One of the Largest Stars Known has Surprise Partner

One of the largest stars ever found, a behemoth 1,300 times the diameter of the Sun and about 1 million times as bright, is part of a surprise binary star system, astronomers have learned.

Writing in the journal Astronomy & Astrophysics, the researchers report their discovery, which was made with the European Space Observatory's Very Large Telescope Interferometer (VLTI).

Astronomer Olivier Chesneau of the Côte d'Azur Observatory in Nice, France, and a team of international colleagues found the binary system around a yellow hypergiant dubbed HR 5171 A.

The star is the largest yellow star known and in the top 10 of the largest of any known star. It is 50 percent larger than the famous red supergiant star Betelgeuse.

"The new observations also showed that this star has a very close binary partner, which was a real surprise," Chesneau said. "The two stars are so close that they touch and the whole system resembles a gigantic peanut."

To make their observations, Chesneau and his colleagues used a technique called interferometry, which combines light data collected from multiple individual telescopes, effectively creating a 140-meter telescope to peer deep into space.

In their researcher, Chesneau and his collaborators used 60 years worth of astronomical data to see how the newfound binary system behaved in the past.

Yellow hypergiants such as HR 5171 A, and the more well-known Rho Cassiopeiae, are very rare. Only about a dozen are known in the Milky Way. Despite being 12,000 light years away, HR 5171 A can be seen with the naked eye in the constellation of Centaurus.

Currently, the star is in a transitional period where it is unstable and changing rapidly. Over the last 40 years, the star has been expanding, but cooling as it grows.

The companion star of HR 5171 A is slightly hotter and makes an orbit around the star every 1,300 days, the astronomers learned.

"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," Chesneau said.