You’d think that a white dwarf star is the end of a stellar life. It’s all downhill from there. A long, slow cool down towards the end of everything. But in some situations, even dead stars can get exciting again, briefly becoming some of the brightest objects in the Universe. And just maybe, the last exciting thing that ever happens in the Universe.
Ep. 579: White and Black Dwarfs
Old Star’s ‘Rebirth’ Gives Astronomers Surprises (Science Daily)
Classical Novae (Swinburne University)
VIDEO: Conservation of angular momentum (Khan Academy)
Quasar (Swinburne University)
Active Galactic Nuclei (Swinburne University)
How common is Common Envelope evolution? (Astrobites)
Roche Lobe (Swinburne University)
Type 1a Supernova (Swinburne University)
Chandrasekhar Limit (Swinburne University)
Ep. 490: What’s New With Supernovae?
‘Black dwarf supernova’: ISU physicist calculates when the last supernova ever will happen (Illinois State University)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 580: Exploding Dwarfs. Welcome to Astronomy Cast, our weekly facts-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, is Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute and the Director of Cosmo Quest. Hey Pamela, how you doing?
Dr. Gay: I’m doing well-ish. How are you doing?
Fraser: I am also doing well-ish. Apparently we only have three cases of the ‘rona here on our island, which is mind-blowing.
Dr. Gay: Oh, wow.
Fraser: For some reason, I guess because we’re on an island, an island in Canada, we somehow figured out how to minimize the effect. Now, we’re all still taking precautions, but it definitely feels really hopeful from that front.
Dr. Gay: Yeah. Yeah, I live in the Midwest.
Fraser: Yeah. Yeah, I know, I know, I know, I know. You’re not doing quite as well for you guys. How’s your weather, then?
Dr. Gay: It’s actually that beautiful golden time of fall when the leaves are just that hint of orange here and there, and yesterday I went out and I lay in a sunbeam in the hammock. My dog joined me. We did not break the hammock. That was glorious. I don’t leave my yard anymore.
Fraser: It is my favorite time of the year, by far. My wife asked me, “What’s your favorite time of the year?” I’m like, “September, October.” And for her, it’s one of her least favorite times of the year. Not because the weather is bad, but because winter is coming. And so, for her, it’s just the fact that summer is ending and winter is coming. It just makes her too sad. But for me, I just love this weather. Yeah, it’s the best.
Dr. Gay: I am about to be overwhelmed by squash. That’s the one drawback.
Fraser: You know my rule, right? If you have to buy your own zucchini, you don’t have enough friends.
Dr. Gay: Yeah. Yeah. I have a squash problem.
Fraser: Yeah, no, get cooking. That’s awesome.
Dr. Gay: And there shall be a lot of frozen soup. A lot of frozen soup.
Fraser: Oh, that sounds great. All right. So, you think that a white dwarf star is the end of a stellar life, and it’s all downhill from there. A long, slow cool-down towards the end of everything. But, in some situations, even dead stars can get exciting again, briefly becoming some of the brightest objects in the universe, and maybe – just maybe – the last exciting thing that’ll ever happen in the universe.
Pamela, last week we covered white dwarf stars top to bottom, front to back, beginning to the end, from the formation to their cool-down to the background temperature of the universe.
Dr. Gay: But only for those in isolation.
Fraser: That’s right, yeah.
Dr. Gay: We were talking about boring isolated ones.
Fraser: On their own, white dwarfs will cool down. But if they’re in other situations, things can get exciting again.
Dr. Gay: And there are so many different ways they can get exciting. There was one time in graduate school when it clouded up, and so I couldn’t use the telescope, and I was stranded at MacDonald Observatory, and I found a book on cataclysmic variable stars; a conference proceedings, of all things. And I just poured through it, learning about all these really cool things that I just didn’t know enough about, because I was a first-year grad student.
And it turns out that you can get surface explosions on white dwarfs. You can exploding discs around white dwarfs. And the white dwarfs themselves can reignite, form common envelopes, and completely explode in a variety of fascinating and awesome ways. They just explode.
Fraser: Right. Yeah, in the end it’s all about explosions. But so, what are the environments that they’re going to be in that are gonna give them some kind of interaction?
Dr. Gay: They need a companion that is close enough that gravitationally they can steal material off of it. And the phase of their companion star is part of what determines what kind of a situation they’re in, because it’s easier and harder and has different densities of things that you’re stealing along the way, depending on that stage of what your companion is. So, you can imagine a scenario where you have that 8 solar mass star that formed, lived its life, underwent mass loss, poofed off its atmosphere –
Fraser: That’s a technical term.
Dr. Gay: Yes, we’re gonna go with that.
Dr. Gay: And left behind that 0.9 solar mass white dwarf core next to its smaller 3 solar mass companion. Well, when that 3 solar mass companion gets bigger as a red giant star, in the end of its own evolution, that expansion outwards just might put its atmosphere in harm’s way, causing it to get gravitationally pulled off; in which case, you can get mass transfer, and end up in this weird situation where you can end up changing the evolution of this system where that white dwarf can eventually end up sometimes even getting reignited, becoming a new star as it gets covered in the material from the neighbor, leaving a white dwarf. And this is just one of those fascinating trade-off things that can happen.
Fraser: Right. I kind of imagine this situation where, as you said, you’ve got this white dwarf, and then you’ve got this red giant star beside it. The red giant has this big envelope of gas, and then the white dwarf is kind of unwinding the material and wrapping it up around the exterior of the white dwarf. And that can only go on so long, until you get a nova.
Dr. Gay: And this is where different kinds of things can occur. So, it doesn’t always have to go supernova. That’s one of the cool things.
Fraser: Right. You could start with just going regular nova.
Dr. Gay: Yeah. So, you can have a situation where you have what’s called a classical nova. This is where material is pulled off of the companion star. It spirals in. It’s always a spiral. Conservation of angular momentum just requires this. It builds up on the surface of the white dwarf, and eventually it builds up enough that it detonates in a nuclear explosion. And this can happen over, and over, and over. And so, this pattern of how it flares up: That’s the cataclysmic nova.
Now, you can also end up with situations where the material that’s spiraling in, is spiraling in so fast and building up an accretion disc so big that the accretion disc itself hits densities where the accretion disc explodes in a thermonuclear explosion.
Fraser: Now, is that sort of similar to – I mean, we think about the accretion disc around a black hole, which can be giving off a tremendous amount of X-ray radiation. Or even the smaller black holes, but also the super massive black holes. And when we talked about this – that around a black hole, you can get to the point where it’s starting to act like the interior of a star. Same situation?
Dr. Gay: It’s the same situation, but the ability of that central mass to hold on, that’s where things differ.
Dr. Gay: With that white dwarf accretion disc, you can end up with it cataclysmically exploding, and blowing out material, so that it settles back down and lets the material build up a second time. So, you can have this cyclic kind of situation going on. Now, with a black hole, it’s big enough that it just holds on to the material. So, the material is in that accretion disc, going, “Hi, we’re really dense. We’re gonna generate out own nuclear thermoreactions here. Nucleosynthesis shall be a thing.”
And the gravitational pull that’s holding the accretion disc in place, fighting against that light pressure being generated in the disc, it balances. The disc can be held on to. Quasars, active galactic nuclei. These are situations where you have a stable – while it’s there – accretion disc giving off light. White dwarfs, they’re like solar mass objects; 0.8 solar mass, 0.9 solar mass objects. They just don’t have the gravity. So, when their accretion discs blow up, they blow apart.
Fraser: Right. Right. And it doesn’t just happen bit by bit. It just happens…
Dr. Gay: Because kablooey.
Fraser: Kablooey. The whole thing.
Dr. Gay: We’re still working out the details. Yes, it pretty much goes kablooey, and then has to start over, rebuild back up again.
Fraser: Yeah, yeah, yeah. So, you get this blob of stuff; this stolen stuff, this thread of red giant or companion star whatever, that is winding around the outside. And it crosses some limit, and that entire blob ignites, burns off the surface of the star; but a little bit gets added –
Dr. Gay: Or the accretion disc.
Fraser: Right, of the accretion disc. But a little bit does get added to the star, right? In terms of mass.
Dr. Gay: Yes. Exactly.
Fraser: Yeah, okay.
Dr. Gay: Yes. And so, what I find particularly fascinating about this is they can evolve over time exactly how they’re interacting. So, you can have situations it will go through – it’s called a common envelope period – that is the result of the one star expanding out enough that it’s outer envelope ends up around the white dwarf. The white dwarf is like, “Okay. Got material coming in, got material coming in. I’m gonna blow some of this off.” And also just normal red giant behavior, it’s gonna poof out its atmosphere.
And so, you can get left behind with a double degenerate system, with two white dwarfs; you can end up with mass exchange between the two objects where what was a white dwarf will return to being a star again, essentially. And through these complex evolutionary systems that we can’t fully model yet, because this is where you need three-dimensional magneto-hydrodynamic equations dealing with degenerate gas. And computers take one look at what’s needed, and they’re like, “No. We’re discussing this based on orbital energies and allowed energies in the systems.”
So, we have ways to get at it; but because the common envelope phase would be so short relative to the rest of the things that the stars undergo, we just don’t see this. So, we know it’s something that should be happening. We know it’s how you explain some of the binary stars that we see later. It’s how you get two stars close enough for a lot of the things we see.
Fraser: But I’m just trying to imagine, like really paint a picture here. If you could hover outside and see these stars interacting, what would you see?
Dr. Gay: It would start out as two stars fairly far apart.
Dr. Gay: One of them would bloat out massively. So, our own sun will get close to the radius of Jupiter’s orbit when it bloats up. It bloats up, and the outer parts of the atmosphere are like, “Oh, gravity between this white dwarf and the core of the red giant is kinda equal over here, folks.”
Fraser: I choose the white dwarf.
Dr. Gay: And this is what’s called filling the Roche lobe, and Roche-lobe overflow. Because Roche was the scientist who figured out the gravitational modeling for this. And when the Roche lobe is overflowed; when the material in that expanding red giant moves past the point of the greater gravity being the red giant star, that white dwarf can start to grab on to that material.
Fraser: Right. It’s like a hill, and downhill is actually now towards the white dwarf, as opposed to the center of the red giant.
Dr. Gay: Now, when this starts to happen, you get all sorts of weird other physics happening, due to interactions between magnetic fields, due to interactions with gravity, and this can cause the two stars to migrate together. Now, if you start bringing that white dwarf closer to that expanded red star, you’re gonna accelerate the mass loss, and you’re potentially gonna end up with the white dwarf in the outer atmosphere of that red giant, where it can now start grabbing material.
Fraser: Now it’s in a buffet, right? All-you-can-eat buffet.
Dr. Gay: Yeah, it’s in not all-you-can-eat, because there’s a limit most of the time.
Fraser: Limit of your accretion disc.
Dr. Gay: One of the saving graces of these kinds of systems is in most but not all cases, the white dwarf came from the more massive object. So, whatever it’s feeding off is smaller than it started, except in weird cases where things come together later in their lives. May/December relationships in stars do not work.
Fraser: Right, so the big star, now the dead star, is feeding on the younger star, which is also the smaller star. And so, the opportunity for mayhem is less.
Dr. Gay: Sort of. They’re the exact same age, but the less evolved and more evolved. Not all stars grow up at the same rate. They can be born at the same time, but the more massive star, well, it lives faster and harder.
Fraser: Right. Yeah.
Dr. Gay: So, you can have this common envelope period. You can end up afterwards with two white dwarfs, because they’ll expel their shared envelope. Now, in other cases, you can for instance have a white dwarf and a red dwarf companion, and it’s grabbing material off that red dwarf companion. You can have all sorts of different kinds of combinations. You can have a main sequence star and a white dwarf.
And the key is, the results are essentially fourfold. If you have a binary system that is widely spaced, they just orbit and ignore each other. This is fine. You can have a situation where you have a low-density accretion disc that is allowing material to spiral all the way down to the surface of the white dwarf, where it will build up on the surface of the white dwarf until there’s enough of it that it goes boom.
Fraser: Right. Now, is that the material going boom, or the star going boom?
Dr. Gay: It’s the material on the surface of the star. So, this is Situation 2; this is the classical nova situation. Situation 3 is you have a big old honkin accretion disc, and the material is building up in the accretion disc, until the accretion disc gets big enough and dense enough that the accretion disc goes boom.
Fraser: That’s so crazy! Just imagine, you’ve got this disc of material orbiting around this star. Part of it is winding up onto the star, but the rest of it is just swirling around on the outside.
Dr. Gay: It can’t shut angular momentum fast enough to get to the surface, and so, it’s trapped in the disc with all of its particle friends.
Fraser: We talk about this a bit, that the sun is the hardest place to reach in the solar system. People think like, oh, the sun, just shoot something towards the sun. If you shoot something towards the sun, you will miss. If you’re on the earth and you want to get to the sun, you have to shed 30 kilometers per second of orbital velocity to be able to drop down onto the sun. And so, this material is swirling in. It’s got mountains of angular momentum, and the only way it’s getting rid of it is just by bumping and bouncing and slowly getting that speed down til it can start to actually drop onto the surface of this star. And so, what you’re saying is that the easier pathway is for it to just pile up in the disc, and then explode.
Dr. Gay: Yes.
Fraser: What does it do to the disc?
Dr. Gay: The disc is greatly reduced. It’s not necessarily gone way. And this is one of the things that is ridiculously hard to observe, because you want to look at the system while it’s what’s called quiescent. While it’s quiet, without the nova activity going on. Because otherwise you’re going to blow out your detectors.
Dr. Gay: So, you can look at it in radio while it’s active. You probably don’t want to use the Hubble. So, the Hubble will actually work with organizations like the American Association of Variable Star Observers; the Center for Backyard Astrophysics; all of these different amateur groups, to have them monitoring the stars to make sure that they don’t flare up while Hubble’s preparing to look at them.
Fraser: Right. So, you’ll actually burn out detectors on Hubble if you get it at the wrong time.
Dr. Gay: You could avoid doing that. So, we’re still working to figure this out. And it’s happening at so high a resolution that you can’t catch all the details. But it appears that you have much, not all, of the accretion disc gets blown to other places, and then it rebuilds.
Fraser: Right. But some of it – like, a little of it – is making its way to the star itself, and that’s important.
Dr. Gay: Yes. Yes.
Fraser: And that’s important for another way that they explode.
Dr. Gay: Exactly. So, these systems are basically time bombs, and a star that ends up in these accretion scenarios is eventually – if its companion is large enough, – going to just flat-out become a type 1a supernova, or a type 1ax supernova. The limiting factor here is how big is that companion. If the companion isn’t big enough, you end up with two white dwarfs that are both sitting there going, “Hi, I’m too small to become a supernova,” but are bigger than they started when they formed.
Dr. Gay: If that companion is sufficiently large and the process goes sufficiently quickly so that you don’t have a myriad of things that disperse the matter going on, then you can build up past what’s called the Chandrasekhar limit. This is the point at which the electron degeneracy pressure, which we talked about in the last episode, is no longer able to support the star. So, it’s not that you’ve built up too much stuff on the surface and it undergoes thermonuclear reactions. It’s that you’ve built up too much stuff, and the electrons are no longer able to hold each other apart against the inward push of gravity, and the star becomes unbalanced.
Fraser: But isn’t that just a neutron star? Shouldn’t it just turn into a neutron star at this point?
Dr. Gay: So, the issue is that when the electrons and the protons in the atoms that make up the white dwarf combine to form neutrons that have the potential to form a neutron star, other things are released, including energy and photons. Other particles. And as these neutrinos, these photons, all of this stuff radiates away, the force of all of that stuff getting released, that’s what tears the star apart.
Fraser: Right. Right. And it’s not – like with the nova, you get these explosions off the surface. In the process of these type 1a supernovae, the entire star is just completely obliterated in one moment.
Dr. Gay: And the theoretically awesome thing about this is because this consistently happens at the same mass point in the growth of a white dwarf, in theory all of these should give off the exact same amount of light, because you have the same essentially number of atoms that are combining into – protons and electrons become neutrons, and release energy.
Now, the quandary with this is, you can imagine, if all the stars are all made of carbon and you add stuff onto them, all these carbon white dwarfs exploding do the exact same thing. You can imagine all the helium white dwarfs exploding do the exact same thing. You can see how this should give off the same amount of light; you should see these same types of curves.
And the caveats start to become, well: What if having impurities in the star leads to different things happening? What if the impurities cause some of the energy to go into other processes than making a supernova? Does this change the luminosity? You also have weird stuff that happens, like we have seen periodically what are called now type 1ax supernovae, where the white dwarf doing the exploding happens to be inside of another star while it does this.
Fraser: We did a whole episode on this.
Dr. Gay: And so, go back and listen to that episode. I know that there’s weird exceptions out there.
Fraser: Right. Right. But so, these things are so bright, and so we see them but we see them because they’re so bright. They’re actually very rare to have this situation happen. I mean, one per galaxy per several hundred years, right?
Dr. Gay: And that’s for the type 1ax. Type 1as are estimated to be more common than that. Supernovae in general are about one per 100 years. The 1ax are super rare because to get them we think in most cases, if not all cases, you have to have a star that becomes a white dwarf meet up with a star that forms substantially later than it did. So, this May/December binary system where the lower-mass star formed so much further back in time that it evolves before its higher-mass companion. And because there’s this high mass companion that has the potential to do something potentially even more dramatic than becoming a white dwarf, their interaction can lead to a perhaps even double detonation in some cases.
Fraser: Wow. That’s incredible. All right, so, when we were leading up to this show, we talked about – I mentioned in the intro – there’s another method that dwarf stars can explode, and you must have missed the paper.
Dr. Gay: I totally did, and –
Fraser: Yeah. So, this is my chance to explain something to you.
Dr. Gay: Research is ongoing. And –
Fraser: Oh, totally. And this is purely theoretical, because the timescale is crazy. But there’s a researcher out of Illinois State University, Matt Kaplan. And the theory goes that if you wait long enough – if the white dwarf is gonna turn into a black dwarf, it’s gonna cool down to the background temperature of the universe. And for most of those, they’re just going to just –
Dr. Gay: Hang out.
Fraser: – be frozen forever and hang out. And maybe if protons decay, then it will eventually tear itself apart atom-by-atom. But also, there’s a possibility that you can have quantum tunneling happening inside white dwarfs, or I guess at this point black dwarfs, of a certain mass. And they will be changing the structure inside the star over incomprehensible periods of time. And eventually – so, it’s a 10 to the power of 1,100 years. So, if a google is a 10 to the power of 100 years, this is 10 to the power of 1,100 years, which is a 10 –
Dr. Gay: A lot.
Fraser: – followed by 1,100 zeros, then you will get this thing eventually explode. And that’s when the first ones will go. But you’ll still get this quantum tunneling in the smaller and smaller white dwarfs. And so, the last ones will go off at 10 to the power of 32,000 years.
Dr. Gay: Now, does this ignore the possibility of proton decay?
Fraser: Well, that’s what I was saying. Yes, so, if protons don’t decay, then the last interesting that will happen – way after the black holes have all evaporated – the last interesting thing that will happen in the universe is the white dwarfs will explode as supernovae because of this quantum tunneling in their interiors, which was transforming them from the inside out, until enough of them is unstable again, and they explode. So, it’s like the last interesting thing to happen in the universe.
Dr. Gay: And this is where you see one of the real differences between what Frasier and I choose to read is I would have looked at that and gone, “We don’t even know if protons decay. I’m not going to bother reading this.”
Fraser: Yeah, if there’s not a working mission to search for them, I’m not interested.
Dr. Gay: And there’s so much literature out there. There’s so much research being done that we each have to make our own choices on what we read and don’t read, and it’s the fact that we have this balance between making different choices that allows us to bring you today’s episode.
Fraser: Yeah, you want to learn about different ways to communicate astronomy in citizen science, and I want to learn about how the universe is gonna end.
All right, Pamela, now, normally this time of the show we thank all the patrons. And here’s the short version: Thank you, patrons. But we wanted to take a second to remember one of our community who passed this week.
Dr. Gay: So, those of you who watch over on Twitch, you’ve been able to do that because of Paranor001, Tim Hawkins. He joined our community when we joined Twitch about two years ago. He was one of the very first people I made a mod that was new to our community. And he was always there for us, always offering suggestions. He was the first person to tell me when my audio levels were wrong. And he was there to help with lighting, and eventually he became a core member of our community, helping Simocast, and helping us plan a lot of the things we do; and he and I talked almost every day.
Fraser: And I’m certain, if you go back through all of the moderator comments that we’ve had in the Slack chat, in YouTube, the vast majority of them are his. So, he was one of the – just as you say – cornerstones of this community, and made really a huge effort to help us communicate a base in astronomy to a wider audience.
Dr. Gay: And he’s always been here. Several months ago, he basically did a, “Look, you know, I live alone. You know I don’t leave the house very often. If you ever don’t hear from me for a few days, here’s a real-life person you can contact.” And when he went dark on Sunday and didn’t come back after his normal nap, we reached out and we found out, yes: When they went to check out on him, he had passed.
And now we’re trying to find the right way to commemorate him. There are over there on the Cosmoquest Discord many different things that people have written and shared, and one of the last things he had been working on with us was plans that during this years Hangout-a-thon, we’re going to do a scale model of the solar system in Minecraft.
Fraser: Oh, that’s cool.
Dr. Gay: He got me Minecrafting last week. That was the last thing he and I did together.
Dr. Gay: And so, on the weekend of October 24th, 25th, as part of our Hangout-a-thon, we are going to be building a scale model of the solar system. And if you’d like to be part of that, go on over to cosmoquest.org, and this weekend – the Saturday and Sunday before this episode gets published on iTunes and Google Play and Spotify and all those other places – we’ll be adding all the links, so you can sign up to be part of that. And he always described himself as a madman without a blue box.
Fraser: Right. That’s a Doctor Who reference.
Dr. Gay: And we’re gonna make sure that blue box is embedded in the solar system.
Fraser: Yeah. Thank you, Tim. We miss you already. And thank you, Pamela, and we’ll see you next week.
Dr. Gay: We’ll see you next week.
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