Behold HR5171A, a massive yellow star that's an unfathomable 1,300 times the diameter of our sun. It's part of an exotic binary system that was spotted by an international team of astronomers who say it's now among the ten largest stars ever found.
HR5171A just hit the record books for being the largest yellow star we know of. It's 50% larger than the red supergiant Betelgeuse and about one million times brighter than the Sun. It measures a breathtaking 1.2 billion miles (1.8 billion km) in diameter. That's roughly the distance of Saturn to our sun, about 12 AU.
The system is located about 12,000 light-years away, but remarkably, it's so bright that it can just be made out by the naked eye. Astronomers have known about it for at least four decades, but new observations made by the ESO's Very Large Telescope (VLT) resulted in refined measurements.
As you can see from the ESO's artistic impression, the hypergiant has a very close binary partner. Like, really close. The two stars are actually touching each other, which is why the binary system resembles a kind of distorted peanut. It's an eclipsing system in which the smaller star is passing in front of and behind the larger one as it orbits — in this case once every 1,300 days.
Observations over the past 40 years show that HR5171A has been getting bigger, cooling as it grows. It's rare for astronomers to catch a star during this brief evolutionary phase as it undergoes a dramatic change in temperature.
The field around yellow hypergiant star HR5171. Credit ESO.
Yellow hypergiants are extremely rare. Astronomers have only been able to catalog about a dozen or so in the Milky Way, the best example being Rho Cassiopeiae. They're among the biggest and brightest stars in the cosmos, and are at a stage in their lives when they're highly unstable and rapidly changing. This instability causes the star to expel material outwards, which forms a large extended atmosphere around the star.
[ ESO ]