This deserves a “whoa”: astronomers have found a sixfold (six) galaxy where, if you look at it for a few days, every star in it will at some point undergo a solar eclipse.
Whoa.
Multiple stars are just intrinsically cool: Unlike our sun, which travels alone through space, are multiples where two or more stars orbit each other in a stable, gravity-bound system. Half of the stars in the Milky Way are in multiple such systems. Most are binaries (two stars orbiting each other) and some in trinaries (three stars). Even less are in higher order systems.
That’s the first thing that makes TYC 7037-89-1 special: it’s a sextuplets, a six-star system. It is a little over 1,900 light-years away, which is quite a distance, but bright enough to be detected by TESS, the Transiting Exoplanet Survey Satellite. TESS scans the sky and measures the brightness of stars to search for traversing exoplanets that create mini-eclipses on their host stars and reveal their presence.
But it can also find many other interesting things. TYC 7037-89-1 looks like one star in TESS data, but one that changes brightness – one variable star. The astronomers who found it looked in TESS data for stars that change their brightness in a certain way, indicating that they are multiple galaxies.
What they were looking for is obscuring binaries: Stars that not only revolve around each other, but also stars whose orbits we see almost rim-shaped, so that the stars seem to pass in front of each other. When that happens, the couple’s overall light goes down a bit in a characteristic way. The 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-star systems or more.
And that’s what the second cool thing about TYC 7037-89-1 brings up: it’s not just six stars all orbiting the Earth, but they’re arranged in binary files: one pair of stars orbits another pair of stars, and a third pair revolves around them both!
The binary pairs are named A, B and C in order of brightness, and each star in them is numbered 1 or 2 (again in order of brightness). The two inner binaries are then A (consisting of stars A1 and A2) and C (C1 and C2), turned further out by the double star B (B1 and B2). A and C are about 600 million kilometers apart (roughly the distance from Jupiter to the sun), and take about 4 years to orbit each other – 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 one period.
And that now brings out the coolest thing about this system: All three star pairs are eclipsing double stars! We see all three binary orbits almost edge-on. A1 and A2 undergo mutual eclipses every 1.57 days (A1 eclipses A2, then a half orbit later A2 eclipses A1), so they are very close to each other. C1 and C2 rotate every 1.31 days, and B1 and B2 take 8.2 days to complete.
Since each star in a particular pair obscures the others, we can learn important things like how big the stars are, how hot they are, and more by measuring how long the eclipse lasts and other parameters (including taking spectra). And that pays off another surprise: all three binaries are very similar. They 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, the smaller stars are about the same: about 0.6 times the mass of the sun and 0.6 times its diameter. They vary a bit, but the thing is, they’re pretty close, which is odd.
This kind of system is just ridiculously unlikely. Models of how stars are formed show that sextuples are much more likely to consist of two trinary systems orbiting each other, not three binaries. So that’s rare enough, but it seems impossible to show all three binaries edge-on.
… “seems.” In fact, it is likely that they were formed from a swirling disc of material, each star collapsing from it. Therefore, it is likely that the three orbital planes of the binaries are the same. So when we see one edge-on, we see all of them edge-on, or almost like that. That doesn’t make it as unlikely as you might think they black out all three.
I will also note the trajectories of the binaries around each other are not sharp. We see the orbit of A and C around each other from an angle of about 40 °, just as we see the individual stars in the binary stars side by side. However, the slope of B’s orbit around them is not well limited by the observations.
Hopefully, a longer term study of this system will provide more information on how they are formed. We don’t really know much about multiple systems like this one, so it would be quite interesting to understand what conditions they form under.
I know, this causes a headache. So many tracks, angles, stars … Sometimes nature is complex and difficult to keep up with. If it helps, I’ll describe a similar fictional system that played a key role in season one of Star Trek: Picard. And more systems are known that are similar to TYC 7037-89-1; For example, CzeV1640 is a quadruple system with two pairs of obscuring binaries. Nature is complex, but sometimes economical, and uses the same idea over and over again.
But oh my, I would like a ship Company straight away! To see something like that up close, look how these six stars – six! – dancing around each other …
Strange new worlds indeed.