on the Kissing Stars

Let’s talk about the Kissing Stars, a very massive binary system in the Large Magellanic Clouds that’s going to do SOMEthing exciting soon. Well, “soon” as in a couple hundred thousand years; this IS astronomy we’re talking about.

Artist’s impression of the hottest and most massive touching d
Artist’s Impression of VFTS 352 (ESO/L. Calçada)

There are over a thousand of these known (compared to millions of more ordinary stars studied), so while they’re rare, they’re not unheard of. The fact that such stars are peanut-shaped is also pretty well known, and because of that peanut shape the light from the stars varies in a very particular easy-to-spot way: There’s a LOT of light when you’re looking sideways at the peanut, and less when you’re staring down either end of the peanut. Unlike other eclipsing binaries, there are no sharp dips in the light at any point because the visible area of the peanut changes continuously. These are called W Ursa Majoris variables, named for the star W Ursa Majoris (handy, that). Interestingly, the paper itself never calls the system that, so my knowledge about binary star naming conventions may be out of date.

How does the peanut shape happen? Imagine you’re floating stationary in space between two stars, and you have a tiny rock. If you’re very close to one of the stars, it’ll end up falling into that star. But there’s a point in the middle between the two where the rock will just… float motionless, because the gravity exactly balances out between the two stars. Anything near that exact point will still fall into one star or the other, but slower because of the pull of the other star.

That’s more or less what’s going on with binary stars like these, because stars are not solid objects. Their plasma is slightly pulled toward the other star – it’s the same mechanism that produces tides on Earth due to the Moon – which means that as the stars get closer to each other, or they expand outwards (as stars do at the end of their lives), they become increasingly teardrop shaped. Basically, there’s a shape (called the Roche lobe) that describes the maximum distance at which something is bound to only one of the two stars. If one of them expands larger than the Roche lobe, it’ll start leaking material into orbit around the pair, and into orbit of the other star. That’s something that happens a lot to binary systems at the end of their lives – the more massive one runs out of material to fuse and swells up, dumps a lot of material onto the other one, and slightly prolongs its own life at the expense of shortening its partner’s. If BOTH stars fill the Roche Lobe, they end up merging into an overcontact binary that looks like a peanut, where the outer layers actually don’t belong to either star in particular.

What’s unusual about VFTS 352 are their masses. VFTS 352 is the most massive overcontact binary known – 22 and 25 solar masses. Both are large enough to explode as supernovae (and rather soon, because stars that massive don’t last long). They’re not the MOST massive stars known, there are a number of stars that genuinely seem to have masses over 100 solar masses (a few in the Tarantula Nebula, no less, the same location as VFTS 352). But in this case, the fact that this system is an overcontact binary means they may turn into twin black holes in a very tight orbit – currently, the stars orbit every 1.1 days. Astronomers who care about gravity waves love binary black holes, and this is a rare chance to see what they look like before they form. They are probably not going to explode in our lifetimes, but it’s worth studying them to learn more about systems like them. The paper argues that overcontact binaries should be a common thing for very high mass stars – they usually come in pairs (or more), so there should be plenty of systems like this, only we haven’t seen many of them yet (perhaps there’s a reason). Alternatively, the two stars might merge into one gargantuan star…

Do we need to worry about the supernova? Not really; this binary system (and the Tarantula Nebula it’s part of) is in the Large Magellanic Cloud, a small galaxy orbiting the Milky Way. It’s 50,000 parsecs away, so it would just produce an exciting light show.

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