Monster yellow star is 1,300 times wider than the Sun

Astronomers have discovered that a star that is just too faint to be seen with the unaided eye is actually one of the biggest and brightest ever identified in our galaxy.

An artist’s impression shows the yellow hypergiant star HR 5171. Credit: ESO
An artist’s impression shows the yellow hypergiant star HR 5171. Credit: ESO

The hypergiant is the largest known yellow star, 1,300 times the diameter of our own Sun and around one million times brighter. It is also one of the top ten largest stars known.

Catalogued as HR 5171, the star lies in the constellation of Centaurus, close to apparently bright stars Alpha and Beta Centauri and Crux, the Southern Cross. It is so far away, at a distance of nearly 12,000 light-years from Earth, that you need binoculars to see it.

The star’s true nature was revealed when astronomers combined a number of telescopes that work together as the European Southern Observatory’s Very Large Telescope Interferometer. They were surprised not just by its size but to find that it is virtually welded to a stellar companion.

The research team used not just observations from ESO’s telescopes in Chile but also those made over 60 years by observatories around the world, but also including those made by amateur astronomers. HR 5171 is a variable star that is also known as V0766 Cen, and the American Association of Variable Star Observers (AAVSO) gives its magnitude range as from 6.17 to 7.5.

In particular the team praised the observations of amateur astronomer Sebastian Otero between 2000–2013 as being in “excellent” agreement with professional measurements of the star’s brightness.

Olivier Chesneau, of the Observatoire de la Côte d’Azur, Nice, France, led the international team that investigated the star. Their results show that the main component of the binary system, labelled HR 5171 A, is 50 per cent bigger than the famous red supergiant Betelgeuse.

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

He added: “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.”

Analysis of the star’s changing brightness shows that HR 5171 A has been growing in size over the past 40 years, and is cooling down as it expands. Its companion star, which is only slightly hotter than the primary star’s 5,000 Celsius, orbits it every 1,300 days.

Only a dozen or so yellow hypergiant stars known in our Milky Way galaxy, with the best known being Rho Cassiopeiae in the northern sky. They are among the biggest and brightest stars known and at a highly unstable stage of their lives.

HR 5171
HR 5171 is the brightest star just below the centre of this wide-field image. Credit: ESO/Digitized Sky Survey 2

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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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