Yellow hypergiant star has conjoined twin

Artist's impression of the yellow hypergiant star HR 5171

An artist’s rendering of yellow hypergiant star HR 5171 A and its orbiting partner, to which it is connected; photo courtesy of the European Southern Observatory

An international team of astronomers has discovered that one of the rare yellow hypergiant stars is bigger than first thought and has a smaller, orbiting star that is attached to it, something akin to conjoined twins.

Using European Southern Observatory's Very Large Telescope Interferometer, scientists, led by France's Olivier Chesneau, found that the yellow hypergiant star “HR 5171 A” was 1,300 times the diameter of the sun, making it the largest yellow hypergiant star among the dozen known in our galaxy. It also is one of the top 10 largest stars known to exist.

But the biggest surprise was its "very close binary partner."

"The two stars are so close that they touch and the whole system resembles a gigantic peanut," Chesneau said. "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."

yellow hypergiant star

Yellow hyper giant star HR 5171 A is 1 million times brighter than the sun and is among the top 10 biggest stars known to exist in our galaxy. Photo courtesy of European Southern Observatory

Remarkably, this yellow hypergiant star is about 1 million times brighter than the sun. More from the European Southern Observatory:

Despite its great distance of nearly 12,000 light-years from Earth, the object can just about be seen with the naked eye by the keen-sighted. HR 5171 A has been found to be getting bigger over the last 40 years, cooling as it grows, and its evolution has now been caught in action. Only a few stars are caught in this very brief phase, where they undergo a dramatic change in temperature as they rapidly evolve.

By analyzing data on the star's varying brightness, using observations from other observatories, the astronomers confirmed the object to be an eclipsing binary system where the smaller component passes in front and behind the larger one as it orbits. In this case, HR 5171 A is orbited by its companion star every 1,300 days. The smaller companion is only slightly hotter than HR 5171 A's surface temperature of 5,000 degrees Celsius. […]

This new discovery highlights the importance of studying these huge and short-lived yellow hypergiants, and could provide a means of understanding the evolutionary processes of massive stars in general.

The European Southern Observatory said the yellow hypergiant star HR 5171 A is changing very rapidly and has been caught during a very brief phase of its life cycle.

A video example of the yellow hypergiant star and its orbiting partner:

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