This deserves a “whoa”: astronomers have found a system of six (six) stars in which, if you follow it for a few days, every star in it will suffer an eclipse at some point.
Whoa.
Multiple stars are only intrinsically cool: unlike our Sun, which navigates space alone, multiples are where two or more stars orbit each other in a stable, gravitationally bound system. Half of the stars in the galaxy are in several such systems. Most are binary (two stars orbiting each other) and some binary (three stars). Fewer are still 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 is just over 1,900 light-years away, so at a fair distance, but it is bright enough to be detected by TESS, the Exoplanet Survey Transiting Satellite. TESS scans the sky by measuring the brightness of the stars to look for exoplanets in transit, which make mini-eclipses on the host stars, revealing their presence.
But he can also find many other interesting things. TYC 7037-89-1 looks like a star in TESS data, but one that changes its brightness – a variable star. Astronomers who have found it look in the TESS data for stars that change brightness in a certain way, indicating that there are several stellar systems.
What they were looking for are eclipsing tracks: Stars that not only orbit each other, but also those that we see orbiting near the edges, so that the stars seem to pass in front of each other. When this happens, the total light in the pair decreases slightly in a characteristic way. Astronomers have created automated software to search for such stars, and out of nearly half a million, they have found 100 that appear to be systems of three stars or more.
And this is what brings the second interesting thing about TYC 7037-89-1: they are not just six stars all orbiting in all directions, but they are arranged in binaries: a pair of stars orbit another a pair of stars, and a third pair orbits both!
The binary pairs are named A, B and C in the order of brightness and each star in them receives the number 1 or 2 (again in the order of brightness). The two inner binaries are then A (consisting of stars A1 and A2) and C (C1 and C2), orbiting further from binary B (B1 and B2). A and C are separated by about 600 million kilometers (about the distance of Jupiter from the Sun), taking about 4 years to orbit – 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, taking 2,000 years to complete a period.
And that now brings the coolest thing about this system: All three pairs of stars are eclipsing binaries! We see all three binary orbits near the edge. A1 and A2 undergo reciprocal eclipses (A1 eclipses A2, then half an orbit later A2 eclipses A1) every 1.57 days, so they are very close. C1 and C2 orbit each other every 1.31 days, and B1 and B2 last 8.2 days.
Because each star in a given pair eclipses the other, measuring how long the eclipse lasts, as well as other parameters (including spectrum capture), we can learn important things like how big the stars are, how hot they are, and more. And this gives way another surprise: All three binaries are very similar. There 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. Also, in each of them, the smaller stars are about the same as them: about 0.6 times the mass of the Sun and 0.6 times its diameter. They vary a little, but the idea is that they are quite close, which is strange.
This type of system is just ridiculously unlikely. Star-forming models show that sextuples are more often made up of two trinary systems orbiting each other, not three binaries. So it’s pretty rare, but seeing all three binaries seems impossible.
… “pare.” In fact, they probably formed from a spinning disk of material, each star collapsing from it. Because of this, the three orbital planes of the goods are likely to be the same. Therefore, if we see an edge, we see them all on the edge, or so. This does not make it as unlikely that one might think that all three are eclipsing.
I will also note the binary orbits one around the other they are not the edges. We see the orbit of A and C around each other from an angle of about 40 °, even as we see the individual stars at the binary edges. However, the inclination of B’s orbit around them is not well limited by observations.
We hope that the longer-term study of this system will produce more information about how they were formed. We don’t know much about more systems like this, so understanding the conditions under which they are formed would be quite interesting.
I know, that causes a headache. So many orbits, angles, stars … Sometimes nature is complex and it’s hard to keep up. If it helps, I describe a similar fictional system that played a key role in the first season of the year Star Trek: Picard. And several systems are known a bit like TYC 7037-89-1; for example CzeV1640 is a quadruple system with two pairs of eclipsing tracks. Nature is complex, but sometimes frugal, always reusing the same idea.
But, oh, I’d like a ship like that Business right now! To see something like this up close for me, watch these six stars – six! – dance around each other …
Strange new worlds really.