Arcane supernova in the sky

Stars like the 1181 AD 'Guest Star' are no longer visible, so how can we work out what they were?...
27 September 2021

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Question

In 1181 AD ‘Guest Star’ appeared for 185 days before slowly diminishing in brightness. Earlier this year researchers were able to classify that the object in the night sky was a supernova. How do we go back and identify these astronomical events?

Answer

University of Cambridge's public astronomer, Matt Bothwell, does some sleuthing of astronomical proportions...

Matt - Fundamentally, what we have to do is just look at the patch of sky, which isn't always easy, right? Because these ancient peoples a thousand years ago, didn't always have very accurate ways of telling where things were in the sky. So we to kind of take their descriptions, look at the patch of sky and just try and piece things together by looking for whatever might've been left behind. The 1181 Guest Star has historically been kind of tricky. It was probably some kind of supernova. When something appears in the night sky and then lasts for a few weeks or something, and then fades away, some kind of supernova is normally a good guess. This one was a bit weird though, because it took months to fade away, which is much, much longer than normal. There was only one thing in that patch of sky that we used to know about that could have come from a supernova. It was a pulsar; the compressed core of a dead star called 3C85.

Matt - The problem is, when we aged it, it came to about 7,000 years old. So not a good candidate. It was a much, much older supernova. This new result, which got everyone really excited and has solved this 900 year old mystery, is that we found the smoking gun. We have found this new supernova remnant, and we combined it with a distance measurement taken from this satellite called Gaia. And we've worked out that it's about a thousand years old, which makes it perfect, right? It's a supernova remnant. It went off around a thousand years ago. And now we know these things, we can go even further and, and investigate it. So people at the time noticed that this thing was about the same brightness as Saturn, which is a pretty good yardstick. So now we know how bright it looked and how far it is away, we can work out how intrinsically powerful this thing was.

Matt - And it turns out it was actually pretty wimpy as supernovae go, which is kind of interesting. It was a wimpy little explosion, but it also took six months or something to fade away. That's a clue that it was something very, very rare. So our best guess now is that it was two white dwarfs crashing together and they've left behind something called a Wolf-Rayet star, which is a very, very hot star full of all kinds of interesting heavy elements. So yeah, it's going to need a lot more investigation because this was a very, very rare event, which has left behind something very exciting.

Chris - This is like stellar or cosmic archaeology almost isn't it?

Matt - It literally is, yeah. So all astronomy in some ways is the study of the past because the signals that we're seeing from the Universe have taken thousands or even millions of years to reach us. But this is taking that to another level because we literally are doing archeology. We're looking in the sky and then we're trying to put the pieces together and work out what's happened a thousand years ago.

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