Ep. 563: White Dwarf Mergers

Posted on Mar 26, 2020 in podcast, Stars, Stellar Evolution | 2 comments

White dwarfs are usually about 60% the mass of the Sun, so it was a bit of a surprise when astronomers found one that was almost exactly twice that. What happens when white dwarfs merge?

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Show Notes


Fraser Cain:                 Astronomy Cast episode 563. White dwarf mergers, for real. 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 Cosmos Quest. Hey Pamela, how are you doing?

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

Fraser Cain:                 Great. I mean, after we had our group therapy session, I’m ready to get back to work. Ready to science.

Pamela Gay:                Good. It’s a world where we have to find lightness wherever we can. My current favorite trope of the moment is people taking animals to visit animals at zoos and aquariums. There’re so many bored zoo keepers right now producing the most amazing stuff.

Fraser Cain:                 That’s amazing.

Pamela Gay:                And I can only hope that we can be part of the bringing people joy wherever they are at this moment.

Fraser Cain:                 And for me, I have been trying to learn Chinese for fun and it is hard and really tough. So, if you want to just make your brain hurt, go learn a new language during this time. Don’t make me practice it in front of you. Okay. White dwarfs are usually about 60% the mass of the sun. So, it was a bit of a surprise when astronomers found that one was almost twice that mass. So, what happens when white dwarfs merge? All right Pamela, before we get into what happens when white dwarfs merge, let’s just find out what white dwarfs are.

Pamela Gay:                So, stars that are fairly average in mass, these are things that are probably under six solar masses. There’s a lot of arguing on exactly how big the progenitor star that original star might be, but stars that are fairly boring in their day-to-day lives will undergo massive amounts of mass loss as they evolve. And in their final days when they’re, what we call super giant stars, red giant stars, they’re going to start exhaling their atmosphere forming amazingly beautiful and occasionally super strange planetary nebula around them. And when they’re done exhaling that atmosphere, what’s left behind is a white dwarf star. And in the final stages of that star’s evolution, it gives up on generating its own energy through nuclear fusion processes.

                                    It ends up with a temperature that no longer has sufficient density, no longer has sufficient fuel, no longer has sufficient temperature to take the stuff it’s got, fuse it and release new energy. And when that energy production shuts down, you’re often left with a core of carbon, a core of helium, and that leftover core is sitting there going, “I’m hot, I’m really, really hot.” It often is giving off the majority of its light in the ultraviolet. And this ultraviolet light shines out into that exhaled atmosphere that has previously been let go and lights it up. Over time, that leftover core is going to cool. It’s going to change color, that planetary Nebula is going to fade away.

                                    But throughout the entire rest of its existence, that white dwarf core that is left behind, even when it becomes brown or black or whatever color it fades into, it’s going to be supported through what’s called electron degeneracy pressure. Previously, this star had had light pushing outwards supporting the outer layers of the star against the crush of gravity. But when you turn off that energy generation, gravity tries to crush everything down. But at a certain point, physics kicks in and within a certain pressure regime, the electrons surrounding all the atoms in that white dwarf are able to go, “Pauli exclusion principle, don’t come near me.”

                                    And this Pauli exclusion principle basically says you can’t have two identical electrons and the same energy level around an atom. And because all of these atoms are sharing their electrons, these electrons end up forming essentially a crystal in structure and they’re making sure all the energy levels are filled, all the spins are correct, and this electron degeneracy pressure is what is now supporting these stellar remnants from the crush of gravity. But electrons can only support so much pressure before the pressure on them is greater than their repulsive force between them. And when that repulsive force between them gets overcome, they go, nope, and the electrons and the protons combine. Neutrons are created, energy is released.

                                    Sometimes you leave nothing, sometimes you leave a neutron star, which is supported through a neutron degeneracy pressure, which is the exact same thing as electron degeneracy pressure, but now it’s the neutrons that are crying, “Stay away from me.” It’s physics in its most elegant. And one of the most common assignments to give advanced undergrad and baby graduate students is to calculate the masses of a white dwarf. And what is awesome is this idea of what is the limit on the mass of a white dwarf was figured out by Chandrasekhar while he was on a boat from India where he did his undergraduate studies to England where he did his graduate studies.

                                    So, this is really the ultimate graduate student problem that was originally figured out by a graduate student who went on to get multiple Nobel Prizes as you do.

Fraser Cain:                 All right, so I mean, we don’t want to go too deep down the rabbit hole, but how does one do this calculation? I mean, in general, what are the things that you’re thinking about when you’re trying to figure out how big a white dwarf is, how massive it is? You’re thinking about like the star, like how much star was there before?

Pamela Gay:                Well, it is a function of how much star remains. What is the leftover core of the star that exerts gravitational force inwards? See, you look at the core of the star as the worst case, this is where things will fail. So, you figure out, what is the pressure exerted by all the mass above you pushing down upon your electron head. Then you look at, well, electrons repel other electrons. This is where the whole electron degeneracy pressure comes from. And so, you know, okay, you have the electrons closest to you, you have the electrons a little bit further away, and you can calculate out what are, and it’s always calculus based. It’s calculus all the way down here.

                                    You then start figuring out, okay, what is this force in this degenerate gas that is acting against you as all of these electrons are going, nope, nope, nope, pushing against each other. So, you have the repulsion of the electrons against each other as they maintain all their specific energies and alignments. You have gravity pushing in. And at a certain point, the electrons will get closer and closer and closer as gravity gets stronger and stronger and stronger. And when the electrons get too close, this is when everything fails and that’s when your star goes boom.

Fraser Cain:                 Right. And that’s where you get a supernova, a type 1A supernova, which was that 1.4 times the mass of the sun. So, let’s talk about colliding white dwarfs. If you’ve got one white dwarf with say 60% the mass of the sun and you smash it into another white dwarf with about… so what would have led up to a scenario where you could make this happen, the smash happen out there in the universe?

Pamela Gay:                So, when you say smash, I know my brain goes straight to a game of good old-fashioned marbles where you’re flicking these objects across the universe at one another. And that is generally not how things work, but it’s a fabulous mental image. More accurately, you have one of two scenarios. You either start out with a binary star system with two fairly normally masked stars and one evolves first and bloats up and steals matter from the other stars. It goes and then spews mass out forming this planetary nebula around them both. Then the other star finishes its evolution out and you end up with these two white dwarf stars and this remnant of their two atmospheres and they’re not in a nice forever orbit.

                                    Instead, they’re spiraling in towards each other. And this kind of a situation where you have two extraordinarily dense objects, the one thing we haven’t done is give any of you a sense of scale. White dwarfs are generally about the size of the moon. So, we have from half a solar mass to more than a solar mass of material crammed down to the size of the moon so they can get pretty close. So… go ahead.

Fraser Cain:                 And you mentioned sort of just briefly, right, they can be incredibly close, and so you mentioned that they are in the envelope, they are in the atmosphere of material that they had blown off. So that’s acting, I guess like a friction as they-

Pamela Gay:                Drag.

Fraser Cain:                 … whirl around each other, but there are other forces that are helping to bring them closer and closer together as well. Right?

Pamela Gay:                Yeah. So, they’re also gravitationally radiating away energy. And this is one of the most exciting things about this. And we didn’t really know how many of these suckers were out there until Gaia launched. And Gaia is finding white dwarf binaries. And when LISA, the gravitational wave detector that we’ve been talking about for decades now, when and if it finally ever launches from its orbital position, it will have the sensitivity to detect the gravitational radiation, the waves of gravity coming off of these spiraling systems as they fall into one another.

Fraser Cain:                 And I want to just take a moment here and just wrap our heads around just that idea, which like when you think of a boat moving through the water, you can imagine the waves that are coming off of the boat and they’re being transferred. And really that’s where the fuel of your boat is going, is to create those waves that are rippling out behind your boat so that your boat can move. And when any of us are moving around in space and time, we are leaving a wake of gravitational waves as we go. And that is we are converting just a tiny little bit of our kinetic energy into these gravitational waves that are then transmitting out into the universe.

Fraser Cain:                 And so, you can imagine in your mind these two white dwarfs whirling around each other, leaving ripples of gravitational waves in the fabric of space time. And as they do so, they lose kinetic energy and whirl closer and closer to each other.

Pamela Gay:                And these systems are in some ways, the best word I can come up with is very intimate in how they evolve because they can, depending on the geometry of the system, literally take turns being inside the envelope of the other star as they go from giant star to giant star. They shed and gain material from one another, passing it back and forth. And it’s really a fascinating story of what forms and what dies can go through many stages of sharing material back and forth in these binary systems. What’s mine is yours, what’s yours is mine is really the tale of the stars in white dwarf binaries.

Fraser Cain:                 So, you’ve got various forces that are conspiring to bring these two white dwarfs closer and closer together and eventually they merge.

Pamela Gay:                It’s true. And if you have two tiny enough white dwarfs, when they merge, they simply become one very chunky white dwarf sitting out there refusing to explode, being hot and bright in a-

Fraser Cain:                 But then you get something, don’t you? I mean, you say refusing to explode so it doesn’t go full supernova, but it does, it’s got to do. I’ll bet you we could detect it happening, right, if we looked out into the universe.

Pamela Gay:                Well, it depends on how it happens. I mean, if it goes through a common envelope phase where they bloat back up into having a shared envelope around them as the nuclei merge together, it’s going to be nothing extraordinarily dramatic. If you have two degenerate stars that simply spiral together without reinflating that common envelope, then you can get a flash of light. But it’s not the supernova that we’re used to when we talk about white dwarfs.

Fraser Cain:                 Right. Because it’s just a factor of scale, like supernova are ridiculously extreme and catastrophic while regular, plain old novae, which we can see are-

Pamela Gay:                They’re a thing.

Fraser Cain:                 … mildly catastrophic.

Pamela Gay:                They’re a thing. But what gets me about this research is we’ve been telling ourselves as astronomers this story for the past 30, 40 years, that type 1A supernovae are all identical. That type 1A supernova come about when a more massive star evolves faster, becomes a white dwarf, steals matter from its lower mass main sequence companion, exceeds the Chandrasekhar limit of that 1.4 solar masses and goes boom. It’s a nice elegant story. And it is off of this story that assumes that all type 1A supernova start with the same first initial amount of mass.

                                    That has led us to saying, “Hey, if you blow up the same amount of star every time, these should all give off the same amount of light. Let’s use these as standard candles to measure the size of… not the size, to measure the expansion of our universe. It’s a great story. But not all type 1A supernova are the same. And this is getting clearer and clearer every new set of papers on these things.

Fraser Cain:                 Right. And so, the original idea was that you were taking exactly 1.4 times the mass of the sun in the form of degenerate matter. You were lighting it on fire supernova style, and it was releasing the exact same amount of energy. So, does the merger of potentially two white dwarfs create a different precursor event than say, one white dwarf that happily fed off of its companion star until it hit 1.4 times the mass of the sun? Are they two different kinds of explosions?

Pamela Gay:                Oh, it’s way more than two. So, one of my favorite recent examples, there’re so many ways that this story goes wrong. So, let’s look at each of the parts of the story. So, the first part of the story is you have a white dwarf star and a smaller didn’t evolve as fast, still main sequence companion that is sucking the material off of to hit the place where it goes boom. Well, what if you’re looking at a star in an extraordinarily dense environment like a globular cluster where that white dwarf is suddenly best friends with a much bigger younger star that it can now steal matter off of. And because that other star is massive, it can fall into that star’s atmosphere.

                                    And while in that other star’s atmosphere, it exceeds its Chandrasekhar limit, it can cause what’s called a double detonation, where its core goes kaboom. And the shockwave from its core going kaboom triggers the, well, core of the star that it is now inside of it to also go kaboom.

Fraser Cain:                 Right. And it kind of looks like a type 1A supernova.

Pamela Gay:                Well, technically a totally is a type 1A supernova.

Fraser Cain:                 Totally is.

Pamela Gay:                Because technically it’s still a white dwarf going boom.

Fraser Cain:                 Right. But if you took your, as I said, your 1.4 times the mass of the sun, lit it on fire inside 10 times the mass of the sun, you might see something that’s a little different than what you were expecting to see before.

Pamela Gay:                And this happened with supernova 2006gy, which is now my favorite supernova [crosstalk 00:19:31].

Fraser Cain:                 Yeah. And we did a whole episode on this. So, if you want to go back and listen to that episode, we went into gruesome detail about this event.

Pamela Gay:                Yes. So that is one of the ways things can go rapidly wrong. Then the next is, so you have a double detonation of the white dwarf detonating the star that it’s inside of, double kaboom. You can also have what’s called a double degenerate detonation. This is where essentially your two merging white dwarf stars are able to detonate one another. So, the one will start to go kaboom, it’s shockwave will kaboom the other and double detonation, double degenerate detonation [inaudible 00:20:21].

Fraser Cain:                 Right. So, in other words, if that other white dwarf wasn’t quite there, the shockwave compressed parts of the white dwarf causing it to undergo that carbon fusion and then it was game over for the whole star at maybe a mass that it shouldn’t have exploded at.

Pamela Gay:                Exactly.

Fraser Cain:                 And there goes your standard candle again out the window. It can’t be trusted.

Pamela Gay:                Oh yeah. No. And it’s now looking like the majority of white dwarfs are in these weird double degenerate systems.

Fraser Cain:                 I mean, right now obviously astronomers use spectroscopy to analyze the chemical signature of various events. They time how long the thing took to brighten the light curve. Can they start to classify all of these different kinds of objects into what they now think they are and then return to a new set of standard candles?

Pamela Gay:                Not yet. And here’s where life is not always as good as you would like it. The primary way that we discover supernovas is we have research teams all over the world that are out night after night looking at field after field trying to find the things that go flicker, flare and move in the night. These are the transient objects. And when you find a transient object, you sit on it, you look to see how does it change? Did it move from point A to point B? Is it casually pulsating like a nice normal variable star? Did it go kaboom and begin to fade away? And we classify objects based on the pattern of their brightness and if they move.

                                    Now to differentiate between all these different kinds of type 1A supernovae, if you know exactly where they are, if you know what galaxy and how far away that galaxy is from some other methods, Cepheid variables or something for instance, you can start to say, “Ah, this is unusually bright. This is a weirdo.” But if you don’t know where it is, you can’t know if it’s weird or not if it follows the shape of getting brighter and fading away that says, “I am a type 1A supernova.”

                                    Now, in ideal circumstances, these suckers are close enough that after they’ve gone boom and after they’ve been discovered, someone with a massive telescope has pointed their massive telescope at the supernova and measured the spectral signature, has looked to see how much nickel, how much iron, how much, how much, how much through all the different elements to see what was illuminated with what particular energies at one point in time as the supernova and its remnant evolved. And this can start to flag, oh, this is weird. And that, as we talked about it in the earlier episode, is how we figured out 2006gy was what it was, is looking at the chemicals.

                                    But in order to see the barcode of nickel, in order to see the barcode of titanium, you need to take the light from the supernova and spread it out into a massive rainbow where you can see every single finest gradation of color. And you have to have a really bright object to be able to spread the rainbow out that much and still see the light. If you’re-

Fraser Cain:                 Or a really powerful telescope.

Pamela Gay:                But the farthest corners of our universe defeat the biggest telescopes because we just don’t have telescopes yet that are big enough. This is why we keep building bigger and bigger telescopes.

Fraser Cain:                 Right. But like James Webb for example, would be helpful.

Pamela Gay:                It would.

Fraser Cain:                 Right? Super Hubble, mega Hubble, double Hubble.

Pamela Gay:                JWST isn’t a mega Hubble. And one of the issues-

Fraser Cain:                 No, no. That’s why I’m saying, I’m saying, let’s get a mega Hubble. Let’s get a double Hubble.

Pamela Gay:                So where even these space telescopes fail is the specter graphs that we’re using. They’re often what are called Kude spectrographs. You take the light from the telescope, you funnel it either with a set of mirrors or most often nowadays, a bunch of fiber optics. You take that light and you move it from the telescope into a basement optical room, or at least a lower level optical room. And you have several tons of equipment that is used to produce that gorgeous spectrum. And we can’t launch an entire basement of optical benches. So, the stuff that we’re doing with these massive telescopes on the earth, we can’t do from space.

Fraser Cain:                 But you can see, yes, and I mean, you could make a bigger [inaudible 00:25:42]. I mean, this is one of the things that you could theoretically bolt to the bottom of the European extremely large telescope and try to get a better view of it.

Pamela Gay:                Yes. That’s the bigger telescope we need.

Fraser Cain:                 Right. And it’s coming. It’s coming like six years away. It’s really fascinating. It’s this idea of like classification, right? You see a bunch of stuff and you go, “That’s a bird.” And then you’re like, and there’s one up in the air and there’s one that’s three meters tall and it’s chasing you and you’re finding, right, and there’s ones that are swimming around in Antarctica and you’re just like, “Bird, bird, that’s a bird.” And then you’re like, “No, no, wait. Actually, they’re different kinds of birds.” And it’s that same classification that you can just imagine once astronomers, because these are thought to be standard candles.

                                    Once astronomers can work out which is what and what is which, it might come all the way back around to now you have even better standard candles than you did before that you know which side of the galaxy was the far side of the galaxy or the close side of the galaxy, was it right? How are these galaxies interacting? You can use this with more precision and it’s discovering that in fact this thing that you thought was just bird actually comes in different flavors.

Pamela Gay:                And this is where some of the really cool stuff about chemistry matters, where it’s starting to get down to the, you can tell what a flamingo eats based on its color because that pink comes from shrimp. Well, with these supernovae, what they’re made of appears to tell you how bright they should be, how luminous they should be. So currently when we’re using the standard candles, we assume that on average at a given age in the universe, all supernovae are the same. And yeah, this would maybe a little bit brighter, that would may be a little bit fainter. But on average at all times, all type 1A supernova average out the same.

                                    So now what we need to figure out is does that law of averaging work out or is it like, well, here I’m going to switch from your bird analogy to humans because this one we actually have data on. Over time, human beings have gotten taller and taller and taller. So, if you’re looking at a skeleton, you can’t say this person was under average height comparing them to modern humans because our average has changed over time. George Washington was my height and I am short compared to most modern men, but George Washington was a tall man for his time. Now with these type 1A supernova, our populations of stars have changed. Globular clusters have had time to have more random binaries form.

                                    We’ve had the chemistry of the universe evolve and all these differences from one time period to the next could be changing with the average type 1A supernova looks like, but we don’t know perfectly.

Fraser Cain:                 Yeah, absolutely fascinating and it was a wonderful discovery that they did find these examples of potentially merged white dwarfs and this is the beginning of another time when you thought it was kind of figured out and suddenly it’s a brand-new field of discovery. More questions than answers. Pamela-

Pamela Gay:                And Gaia is going to point us where to look and someday maybe, maybe we’ll be able to see them with more detail than Gaia allows.

Fraser Cain:                 Have I mentioned how much I love that mission?

Pamela Gay:                Yes.

Fraser Cain:                 I love that mission.

Pamela Gay:                It’s my favorite. We just need LISA and JWST to finish answering this question.

Fraser Cain:                 Do you have some names for us this week?

Pamela Gay:                I do. I would like to thank so many people, so many people for making our show possible. There are four screens worth of you now and it is amazing. So, the people I am going to say thank you to today are Christian Books, Cassinia Penn Flianco, Shannon Humber, Dwayne Isaac, Thomas Tubman, Eric Faringer, Rachel Fry, David Gates, Justin Proctor, Frederick Yogad Jack, Claudia Mastroianni, nerdy dude, Iran Sergei. Sorry. I know you. Egan… I just can’t say it today. Iran, I’m sorry — Arthur Lats Hall, William Andrews, Tim Garish, Omar El Riviero, Paul L. Hayden, Michelle Cullen, Brett Crimap, mark Steven Reznak, J. Alex Alexanderson, Jeremy Kerwin, Mark Grundy, William Lauer, Bruno Lights, Joe Wilkinson, Brian Kilby, Dustin A. Ralph, Jessica Feltz, Marco Rossi, Dave Lackey, and Gillian Rhodes.

Fraser Cain:                 Wow.

Pamela Gay:                Thank you all so much. You keep this science flowing.

Fraser Cain:                 A special thank you I think right now definitely for us at Universe Today, in this last week, our traffic has dropped. I know things are a little weird for Cosmic Quest as well. I’m spending all the money that I have to employ as many writers as I can afford. So, the more you donate, the more you Patreon Universe Today, the more people that I can keep employed and similar for the work that’s being done with Cosmic Quest.

Pamela Gay:                And there are some people in our Cosmic Quest family who, because they had other jobs as well, let’s face it, a lot of us work multiple jobs. They were able to say, “Okay, right now it’s all right if you don’t pay me as much and I’m going to volunteer a lot of time,” and there’s other jobs are going away right now. So, we have people that, you’re making all the difference in the world. Thank you.

Fraser Cain:                 Thank you. Thank you. Thank you. Pamela, stay safe.

Pamela Gay:                I will. You too.

Fraser Cain:                 I want to make sure that we can continue this for decades to come. So, don’t let anybody into your house and don’t go anywhere.

Pamela Gay:                Well, I have not left my yard since February 28th-

Fraser Cain:                 Perfect.

Pamela Gay:                … and I plan to stay here until there’s a vaccine, I think.

Fraser Cain:                 That sounds good. All right, we will see you next week.

Pamela Gay:                Bye. Bye.

Speaker 3:                   Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Frazier Cain and Dr. 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 PM Eastern, 12:00 PM Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Searle and the show was edited by Susie Murph.

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  1. Thanks for the chance to learn about something other than Covid

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  2. Not really an astronomy-related note, but just to set the record straight: Pamela stated (more or less) that George Washington would not be considered tall by today’s standards; that he was basically her (Pamela’s) height. By most accounts, Washington was 6’2” (so was Jefferson), which was tall then but is still considered tall by 2020 standards. It is true, however, that the average American today is taller than in the late 18th century—which I think is the point she was trying to make.

    In any case, great podcast and keep up the good work!

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