In 2013, a jumbo star 1,300 times wider than the sun stunned astronomers. They say it’s the largest yellow hypergiant star ever found.

Stars are as diverse and complex as anything else in our universe. They can be classified into many different types – from white dwarfs to hypergiants. But this time we’re going to talk about the largest yellow hypergiant star ever detected.

Yellow hypergiants are very rare, with only about a dozen known in Milky Way Galaxy. The best-known example is Rho Cassiopeiae.

Not only are these stars the largest, but they are also the brightest stars known. They are at a stage of their lives when they are unstable and changing rapidly. Thus, these gargantuan objects expel material outwards, forming a large, extended atmosphere around the star.

Astronomers found this hypergiant star, dubbed HR 5171A, in 2014, using ESO’s Very Large Telescope Interferometer. The star lies about 12,000 light-years away from Earth in the constellation Centaurus.

HR 5171A is part of a binary system, together with HR 5171B. The companion star orbits so close that it touches the main star and the whole system resembles a gigantic peanut.

The distance from center-to-center for the system is about 10 AU’s. Whereas the surface-to-surface distance for the A and B components of the system are “only” about 2.8 AU’s apart.

However, the expanded outer atmosphere of the bloated primary contacts the secondary, meaning these two massive stars are in physical contact.

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By finding out the orbital period of a secondary star, astronomers can measure the mass of the primary using good old Newtonian mechanics. Some extra astrometry used to measure its tiny parallax can enable the researchers to pin down the incredible size and distance of HR 5171A.

The binary system weighs in at a combined 39 solar masses and has a radius of over 1,300 times that of our Sun.

The gargantuan object is 50% larger than the famous red supergiant Betelgeuse — and about one million times brighter than the Sun.

If we were to replace the Sun with HR 5171 A, the yellow hypergiant would extend out over 6 astronomical units (AU) past the orbit of Jupiter.

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Dr. Olivier Chesneau, the lead author of the study, and his colleagues used the Interferometry technique to combine the light collected from multiple individual telescopes, effectively creating a giant telescope up to 140 m in size. These 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.

HR 5171B orbits its host star every 1,300 days. Such a system is known as an eclipsing binary. Famous examples of similar systems are the star Algol (Alpha Persei), Epsilon Aurigae, and Beta Lyrae. 

The companion star for HR 5171 is also a large star in its own right at around six solar masses and 400 solar radii in size. It is only slightly hotter than its main star’s surface temperature of 5,000 degrees Celsius.

The binary star system lies really far away from us but the keen-sighted can still see it with the naked eye in the southern hemisphere.

Astronomers have found HR 5171 A has been getting bigger over the last 40 years, cooling as it grows. It’s like scientists have caught its evolution in action. We have only discovered a few stars in this very brief phase, where they undergo a dramatic change in temperature as they rapidly evolve.

The companion we have found is very significant as it can influence the fate of HR 5171 A, for example, stripping off its outer layers and modifying its evolution, said Dr. Chesneau.

Studying these huge and short-lived yellow hypergiants can help astronomers understand the evolutionary processes of massive stars in general.

The researchers revealed their results in a paper published in the journal Astronomy & Astrophysics (

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