Betelgeuse, in the constellation Orion’s “shoulder”, has been considered by many to be quite a large star, being a red giant. Nearby neighbor Sirius is our best known example of a binary star; two stars orbiting each other in one system, but a star has now been found that is 50 percent larger than Betelgeuse, and has a smaller sister star locked so closely in orbit with it that they are touching as they circle each other. This new binary star system features one of the largest stars recorded thus far in our galaxy, using one of the most powerful telescopes available on Earth.
HR5171A is a yellow hypergiant. These stars, even larger than the famous red giants, are the rarest of suns. This particular star is 1,300 times the size of Earth’s sun, and is a massive 12,000 light years away. This makes the newly discovered giant a distant note in the night sky. In comparison, the binary star Sirius is a cozy next-door neighbor at 8.6 light years away, and Betelgeuse is just in the next town at 642.5 light years away.
Of course a very powerful and well-engineered telescope is needed to sight star systems at that distance. The astronomers who discovered HR5171 used the VLTI (Very Large Telescope Interferometer) array on Paranal Mountain in Chile. The VLTI was built by the European Southern Observatory to power such very distant viewings, and includes four mobile telescopes, each with mirrors over eight meters in diameter.
The array also includes four mobile auxiliary telescopes at just under two meters apiece. When used together, the entire array becomes the VLTI, which can ‘infer’ details upwards of 25 times finer than the telescopes can discern individually. The light beams from each telescope are passed between them using underground tunnels and precise calculations, and are precise enough that the VLTI can be used to construct images with a higher resolution than any other telescope array in existence.
The VLTI’s individual telescopes are named Yepun, Melipal, Kueyan, and Antu, and on their own can discern objects four billion times fainter than a human can see unaided. Combined together, they created a telescope 140 meters in diameter, and are able to parse enough detail that astronomer Olivier Chesneau of France could actually characterize the relationship between HR5171A and its partner as looking like “a gigantic peanut” rather than two stars.
Only a dozen yellow hypergiants have been discovered in the Milky Way galaxy thus far, with 21 known in total in the visible universe. HR5171A is the only one astronomers have identified as being part of a binary system. Scientists involved in the study have spent a cumulative sixty years finding and observing this star, and have finally made the determination of its binary nature now they have access to this startlingly powerful and high-tech new telescope array.
The discovery may be just in time, at least as lifetimes of stars are counted. Suns only become yellow hypergiants for a short time near the end of their lifespans, right before they burn up all their remaining fuel and wink out. As such a star dies, it blows out vast quantities of gases, creating a nebula as its tombstone. HR5171A is moving along this path fairly quickly, as scientists observing the star have seen it growing larger and noticeably cooling in the last 40 years.
Astronomers posit that the younger companion star, HR5171B, may be hastening the process by feeding off of the larger star’s gases, stripping off layers of the dying hypergiant in the process. HR5171B orbits its larger neighbor every 1,300 days to cross in front of the giant, giving scientists both a visible reaction to prove the binary nature of the system, and a good idea of just how large is HR5171A that it causes its partner to describe such a large, tight orbit when they are touching the entire time.
With their combined radiation, these two stars give off enough light to shine a million times brighter than Earth’s sun. Even at this distance, they can just barely be seen with the naked eye. The binary system exists in the constellation Centaurus.
Previous research on the star was done using the South African Astronomical Observatory, the Anglo-Australian Telescope, and the Gemini South Telescope, but the observations with the VLTI have clarified the nature of the phenomenon. The newfound hypergiant sun could not have been proven to be a binary star without the powerful new telescope. Though HR5171A is not the largest yellow hypergiant known, it is one of the largest, and the only one proven to have a binary nature.
By Kat Turner