Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 261 | Sanjana Curtis on the Origins of the Elements

Episode Date: January 8, 2024

In mid-20th-century cosmology, there was a debate over the origin of the chemical elements. Some thought that they could be produced in the Big Bang, while others argued that they were made inside stars. The truth turns out to be a combination of both, with additional complications layered in. Some of the elements of the periodic table come all the way from the Big Bang, but others are made inside stars or in stellar explosions. But still others are made by cosmic rays or when neutron stars and black holes merge together. We talk to nuclear astrophysicist Sanjana Curtis about all the different ways that the universe is cleverly able to produce various elements. Blog post with transcript: https://www.preposterousuniverse.com/podcast/2024/01/08/261-sanjana-curtis-on-the-origins-of-the-elements/ Support Mindscape on Patreon. Sanjana Curtis received her Ph.D. in physics from North Carolina State University. She is currently a National Science Foundation postdoctoral fellow at the University of California, Berkeley. Her research involves nuclear astrophysics, especially the production of heavier elements in supernova explosions and neutron-star/black-hole collisions. She is also active in science communication, including at her TikTok channel. Website Google Scholar publications

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podcast. I'm your host, Sean Carroll. I don't know how many of you remember or have this recollection, but back in the early days of, like, blogging and social media, there was this question of whether or not there was still a place for science journalism in the world, since after all, you had scientists who could now just start up their blog and talk about whatever it is they wanted to do. And the answer is, yes, you still do need science journalists, and you need political journalists and legal journalists and economic journalists, you need journalists who are experts in what they're doing. If only because professional journalists have an obligation to be fair, to think about everything that is going on in the field they're covering, and explain it to the people who are reading or watching or listening

in an unbiased, overarchingly fair way. Whereas the individual researchers, they're going to talk about what they personally think is cool. So in fact, I think you need both. It's great to have professional researchers being able to reach broad audiences in whatever way they can, you also need professional journalists to set a wider stage and make sure that nothing falls between the cracks. So I say this because I'm a person who is a professional researcher, but I also have public outreach, public-facing aspects of what I do, including this podcast, Mindscape. And I have a weak spot for special. speculative, big picture, conceptual kinds of things, right? The origin of the universe, the nature of life, what intelligence or complexity are, the foundations of quantum mechanics, stuff like that. And I say this because I hope people don't get the impression that most of science is like that, because it really isn't. I happen to be interested in that stuff, but most of science, and really, really good, important, fun, exciting parts of science are much more great.

right? They're much more close to the data, to known laws of physics. You can know the fundamental laws of physics. And that still leaves an enormous amount unknown in terms of how those laws play out, whether it's in biology or, for that matter, in nuclear astrophysics. And that's what we're talking about today, nuclear astrophysics. You might wonder what the intersection of those two words, is astrophysics, of course, the whole universe, stars, galaxies, et cetera, well, where did those nuclei come from that make up the heavier elements that make up you and me and the earth and things like that? From astrophysics, from stars mostly, but as we will see, not only from stars. So our guest today is Sanjana Curtis, who is an astrophysicist that you see Berkeley, and she is

nuclear astrophysicist. She studies how heavier elements are created in stellar explosions and other environments in the universe. She's also extremely effective as an outreach communication person. She started a new TikTok series, believe it or not. I can't do this. I'm too old for that, but Sanjana is hip to what the kids like these days. And she started a series called Stardust that in little tiny TikTok videos, but very carefully produced, explains the principles of nuclear astrophysics,

and, because it's fun and cool, goes into how those elements, that you make in the stars show up in different ways here on Earth. So you can hear about biology, you can hear about archaeology, you can hear about all sorts of fun things that these nuclei end up in here on Earth, as I said, in very grounded scientific context. And I have to say, as someone who was an undergraduate and graduate student in astronomy, nuclear astrophysics is super cool. You know, the story of how all of the elements in the periodic table get to be made over the history of the universe is almost suspiciously rich. You know, you might think, if you were thinking like a physicist, that there was a mechanism that made all the elements,

you know, maybe 99% of them and then there were some little tiny things. But that's not how it actually is. There are many different channels, different mechanisms in the universe, from the Big Bang to cosmic rays, to supernovae, and more that all are important for explaining where the different elements in the pre-eotic table came from. So this is a little bit of a journey through real-world astrophysics, real-world in the sense that we have data and are testing our theories, but not that we know all the answers yet. As you'll see, there's some good questions still remaining. So let's go. Sanjana Curtis, welcome to the Mindscape Podcast. Thank you. I'm really excited to be here, Sean. As I just said, as we were talking before,

this is one of the rare pleasures for me where I get to, sorry Mindscape fans, I know this is audio only to you, but a very cute kitty cat just walked in front of Sanjana here in the video. So we get extra treats that you don't get. That's my cat cinnamon. She has to make an appearance at these things. Whenever I start talking, she's here. My two cats are very ground-based cats. They do not jump up very easily. So anyway, as I was saying, this is a happy occasion for me because we get to talk about something I know a little bit about,

but I will try to channel the people who don't know anything about it. So let's start super duper simple. We're interested in how elements are made. What are these elements of which you speak? Tell us a little bit about the periodic table, how it works, how we should think. We've all seen it, but, you know, how should we conceptualize it? How should we look at it and get things out of it? Right.

So the periodic table, all the 118, I believe, elements. The origin of those elements is what we're interested in the field of nuclear astrophysics. And the way I think about it usually is in terms of the astrophysical sites that operated and created new nuclei and how sort of the periodic table builds up as a result of those astrophysical, nuclear reactions in various astrophysical sites. And so, for example, you have the lighter elements, hydrogen and helium. There's a particular type of nucleosynthesis that produces those. And then when you're looking at things like carbon and oxygen, elements that are essential to light, there's really stars that are doing nuclear reactions to make those. And then you start thinking about the iron group of elements. So iron and copper and zinc.

And there you need really energetic supernovae explosions. And then you go even happier. And there's even less of those elements, generally speaking, in the universe. and things like gold, our precious elements come from an entirely different type of nuclear synthesis sites. So in my mind, I think, of the periodic table in terms of the different astrophysical processes and sites that have built it up. That's great. And just to be super duper clear, nuclear astrophysics being your central concern means that you don't actually care about atoms. It's just the nuclei that matter to you. I mean, the periodic table is organized on the basis of the chemical structure.

And you're not into the whole chemical thing. No, not so much. So the second molecules begin to matter. That's not really my realm of interest. Of course, I'm interested in it as any human being is and as chemists are in terms of the properties of the atoms themselves. And, you know, where you have metals and non-metals and noble gases and metal. and metalloids. So the various columns of the periodic table

tend to share certain properties, things like that. That's not really my research. My research is more about how these elements came to exist at all. Exactly, right. And there's a longstanding joke among non-astronomers how astronomers count elements by going hydrogen, helium, metals. But you don't get to do that, right? Anything heavier than helium.

is a metal for most astronomers. Yes, that's exactly correct. That's just how we think of metallicity, which is a quantity, is just the amount of metals, and it's just anything that's not hydrogen or helium is a metal to me. I'm sure Kammoths are not a fan

of this kind of nomenclature, and I completely sympathize. The other thing, just to get our audience on the right wavelength here, I bet a lot of people think that the universe is almost all hydrogen or maybe almost all helium. I mean, how much do the other elements, the metals, etc., matter to the evolution of the universe in terms of stars, galaxies, blowing up things?

Oh, they matter quite a lot. I mean, it is correct to say that most of the stuff in the universe, the baryonic matter anyway, it's hydrogen and helium. But the way stars form and then explode and injects energy as well as new elements really shapes how galaxies evolve, just by changing the properties of the material that you're looking at, and also the energetics. So a supernova can shove material out of a galaxy even, potentially. And so it really matters in terms of when you're starting to think of the nuclear history of the universe, how structures formed. It really matters, stars formed and interacted with everything and the feedback also. I bet that a lot of how we get an image of the universe comes from the pictures that we get of it,

and there's still pictures generally, right? Which makes sense because the timescale on which these things happen, the timescale in which a galaxy effectively evolves is much longer than a human lifetime. But it does evolve, and I think maybe this is an important thing to get across, that if you took a time lapse of a galaxy, which we can't do, but if you did, there'd be all sorts of churning and exploding and interesting things happening. Yeah, so there's actually this field called Galactic Archaeology. And so you can almost think of astronomers as archaeologists.

As we look at older and older stars or older galaxies, we can kind of get snapshots of what was going on a long time back in the universe and then we come to present day and the solar system itself. Okay, well, let's do exactly the, that then. I think that we can start at the Big Bang. Tell us a little bit. As a nuclear astrophysicist, how do you think about the whole phrase, the Big Bang, and its aftermath? Yeah, so as a nuclear astrophysicist, the part of the Big Bang that I begin to get interested in is once the Big Bang has occurred and now the universe has mostly quartz and gluons. And as it expands and cools,

neutrons and protons begin to condense out. And of course, there's a lot of nuclear reactions happening, and a lot of it depends on the temperatures, and the way radiation is interacting with the matter. But sort of broadest strokes, you have neutrons and protons beginning to condense out very short timescale after a big band. And then you end up with a certain fraction of neutrons versus protons,

and this kind of thing is important for what kind of elements you can meet. But basically maybe seconds after the Big Bang or so, the elements that are produced are hydrogen, helium, teens, a bit of lithium. So this is all that really comes out of Big Bang nucleosynthesis. And now you have a universe full of hydrogen and helium. And then we can begin to form stars at all the sense. And then that's the portion. That's where my active research area is, is basically stars and supernovae and the aftermath of those. So it's kind of an interesting race, as I understand it, because if you have a hot but not too hot box of plasma, you would like to make iron, right?

Isn't that the curve of binding energy? Maybe fill us in that story. Yeah, yeah. So the binding energy really wants everything to be iron 56. However, the Big Bang Nuclear Synthesis, okay, the temperatures and the densities that exist during Big Bang nuclear synthesis are not conducive to having this whole set of nuclei being produced. And also, there's a funny thing where there's a gap in stable nuclei. So it's very difficult for Big Bang Nuclear Synthesis to produce lithium beryllium and boron. So these three elements don't really, well, there's a tiny bit of lithium,

but there's a gap there where there's not a lot of stable isotopes that you can get at in the nuclear reactions that are possible to occur in Big Bang nucleosynthesis. So what needs to happen really is you want to be fusing all this helium that you've made to make carbon. But to do that, you really need the conditions that exist in stars. I think I remember that George Gamov and his friends when they were inventing Big Big Mac nucleosynthesis really hoped that they were going to be able to make the entire periodic table.

But basically, the universe just expands too fast and cools off and there's not enough time for the reactions to happen. Exactly. Yeah, yeah. So, I mean, the reaction rates, and even making the carbon in stars, that had to be really figured out to get to the correct state of beryllium. You need to hit enough. You need to fuse two helium nuclei to make beryllium. And then you need to hit it with another helium to get to carbon.

And so then it's a very sensitive thing where you need to have enough beryllium that's not decayed away and still have a reaction rate that gets you that carbon. Because it is very intricately connected to the specifics of nuclei and how heavy they are, because you can't just take a helium and add one more proton to it and one more proton to it and one more proton to it. No, no. So, yeah, I mean, you can add a proton to the helium, but then what you end up with is an unstable isotope of maybe beryllium and then it very quickly will decay away. And so there's nothing that you can really do with that. Most concerts, you're in a seat. You're watching. Downtown Rocks at Fremont Street Experience is something else entirely.

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So you're stuck coming out of the Big Bang with a bunch of hydrogen and helium. And then I guess you make stars. So when I was in graduate school, which was a long time ago, the whole process by which stars were made was still pretty murky, I think, in the minds of astrophysicists. And interestingly, in the late universe, it does seem to care about the existence of heavier elements. they're very useful for making stars.

So how does that even happen? Do we know how you make stars in the first generation where it's just hydrogen and helium? That's a really interesting topic, but it is similar to what you said, where we don't really have a clear answer yet. So, for example, is something that you might call the initial mass function of the first stars, meaning if you were to bin these stars like mass, how many stars have? how many stars have like 10 times the mass of the sun or how many of them have 100 times of the mass of the sun. And people have done, and this is sort of broad strokes knowledge that I've gained from reading other people's favors, is if you do these simulations and yes, metals are important for cooling and the way things evolves, there are different predictions for what should happen.

So some people think that you might end up with stars that are hundreds of times, the mass of the sun, and those stars will undergo not core collapse supernovae, which I study, but something called parent stability supernovae. And then those particular supernovae have a very specific nucleosynthetic signature. Or you might end up with low mass stars as well. So it's really not very clear what the masses of these stars are going to be when you form the first generation of stars, and nor have we observed a star that's a population three star. Right. Astronomers count stars backwards, so the population three is the first population

chronologically, right? Yes, just for fun, you know. It makes sense. They count from the present day. They count backwards. So I know that some people like Katie Fries have suggested that dark matter actually plays a big role in forming that first generation of stars. I have no idea of whether that's plausible or not. I guess it would have to do with the way, like the halos form and then the galaxies are born in those halos. And so I'm sure there's an interplay, but I am not sure how that goes. I don't think anyone is.

Okay, that's fair. Okay, so we get some stars. Do we at least know when, like how many years after the Big Bang, that first generation really comes to life? There were dark ages, right? Yes, the dark ages. and then there's this epoch of re-ionization. I don't know if I have the number off the top of my head, Z something. You might know better than me, to be honest.

A long time after the first minute when we did big bag nucleosynthesis. There's a long stretch when it was just hydrogen. There's a long stretch of the dark gauges. Yes, and then stars form. And then good. Now we can make the heavier elements, right? I mean, so the short lesson here, if people are going to stop listening right now,

most of the elements are made in stars, not in the Big Bang. There's some miscellaneous ones made by even cooler mechanisms, but it's stars doing most of the work for anything heavier than helium. That's right. Stars and even stellar corpses, so neutron stars, are doing the rest of the work. Okay, so what's the first thing that happens in the newly stiliferous universe that spurts heavier elements out there into the cosmos?

The first thing that happens, I mean, one could argue it's these pair instability supernova. So what does that mean a pair instability supernova? Okay, so the pair instability supernova basically stars hundreds of times the mass of the sun, where in their core, the conditions are such that you can reach a regime where runaway pair production basically begins to happen. So pair production, meaning that you have electrons and positrons colliding together to make gamma rays. And that reduces the radiation pressure. And so it goes, sorry, when the gamma rays collide to also do the opposite reaction, that reduces the radiation pressure in the

core of the star and the radiation pressure is what's keeping it up. And so when this parent's ability is encountered, the core of the star begins to basically lose all support and it will trigger nuclear reactions as it collapses and the whole star blows. This is kind of fascinating. Like, I'm going to confess right here to all the millions or hundreds of millions of Mindscape listeners. I did not really know about this kind of supernova. So you have a big star, much, much bigger than the sun, and it's held up by radiation

pressure. So literally the photons that are being made inside are keeping up the star. And it gets so intense that the photons start bumping into each other and making electron positron pairs, which lowers the pressure and the star collapses. Yes, and the star collapses and then as the temperatures and densities rise, you get nuclear reactions happening and that kind of undergoes a runaway because the star continues to collapse. So the pair instability has nothing to do with the star splitting into. It's gamma rays, its photons making pairs. Yeah. Yeah, yeah. And is this, we've heard of things like type

one type two supernova and things like that. Is this one of those? This is a separate category. So this is a type of supernova that's kind of a theoretical, a very theoretically motivated supernova, but to my knowledge, we have not observed a supernova light curve that we might say, look, this is a parent instability supernova. These explosions are meant to be very, very energetic. And so it could be that they produce a particular class of supernovae called superluminous supernova. And so they're, as the name suggests, way more luminous than your standard type 1, type 2 supernova. Okay, cool. I feel a little bit less bad now. Yeah. And the latest kind of thing on that is people look for signatures of nucleosynthesis in these parent stability supernovae in old stars to kind of show that they might have occurred.

So it's something that's not been observed yet, but from the theory side has a strong motivation that should exist. And is it the general idea that when these stars explode and populate the region around them with heavier elements, were those heavier elements already there having been created in the center of the star and they're just being mixed into the environment, or are they actually being created in the explosion? Yeah, so they are being created in the explosion, as well as there's some material that, some of the elements were already created during the life of the star. So in a supernova, what comes out is a mixture of what we would call

the hydrostatic nucleosynthesis. So nucleosynthesis that happened when the star was happily fusing hydrogen to helium to carbon to oxygen. and then explosive nuclear synthesis. So really elements that were produced during the explosion itself by, say, explosively burning oxygen to silicon. Burning being here used in a very loose sense. In the nuclear, yes. Nuclear fusion sense, right.

Okay. Okay, so that was a very helpful to me discussion of the parent stability supernovae, but they're not the most common ones, like you said. So what are the kinds of supernovae? explosions that are really doing the heavy lifting metal-wise? Right. So the core collapse supernovae are one of the big ones, and especially early in the history

of the universe, core collapse supernovae are exclusive deaths of stars that are more than 10-ish times the mass of our sun. And so these stars are able to fuse hydrogen to helium to carbon to oxygen, all the way to making an iron core. And as you touched on before, because of the binding energy of iron, it's no longer possible to fuse iron to generate energy and keep the core of the star stable.

Right. And so then once the iron core grows to a certain mass, the core begins to collapse. And there's a whole sequence of, a complicated sequence of events that occurs for this collapse to turn into, an explosion of the star. And so those are core collapse supernovae.

They're also, so if people are familiar with the observational categories of type 1 and type 2, type 1, B and C as well as type 2 supernovae, our core collapsed supernovae. Oh, that's very messy. So we'll get to them later, but you got to fill in what type 1A is. Oh, yes. So then the type 1A's are basically very interesting. explosions of white dwarfs. I'm saying this very carefully because one of the things that we don't know for sure is the progenitor systems of type 1A supernovae. So is it a, it's some sort of an

explosion of a white dwarf, but what is, is it a chandr shaker mass white dwarf or is it a subchandr shaker mass white dwarf? There's some sort of binary. here where the white dwarf is either merging with another white dwarf to create this thermonuclear explosion. That's really the distinction. It's a thermonuclear

explosion. Or is it some sort of main sequence star? That's the companion of the white dwarf that it's accreting material from and then that's triggering a runaway thermonuclear reaction of the white dwarf. So the progenitor

system of these has been sort of an open question, and many people have many ideas on how exactly the stars blow up, and what exactly is the binary that we're looking at. And among other reasons, the reason why we need to understand this is because these type 1A supernovae were the first evidence for dark energy. That's right. The trying to constrain the expansion rate of the universe relies on, is it the Phillips relation, I believe? of how you can standardize the light curves of type 1a supernovae.

Yeah, I think this is, since I was in astronomy undergraduate and graduate student, I've seen this up close, but it's kind of miraculous to me that we can say so many true things about these systems in the universe without truly knowing what they are or what's going on, right? I know, it's a bit unnerving, but it's also amazing. Okay, so I think we'll get back to the type 1A supernova, the white dwarf's exploding. But let's say more about the core collapse supernova. I'm going to guess roughly that this is what most people have in the back of the minds when they think of the supernova. It's sort of used up all its fuel, it collapse, and then it bounces.

And then what happens? There's a lot of nuclear reactions going on. It's a very intricate thing. I mean, this is full employment for people like you. Yeah, yeah. Lots of nuclear reactions going on, a lot of nuclear physics going on, even with particles like neutrinos. And so it is truly messy, but that's kind of the fun of it, too. So the general sort of schematic picture is that the iron core begins to collapse. However, it does not

collapse indefinitely. So at some point, you hit very, very high densities, nuclear densities in the core as the core collapse proceeds. And around this point, the repulsive part of the strong nuclear force kicks in. And it sort of tries to stop the collapse. And what happens as a result is the portion of the core begins to turn into a neutron star, and it finds a new stable configuration. And so it sort of goes back into this sort of stable configuration that results in a bounce shock, what is what you would call, and that bounce shock begins to go through the rest of the iron core and kind of goes out and out.

And it's breaking up the iron that's already been created during the life of the star into smaller nuclei as it does so. So it's going through the core, but it's also losing energy as it does so. And so sort of it goes out a certain distance into the star and it begins to stall. And this has been for decades the core collapse supernova explosion mechanism problem is how do you get the shock that was launched as a result of the collapse to actually continue going through the rest of the star and blow up the star? Okay. So this stalled shock issue. and the answer that a lot of people are looking at and it's sort of working now in multidimensional simulations

of core collapse supernovae is you have to rely on neutrinos heating the material behind the shot. And yes, so as this neutron star is forming, it's so hot and a lot of reactions are happening and you have a flux of these subatomic particles called neutrinos coming out of the neutron star. and they're carrying away a lot of the energy of the collapse. And so if you can guess even a little bit of the neutrino energy to deposit,

so for these neutrinos to interact with the material that is behind the supernova shock and deposit their energy there, you can reenergize the shock and get the explosion to occur. And so this is a neutrino-driven mechanism of core collapse supernovae that most people agree is sort of working. Do you ever feel like you're drinking from a firehouse? Paycor's intelligent HR solution empowers leaders to turn down the pressure. Their unified platform includes payroll, talent management, compliance software, and a lot more, connecting you to the people, data, and expertise you need to drive long-term business results.

Visit paycourt.com slash leaders and go from work flood to workflow. That's paycore.com slash leaders. And it is kind of fascinating because we are told, if we hang out on the wrong street corners, that, you know, the sun is emitting neutrinos and they're passing through our bodies all the time. But they just interact so weakly that they're irrelevant. Right. But this is an example of where they're super relevant, just because the energies are so high, the densies are so high. Neutrinos doing a lot of the work.

Yeah, that's amazing. And this is actually the thing that every. everybody asks me the first time I say the neutrinos interact because, you know, the fun fact that we all grow up with is neutrinos are passing through your hand and trillions of them pass through you and you just never know because they so rarely interact. But here is the sort of extreme of nature where the densities and the energies and the temperatures make it so neutrino matter interactions become very important. And we saw the neutrinos from Supernova 1987A back in the day. That's right. Were you sort of in the field then? I was an undergraduate when that happened. Oh my gosh.

How amazing. No, but it was very exciting. Yeah. Yeah. We're not around yet. Wow. And we've been waiting ever since for anything like it. But yeah, whatever, I don't know, two dozen neutrinos that were detected across.

It's a small number of neutrinos. The number of neutrinos is much smaller than the number of papers written about those neutrinos. Let's put it that way. But like you say, we're preparing. We have a whole network ready if another supernova goes off in our galaxy or nearby. So that would be very exciting. So do these neutrinos help us with making heavier elements? Do they play a role there?

They do play a role. So making heavier elements, once you begin to get to maybe the, upper iron group of elements. Things are a little bit fuzzier. And once you get beyond zinc, they're even fuzzier in terms of exactly what's going on in a core collapse supernova. So for the longest time, people thought that you can make, maybe you can make everything up to zinc and even beyond. So you're gallium and germanium and gold and platinum in a core collapse supernova. But as we've studied these more and simulated them more and actually tried to do the neutrino physically, correctly, it does not seem as though they produce any substantial amounts of the very heavy

elements. So core collapse supernovae may be able to produce maybe elements a little bit beyond zinc, but not the two-two heavy elements. And neutrinos matter here because neutrinos, as they get absorbed and emitted, they change the neutron to proton-to-proton ratio. off the material. And the neutron to proton ratio is something that's really critical in terms of making heavier elements because beyond a point to make heavy elements, you have to capture neutrons. When you have too many protons in the nucleus, it's no longer favorable to be capturing another proton on top of that. So neutron capture produces a lot of the truly heavy elements. And whether you have enough neutrons to capture or not, that becomes the question.

Okay. So it sounds like there's a lot of uncertainties still in where sufficiently heavy elements come from. I mean, just so people know what's going on here, iron is 26 protons, if I'm remembering correctly. So, and that's where, that's like the lowest energy that protons and neutrons can settle into. So anything heavier than that, you had to do some work. to make it. And you mentioned, I guess, zinc, which is just a little bit beyond iron. Yes, that's right. So if you have some amount of neutron rich material, it's possible to make zinc. But it's just a lot of balancing a lot of different reactions at the end of the day and the cross sections of those reactions.

So you can get a little bit, a little bit beyond iron, but to truly go much higher. Because we do go much higher. Like lead is like 82 protons or something like that. And that comes from the universe, right? Yeah, zinc is 30. So that's where I'm stopping. But even for zinc, there's a lot of uncertainty in terms of where it came from, all of the zinc in the universe.

So should I think of then like these iron and iron adjacent elements as mostly coming from Core Collapse supernova? Actually, mostly coming from type 1A supernova. Oh, okay. All right. Let's talk about their role then, maybe. It's a combination of these two that really produces most of the iron group elements. It seems like a weird coincidence. I shouldn't presume.

Both kinds of supernovae, the Core Collapse Supernovae and the Type 1A supernovae are playing a role in making iron and heavier than iron elements. In making iron, and heavier than iron is mostly than it really depends. The balance really depends. Once you get to nickel and zinc, there's a lot of uncertainty there. Some people think that they have to be made through maybe neutron capture process, like the S process. What is that? Okay, okay. So for making heavy elements, there's two types of neutron capture process.

that people think about. One is the slow neutron capture process, the S process, and the other is the rapid neutron capture process, the R process. And it is what it kind of sounds like. Basically, when you capture a neutron, you make an unstable isotope, and then you can undergo a beta decay, and then you end up with basically something that's one proton, heavier, a nucleus that's one proton heavier. So if you capture one neutron and you undergo a beta decay, you've made a new element. That's plus one. So beta decay is the neutron is decaying into a proton. The nucleus that has captured a neutron is decaying into a nucleus with plus one proton. Fair enough. And so then it's, then it's the question of how quickly can you capture neutrons versus how quickly is the beta decay happen. So if you're very slow, capturing neutrons, you kind of stick close to the stable elements and sort of have a very nice little ladder that you're climbing very close to stable elements, and that's the slow neutron

capture. And this is believed to happen in sort of lower mass stars in the envelopes of like AGB stars or something like that. So slow neutron capture can happen. What is an AGB star? an asymptotic giant branch star. So basically, a lowish math star can still make heavy elements. It's just a completely different process that's happening, the slow neutron capture process. The rapid neutron capture process, the R process, that's where you capture a whole bunch of neutrons and you beta decay back to truly heavy elements.

So you move very, very far away from the state. elements in terms of making isotopes that have just been engorged almost. Their nuclei have been engorged with neutrons, and then they're going to undergo beta decay back to stability to make heavy elements. And this, our process is where, so this is part of my research also, so core collapse supernovae and our process in neutron star mergers or neutron star black hole mergers. So there you do need something that's a. bit more extreme, you need a very high number of neutrons to enable the art process, basically.

Can you just give us a quick feeling then how much of the heavy elements we see around us actually didn't come from explosions at all, but just from like really very persistent grunt work on the part of these low-mass stars? Yeah, so it goes about half and half. in terms of the heavy elements. But it differs from element to element as well. So there are elements that you can't actually access by doing the S process. And so there are process only. And there's also something called the P process, the proton capture process.

This is the thing, you know, people think that this is the broad stroke of the periodic table. Okay, it's solved. You know, what else is there to figure out? No one is going to come away from this, thinking that. But then the other thing that you mentioned that I didn't want to let go by is that I don't think everyone is familiar with. It's not just exploding stars. It's not either core collapse supernovae or white dwarfs creeding from their backgrounds and exploding. There's this important whole other phenomenon of coalescence of stars, whether it's two neutron stars,

neutron stars black holes and maybe again, I'm just thinking people's intuitive picture of the universe, maybe people don't appreciate how often that happens. Yeah, yeah, the rates of these events are another big uncertainty that people are trying to get at through these gravitational wave detection. So we're narrowing it down slowly. But yes, how much heavy elements, how much of heavy elements are produced in one neutron star merger, combined with the rates of these events, actually gets you to present abundances of heavy elements. Yeah. So what exactly happens in the process of two neutron stars merging? Yeah. So the neutron star binary has formed is the point where I will start, because there's a lot to understand in terms of binary style, stellar evolution as well.

But the binary exists and the stars are sort of losing some of their, losing energy through gravitational wave radiation and spiraling towards each other. And so at some point, and this will take eons to be evade, at some point the stars will get so close to each other that they will begin to rip each other apart. and neutron stars are going to start tidally disrupting each other and merge together. And so as this happens, some of the material will get thrown out of the system in mainly the equatorial region through tidal forces. there's also depending upon depending upon kind of the way these stars shear against each other there's going to be additional physical processes

going on there's maybe shock heated ejecta in the polar region and so there's many ways as these stars merge together that they will throw material out into space and once they merge together what happens is also this post-merger evolution is a big open, uncertain area that people are studying. And so depending upon the masses of the neutron stars, you might get a remnant that's so massive that it will collapse to form a black hole. And so if you have a black hole, it could just be a black hole or it could be a black hole with an accretion disk of material around this.

and that accretion disk also has a lot of important physics going on with respect to magnetic fields and turbulence and just viscous heating. And so that accretion disk also ejects material and it can unbind over a certain amount of time. Part of it gets accreted by the black hole. Part of it gets ejected into the space. And so long short, the neutron stars merged together. They form either a black hole, a black hole within a cremate. disk or they form a third possibility where depending upon the masses of the neutron stars and the way angular momentum is being transported, you can end up with a temporarily stable neutron star.

And so short-lived neutron star that has its own set of processes of mass ejection and eventually collapses to form a black hole with a disc around it. So there's a lot of channels through which the binary might go after merging together. For nuclear synthesis purposes, this whole time, whatever is going on, material is being ejected from the system. And all of this material has different types of properties conducive to making different types of elements. And again, I shouldn't say again in this case, are neutron stars 100% neutrons? Or do they have an atmosphere where there are protons as well? Yeah, so they're not 100% neutrons.

They're largely neutrons, but there's some structure to them as well. And I think it's questionable what percentage protons are there, but there's some. I guess a very naive question or worry that one might have that I can already see the answer to is, how do you make nuclei if all you have are neutrons? Don't you need protons too, but I guess the decaying takes care of that pretty quickly. Yeah, yeah, exactly. Yeah. And so, but I would guess, less naively, that if I start with just a bunch of neutrons, I'm going to be making light elements again.

I'm going to start from, you know, one or two neutrons at a time. So how do I work my way all the way up to, am I working my way all the way up to very heavy elements this way? You are, you are. So there's, I mean, this is going to be too convoluted to get into, but the basic idea is you'll form, you'll form elements up to ion. And those will serve as seed nuclei, where you can call this seed nuclei for neutron capture to make the inhibitor elements. And so that's what's going on. It's kind of, yes, you start out with some fraction of neutrons and protons, and then you, depending upon the temperature. and the densities and the interaction rates, you form some fraction of heavier elements,

and then there's still a lot of neutrons remaining, and then you can capture onto those, say, iron to make things like gold. Of course, like, it's not one-to-one, and you go through a bunch of intermediate nuclei. It's really, when you think about a nuclear synthesis, there's this whole set of two-body, three-body nuclear reactions that are possible. And depending upon the temperature and the density and the electron fraction and entropy, the rates of these reactions are different. And so you have to, but for neutron capture, you can just think about it in terms of maybe the number of seed nuclei that are produced and then the neutron-to-seed ratio.

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Visit paycourt.com slash leaders and go from work flood to workflow. That's paycourt.com slash leaders. So this is actually, again, I'm going to admit, it's super helpful. I've never quite understood this stuff. So as a cosmologist, I understand the Big Bang thing. And there you make your helium and then you dilute away so much that there's not really enough time to do anything else. Here, I guess you're saying that there's enough density and it's packed into enough place that you are able to get up to iron. And once you have iron in this super neutron-rich environment, you can just grow, right?

you can just like you say, grab a neutron, beta decay, grab another neutron, and climb up the ladder. Yeah, climb up the ladder, yeah. But that, okay, I've oversimplified it because I'm betting that, in fact, you have some incredibly intricate reaction network that you have to put on a computer. Yes, it's thousands and thousands of isotopes. And, you know, we can, to try to understand them, we sit and make these movies where you can see the flux. of one element churning into another element or the flux of the reaction rate basically from one element to another element

and kind of, and as you watch this movie, you'll see it just goes absolutely bonkers in the way things are interacting. Of course, there's like some reactions that are way more favorable than others, but there is a lot going on. So truly you need these thousands and thousands of isotope networks that you have to solve. Do you ever watch one of these movies and think that if the mass of the down quark had been

a little bit different, it would have been a very, very different story, and this is evidence for the universe being fine-tuned? Oh, my gosh. I don't know about the fine-tuning piece of it, but yes, if the nuclear properties were different, things might have looked completely different. And that, and, you know, who knows if we'd be here in this way, discuss it. But yeah, I mean, this I think about a lot, actually, because nuclear astrophysics is truly special in some ways,

or this is just my bias, too, that it's truly interdisciplinary. It's truly a lot of trying to figure out properties of nuclei and particles and exactly how those will translate. to the making of elements in this whole range of possible astrophysical conditions. And, you know, when I, when I'm doing a supernova simulation, yes, this is a very large-scale event, right? Like a star tens of ten times the mass of the sun or twenty times of the sun, but it's coming down to the neutrino matter interactions. And that's kind of a funny connection between the scales of these things,

about as well. There's a kind of intricacy about it. It kind of, you know, the line of stable nucleides doesn't go on forever. We don't have, you know, a nucleus that is 10,000 nucleons in it, but it goes up pretty far, and it seems like almost a little delicate if things were different, it wouldn't even go up that far. So I do, you know, think it's at least worth wondering about why the laws of physics allow for that kind of richness. Yeah, yeah, for sure. And speaking of this is just something, you know, I get a lot. People ask whether there are even heavier elements in space that we have not found on.

Yeah. Maybe. And I, my answer is usually like, this is a nuclear physics problem to some extent. You know, like it's not just that we're not looking hard enough. Whether an element can exist or not is a nuclear physics problem. And so when you get to truly, truly heavy elements, then you have to start to figure out. I mean, some people think there's maybe at 150 protons or something.

There's an island of stability where super heavy nuclei can exist. But if those are made somewhere in the universe, then it's the question of can we observe them? But I guess even if they did exist, you may have already given us a reason to expect that they're not that abundant, right? Because the elements you make, you have to get there step by step. And if there's a gap in between, then it is an island you can't get to. Yeah, exactly. So it's like a truly interdisciplinary kind of figure out the nuclear physics,

figure out the environments in which this nuclear physics has to be applied, and then figure out if we can even expect to tell what is going on just from, you know, the ways in which astronomers gather information through photons, through gravitational waves, through neutrinos, and abundances, yeah. Okay, so I don't want to forget to ask because you mentioned neutron star and neutron star mergers. Those are obviously very important, but also neutron star black hole mergers. And again, just the super naive thing is, why doesn't the neutron star fall into the black hole? It's not all naive because that is what happens most of the time.

Okay, good. So it's, that one's a bit more kind of, we have not really observed a nuclear synthesis signature from those, I would say. It's kind of an idea where it's possible to do nuclear synthesis, and the way that would work is the black hole and the neutron star mass would have to be sort of close-ish to each other. So the mass of the black hole can't be so large that it swallows up the neutron star hole. And then it also depends on if the black hole is spinning and all of these things together and the equation of state of the neutron star. And so sort of the mass ratio of the black hole neutron star binary,

the equation of state of the neutron star, which is the mass radius relationship of the neutron star, as well as the spin of the black hole come together to see whether a black hole can disrupt a neutron star as it's approaching for merger or not. So part of the neutron star will get eaten, but part of it may get tidily disrupted and settle in an accretion disk around the black horn. And there you can make heavier elements maybe?

Yes, and there you can, over time, eject part of the accretion desk and make heavier. So there's a lot of ways. to make heavier elements. And I guess the last one that I know of that we haven't mentioned yet is through cosmic rays. Cosmic rays, yes. So I usually think of cosmic rays

as producing lithium, beryllium, and boron. Okay. That's very possible. Yeah. So I mean, I shouldn't, I said heavier elements, but I meant heavier than hydrogen. Oh, heaviness hydrogen, yes, yes. Okay, so some of our elements are made by spolation, I guess.

Yeah, but not the super heavy ones, not the buns heavier than iron. Yeah, yeah, that's right. So I guess to kind of recap our discussion so far, it would be hydrogen, helium, and a tiny bit of lithium in the Big Bang. And then the rest of the lithium, there are helium and boron in cosmic rays, spolation processes.

So this is just where cosmic ray protons mostly are breaking apart nuclei of carbon and oxygen in the intercourse. stellar medium. And so then there's still questions. So, okay, lithium, maybe some of it comes from a particular type of eruption called novi and not really. So there's always questions when it comes to specific elements where they come from. So, okay, we've got our Big Bang, we've got our cosmic respiration, and then stars. So now you have carbon and oxygen and nitrogen and everything, maybe even up to silicon and up to iron. And then in the supernovae really, most of the star's core,

even if it has made iron, turns into a neutron star or it gets once again broken up. So the iron that comes out in the supernova is not really, the iron that was made during life of the star. So explosive nucleosynthesis, burning off silicon to iron happens, burning of oxygen to silicon, these kind of things. you have the iron group of elements. So now you have done with Big Bang, Cosmic rays, stars, and

supernovae, maybe everything up to iron. And then the question is, okay, what now? And there, this S process in low mass stars does part of the creation of elements heavier than iron and then our process. So these neutrons from mergers and neutrons for black hole mergers, maybe other kinds of supernovae, it's really not super, super solved as a problem. So these types of events create the rest, and it would be remiss not to mention that we have been trying to create humans, have been trying to create elements too. And so some of the elements are basically human-made elements like Berkillium where I'm sitting right now in Berkeley was created here. And okay, I guess the other thing I wanted to help our listeners with is a feeling of scale.

I mean, both in terms of a supernova explosion where a lot of these are made, I guess. Let's include, sorry, is it correct to include these neutron star merger events in the world of supernovae or are they separate things? They are kind of their own thing, but I think it's fine to include them in the sense that it's sort of a dynamic event. Okay. Big explosions. Yeah. So these big explosions I would tend to think of as being located somewhere in the galaxy or someplace like that.

How efficient are they at spreading all these heavier elements across a galaxy? Yeah, that's another sort of area of study almost. So there are relationships that people have tried to derive, depending on how much energy is output in a supernova and how much of the material it can mix in. So the supernova ejecta will mix in. And so there are relationships. So if you have a very energetic, this might be intuitive,

if you have a super energetic explosion, you can just mix the stuff that you've ejected way farther out. than if you have a lower energy explosion. And so that kind of is this field of galactic chemical evolution where people try to figure out if a supernova goes off, you know, if 10 supernova go off and all these parts of this galaxy, what does a galaxy look like down the line? And I guess this is also should be obvious,

but maybe there are some details that are interesting. Here on Earth, it seems like we have a lot of iron. We even have things like copper and nickel and things like that. So did these mostly come from exploding stars one or the other? Yes, yes, that's right. And so then once you start thinking about the Earth itself, it gets a bit messy in terms of now you have to think about, you know, the planets, formation, and so on.

When I think about the composition of the solar system or when I say a composition of the solar system and trying to understand that, that really means composition of the sun. The planets don't matter, basically. Yeah, because all of the mass in the solar system is mainly sitting in the sun. And so for the Earth, then the geological processes and things like that begin to matter as well. Okay. Like, in terms of deciding in which layer of the Earth, various elements, not in terms of producing new elements.

Right. Yeah. And, well, we can wrap up with, I do want to mention again. I did mention the intro, but I'll mention again your TikTok series where you're explaining some of this in a completely different medium. But one of the fun things is you not only explain the nuclear astrophysics and the R process as process stuff, but every element kind of tells a story, right? And you certainly given yourself the task of having fun things to say. about strontium and boron and things like that. Maybe you can just share with us a couple of fun things about your favorite elements.

Oh, wow. Yes. So I kicked off this series, which is called Stardust, and I'm posting it on TikTok and also on YouTube with Strontium. And part of the motivation was because there was in 2017 a detection of a neutron-strand merger and the light from a neutron-strand merger. And in that light, one of the elements that astronomers have confidently detected, again, there's arguments, of course, around every single thing in astronomy, but is the element strontium. And so I was thinking about strontium that year a lot, and it started popping up in places that you wouldn't usually as an astronomer think about. So, for example, there's marine organisms that make their skeletons out of strontium sulfe.

Okay, I did not know that. Strontium isotopes can tell you what region a wine came from. So there's varying levels of strontium isotopes in the soil in different parts of the earth. and when plants grow or an animal like a mammoth eats, those strontium isotope ratios get encoded in the grapes, for example. When you make wine out of those grapes, there's the strontium. So it's a lot of fields that you wouldn't even think about use these isotopes. So wine fraud can be prevented by looking at strontium isotope ratios in your wine.

But I would say my favorite one is how you can figure out the region in which of woolly mammoth lip by looking at the tusk of the mammoth and the layers in the tusk and the strontium isotopes ratios encoded in those layers. So mammoth tusks grow in layers like tree rings almost. And so every layer encodes the ratio of isotopes from the region in which the mammoth was living and eating. What is it specifically about strontium that makes that so useful rather than some other element? Oh, yes. So strontium and I believe rubidium, there's a particular relationship in the way these elements decay. So there's two

stable isotopes. I'll have to look at the details because I don't remember off the top of my head, but there's some sort of a relationship between the stable isotopes of strontium and the way the decay of other isotopes forms those. That makes it a good tracer of time and region and things like that. Okay, so something that just the decay rates and things like that.

So I guess the final question then is that this leads on from that. You already mentioned how interdisciplinary the field is. Like if there's a young person here who wants to become a nuclear astrophysicist, what is included in the list of things you have to know? I mean, it sounds like there's some particle physics, there's astrophysics, there's, I mean, nuclear physics pretty obviously, and maybe some other things I'm not even thinking of.

You know, I'll tell you what first drew me into nuclear astrophysics, and it was listening to a talk where somebody was describing their supernova simulations. And basically, it turns out that to correctly understand a supernova explosion, and this is true of very many things, but especially in this case, it becomes very obvious, you need to understand basically all of the types of physics that exist. So you need general relativity. You need nuclear physics. So you actually need to understand things like quantum as well.

And then, of course, it's radiation and radiation transport, this kind of thing. And so electromagnetism as well. So it kind of just brings everything together into this very complex, messy system. And now you're trying to extract information out of it. about how stars live and die, as well as how dense matter works, conditions that we're not able to access here on Earth. Do you have a specific research goal that you're hoping comes true over the next number of years? Like, is there a burning question you most want to answer?

Yes, there's two. So one of the things I'm really trying to understand are the transient, that come from neutron stram mergers. So what I mean by that is there's light that is produced when a neutron star merger makes heavy elements. Those elements decay and the radioactive decay of those elements powers a light, a signal basically, and it's called the kilonova signal, similar to a supernova just for the merger case. and to truly understand how Kilanovi work,

you really have to get a handle on every single way a merger could produce material, the properties of that material, and then how photons travel through that material. And so this very last piece of doing the multidimensional radiative transfer calculation, sorry for all the jargony words, but that is what I'm trying to do. And the goal is to really figure out, are neutron surmerger is the only source of our process elements? Is this truly the only size where rapid neutron capture happens

or needs to happen to explain the abundances of these heavy elements in the universe? My guess would be probably no, but it would be good to understand. And then sort of from in the, opposite direction from the core collapse supernova side, I want to try to understand the properties of the first stars, for example. So it's very difficult to make connections from a star that's maybe 20 solar masses all the way to what kind of elements, exactly what amounts of different elements it produced and the type of light curve that you should see. We don't even know if it should necessarily explode or not.

So this is one of the big open questions. It's like which stars explode and which don't. And so hopefully by understanding the chemical signatures of explosions, I can try to work towards understanding the properties of the stars that exploded themselves. Well, that should keep you busy for a while. I like that. I won't keep you here any longer with all that on that. the plate. So Sanjana Kurnas, thanks so much for being on the Bineskate podcast. Thank you.

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