TIC 168789840, also known as TYC 7037-89-1,[5] is a stellar system with six stars.[6][4] Three pairs of binary stars circle a common barycenter. While other systems with three pairs of stars have been discovered, this was the first system where the stars can be observed eclipsing one another, as their planes of rotation point approximately towards the Earth.[5]
Discovery
Astronomers used the orbiting TESS telescope to determine TIC 168789840 was lined up so all six stars' eclipses could be observed.
The Transiting Exoplanet Survey Satellite identified that the star system consisted of six eclipsing stars.[7] The discovery was announced in January 2021. It is approximately 1,900 light-years (584 pc) from Earth, in the constellation Eridanus, west of the river asterism's sharpest bend, Upsilon2 Eridani, often called Theemin.[8] To be seen the group needs strong magnification from Earth as is much fainter than red clumpgiant star Theemin and is about nine times further away.
Two sets of the binaries co-orbit relatively closely, while the third pair of stars takes 2,000 years to orbit the entire system barycenter.[6][10][11] The inner A pair and C pair orbit each other in 3.7 years. These are, as taken from the paired B stars, about 250 AU away (specifically the mean telescopic separation is 423 mas) and the three lettered pairs, as groups, have been resolved (the three gaps made out). From A pair to C pair is calculated to be 4 AU (∼7 mas) apart, which means this gap should be resolvable using speckle interferometry which has not yet been achieved.
Note, the three binaries (here close pairs) A, B, and C are resolved only as systems, the pairs being just 6.9 R☉ (∼0.054 mas), 21.4 R☉ (∼0.168 mas), and 6.1 R☉ (∼0.047 mas) apart, respectively.[4]
The stars in the A and C systems orbit each other roughly every day and a half, and the two binaries orbit each other about every four years. The B binary’s members circle each other about every eight days, but the pair is much farther away, orbiting around the inner systems roughly every 2,000 years.[5]
Stellar characteristics
The primary stars of all three close binaries are slightly hotter and brighter than the Sun,[7] while the secondary stars are much cooler and dimmer.[4] Because the two closely bound pairs are so close, only the third, more distant pair could have planets.[6] The primaries are all beginning to evolve away from the main sequence, while the less massive and longer-lived secondaries are all still firmly on the main sequence and fusing hydrogen in their cores.[4]
See also
Castor (star) – the second-brightest (apparent) "star" in Gemini, likewise a (double-double)-double system
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Robin George Andrews (2021-01-23). "Six Stars, Six Eclipses: 'The Fact That It Exists Blows My Mind'". The New York Times. Archived from the original on 2021-01-28. Retrieved 2021-01-29. But only one of the pairs could have any planets. Two of the system's binaries orbit extremely close to one another, forming their own quadruple subsystem. Any planets there would likely be ejected or engulfed by one of the four stars. The third binary is farther out, orbiting the other two once every 2,000 years or so, making it a possible exoplanetary haven.
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Natali Anderson (2021-01-25). "TESS Discovers Sextuply-Eclipsing Six-Star System". Sci-News. Archived from the original on 2021-01-26. Retrieved 2021-01-29. 'Prior to the discovery of TIC 168789840, there were 17 known sextuple star systems according to the June 2020 update of the Multiple Star Catalog,' lead author Dr. Brian Powell of NASA's Goddard Space Flight Center and colleagues wrote in their paper.
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