Ep. 558: Supernova SN 2006gy

Posted on Feb 5, 2020 in podcast, Stars | 0 comments


We’ve been following this story for more than a decade, so it’s great to finally have an answer to the question, why was supernova 2006gy so insanely bright? Astronomers originally thought it was an example of a supermassive star exploding, but new evidence provides an even more fascinating answer.

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Fraser:                         Astronomy Cast, Episode 558. Finally explaining Supernova 2006gy.

                                    Welcome to Astronomy Cast, where we take a fact-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Doctor Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest.

                                    Hey Pamela, how you doing?

Pamela:                        I’m doing well. How are you doing?

Fraser:                         Doing very well as well. Well, as well. Anything new happening in Your-verse, the CosmoQuest-verse?

Pamela:                        It won’t stop snowing.

Fraser:                         Yeah, we got a few extra snowfalls too.

Pamela:                        So, we are plugging away every winter’s day on our new code set. One of you out there, you know who you are, actually came over and joined us on Discord to help us out with the coding and that made me super excited. So, if any of you want to donate some time and join our opensource project, we’re doing a complete rebuild and you are all welcomed. And I get super happy if you say you understand JavaScript.

Fraser:                         Complete rebuilds are always fun. And by that, I mean never.

                                    We’ve been following this story for more than a decade, so it’s great to finally have an answer to the question, why was Supernova 2006gy so insanely bright? Astronomers originally thought it was an example of a super massive star exploding, but new evidence provides an even more fascinating answer.

                                    All right, Pamela, do you remember where you were when Supernova 2006gy exploded? It was actually before our time on Astronomy Cast, wasn’t it?

Pamela:                        Yeah, so I would have just moved here to Illinois. I would have been a baby professor. We would have just started the show. This was an end of the year supernova. And so, I don’t know exactly where I was, but it wasn’t too far from here.

Fraser:                         I don’t remember the story. You know I had to go and look in the archives at the time when we reported on it and then all the various updates that we reported on this story. And it’s one of those things, though, where when you do kind of look back at the story with all of the knowledge that we now have, you can see the whole thing unfolding bit by bit.

                                    And so back in 2006, as you say, at the end of the year, astronomers saw a supernova that was unlike anything that they had ever seen. So, what was wrong with it?

Pamela:                        Well, not necessarily wrong, but the thing that was initially fascinating about it is it was just way brighter than we had previously seen. Once you corrected for distance, it had the highest luminosity up until that date. So, with massive amounts of energy coming out of it and with a crazy light curve, it was initially assumed that this was what’s called a Type II supernova. The explosion of a massive star that’s going to leave behind a neutron star or black hole.

                                    But different kind of supernova are supposed to increase and decrease in brightness in very set ways. And while this started out looking like a Type II supernova, it didn’t stay looking as Type II supernova.

Fraser:                         So, let’s talk – I mean we’ve done whole episodes on all the different flavors, but let’s go back and give people the quick overview of the different kinds of supernova that you can expect to see out there.

Pamela:                        So, broad physics case, you have supernova that are single stars that are generally massive and when they run out of fissionable materials in their core that can generate new energy, they stop producing light pressure that supports the outer layers of the star and they collapse under gravity. And all of that collapsing material triggers a new round of thermonuclear reactions. The star now explodes outwards as a supernova. So, that’s one model of behavior.

                                    Now, the other model of behavior is you have something like a white dwarf that is made out of, what we call, degenerate matter. A white dwarf is an object roughly the size of the earth that has the mass of the sun. And when you cram all of that mass into so small a volume, you end up with the electrons having to arrange themselves in a very specific way to avoid breaking the Pauli Exclusion Principle.

                                    So, all of the electrons are like, you, okay, you be in this level, you be in this level, we’ll spin in these particular ways. And this electron degenerate gas is as tight a gas as you can form out of the electrons. And if you compress it too far, the electrons can no longer support each other. Pauli Exclusion Principle breaks; those electrons and the protons that are associated with end up merging and forming neutrons, everything goes badly. There’s a lot of energy released.

                                    And this is the other way you can get a supernova is by piling too much mass on one of these electron degenerate gas white dwarf stars defying the Pauli Exclusion Principle and exploding that white dwarf. That’s a Type Ia supernova. Now, in general, because Type Ia’s all detonate at the same amount of mass, they’re supposed to be the same amount of light.

                                    Now, the names for those massive stars that are exploding are all over the map. There’s Type II, there’s Type 1c, there’s all these different letters added on, but they’re all massive stars.

Fraser:                         So, okay, so just to sort of follow the mystery. Astronomers saw what was inherently the brightest supernova they have ever seen?

Pamela:                        Yes.

Fraser:                         But so, then it couldn’t have been a white dwarf because they’re not that bright?

Pamela:                        Exactly. At least that’s what was initially thought.

Fraser:                         Right.

Pamela:                        So, you have – their initial thinking, and I’m going over this so I can make sense of all of this later because no physics is harmed in this episode. So, you have a white dwarf worth of energy tied up in the mass. And that amount of energy when it goes boom always releases the same amount of energy.

Fraser:                         Right, right. And the sort of example, right, I mean, essentially, it’s like one big diamond the size of the earth. It is this carbon, as you say, this carbon lattice. And the moment it crosses over this line where the thing collapses inward, the whole thing just turns into carbon burning and it’s just gone.

Pamela:                        Yeah.

Fraser:                         Like, kaboom, and it’s gone. Because the whole thing – suddenly a lifetime’s worth of fusion happens in an instant. Every atom in the entire star proceeds to do carbon fusion and the whole thing just goes kaboom and you get this wonderful standard candle the astronomers use to measure the size of the universe.

Pamela:                        And there’s different kinds of supernova. All give off their own signature display where you get one set of elements being brought into creation by a Type 1a. Another set of elements being brought into creation by a Type II, by a Type 1c. All of these different kinds because they have distinct nuclear reactions that go on produce a distinct set of elements and also related elemental lines in their spectre.

Fraser:                         Right. Okay, so Type 1a supernova, exploding white dwarf ruled out. So, now we’re looking at a Type II supernova, but why did they not think that it was a core collapsed supernova? Why didn’t they think it was something – I mean it was more energy than the brightest supernova that had ever been seen, so couldn’t that just be like a monster star that core collapsed?

Pamela:                        Well, so, they initially did think that it was a monster star that had core collapsed. Now, the issue became a few hundred days after its initial explosion, when they were looking at it, what they saw was it had, one, decreased in mass – not mass, decreased in luminosity in ways that didn’t match with the Type II model.

                                    The other thing that they saw was this spectre had a bunch of really weird atomic lines in it that for a long time scientists couldn’t figure out. And it was only in a recent paper by Anders Jerkstrand, Keiichi Maeda, and Koji Kawabata, that they were able to piece together that these really weird atomic lines that were coming from the area of excited material around that supernova explosion were neutral iron.

Fraser:                         Okay. What’s wrong with neutral iron?

Pamela:                        Well, neutral means that your little happy iron atom hasn’t had any of its electrons stripped off. And you’re always gonna have some of that hanging out around a supernova, but the strength of these lines corresponded to half a solar mass or so, between a third and a half a solar mass of iron in the vicinity around this star.

                                    And that amount of iron at the temperature that made sense with neutral iron isn’t something that you’re gonna get with core collapsed star. The temperature was off.

Fraser:                         Okay, right. And so, astronomers now had to – because originally they were thinking that it was something like Anacarana, right?

Pamela:                        Yeah.

Fraser:                         Like, there was gonna be a star with a 100 to 200 times the mass of the sun had exploded. And as you said, they looked at it and it was visually bright. But like when they looked at it with, say, the Chandra X-ray Observatory, it wasn’t intrinsically x-ray bright in the way that you would expect one of these monstrous stars. So, it’s always this mystery.

                                    Okay, so now you see all of this neutral iron in the vicinity, so what does that mean?

Pamela:                        Well, so that seems to imply – well, expletive. The only way you get that is with that Type 1a supernova that we were talking about.

Fraser:                         Wait, we just ruled that out.

Pamela:                        Right. And this is the problem. And this is a very short and brilliant research paper that pieced together a lot of the research about this to come to a really clean conclusion. So, looking at this and seeing, wow, there must have been all of this material that came out of this. How do you do that? What is the temperature that corresponds to it? Well, what they looked at was – and here I’m gonna read from the paper.

                                    A core collapsed supernova would produce too little, and I’m paraphrasing, too little nickel. It would only have produced a tenth of the solar mass of nickel that would have then decayed via cobalt to form that iron. So, it can’t be that and it couldn’t have expelled that much mass.

                                    And so, if you instead look at, well, what about a pair-instability type explosion? Well, they’re you get –

Fraser:                         Which is a variety of those super mega heavy stars.

Pamela:                        Right.

Fraser:                         Where they get to a different kind of – essentially, from what I understand, it’s like they get to a point where they contract inward almost in a moment and then bounce as a supernova and tear themselves apart completely.

Pamela:                        Right. And to get what they were seeing would have required a 90 solar mass helium core.

Fraser:                         Right.

Pamela:                        And that’s not a thing, really.

Fraser:                         Right, okay.

Pamela:                        So, that didn’t work. So, how do you get this much energy out of a white dwarf, which is the only way they could explain the amount of iron they were seeing.

Fraser:                         Okay, okay. What do they propose? What do they think happened?

Pamela:                        Kinetic energy.

Fraser:                         Kinetic energy?

Pamela:                        So, you have all the energy that is in the mass of the star. That’s contributing to the supernova. That’s what we’re used to in the Type 1a. But if you have a fastmoving white dwarf star that has kinetic energy, this is the same kind of energy that causes massive craters to form when asteroids strike objects. It’s that kinetic energy getting transferred into other forms of energy. It causes things to go boom.

                                    Well, in this case what we’re seeing is a white dwarf in a binary system that is undergoing a merger. So, here it’s like we’re gonna pull up every possible bell and whistle to make this happen.

Fraser:                         Right. This is the most extreme, bizarre, crazy system you can possibly imagine.

Pamela:                        This is awesome.

Fraser:                         Yep.

Pamela:                        And as the white dwarf goes into the envelope of its companion star, which is just a like regular giant star.

Fraser:                         Right, like a red giant star.

Pamela:                        Yeah.

Fraser:                         Of what our sun will do when it dies.

Pamela:                        It is able to shed out all of this material from the outer envelope to make a tight circumstellar medium around the star.

Fraser:                         I’m imagining a – sorry, like I’m imagining like a car tire going through a mud puddle. Right?

Pamela:                        Or the way I imagine it –

Fraser:                         Right? Or just like spraying out water as it’s just carving through this puddle.

Pamela:                        So, maybe because I’m a woman, I think of it as stirring something too quickly and you spray material in all directions. So, you have this white dwarf plowing through the outer layer of its companion. They’re now like a single shared envelope system; it’s in the process of merging. And somewhere along the lines this white dwarf star is like and there’s too much mass on me.

Fraser:                         Right. But that’s what they do, right? I mean that’s how you get a Type 1a supernova is you feed a white dwarf 1.4 times the mass of the sun slowly, carefully, and then it finally explodes. But in this case it just – it was force fed as it dove through the envelope of this red –

Pamela:                        And it got so much angular momentum.

Fraser:                         Right.

Pamela:                        So much angular momentum.

Fraser:                         Right.

Pamela:                        And it’s the transfer of angular momentum and the shedding of its orbital velocity. All of this energy was where all that excess energy came from. And so, this raises this fascinating, well, expletive. Part of the variation that we’re gonna see in Type 1a supernova is going to be driven by are they – what is their velocity energy that they’re getting rid of? What is this kinetic energy that’s going into these explosions?

Fraser:                         Oh, so you think this might have implications for using white dwarfs as standard candles just in general?

Pamela:                        Yeah.

Fraser:                         Hmm, interesting.

Pamela:                        So, one of the things they hint at is this could be an explanation for other weird supernova we haven’t been able to understand. And if we’re seeing on a regular basis, by which I mean it’s astronomy. If you see something six times, it’s a trend.

                                    So, if we’re seeing this ever so rarely with this huge and dramatic effect, how often are those small deviations from the mean caused not just by the environment that the star is in, but by also the energy that is in the star system. We know all of these are binary. That’s how they happen.

Fraser:                         Right, right. We know that white dwarfs exploding as Type 1a supernova, which is the standard candle, they are all a – each one of them is a, as I said, you’re sipping away at some of the star’s juice and eventually you explode.

Pamela:                        And so, if you start –

Fraser:                         Oh, go ahead.

Pamela:                        — with the idea that the standard candle means every single one of these should explode with the same luminosity, the same number of lumens, that same candle brightness, but then it turns out that sometimes they have this secondary source of energy, that adds a fascinating source of noise.

Fraser:                         Right, absolutely.

Pamela:                        And so, I love the implications of this research.

Fraser:                         Right. And so, when you think of how astronomers are using these standard candles to measure the distance to various galaxies across the universe and of course the finding that these stars are farther away than they should have been, then this is dark energy. I mean this whole idea of dark energy. Although, I mean it’s exciting, but then at the same time you would expect it to be random, right?

Pamela:                        Yes.

Fraser:                         There wouldn’t be some reason why the stars that are farther away are extra far away while the stars that are closer are not. Like, you wouldn’t have that direct correlation between them –

Pamela:                        Right.

Fraser:                         — feeding in strange ways, but you would definitely – it could definitely have an implication on just your understanding of what – how bright these standard candles are.

Pamela:                        And this is a random source of noise. And you have to assume that while it may not be equal on both sides of the skew, there’s probably not a reason that it would change over the history of the universe.

                                    I wouldn’t be surprised if we see dark energy go away as our research evolves because we are finding that there are systemic effects where the populations that white dwarfs are forming from change over time. And we’re starting to see hints that different populations of stars produce white dwarfs of different luminosities. This isn’t the effect that’s gonna make dark energy go away.

Fraser:                         Right.

Pamela:                        This is just gonna create noise in our measurements that is gonna make measuring those other effects that much more difficult.

Fraser:                         Right. But it is gonna be awesome noise. So, just to sort of like rewind and sort of understand this story, right? You had these two happy stars in a binary relationship, two main sequence stars, one was a little more massive than the other. And at some point, the more massive star died.

Pamela:                        Yes.

Fraser:                         Bloated up as a white dwarf, puffed out its outer layers, and then – puffed out as a red giant, sorry, bloated up, threw out all its outer layers, and then collapsed down into a white dwarf. And I guess by doing so it sort of changed the gravity dynamics of the system. And so now you’ve got a white dwarf orbiting around a main sequence star and then X billion years later the second star puffs out as a red giant, eats the white dwarf, the white dwarf just careens into it, gobbles up enough mass. And apparently it only took like a hundred years.

Pamela:                        Yeah. That was awesome.

Fraser:                         When this actually happened. Yeah. And then it wrecked the star. So, it’s not like it sort of plunged in and just sort of gently faded away down to the core of the star. It just – you know like I said, like a car going through a mud puddle or – someone mentioned in the chat, right, turning on your blender without the lid.

Pamela:                        And what’s kind of cool is they can actually use that iron – the iron lines keeping so important. So, they were able to figure out when the circumstellar material must have been admitted to get all the velocities of everything going correctly. And so, they figured out that that must have been emitted in the past 100 to 200 years.

                                    So, that actually – they went from thinking that the merger took between 10-ish order of to 200-ish order of years. But then you look at that iron and that material that would have been created by the infalling white dwarf. And that tells you that this was a hyper giant because of the infall times for the different kinds of scenarios. So, they were able to figure out what the white dwarf fell into by when that circumstellar material was put into place.

Fraser:                         And so when you’ve got this white dwarf star exploding in the wreckage that it caused as it spirals into the red giant, then the energy and the material blasting out of this white dwarf collided with this material and that’s what caused the brightness, that’s what caused the luminosity is the collision between the white dwarf and the material around it?

Pamela:                        That’s what caused the iron that we saw. So, the brightness of the supernova came from you have a white dwarf star that is flying around inside what’s called a common envelope. So, you have a star that now has two nuclei. One that is that hyper giant, it started out there nuclei. And the other that is this infalling white dwarf. And it is the combined kinetic energy of the white dwarf and the mass energy of the white dwarf that goes into all of the nuclear kaboom parts. That combined kinetic energy and the regular kaboom energy, that’s what created the amazing –

Fraser:                         Right, okay, okay.

Pamela:                        — initial luminosity. It radically faded so that when they were looking at it 400 days later, it was 100 times less bright than expected.

Fraser:                         Right.

Pamela:                        And what they were seeing at that part of the time was the illuminated material that had been shed, which included the neutral iron lights.

Fraser:                         So, a better analogy might be like when two black holes collide with each other and you get this gravitational waves and people say ten times the mass of the sun was released in gravitational waves. It’s not like the black holes got less massive. It’s that the kinetic energy of their collisions was turned into the gravitational waves that rippled throughout the universe. And so, in a similar situation you’ve got the kinetic energy from this white dwarf spiraling inward that is then able to translate that into additional energy, an extra one/two punch for its supernova.

Pamela:                        And one of my favorite parts about this paper is I was extremely eager after hearing the question from one of our audience members Bad Panda Bear. He asked, well, what was the result of this? And in trying to find out, the authors of the paper were like, we haven’t been able to bottle situations like this. They’re just too complicated.

Fraser:                         But did the red giant exist after the white dwarf exploded?

Pamela:                        Can’t tell yet.

Fraser:                         Yeah.

Pamela:                        And so, this is a class of objects. This isn’t just one object. This is a class of objects shared common envelope systems, merging binaries that go boom. And in some cases the going boom is gonna be mediated by gravitational waves, in some cases it’s just gonna go boom, and trying to understand the timing of this, how quickly they go, how slowly they go, what’s left. We need better computers.

Fraser:                         Yeah.

Pamela:                        So, they’re still more to come on stories like this.

Fraser:                         Right.

Pamela:                        We don’t yet know how this one ends. We just are understanding how it began.

Fraser:                         Or way bigger telescopes because this thing is tens of millions of lightyears away.

Pamela:                        Well –

Fraser:                         So, there’s no observing it, right? There’s only – there’s observing the explosion, but you can’t point a telescope with that level of precision to see what’s there now or what was there before.

Pamela:                        Once the nebula gets a little bit bigger you’ll be able to see what’s left in the center.

Fraser:                         Oh, good point.

Pamela:                        We’re just not there yet.

Fraser:                         Yeah, yeah. So, James Webb, this will be one of your future targets.

Pamela:                        Oh, you’re so optimistic.

Fraser:                         But it’s great when you think about – it’s gonna launch. It’s so great when you think about just all of these combinations that there are every – you can mix and match black holes, neutron stars, white dwarfs, red giants, super giants, in every combination.

Pamela:                        Black holes do not form common envelope stars.

Fraser:                         No, but they sure can – what happens if they hit one, right? Like, I mean the point is something is observed when these monsters interact with each other.

Pamela:                        Yes.

Fraser:                         And so right now you’ve sort of got, on the one hand, you’ve got this imaginary collection of different ways these different objects can come together. And on the other hand, you’ve got all of these observations that are across this entire spectrum of what people have seen. And it’s like this mix and match where you’re like, okay, is this that, is that this? And so I look forward to them continuing to try to figure out which causes what.

Pamela:                        And the cool thing that we’re just starting to statistically learn – and this is the last cool thing for today, I promise. We’re finding that more binaries than we used to think become binaries late in life. So, you can have systems that have an involved low mass companion and then not yet evolved high mass star.

Fraser:                         Right.

Pamela:                        And so this potential of having white dwarfs around truly massive stars is gonna lead to even a larger diversity of objects being possible.

Fraser:                         So cool. Pamela, do you have some names for us to celebrate here on Astronomy Cast?

Pamela:                        I do. Astronomy Cast is entirely a listener supported podcast. We are here because of you, you and your patronage on Patreon.com/Astronomy Cast allow us to pay our servers, pay our software, and pay our Susie. So, thank you for everything you do.

                                    And this week I’d like to thank Bryan Kilby, Jessica Phelts, Omar Del Riverio, William Loward, Joe Wilkinson, Bruno Lets, Marco Larosie, Dustin A. Ralph, Mark Grundy, J. Alex Anderson, Jeremy Kirwin, Mark Steven Rasdack, Tim Garrish, Paul L. Hayden, Brent Kronop, Eron Sigev, Arthur Latts Hall, William Anders, Jack, Joshua Pierson, Justin Proctor, Fredick Saje, Claudia Mastrioni, Rachel Frye, David Gates, Dwayne Isaac, and Thomas Tubman.

                                    Thank you all. You make us happen.

Fraser:                         Thanks everybody and we’ll see you all next week.

Pamela:                        Buh-bye.

Female Speaker:          Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary of Science Institute, Fraser Cane, and Doctor Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at Info@AstronomyCast.com. Tweet us @AstronomyCast, like us on Facebook, and watch us on YouTube.

                                    We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Sorrow and the show was edited by Susie Murph.

[End of Audio]

Duration: 31 minutes

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