The European Space Observatory's Very Large Telescope (VLT) has spotted a massive yellow star with a diameter of more than 1,300 times the size of the Sun. The star is also a part of a binary system, with a companion star orbiting so close that it is actually in physical contact with the giant.
The truly massive nature of the stellar behemoth, imaginatively named HR 5171 A , was discovered due to detailed imaging by the ESO's Very Large Telescope Interferometer (VLTI).
The VLT itself is not a single telescope, but comprises four Unit Telescopes, each boasting an impressive 8.2 m (26 ft) diameter main mirror. In addition to this, the VLT has four mobile 1.8 m (5.9 ft) auxiliary telescopes. Interferometry essentially allows the light collected from all four main telescopes as well as the auxiliaries, to be combined, creating one giant 140 m (459 ft) telescope.
HR 5171 A  is the largest of a very rare category of star known as a yellow hypergiant. These stars are generally extremely unstable and represent some of the brightest stars that we know of. The unstable nature of the star manifests physically with the regular expulsion of massive amounts of stellar material, causing the star to have a large extended atmosphere.
HR 5171 A  is roughly 50 percent larger than the red supergiant Betelgeuse. Sitting 12,000 light-years away from earth, HR 5171 A  has an astonishing diameter of more than 1,300 times our own star, and is over a million times brighter.
However, even with the yellow hypergiant's incredible size making it one of the top ten largest stars ever discovered, it cannot compare to some of the biggest stars on record. For example, the red supergiant UY Scuti is a veritable leviathan with a diameter of 1,708 times that of our Sun. It must be noted that references to the size of these monster stars are subject to significant margins of error. This is due to the fact that the size of the stars cannot be discovered via direct observation and must instead be inferred with measurements of brightness, temperature and distance.
It is not just the sheer size of HR 5171 A  that makes it such a rare and beautiful find. Regarding the unique nature of the star, Olivier Chesneau of the Observatoire de la Côte 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," going on to say that, "The two stars are so close that they touch and the whole system resembles a gigantic peanut."
The existence of the companion star was verified with observations from other observatories, with data suggesting that the small star orbits its massive parent once every 1,300 days.
HR 5171 A  has actually been observed for around 60 years prior to being imaged by the VLTI, but no-one had previously known how truly massive it was. It's also been growing over the last 40 years, with the star cooling as it enlarges. Further observations of the changing star will undoubtedly grant a fascinating insight into the evolution of these stellar giants.
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