Washington: Researchers using ESO’s Very Large Telescope Interferometer (VLTI) have found the largest yellow hypergiant star HR 5171 A – 1300 times the diameter of the Sun and much bigger than was expected.

This makes it the largest yellow star known. It is also in the top ten of the largest stars known — 50 per cent larger than the famous red supergiant Betelgeuse — and about one million times brighter than the Sun.

Olivier Chesneau (Observatoire de la Cote d’Azur, Nice, France) said that the new observations also showed that this star has a very close binary partner, which was a real surprise, asserting that 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 spanning more than sixty years, to see how it had behaved in the past.

By analysing 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 1300 days. The smaller companion is only slightly hotter than HR 5171 A’s surface temperature of 5000 degrees Celsius.

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