The Star That Disappeared, and Then Came Back

Welcome to Three Alpha! In this edition of the newsletter we’re focusing on the story of a disappearing star. Read on for more…

What could make a star disappear from the sky? It would have to be some big, destructive event, such as the formation of a black hole. But what if the star then came back after a few months, returning to the sky as though nothing happened? That’s precisely what happened to a star known as ASASSN-24fw in 2024, and two recent papers published in The Astronomical Journal (Zakamska et al.) and Monthly Notices of the Royal Astronomical Society (Shah et al.) attempt to get to the bottom of it.

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This isn’t the only time a star has temporarily vanished. Famously, Betelgeuse underwent an episode now referred to as the “Great Dimming” in early 2020. Other stars repeatedly dim, either unpredictably or at regular intervals. That includes stars in binaries that are eclipsed by their partners, and others that have more exotic explanations. For example, Boyajian's Star has been observed to dim at irregular intervals in a process thought to be caused by orbiting debris. The Betelgeuse Great Dimming was thought to be caused by material being ejected by the star itself. The pattern here is that stuff gets in the way of the star, and blocks our view of it for a while, until the stuff moves out of the way.

A major clue about what’s happening in the case of ASASSN-24fw is that this wasn’t the first time it had faded. The same thing happened in 1981, and before that in 1937. That means this happens every 44–45 years and suggests it could be caused by something in orbit around the star.

The two recent research papers both support the idea that the dimming was caused by the star’s light being blocked by a disk of debris or a large ring system around an object orbiting the star. They both suggest that the object is a large planet or a brown dwarf. They disagree on the mass of that object, but it is likely a few times the mass of Jupiter, and the ring system extends to about a hundred times the radius of Saturn’s rings. Every time the object passes in front of the star (from our perspective) the rings block most of the star’s light and the star seems to all but disappear.

So what created those rings? This is something else the two papers agree on. Young stellar systems are known to have disks of gas and dust. After all, that is how stars and planets form. And these disks can stick around for a while after the star and its planets have formed. But, ASASSN-24fw is not young (it is probably at least 2 billion years old) and it is unlikely that a disk would have survived for that long. That leaves another possibility: a more recent collision between two planets, which destroyed them and left behind debris, which then coalesced into a disk around the large planet or brown dwarf.

That means, every time ASASSN-24fw dims, its light is being blocked by the remains of two long-dead planets. We’re seeing the shadow of debris left over from the mutual destruction of two worlds which no longer exist. The fading, it turns out, was caused by a big, destructive event, but not one that just happened.

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].