261 | Sanjana Curtis on the Origins of the Elements

0:00:00.1 Sean Carroll: Hello everyone. Welcome to the Mindscape 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.

0:00:54.1 Sean: Whereas the individual researchers, they're gonna 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 am 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 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.

0:01:52.6 Sean: 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 grounded, 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, [laughter], 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 UC Berkeley, and she's a nuclear astrophysicist.

0:02:51.8 Sean: 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 archeology, 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 contexts. And I have to say, as someone who was an undergraduate and graduate student in astronomy, nuclear astrophysics is super cool.

0:03:51.2 Sean: 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. [chuckle] You might think if you were thinking like a physicist, that there was a mechanism that made all the elements, 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 supernova and more that all are important for explaining where the different elements in the periodic 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.

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0:04:58.9 Sean: Sanjana Curtis, welcome to the Mindscape Podcast.

0:05:00.6 Sanjana Curtis: Thank you. I'm really excited to be here, Sean.

0:05:03.4 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...

0:05:17.0 Sanjana: That's my cat cinnamon. She has to make an appearance at these things. Whenever I can talk and she's here.

0:05:20.8 Sean: My cats are in the back on the floor. 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 how should we conceptualize it? How should we look at it and get things out of it?

0:05:57.2 Sanjana: Right. So, the periodic table, all the 118 I believe, elements, the origin of those elements is what we are 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 life, 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 supernova explosions. And then you go even heavier, and there's even less of those elements, generally speaking, in the universe. And things like gold are precious elements come from an entirely different type of nucleosynthesis 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.

0:07:20.4 Sean: 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. The periodic table is organized on the basis of the chemical structure, and you're not into the whole chemical thing.

0:07:41.7 Sanjana: 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 where you have metals and non-metals and noble gases 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.

0:08:16.4 Sean: Exactly. Right. And there's a long-standing joke among non-astronomers how astronomers count elements by going hydrogen, helium metals. [laughter] But you don't get to do that, right? Anything heavier than helium is a metal for most astronomers.

0:08:32.7 Sanjana: Yes. That's exactly correct. That's just how we think of metallicity, which is a quantity, it's just the amount of metals. And it's just anything that's not hydrogen or helium is a metal to me. I'm sure chemists are not a fan of this kind of nomenclature, and I completely sympathize.

0:08:51.1 Sean: 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, etcetera, matter to the evolution of the universe in terms of stars, galaxies, blowing up things?

0:09:12.7 Sanjana: Oh, they matter quite a lot. 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 inject 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.

0:10:04.9 Sean: 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 in 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.

0:10:33.7 Sanjana: 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.

0:10:57.1 Sean: Okay. Well, let's do exactly 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?

0:11:10.4 Sanjana: 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 quarks 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 the Big Bang. 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, teensy bit of lithium.

0:12:05.2 Sanjana: 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 out of that. And then that's the portion, that's where my active research area is, is basically stars and supernovae and the aftermath of those. Yeah.

0:12:24.0 Sean: 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.

0:12:42.1 Sanjana: That's right. Yeah. So the binding energy really wants everything to be iron-56. However, the Big Bang nucleosynthesis, okay, the temperatures and the densities that exist during Big Bang nucleosynthesis 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. It's very difficult for Big Bang nucleosynthesis to produce lithium, beryllium and boron. 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. 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.

0:13:51.2 Sean: I think I remember that George Gamow and his friends, when they were inventing Big Bang 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.

0:14:06.5 Sanjana: Exactly. Yeah. Yeah. 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 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.

0:14:44.6 Sean: 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.

0:14:58.4 Sanjana: No, no. Yeah. 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.

0:15:16.0 Sean: You're stuck coming out of the Big Bang with a bunch of hydrogen and helium.

0:15:19.3 Sanjana: Yes.

0:15:20.8 Sean: And then I guess you make stars. 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. How does that even happen? Do we know how you make stars in the first generation, where it's just hydrogen and helium?

0:15:50.9 Sanjana: That's a really interesting topic, but it is similar to what you said, where we don't really have a clear answer yet. For example, something that you might call the initial mass function of the first stars, meaning if you were to bend these stars by mass, how many stars have 10 times the mass of the Sun, or how many of them have 100 times 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 papers, is if you do these simulations, and yes, metals are important for cooling and the way things evolve, there are different predictions for what should happen.

0:16:36.3 Sanjana: 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. It's really not very clear what the masses of these stars are going to be when you form the first generation of stars, nor have we observed a star that's a population three star.

0:17:14.9 Sean: Right. Astronomers count stars backwards, so the population three is the first population chronologically, right?

0:17:23.5 Sanjana: That's right. Yes. Just for fun.

0:17:23.8 Sean: And they come from us. [laughter] It makes sense. They count from the present day, they count backwards. I know that some people like Katie Freese have suggested that dark matter actually plays a big role in forming that first generation of stars. I have no idea whether that's plausible or not.

0:17:39.7 Sanjana: I guess it would have to do with the way the halos form, and then the galaxies are born in those halos. And I'm sure there's an interplay, but I am not sure how that goes.

0:17:54.1 Sean: 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, after...

0:18:06.3 Sanjana: Yes. The dark ages, and then there's this epoch of reionization. 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.

0:18:18.8 Sean: A long time after the first minute when we did Big Bang nucleosynthesis. There's a long stretch when there was just hydrogen.

0:18:21.8 Sanjana: Yeah. There's a long stretch of the dark ages...

0:18:23.4 Sean: And helium.

0:18:25.4 Sanjana: Yes, and then stars form, yeah.

0:18:27.5 Sean: And then, good, now we can make the heavier elements, right? 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. And there's some miscellaneous ones made by even cooler mechanisms, but it's stars doing most of the work for anything heavier than helium.

0:18:45.0 Sanjana: That's right. Stars and even stellar corpses, so neutron stars, are doing the rest of the work.

0:18:54.1 Sean: Okay, so what's the first thing that happens in the newly Deliver Us universe that spurts heavier elements out there into the cosmos?

0:19:03.9 Sanjana: The first thing that happens, one could argue it's these pair-instability supernova.

0:19:09.9 Sean: So what does that mean, a pair-instability supernova?

0:19:12.5 Sanjana: 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 stability is encountered, the core of the star against basically lose all support and it will trigger nuclear reactions as it collapses and the whole star blows.

0:20:04.7 Sean: This is kind of fascinating. I'm gonna confess right here to all the millions or hundreds of millions of Mindscape listeners, I did not really know about this kind of supernovas. 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.

0:20:25.5 Sanjana: Yes.

0:20:25.5 Sean: And it gets so intense that the photons start bumping into each other and making electron positron pairs.

0:20:32.4 Sanjana: That's right.

0:20:33.0 Sean: Which lowers the pressure. And the star collapses.

0:20:35.5 Sanjana: 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.

0:20:47.9 Sean: So the pair instability has nothing to do with the stars splitting in two.

0:20:52.3 Sanjana: No.

0:20:52.5 Sean: It's gamma rays. It's photons making pairs. Yeah.

0:20:56.5 Sanjana: Yeah.

0:20:57.2 Sean: And is this, we've heard of things like type one, type two supernovae and things like that. Is this one of those?

0:21:03.9 Sanjana: This is a separate category.

0:21:05.4 Sean: Separate category.

0:21:06.2 Sanjana: 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 stability supernova. These explosions are meant to be very, very energetic. And so it could be that they produce a particular class of supernovae called super luminous supernovae and so they're, as the name suggests, way more luminous than your standard type one, type two supernova.

0:21:43.3 Sean: Okay, cool. I feel a little bit less bad now.

0:21:46.3 Sanjana: 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.

0:22:07.6 Sean: And is 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?

0:22:30.3 Sanjana: 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 fussing hydrogen to helium to carbon to oxygen, and then explosive nucleosynthesis. So really elements that were produced during the explosion itself by say, explosively burning oxygen to silicon.

0:23:07.7 Sean: Burning being here used in a very loose sense.

0:23:10.6 Sanjana: In the nuclear, yes.

0:23:11.8 Sean: Nuclear fusion sense.

0:23:12.8 Sanjana: Nuclear reaction sense. Yes.

0:23:14.3 Sean: 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 supernova explosions that are really doing the heavy lifting metal wise?

0:23:29.2 Sanjana: Right. So the core collapse supernova are one of the big ones. And especially early in the history of the universe, core collapse supernova are exclusive deaths of stars that are more than ten-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.

0:24:09.9 Sean: Right.

0:24:10.9 Sanjana: 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 I and type II, type 1b and c as well as type II supernovae are core collapse supernova.

0:24:45.4 Sean: Oh, that's very messy. So just, we'll get to them later. But you gotta fill in what type 1a is.

0:24:48.5 Sanjana: Oh, yes. So then the type 1as are basically very interesting explosions of white dwarfs. I'm saying this very carefully...

0:25:00.7 Sanjana: Yep. [laughter]

0:25:00.8 Sanjana: Because one of the things that we don't know for sure is the progenitor systems of Type 1a supernovae. So it's some sort of an explosion of a white dwarf, but is it a Chandrasekhar mass white dwarf, or is it a sub Chandrashekhar 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, is the thermonuclear explosion. And then 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.

0:26:04.7 Sean: 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.

0:26:13.7 Sanjana: That's right. The trying to constrain the expansion rate of the universe relies on, is it the Phillips relation, I believe?

0:26:22.6 Sean: Yes, that's right.

0:26:23.7 Sanjana: Of how you can standardize the light curve, so Type 1a supernovae.

0:26:27.8 Sean: Yeah, I think this is, since I was an 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?

[laughter]

0:26:43.2 Sanjana: I know, it's a bit unnerving, but it's also amazing. [laughter]

0:26:47.1 Sean: Okay. So I think we'll get back to the Type 1a supernova, the white dwarfs exploding. But let's say more about the core-collapse supernova, I think, I'm gonna guess roughly that this is what most people have in the back of their minds when they think of a supernova. It's sort of...

0:27:02.0 Sanjana: That's right.

0:27:02.5 Sean: 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...

0:27:11.0 Sanjana: There's a lot of nuclear reactions going on.

0:27:13.0 Sean: Intricate thing.

0:27:13.6 Sanjana: Yeah.

0:27:13.8 Sean: This is full employment for people like you.

0:27:15.6 Sanjana: 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. [laughter] 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.

0:28:15.1 Sanjana: 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 now 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 schlock that was launched as a result of the collapse to actually continue going through the rest of the star and blow up the star?

0:29:10.9 Sean: Okay.

0:29:11.5 Sanjana: 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 shock. [laughter] 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 re-energize 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.

0:30:13.8 Sean: And it is kind of fascinating because we are told, if we hang out on the wrong street corners, that, 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.

0:30:27.4 Sanjana: Right.

0:30:28.7 Sean: But this is an example of where they're super relevant just because the energies are so high, the densities are so high. Neutrinos doing a lot of the work.

0:30:37.5 Sanjana: Yeah, that's amazing, and this is actually the thing that everybody asks me the first time I say the neutrinos interact because the fun fact that we all grew 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.

0:31:10.2 Sean: And we saw the neutrinos from Supernova 1987A back in the day.

0:31:15.7 Sanjana: That's right. Were you sort of in the field then? Was that as far...

0:31:20.8 Sean: I was a undergraduate when that happened.

0:31:23.5 Sanjana: Oh my gosh, how amazing.

0:31:24.2 Sean: I was not really in the field, no, but it was very exciting, yeah.

0:31:26.9 Sanjana: Yeah.

0:31:27.2 Sean: You were not around yet.

0:31:28.5 Sanjana: Yeah, wow.

0:31:29.0 Sean: But yeah. [laughter]

0:31:30.1 Sanjana: 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...

0:31:38.2 Sean: 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. [laughter] But we, 'cause like you say, we're preparing, we have a whole network ready if another supernova goes off...

0:31:50.3 Sanjana: Yes.

0:31:50.4 Sean: In our galaxy or nearby.

0:31:53.0 Sanjana: Yeah, yeah.

0:31:53.5 Sean: So that'll be very exciting. So do these neutrinos help us with making heavier elements? Do they play a role there?

0:32:00.8 Sanjana: 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 your gallium and aluminum 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 physics 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 up a little bit beyond zinc, but not the too, too heavy elements.

0:32:56.6 Sanjana: And neutrinos mattered here because neutrinos due to, as they get absorbed and emitted, they change the neutron to proton ratio of 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 neutron, 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.

0:33:39.6 Sean: Okay. So it sounds like there's a lot of uncertainties still in where sufficiently heavy elements come from. 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.

0:34:04.9 Sanjana: Yeah.

0:34:05.2 Sean: And you mentioned I guess, zinc, which is a little bit beyond iron.

0:34:08.8 Sanjana: Yes, that's right. So, if you have some amount of neutron rich material, it's possible to make zinc. But it's just, it's 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, little bit beyond iron, but to truly go much higher, you need to...

0:34:34.4 Sean: 'Cause we do go much higher, like lead is like 82 protons or something like that.

0:34:38.9 Sanjana: Yes.

0:34:39.0 Sean: And that comes from the universe, right?

0:34:40.5 Sanjana: Yes, zinc is 30, so... [laughter]

0:34:43.1 Sean: Yeah.

0:34:43.9 Sanjana: 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.

0:34:52.6 Sean: But so, should I think of then like these iron and iron adjacent elements as mostly coming from core-collapse supernova?

0:35:02.0 Sanjana: Actually mostly coming from Type 1a supernova.

0:35:04.6 Sean: Oh, okay. All right, well, let's talk about their role then maybe.

0:35:10.2 Sanjana: It's a combination of these two that really produces most of the iron group elements.

0:35:17.0 Sean: 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.

0:35:30.2 Sanjana: In making iron and heavier than iron is mostly then. 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 a neutron capture process, like the s-process.

0:35:52.8 Sean: What is that?

0:35:53.2 Sanjana: Or... Okay, okay. So for making heavy elements, there's two types of neutron capture processes that people think about. One is the slow neutron capture process or the s-process. And the other is the rapid neutron capture process, the r-process. And it is what it kind of sounds like. [laughter] 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 new undergo a beta decay, you've made a new element, that's plus one.

0:36:36.7 Sean: So beta decay is the neutron is decaying into a proton.

0:36:39.9 Sanjana: The nucleus that has captured a neutron is decaying into...

0:36:43.0 Sean: Good.

0:36:43.1 Sanjana: A nucleus with plus one proton.

0:36:46.5 Sean: Fair enough.

0:36:47.0 Sanjana: And so then it's the question of how quickly can you capture neutrons versus how quickly is the beta decay... So if you're very slowly 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...

0:37:25.1 Sean: What is an AGB star?

0:37:27.1 Sanjana: An asymptotic giant branch star. So basically, a low-ish mass star can still make heavy elements, it's just a completely different process that's happening, this slow neutron capture process. The rapid neutron capture process, the r-process starts 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 stable elements in terms of making isotopes that have just... Have 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 r-process is where... So this is part of my research also. So core-collapse supernovae and r-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 r-process, basically.

0:38:35.5 Sean: 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?

0:38:53.1 Sanjana: 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 they are r-process only. And there's also something called the p-process.

[laughter]

0:39:15.6 Sanjana: The proton capture process. This is the thing, people think that this is the broad stroke of the periodic table. Okay, it's solved, do you know what else is there to figure out? This is not true at all.

0:39:26.0 Sean: No one is gonna come away from this thinking that, but then the other thing that you mentioned that I didn't wanna let go by is that I don't think everyone is familiar with, it's not just exploding stars. It's not either core-collapsed supernovae or white dwarfs accreting from their backgrounds and exploding. There's this important whole other phenomenon of coalescence of stars.

0:39:54.0 Sanjana: Yes.

0:39:54.1 Sean: Whether it's a neutron, two neutron stars, the neutron stars black holes. And maybe again, I'm just thinking of people's intuitive picture of the universe. Maybe people don't appreciate how often that happens.

0:40:06.3 Sanjana: 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 are narrowing it down slowly, but yes. How much of heavy elements are produced in one neutron star merger combined with the rates of these events actually gets you to present day abundances of heavy elements. Yeah.

0:40:35.2 Sean: So what exactly happens in the process of two neutron stars merging?

0:40:39.9 Sanjana: Yeah. So, the neutron star binary has swarmed is the point where I will start because there's a lot to understand in terms of binary 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 [0:41:11.3] ____. At some point the stars will get so close to each other that they will begin to rip each other apart. So neutron stars are going to start tightly 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 kind of the way these stars shear against each other, there's going to be additional physical processes going on.

0:41:52.3 Sanjana: 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 disc of material around it.

0:42:37.6 Sanjana: 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 story short, the neutron stars merged together. They form either a black hole with an accretion 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.

0:43:25.7 Sanjana: 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 disk 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.

0:43:55.6 Sean: And again, the... 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?

0:44:05.0 Sanjana: Yeah, so they're not 100% neutrons. They're largely neutrons, but there's some structure to them as well. And I think they're... I think it's questionable what percentage protons are there, but there's, so.

0:44:25.8 Sean: 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.

0:44:34.1 Sanjana: The decaying.

0:44:34.4 Sean: Pretty quickly.

0:44:34.8 Sanjana: Yeah, yeah, exactly.

[laughter]

0:44:36.8 Sean: And so, but I would guess less naively that if I start with just a bunch of neutrons, I'm gonna be making light elements again. I'm gonna start from 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?

0:44:55.2 Sanjana: 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 elements up to iron, and those will serve as seed nuclei, what you can call the seed nuclei for neutron capture to make the heavier elements. And so that's what's going on is kind of, yes, you start out with some fraction of neutrons and protons, and then you, depending upon the temperatures 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 the, 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 the 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.

0:46:21.6 Sean: Okay. So this is actually, again, I'm gonna admit it's super helpful. I 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.

0:46:58.5 Sanjana: Yeah, climb up the ladder. Yeah.

0:47:00.4 Sean: But that, okay, I've oversimplified it because I'm betting that in fact, you have some incredibly intricate reaction at work that you have to put on a computer.

0:47:08.5 Sanjana: Yes. It's thousands and thousands of isotopes and 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.

0:47:52.9 Sean: 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've been a very, very different story, and this is evidence for the universe being fine tuned?

0:48:06.3 Sanjana: Oh, my gosh. Oh, 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 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 into the making of elements in this whole range of possible astrophysical conditions. And when I'm doing a supernova simulation, yes, this is a very large scale event, like a star 10 times the mass of the sun or 20 times the mass 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. Think about as well. Yeah.

0:49:19.8 Sean: Well, there's a kind of intricacy about it, a kind of the line of stable nucleons doesn't go on forever. We don't have 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 think it's at least worth wondering about why the laws of physics allow for that kind of richness.

0:49:45.7 Sanjana: Yeah, yeah, for sure. And speaking of this is just something I get a lot people ask whether there are even heavier elements in space that we have not found on earth.

0:50:00.8 Sean: Yeah. And...

0:50:01.8 Sanjana: Maybe. And my answer is usually like, this is a nuclear physics problem to some extent. It's not just that we are 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 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 and...

0:50:42.1 Sean: 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? 'Cause...

0:50:49.4 Sanjana: Yeah, exactly.

0:50:50.5 Sean: 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.

0:50:56.4 Sanjana: 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 the ways in which astronomers gather information through photons, through gravitational waves, through neutrinos, and abundances. Yeah.

0:51:22.3 Sean: Okay, so I don't want to forget to ask, because you mentioned neutron star 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?

0:51:41.9 Sanjana: It's not at all naive, 'cause that is what happens most of the time.

0:51:44.7 Sean: Okay, good.

0:51:46.2 Sanjana: So that one's a bit more kind of, we have not really observed nucleosynthesis signature from those, I would say. It's kind of an idea where it's possible to do nucleosynthesis, 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 whole. 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.

0:52:50.3 Sanjana: So part of the neutron star will get eaten, but part of it may get tidally disrupted and settle in an accretion disk around the black hole.

0:53:00.0 Sean: And there you can make heavier elements maybe?

0:53:02.6 Sanjana: Yes, and there you can, over time, eject part of the accretion disk and make heavier...

0:53:08.9 Sean: 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.

0:53:15.7 Sanjana: Cosmic rays, yes. So I usually think of cosmic rays as producing lithium, beryllium and boron.

0:53:23.2 Sean: Okay, that's very possible.

0:53:26.5 Sanjana: Yeah.

0:53:26.6 Sean: So, I mean, I shouldn't... I said heavier elements, but I meant heavier than hydrogen.

0:53:30.7 Sanjana: Heavier than hydrogen. Yes, yes. So this is just spallation.

0:53:30.8 Sean: Okay, so some of our elements are made by spallation, I guess. Yeah, but not the super heavy ones, not the ones heavier than iron.

0:53:39.4 Sanjana: 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, beryllium and boron in cosmic rays, spallation processes. So this is just where cosmic ray protons mostly are breaking apart nuclei of carbon and oxygen in the interstellar 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 ray spallation, 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 gets once again broken up.

0:54:50.2 Sanjana: So the iron that comes out in the supernovae is not really the iron that was made during the life of the star. So explosive nucleosynthesis, burning off silicon to iron happens, burning of oxygen to silicon, these kind of things. And so 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 it's 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 r-process. So these neutrons from mergers and neutrons from 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 berkelium where I'm sitting right now, in Berkeley, was created here.

0:56:04.2 Sean: And, okay, I guess the other thing I wanted to help our listeners with is a feeling of scale, 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?

0:56:27.1 Sanjana: 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 that's making new elements.

0:56:38.8 Sean: Okay, big explosions.

0:56:39.8 Sanjana: Yeah, yeah.

0:56:40.3 Sean: 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?

0:56:52.1 Sanjana: 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 supernovae 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, if 10 supernovae go off in all these parts of this galaxy, what does a galaxy look like down the line?

0:57:46.7 Sean: And I guess this also should be obvious, but maybe there's 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 way or the other?

0:58:04.6 Sanjana: 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 the planet's formation and so on. When I think about the composition of the solar system, or when I say composition of the solar system and trying to understand that, that really means composition of the sun.

[laughter]

0:58:30.9 Sean: The planets don't matter, basically.

0:58:32.0 Sanjana: Yeah. 'Cause all of the mass in the solar system is at least sitting in the sun. And so for the earth then the geological processes and things like that begin to matter as well.

0:58:44.1 Sean: Okay. And...

0:58:46.2 Sanjana: Like in terms of deciding in which layer of the earth, various elements sit, not in terms of producing new elements.

0:58:55.1 Sean: Right, right.

0:58:55.4 Sanjana: Yeah.

0:58:57.1 Sean: And, we can wrap up with, I do wanna mention, again, I did mention in 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, s-process stuff, but every element kind of tells a story. 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.

0:59:31.2 Sanjana: 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-star merger and the light from a neutron-star 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 sulphates. And...

1:00:31.1 Sean: Okay. I did not know that.

1:00:31.7 Sanjana: 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 live 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.

1:01:53.3 Sean: What is it specifically about strontium that makes that so useful rather than some other element?

1:01:57.9 Sanjana: Oh, yes. So strontium and I believe rubidium, there's a particular relationship in the way these elements decay. So there there's two stable isotopes. I'll have to look at the details 'cause I don't remember off the top of my head. But there's some sort of 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. Yeah, I have to look at the details. Yeah.

1:02:31.7 Sean: Okay, so something like just the decay rates and things like that. Yeah. So I guess the final question then is, this leads on from that, you already mentioned how interdisciplinary the field is. 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? It sounds like there's some particle physics, there's astrophysics, there's nuclear physics pretty obviously, and maybe some other things I'm not even thinking of.

1:03:00.3 Sanjana: I'll tell you what, first drew me into nuclear astrophysics, and it was listening to a talk where somebody was describing their supernova simulation. 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.

1:04:13.9 Sean: 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?

1:04:23.3 Sanjana: Yes, there's two. So one of the things I'm really trying to understand are the transients that come from neutron star mergers.

1:04:35.3 Sean: Transients meaning?

1:04:36.9 Sanjana: 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 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 kilonovae work, you really have to get a handle on every single way a merger could produce material, the properties of the, 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 jargon-y words.

1:05:27.7 Sean: It's good and I love it.

1:05:27.8 Sanjana: But that is what I'm trying to do. And the goal is to really figure out are neutron star mergers the only source of r-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...

1:05:53.9 Sean: We need to find out.

1:05:54.1 Sanjana: It would be good to understand. And then in the opposite direction, from the core-collapse supernova side, I wanna 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, is like, which stars explode and which don't.

1:06:38.1 Sean: Right.

1:06:38.6 Sanjana: 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.

1:06:52.1 Sean: Well, that should keep you busy for a while. I like that. I won't keep you here any longer with all that on the plate. So Sanjana Curtis, thanks so much for being on the Mindscape Podcast.

1:07:01.8 Sanjana: Thank you. This was really, really fun.

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