At 864,000 miles across, the sun’s plenty big, and yet we can easily cover it with the tip of our pinkie finger. Its apparent girth is understandably diminished by its distance of 93 million miles. Now imagine “Big Yellow” HR 5171, a star 1,300 times the sun’s diameter or 1.1 billion miles across. Never mind covering it with a finger or anything else. Mercury, Venus, Earth, Mars and Jupiter would all orbit inside such a star. Not for long of course – each would be incinerated in seconds.
Welcome to HR 5171’s world. This hypergiant star, also known as V766 Cen, is located near Alpha and Beta Centauri in the southern hemisphere sky. It’s not only the largest yellow star known but also makes the Top Ten list of largest known stars.
You’re probably familiar with the supergiant star Betelgeuse in Orion – HR 5171 is half again as big and a million times brighter than the sun. It’s unusual to find such a huge yellow star; most extremely large stars are bloated red supergiants like Betelgeuse. There are only a dozen or so yellow supergiants we know of in the Milky Way galaxy.
Good thing HR 5171 shines from a safe 12,000 light years away. Distance diminishes the might giant to a point of yellow light hovering at the naked eye visibility limit.
Using European Southern Observatory’s Very Large Telescope Interferometer (VLTI), Olivier Chesneau and an international team of collaborators measured HR 5171 diameter with great precision.
Interferometry combines the light collected from multiple individual telescopes to effectively create a giant telescope “mirror” up to 459 feet (140 meters) across. Big enough to resolve the disk of a star provided it’s as enormous as HR 5171.
“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 smaller, hotter companion revolves around the giant every 1,300 days, its gravity stripping material from the primary star. Because a star’s evolution depends on how much stuff it has, losing material to the companion could alter HR 5171’s future such as its potential to become a supernova.