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Largest Ever Yellow Hypergiant Star Spotted

422 Largest Ever Yellow Hypergiant Star Spotted
Artist’s impression of the yellow hypergiant star HR 5171 / ESO
Get ready, there are a lot of surprises and superlatives in this one. Using the Very Large Telescope, scientists have realized that this hypergiant star is actually the largest yellow star ever found -- and they caught it in a very rare moment. 
Scientists and amateur observers have been watching the yellow hypergiant star HR 5171 A for over 60 years. Now, using the European Southern Observatory’s Very Large Telescope Interferometer, an international team of researchers have discovered that the yellow hypergiant is even bigger than expected: It’s 1,300 times the diameter of the Sun and about a million times brighter. They thought it was only going to be 400–700 times that of the Sun.
HR 5171 A easily claims a spot in the Top 10 Largest Stars ever discovered. It’s even 50 percent larger than the famous red supergiant, Betelgeuse, which makes up one of Orion’s shoulders. 
Only about a dozen for so of these yellow hypergiants are known in our galaxy. (You may have heard of Rho Cassiopeiae.) They’re some of the biggest, brightest stars, and we’re seeing them while they’re at a particularly unstable and rapidly changing stage of their lives -- where materials expel outwards, forming an extended atmosphere around the star.
Additionally, this one was found during a very short-lived phase. You see, HR5171 is a massive interacting binary, or a double-star system where the companion star is still in contact with the main yellow hypergiant star. This is one of the few systems caught in this phase of mass transfer.
“The new observations also showed that this star has a very close binary partner, which was a real surprise,” study researcher Olivier Chesneau from Observatoire de la Côte d’Azur in France says in a press release. “The two stars are so close that they touch and the whole system resembles a gigantic peanut.”
The smaller one, which orbits the hypergiant every 1,300 days, is slightly hotter than the  bigger one’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 5171 A,” Chesneau adds, “for example, stripping off its outer layers and modifying its evolution.”
In at least the last four decades of observations, HR 5171 A has been getting bigger, cooling as it grows; only a few stars are caught in this very brief phase. (Is there anything not rare about this star?) It’s nearly 12,000 light years away from Earth, but if you’ve got eagle eyes, I hear it’s visible to the naked eye. 
The findings were published in Astronomy and Astrophysics this month. 
Images: ESO (top) & ESO/Digitized Sky Survey 2 (below)


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