telescopes, effectively creating a giant telescope up to 140 m in size.
• By analyzing data on the star’s varying brightness, the team confirmed the object to be an eclipsing binary system where the smaller component passes in front and behind the larger one as it orbits.
• In this case HR 5171A is orbited by its companion star every 1,300 days.
• HR 5171B is only slightly hotter than HR 5171 A’s surface temperature of 5,000 degrees Celsius.
• “The companion we have found is very significant as it can have an influence on the fate of HR 5171A, for example, stripping off its outer layers and modifying its evolution,” Dr Chesneau said.