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Sci-Tech

Astronomers make a real five-star find

Astronomers have found the very first quintuple star system consisting of two sets of eclipsing binary stars and one solo star.

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Quintuple star system 1SWASP J093010.78+533859.5 in the centre. Centre de données astronomiques de Strasbourg

Five stars are inextricably linked in a star system unlike any that has been seen before.

Located just 250 light-years from Earth in the constellation of Ursa Major, the system 1SWASP J093010.78+533859.5 was the subject of a 2013 study led by Marcus Lohr of the Open University in the UK. At the time, Lohr and his team thought the system consisted of four stars -- two binary stars gravitationally locked together, which would have been rare enough in itself.

Presenting at the UK Royal Astronomical Society's National Astronomy Meeting 2015 in Wales on July 8, Lohr revealed that the system actually consists of five stars -- two binaries and fifth lone star -- the first of its kind ever identified.

The two binaries, as Lohr and his team noted in the 2013 paper, are a type of binary called an eclipsing binary. These are binary stars that orbit around each other so that they eclipse each other in our line of sight from Earth, blocking some or all of the other star's light.

The team identified these stars from archived data from the SuperWASP project. This uses low-coast cameras located in the Canary Islands and South Africa to image almost the whole sky every few minutes. Although the data is low-res, it provides an accurate measurement of the brightness of stars over time.

Eclipsing binary stars, as they pass in front of each other, create a measurable dip in brightness as they eclipse each other's light.

The first identification made in the system was one of the binaries. It is what is known as a contact binary: two stars so close together that they share an outer atmosphere. These aren't uncommon, but the 1SWASP J093010.78+533859.5 contact binary has an unusually short orbital period. The two stars take just six hours to complete one orbital cycle.

Artist's impression of the five-star system. The dotted line shows the orbital path of the two binary systems. Smaller orbits are not shown to scale. Marcus Lohr

Some of the dips in the light curve couldn't be explained by the first binary -- indicating the second set of eclipsing binary stars. This pair was detached, with the two stars separated by a distance of three million kilometres, with an orbital period of about 1.3 days. These two sets of binary stars are orbiting on the same plane, which indicates that they might have originally formed from the same proto-stellar disc.

But there were pieces of spectral data still remaining that didn't fit the profiles, as identified by astronomers at the Korea Astronomy and Space Institute in a paper published in 2014.

"Stationary spectral lines were detected that must have originated from a further, previously unrecognised stellar component," the paper reads.

Further analysis by Lohr's team confirmed, in a study published in June in the journal Astronomy & Astrophysics, the fifth star suggested in the 2014 paper. At a distance of up to two billion kilometres from the detached binary, it was far enough to not eclipse the binaries, but close enough -- as confirmed by an analysis of the spectral data -- to be gravitationally bound in a five-star system.

Although each of the stars are individually smaller than the sun, collectively they are bright enough to be visible in small telescopes. Amateur astronomers on Earth could observe the eclipses.

"This is a truly exotic star system. In principle, there's no reason it couldn't have planets in orbit around each of the pairs of stars. Any inhabitants would have a sky that would put the makers of "Star Wars" to shame -- there could sometimes be no fewer than five Suns of different brightnesses lighting up the landscape," Lohr said.

"Days would have dramatically varying light levels as the different stars were eclipsed. They would though miss out on night for a large part of their 'year', only experiencing darkness (and a night sky) when the stars were on the same side of their world."