Astronomers Take Closer Look at Supergiant Binary Star HR 5171

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Astronomers using ESO’s Very Large Telescope Interferometer (VLTI) have captured striking images of HR 5171, a unique binary stellar system in the constellation of Centaurus.

This artist’s impression shows the binary star HR 5171. Image credit: ESO.

This artist’s impression shows the binary star HR 5171. Image credit: ESO.

HR 5171, also known as V766 Centauri, HD 119796 and HIP 67261, is an eclipsing binary system where the smaller component (HR 5171B) passes in front and behind the larger one (HR 5171A) as it orbits. The two stars are so close that they touch and the system resembles a huge peanut.

Despite its great distance of nearly 12,000 light-years from Earth, HR 5171 can just about be seen with the naked eye by the keen-sighted.

HR 5171 was first studied and classified a few years ago by French astronomer Olivier Chesneau and co-authors when the larger component was found to be something known as a yellow hypergiant.

Measuring over 1,400 times the diameter of the Sun, HR 5171A was not only the largest star of its type ever discovered, but also one of the ten largest stars ever found. It is 50% larger than huge Betelgeuse and about 1,000,000 times brighter than the Sun.

However, ESO astronomer Markus Wittkowski and colleagues have instead suggested that HR 5171A is likely to be in the phase of life just prior to that of a yellow hypergiant: an evolved red supergiant, which is losing mass so fast that it will eventually transition back into a warmer yellow supergiant for a short period of time.

Either way, the star is a true behemoth, and of huge interest to astronomers wishing to understand more about this unusual stage in the life cycle of stars.

This image shows HR 5171 as it was seen over three periods of time. These images actually contain both HR 5171A and its companion -- in the first image the companion is passing behind HR 5171A, but in the second and third images the companion is passing in front and is visible as a bright patch. Image credit: M. Wittkowski / ESO.

This image shows HR 5171 as it was seen over three periods of time. These images actually contain both HR 5171A and its companion — in the first image the companion is passing behind HR 5171A, but in the second and third images the companion is passing in front and is visible as a bright patch. Image credit: M. Wittkowski / ESO.

Dr. Wittkowski and co-authors have now used the VLTI again to study the HR 5171 system in greater detail.

They found HR 5171B to be smaller and cooler than its partner — likely a cool giant or supergiant with a radius of around 650 times that of the Sun.

Close companions are thought to be typical for massive stars and are important in the processes of stellar evolution.

The results will be published in the journal Astronomy Astrophysics (arXiv.org preprint).

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M. Wittkowski et al. 2017. Multi-epoch VLTI-PIONIER imaging of the supergiant V766 Cen: Image of the close companion in front of the primary. AA, in press; arXiv: 1709.09430