14th March 2014

Largest known yellow star is discovered

A mixture of new and old observations going back 60 years has revealed an exotic binary system that includes the largest known yellow star.



Credit: ESO

The European Southern Observatory (ESO)'s Very Large Telescope Interferometer has found the largest yellow star – and one of the ten largest stars ever discovered. This hypergiant has been found to measure more than 1300 times the diameter of the Sun, and to be part of a double star system. The smaller companion is so close that it is actually in contact with the main star. Observations spanning over 60 years – some from amateur observers – also indicate that this rare and remarkable object is changing very rapidly and has been caught during a very brief phase of its life.

Olivier Chesneau (Observatoire de la Côte d'Azur, Nice, France) and an international team of astronomers found that this yellow hypergiant – HR 5171 A – was much bigger than they expected. It is so huge that, if placed in our Solar System, its radius would stretch beyond Jupiter. It is 50% larger than the famous red supergiant Betelgeuse and a million times brighter than our Sun.

"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 astronomers made use of a technique called interferometry to combine the light collected from multiple individual telescopes, effectively creating a giant telescope up to 140 metres in size. The new results prompted the team to thoroughly investigate older observations of the star, to see how it had behaved in the past.

Yellow hypergiants are very rare, with only a dozen or so known in our galaxy – the best-known example being Rho Cassiopeiae. They are among the biggest and brightest stars and have reached a stage of their lives when they are unstable and changing rapidly. Due to this instability, yellow hypergiants also expel material outwards, forming a large, extended atmosphere around the star.



HR 5171 A. Credit: ESO/Digitized Sky Survey 2

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 analysing data on the star's varying brightness, using records 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 5000 degrees Celsius.

"The companion we have found is very significant, as it can have an influence on the fate of HR 5171 A," adds Chesneau. "For example, stripping off its outer layers and modifying its evolution."

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.

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