Get warm and cuddly with the largest hypergiant yellow star known

HR 5171 is a yellow hypergiant, a very rare type of stars with only a dozen known in our galaxy. Its size is over 1,300 times that of the Sun and€” one of the ten largest stars found so far. Credit: ESO

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.

This chart shows the constellation Centaurus the Centaur. HR 5171 is marked with a circle and can be easily seen with a pair of binoculars from the far southern U.S. and points south. Credit: ESO, IAU and Sky & Telescope

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.

Artist view shows the yellow hypergiant star HR 5171 and its close-orbiting companion star. The companion may influence the evolution of the hypergiant by stripping off its outer layers during its 1,300-day revolution. Credit: ESO

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.

2 Responses

  1. If the star is 1.1 billion miles across, its diameter is approximately 550,000,000 miles in diameter which would place it just beyond the orbit of Jupiter. Saturn would be “safe”.

    1. Profile photo of astrobob

      Hi Wade,
      Thank you for pointing this out. “Safe” is absolutely correct. I was thinking strictly in terms of diameter but placed at the center of the solar system the star would “only” reach 550 million in all directions; Saturn and its moons would warm up but presumably remain intact.

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