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It deserves a “whoa”: Astronomers have found a sixfold (six) star system where, if you watch it for a few days, every star in it will at some point undergo an eclipse.
Whoa.
Multiple stars are simply inherently cold: unlike our Sun, navigating space alone, multiples are where two or more stars revolve around each other in a stable system linked by gravity. Half of the stars in the galaxy are in several systems like this one. Most are binary (two stars orbiting each other) and some are trinaries (three stars). There are even less in higher order systems.
This is the first thing that makes TYC 7037-89-1 special: it’s a sextuplet, a six star system. It’s a little over 1,900 light-years away, so a good distance, but it’s bright enough to be detected by TESS, the transiting exoplanet survey satellite. TESS scans the sky by measuring the luminosity of stars to look for exoplanets in transit, which make mini-eclipses on their host stars, revealing their presence.
But he can also find a lot of other interesting things. TYC 7037-89-1 looks like a star in TESS data, but changes its brightness – a variable star. Astronomers who found it are looking in the TESS data for stars that change in brightness in a certain way, indicating that they are multiple star systems.
What they were looking for are eclipsing binaries: Stars that not only orbit on top of each other, but also those where we see their orbits almost edge-on, so that the stars seem to pass each other. When this happens, the total light of the pair characteristically decreases a bit. Astronomers set up automated software to search for such stars, and out of nearly half a million, they found 100 that appeared to be three or more star systems.
And that’s what brings up the second interesting thing about TYC 7037-89-1: it’s not just six stars orbiting all over the place, but they’re organized in binaries: a pair of orbiting stars. another pair of stars, and a third pair orbit them both!
Binary pairs are named A, B, and C in order of brightness, and each star in them is assigned the number 1 or 2 (again in order of brightness). The two internal binary are then A (made up of stars A1 and A2) and C (C1 and C2), orbited further by binary B (B1 and B2). A and C are about 600 million kilometers apart (very roughly the distance from Jupiter to the Sun), which takes about 4 years to get around – this was determined using archival data from other telescopes , including WASP and ASAS-SN. B orbits them both at a distance of about 38 billion km, which takes 2000 years to complete a period.
And that now brings up the coolest thing about this system: The three pairs of stars eclipse the binaries! We see the three binary orbits almost on edge. A1 and A2 undergo mutual eclipses (A1 eclipses A2, then half an orbit later A2 eclipses A1) every 1.57 days, so they are very close together. C1 and C2 rotate in orbit every 1.31 days, and B1 and B2 take 8.2 days.
Because each star of a given pair eclipses the other, by measuring the duration of the eclipse as well as other parameters (including taking spectra), we can learn important things like the size of the stars, their heat, etc. And this gives another surprise: the three binaries are very similar. These are triplets!
In each, the larger star is about 1.5 times the diameter of the Sun, slightly hotter, and about 1.25 times the mass of the Sun. Moreover, in each of them, the smaller stars are roughly identical to each other: about 0.6 times the mass of the Sun and 0.6 times its diameter. They vary a bit, but the point is, they’re pretty close, which is weird.
This kind of system is just ridiculously improbable. Models of star formation show that sextuples are much more often made up of two trinary systems orbiting each other, and not three binaries. So it’s quite rare, but having all three binaries flat seems impossible.
… “seems.” In fact, it’s likely they formed from a swirling disc of material, with each star collapsing. Because of this, it is in fact likely that the three orbital planes of the binaries are the same. Therefore, if we see an edge on, we are all seeing edge on, or almost. That doesn’t make it as unlikely as you might think that the three will eclipse.
I will also note the orbits of the binaries around each other are not at the limit. We see the orbit of A and C around each other at an angle of about 40 °, even as we see the individual stars in the edge binaries. However, the inclination of B’s orbit around them is not well limited by observations.
Hopefully, a longer term study of this system will provide more information about their training. We don’t really know much about several systems like this, so understanding under what conditions they form would be quite interesting.
I know, it causes headaches. So many orbits, angles, stars… Sometimes nature is complex and difficult to follow. If it helps, I’m describing a similar fictional system that played a key role in the first season of Star Trek: Picard. And more systems like TYC 7037-89-1 are known; for example CzeV1640 is a quadruple system with two pairs of eclipse binaries. Nature is complex, but sometimes frugal, reusing the same idea over and over again.
But oh my God, would I like a boat like Business now! To be able to see such a thing up close for myself, look at these six stars – six! – dance around each other …
New strange worlds indeed.
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