Supernovae

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In Act I Scene 5 of Shakespeare’s Hamlet, the title character says to his friend “There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy.” In my years following new discoveries in space science, I’ve found this to be incredibly true—there’s always something out there that we weren’t expecting to find. I just never thought one of them would be a completely new kind of supernova, but that happened this week.

I get asked about supernovae a lot in the Planetarium. There’s something incredibly evocative about the violent death of a star via explosion (though I must point out that our Sun will never go supernova, much to the disappointment of many a young student visiting the Museum). So what is a supernova? How do they happen? And how did we find a completely new type?

Become a kid in a planetarium again and join me as we look into the extremely awesome realm of stars blowing themselves up. Because you deserve a treat!

Big Star Go Boom

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I’ve written before about the deaths of stars in this blog (check it out here if you want some more details on the different ways stars can go), so I’m going steal a bit from that post when it comes to what we call core-collapse supernovae. That’s the kind you get when an enormous star is at the end of its life. At this point it is fusing silicon into iron in its core and once it’s out of silicon it can no longer generate the outward pressure needed to counteract its own gravity.

To quote myself: “Everything collapses inwards toward the core. A whole lot of physics happens, but the basics are that the sudden implosion of so much material into such a small space causes a rebound outwards, generating a massive shockwave. That shockwave intercepts the collapsing outer layers and violently reverses their course, throwing them outwards at ridiculous speeds.

Meanwhile all of this is accompanied by a huge release of energy, hundreds of times what the Sun will produce over the course of its entire lifetime. The brightness of this energy is what we can see as a supernova, some of which can be seen way on the edge of the visible universe. When the light of the explosion fades, the rapidly expanding bubble of the ejected outer layers is visible as a supernova remnant.”

In order for a star to go supernova at the end of its life, it must be at least eight times the Sun’s mass. Not a lot of stars reach this size, so it’s a small percentage of stars that are even able to go supernova (despite the hold the concept of a star exploding has on the public consciousness).

Little Dead Star Go Boom

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Really massive stars are rare, but smaller stars are everywhere. These are ones that die gentler deaths in which they gradually give up their outer layers while their cores shrink down into white dwarfs. These are tiny, fiery little beasts with masses not dissimilar to the Sun’s packed down into something the size of Earth.

The important thing about white dwarfs is that they have strict mass limits. They’re held up by something called electron degeneracy pressure. The electrons in the core’s atoms are pushing against each other (negative charges push away from other negative charges) which counteracts the gravity from the core’s mass trying to pull everything inward. As long as the gravity can be counteracted by this electron degeneracy pressure, the white dwarf is stable.

But wait, there’s a catch! Stars are often not alone out there. Binary, trinary, quaternary, and even higher -nary systems are common, and when one star dies that often leaves its companion(s) still there and still shining. If a white dwarf is close enough to a living companion star, its gravity may start pulling material off the star’s outer layers. That material settles onto the surface of the white dwarf, and the white dwarf’s mass begins to increase.

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When it approaches a specific mass, 1.44 times the mass of the Sun, known as the Chandrasekhar limit, the mass of the white dwarf is pushing down enough (thanks gravity!) to reignite stellar fusion inside itself. It begins to fuse carbon into oxygen, powering the nuclear reactor at its heart back up. Except that it’s no longer being held up by thermal pressure the way it was when it was a proper star—it’s being held up by those danged electrons.

If a star gets hotter, it puffs itself up to maintain equilibrium (this is why stars become giants as they’re preparing to die, their cores are heating up). That’s what the balance between thermal pressure and gravity allows them to do. But white dwarfs are balances between gravity and those electrons, which means the white dwarf cannot expand to cool off as it heats up. It just gets hotter and hotter. Fusion goes nuts inside it. The exact detonation mechanism still isn’t known, but pretty quickly this stellar remnant will reach a point where it explodes.

Sometimes the companion orbiting the white dwarf isn’t a living star but another white dwarf. In that case the two white dwarfs can get so close that they can actually merge into one—and that one will (unless those white dwarfs were unusually small) have a mass above the Chandrasekhar limit and the runaway fusion process will trigger in the merged white dwarf. The outcome is the same: big ol’ boom.

Big Star Make Bad Life Choice

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So those are the two known mechanisms for triggering a supernova: core collapse in a star or runaway fusion in a white dwarf. There are subtypes, but at a basic level those are it. Except of course there was this week’s news regarding a star that appears to have gone supernova not as a result of a core collapse, but as a result of a horrifically destructive case of codependency.

In July 2023 the Zwicky Transient Facility saw a light in the sky, designated SN 2023zkd. AI was able to detect the bright event early enough to allow for other telescopes to be pointed at it while the flare-up was still progressing. Initially it looked like a normal supernova: a extremely bright initial blowup that faded over the course of a few months.

But then it suddenly brightened again. Normal supernovae don’t work like that. The initial blast is the star ripping itself to shreds and that’s that. That was a major clue that something had been going on with this star that was unusual. Astronomers went into archival observations of this star and four that for more than four years before the supernova it had been brightening. That also doesn’t usually happen before a star blows itself up.

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It was ultimately determined that the first flare was, indeed, the star exploding. The second flare came from the supernova’s shockwave hitting a thick disk of gas in the vicinity of the star, material that had been shed off the star in the years before it blew. And that disk, along with the unusual pre-explosion behavior, suggest that something was placing this star under enormous gravitational stress before it blew. And frankly, there’s not much out there that can do that to a star and not be easily visible.

When all the puzzle pieces were put together the answer, naturally, was a black hole. Astronomers suspect that it all started out with a 10-solar-mass star (so, a big’un) and a 10-solar-mass black hole orbiting each other. When they got close enough the star tried to envelop the black hole in its outer layers.

Trying to eat a black hole will always be a terrible life choice unless you are a bigger black hole. The gravitational field of the black hole began to wreak havoc on the star—some of it would have been consumed by the hole while other bits would have been gravitationally ejected to form the surrounding gas disk. Eventually, through exact mechanisms still unknown, the star was destabilized enough that it detonated in a supernova that is the first of its kind we’ve ever seen: death by attempted mutual cannibalism. It’s a heck of a way to go.

More in Heaven and Earth

Not gonna lie, I’m still riding on the delicious high of learning about a supernova triggered by a star and a black hole trying to eat each other at the same time. That’s just awesome. But apart from the sheer amazing absurdity of the situation, it also proves what Shakespeare was saying in Hamlet. There is always more going on out there than we’re currently aware of or understand, even something as big as the way stars can die.

But seriously kids, don’t try to swallow a black hole. It’s just a bad idea.