Yellow Hypergiant HR 5171 A Is the Largest Star of Its Kind

March 12, 2014

Today, astronomers revealed the discovery of the largest hypergiant known, star named HR 5171 A. It is 1300 times larger than Sun and has smaller companion star that orbits it very closely, possibly affecting its evolution.

Yellow hypergiant HR 5171 A and its companion star. Artist's impression. Credit: ESO

There are some very large stars out there, so large that saying "over 1,500 times larger than Sun" doesn't do them justice. So, comparison is in order – if Sun was a sphere only 9 kilometers in diameter, a star 1,500 times bigger would be as large as Earth. We call those stars hypergiants – a class above already big supergiants. There are blue, red and yellow hypergiants.

Today, the European Southern Observatory (ESO) reported that a team of astronomers have found the largest yellow hypergiant star known, named HR 5171 A. Its diameter is 1300 times the diameter of the Sun. To put things in perspective a bit more, if you placed HR 5171 A in the centre of our solar system, its surface would extend beyond the orbit of Jupiter. Although sizes of many hypergiants, whether blue, red or yellow, are not precisely determined, it is likely that HR 5171 A makes the top ten of the largest stars we discovered.

An international team led by Olivier Chesneau of Observatoire de la Côte d’Azur, Nice, France, used ESO's Very Large Telescope Interferometer (VLTI) to find the star. Interferometry is a technique where mulitiple telescopes are used to collect the light, which is then combined to create a single image equivalent to one a giant telescope 140 meters in size would make.

HR 5171 A is 12,000 light-years away from us, but in right circumstances, it is still possible to see it with the naked eye. Hypergiants are not only the largest, but also the brightest stars known. Despite their size and brightness, yellow hypergiants are hard to find because they are rare - only about 12 yellow hypergiants are known in our galaxy. 

This phase where already massive star grows to enormous size is only a short period in its already short life. During this time, which can be as short as a few thousand years, hypergiant ejects material from outer layers, forming sort of stellar atmosphere. By doing this, it loses both mass and temperature very fast, which makes it unstable. Surprisingly, every 1300 days, another, smaller and slightly hotter companion star orbits HR 5171 A so close, that it appears to be touching the hypergiant's atmosphere, adding to its unstability.

"The new observations also showed that this star has a very close binary partner, which was a real surprise," says Chesneau. "The two stars are so close that they touch and the whole system resembles a gigantic peanut. The companion we have found is very significant as it can have an influence on the fate of HR 5171 A, for example, stripping off its outer layers and modifying its evolution."

Needless to say, with so few yellow hypergiants in our galaxy, this find will help us better understand this short evolutionary phase in life of massive stars.

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