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The Edge Episode 23: Cosmology with Sarafina Nance

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Show Notes

Episode 23: Exploding stars and other cosmic mysteries with Sarafina Nance 

Roughly every second, a star explodes. Beyond treating astronomers to a radiant light display, these dramatic supernova events contain vast amounts of information about the origin, behavior, and ultimate demise of our universe. Berkeley astrophysicist Sarafina El-Badry Nance, has dedicated her life to studying really big exploding stars and what they tell us about our ever-expanding universe. She joins us this episode to talk about her own path to star-gazing and the big, existential questions that keep her eyes to the sky.

Further reading: 

This episode was written and hosted by Leah Worthington and produced by Coby McDonald. 

Special thanks to Pat Joseph, Margie Cullen, and Sarafina Nance. Art by Michiko Toki and original music by Mogli Maureal.


LEAH: Fizzle, collapse or explode. For a star meeting its end, those are pretty much the options. Stars have been dying for BILLIONS of years, of course, since way before humans were around to notice. But eventually humans showed up and we did notice. And as humans do, we started trying to make sense of these celestial happenings. 

Allow me to take you back to the year 185 AD. In the western world, the Roman Empire is at its peak, while in the east, the Han Dynasty is just a few decades from collapse. But as power-hungry emperors scope out new land to capture, some scholars are setting their sights further…up, to be precise. 

Greek astronomer and mathematician Claudius Ptolemy has published a groundbreaking text on the positions of stars and planets, including his theory of a geocentric universe—with Earth at the center of everything. Meanwhile, astronomers in China are preoccupied with something else… the sudden appearance of a big shimmering orb, or what seems to be a whole new star in the sky.

For eight months, the “guest star” sparkles overheard, unmoving, before ultimately fading out. It was, modern astronomers believe, one of—if not the—first recorded supernova. To put it simply, the observers had documented the months-long explosive death of a massive star, thousands of light years away.

“The size was half a bamboo mat,” they wrote. “It displayed various colors, both pleasing and otherwise. It gradually lessened.”

Nearly two millennia later, our understanding of supernovae has, well, skyrocketed. Not only do we now know what that guest star was, and that it was much much larger than half a bamboo mat, but we even have photographic evidence of it. In images captured by several space telescopes, and published just last year, you can see a blue and red cloud swirling on a backdrop of distant stars—or the gaseous remains of that history-making supernova.

But these astronomical events make for more than just pretty pictures and epic stargazing. They also have a lot to tell us about our universe—how it behaves, what it looks like, and how it might eventually come to an end. 

And, as it turns out, these supernovae happen a lot. 

SARAFINA: “Yeah, so stars explode every second in the universe. So somewhere in the universe of stars exploding right now. “

LEAH: That’s astrophysicist and science communicator Sarafina El-Badry Nance. Sarafina can’t remember a time when she wasn’t in awe of the cosmos. From her childhood in Texas to, now, her high-tech lab at the Berkeley Astronomy Department, she has dedicated her life to studying exploding stars much like the one first spotted by the Chinese astronomers 1,800 years ago.


LEAH: This is The Edge, produced by California magazine and the Cal Alumni Association. I’m your host, Leah Worthington.

In today’s episode we’re joined by Berkeley astronomer and science communicator Sarafina Nance to discuss the nature, and ultimate demise, of our universe.


LEAH: Sarafina Nance has had an unlikely and storied path. She’s an  Egyptian-American woman in a male-dominated field, the recipient of a preventative double mastectomy, and an outspoken advocate for women’s health. Her memoir, Starstruck, was published on June 6.

She joins us today for a truly mind-blowing conversation—like, really, truly—about really big stars, really big explosions, and really, really big questions about life and the universe. 

LEAH: What is it like to be able to introduce yourself as an astrophysicist?

SARAFINA: It’s pretty cool. I’ve always wanted to do this as my career. I definitely feel a sense of pride in being able to say that. Yeah, it’s pretty cool.

LEAH: I never actually studied astrophysics—I studied biology, and now I’m a journalist—but I always thought if I did something, if I could do anything, I think I would be an astrophysicist.

SARAFINA: Really? Wow. That’s so cool.

LEAH: Yeah, just to hear you say, ‘I am an astrophysicist’ is like, ‘Wow, someone gets to say that. And a woman gets to say that.’

SARAFINA: Yes, yes. We love that. Although we also love women in journalism and biology. So that’s also very cool.

LEAH: We love women everywhere.

SARAFINA: Yeah, yeah, we love women, just period.

LEAH: So, I’m curious if you remember your first memory of really seeing the night sky.

SARAFINA: I remember stargazing with my dad a lot as a kid. We lived in the Texas Hill Country, just outside of Austin. And yeah, we would take our pair of binoculars and go look at the moon and stars, and I just loved it from like, the age of five, I think, and just really couldn’t get enough.

LEAH: What did you love about it? People look up all the time, and they’re like, ‘Oh, it’s pretty,’ but something about it got you…more.

SARAFINA: Even at that young age, I struggled with anxiety. And for me something about the night sky was a reprieve from that very physical sensation of feeling overwhelmed. And I think even at that age, I was like, ‘Oh, my God, there’s so much out there that we don’t understand.’ And the vastness of it all was really exciting to me. And it’s also really pretty. 

LEAH: I remember sitting in my best friend’s kitchen as a kid, and we were trying to figure out how big the universe was. And her dad was a math professor, and he said, ‘The universe is infinite.’ And we were like, ‘That doesn’t make any sense!’ You know, what does that mean? There has to be an end, everything has an end. And I was like, ‘Okay, well, if it ends somewhere, then what’s on the outside of the end? Like there has to be something else!’ And we would just sit there.

SARAFINA: Totally. Yeah, it’s mind blowing. I think what’s neat is that those questions that you and your friend were asking, like, that’s what people’s entire careers are devoted to, is understanding what it means to live in a flat universe. Like, what does that mean? How big is it? Does the universe have edges? You know, those are sort of like, the questions that you can ask when you’re five years old, or you can ask when you’re, you know, 80 years old. And, you know, I think what’s neat is that curiosity just can really never leave you.

LEAH: Can you tell me a little bit about what you’ve dedicated your life to trying to figure out or answer?

SARAFINA: Yeah, so I focus on exploding stars or supernovae. And I’m interested in how they form and also what they can tell us about the universe in general. So I use exploding stars to, currently right now, try to figure out how fast the universe is expanding. And this is a really interesting question. It’s a cosmological question, meaning, you know, it is about the universe at large, which, you know, cosmology covers the origin of the universe, the fate of the universe, sort of these big questions that we’re talking about. We find, as astronomers, tools in the universe to help us solve these questions. And for me, supernovae are sort of the tool that I use to explore those questions.

LEAH: We know so much about the universe, which is incredible to me, because it’s so far away. But why don’t we know how fast the universe is expanding, and what can supernovae tell us about it?

SARAFINA: So there’s this notion in astronomy that the farther out you look, the further back in time you’re seeing because light takes time to travel to us. And so if we look far enough away, we’re seeing the early universe. And the early universe measurements predict a value for the expansion rate of the universe that is different than what we actually measure in our local universe. And those predictions assume what we call the standard model of physics. So it’s sort of you know, what you and I know to be “truth of physics.” And we don’t know why we’re getting these two different numbers based on the prediction of the standard model and what we’re actually observing. So, I am trying to use a sort of independent method, a different type of supernova than what is traditionally used, to try to resolve that tension.

LEAH: What’s different about the supernova that you’re looking at?

SARAFINA: So there are two types of supernovae, generally. There’s a lot of different types, but they can be broadly classified into these two types, which are: Binary stars—so two stars that explode as a supernova. So typically, you know, one can accrete matter onto the other one, making it massive enough to explode. Or, you know, they can sort of run into each other and explode. There’s a lot of different variations. And then the second type is a massive star that explodes at the end of its life.

LEAH: That’s a supergiant?

SARAFINA: Yeah, so it becomes a red supergiant, which is sort of a mass threshold. And the stars that we’re talking about are like 15 to 20 times the mass of our sun. That’s sort of a typical red supergiant mass. And yeah, they sort of run out of fuel at the end of their lives and explode. And these can be calibrated in a particular way to basically measure the expansion rate.

LEAH: So historically, we’ve looked at binary supernovae, and you’re looking at supergiant—red supergiants.

SARAFINA: Yeah, so they’re called core collapse supernovae. That’s what I look at. And, so binary supernovae all explode with the same luminosity, meaning we can determine its distance based on how bright it appears. Core collapse supernovae don’t all explode with the same luminosity. And so we, as observational astronomers, have to try to calibrate their luminosities to then figure out their distance, and from their distance, measure an expansion rate.  

LEAH: Ok, let’s pause for a minute. So, here’s what we know: Exploding stars, or supernovae, are a really important tool for measuring the expansion of the universe. Without getting too much in the weeds, the basic idea is that stars release a certain amount of light when they explode, and we can use that luminosity to estimate how far away they are. So, that’s how we know that the universe is expanding in the first place: We noticed that the light from distant supernovae is much fainter than what we’d expect in a static universe. So we figured out that we can use the positions of these supernovae as reference points for measuring really big—like cosmically big—distances. In fact, Berkeley’s own Saul Perlmutter was part of a team of astrophysicists to receive the 2011 Nobel Prize in physics for groundbreaking work on this exact subject. By looking at dozens of these supernovae, they figured out that, not only is the universe expanding, it’s expanding at an ever-accelerating rate.

So getting back to Sarafina. As she mentioned, there are, generally speaking, two different types of supernovae, one of which researchers, like Perlmutter, have been studying for years. But she’s looking at the other kind, called core collapse supernovae, for several reasons. One is simply that these supernovae are more common, so she has a much bigger sample size to work with. And, from a physics standpoint, the hydrogen-rich atmospheres of core collapse supernovae are just a lot easier to understand. In short: more data, simpler math, and, most importantly, a new model for studying an old question.

SARAFINA: Right now I am using massive amounts of data to try to find, you know, hundreds of supernovae, and then use that large sample size to determine the rate of the expansion of the universe. 

LEAH: We don’t always know when a star is going to become a supernova, right?

SARAFINA: Kind of! So I have historically worked on my favorite red supergiant, which is Betelgeuse. And that was a really interesting case in trying to determine when it’s going to explode, because it has not exploded yet. We know it will explode because of its observables—so its mass, its color, its size. Part of my research was trying to constrain about when, so we found that it won’t explode for another 100,000 years or so. But the uncertainty on that number is like, that’s what we’re trying to, like, make smaller. Is it an uncertainty of 500 years? Is it an uncertainty of 5,000 years? And then when you start to apply it to something that we could see in our night sky—which we could see if and when Betelgeuse explodes—that’s when it starts to be like, man, well, the human lifetime is 100 years, so we probably won’t make it.

LEAH: We’re not going to see it…even with a big margin of error.

SARAFINA: Right, the odds aren’t good. But hey, maybe my models are totally wrong, and it’ll explode tomorrow. We don’t know.

LEAH: What would that look like?

SARAFINA: It’ll be as bright as the moon, and it’ll be visible for a couple of months. 

LEAH: Wow. 

SARAFINA: Yeah, it’ll be sort of like a second moon in our sky that’s visible day and night, which is—I would love to see.

LEAH: Is it possible that we’ll see another supernova of, if not that big, similar size and visibility in our lifetime?

SARAFINA: Yeah, so stars explode every second in the universe. So somewhere in the universe of stars exploding right now. In our galaxy, a star explodes every 50, 100 years. So a star will explode that we will be able to observe. But the question is like, will it have, will it be close enough, like Betelgeuse, to explode, and we’ll be able to see it that well with the naked eye? And that’s less likely.

LEAH: What is the, kind of…I don’t know how to ask this in a way that sounds not—I’m just going to ask it. But you know, that I don’t mean this with any judgment. What is the point of figuring out what the rate of expansion of the universe is?

SARAFINA: That’s not a bad question. I get that question a lot. And my answer might be different than other astronomers’. And, for me, I think there’s sort of this overlap with this existential question of like, where did the universe come from? How’s it ending? Are we alone in the universe? You know, sort of these fundamental questions about where we are and how we got here and where we’re going. And I think that those are questions that people have been asking since the dawn of time, and our tools of answering those questions have just evolved.

LEAH: So, suppose tomorrow, you woke up and your data was immaculate, and all of a sudden, we knew the rate of—the definitive rate of expansion of the universe, what would that be able to tell us? What would we then know more about the universe broadly from that?

SARAFINA: Well, you know, I think our standard model of physics is tied up in this question. So, do we really know what the composition of the universe is? Are dark energy and dark matter real? Is there a modified theory of gravity that we need? You know, there are these, like, bigger questions about how we model the universe that are tied up in this question, in this question and this, like, investigation.

LEAH: So, what is dark energy? And does it exist?

SARAFINA: If you ever interview someone who has an answer to that question, send them my way because I would like to—I would like to know. Um, yeah, I mean, we don’t know. We see its effects, and we see that dark energy basically propels the expansion of the universe. So it’s sort of this, like, push on spacetime that causes the universe to expand faster and faster with time. It’s intrinsic to the fabric of the universe, meaning there’s more and more dark energy the more the universe expands. But we don’t know a lot more than that in a way that’s like communicable, I think. And same with dark matter. I mean, we have theories of it being, you know, particular types of particles. But it’s this…extra matter in the universe that helps explain some of the things that we measure and adds to the composition of the universe, but we don’t yet know what it is. And we can’t see either of these things. They’re dark, meaning they don’t interact with light, they just are there.

LEAH: Dark energy-wise, there’s some stuff, force, energy that is pushing out the universe, and causing it to expand out at an accelerating rate. 

SARAFINA: That’s right. 

LEAH: And we can’t see it. And we only know it exists, because something has to basically account for the calculations, the math that we’re seeing?

SARAFINA: Pretty much yeah. Things like you and me and stars and books and everything we see, know, feel, touch, taste, all of that is ordinary matter. And that’s 5 percent of the universe. The rest is dark matter and dark energy.

LEAH: It’s impossible to try to imagine something that is beyond, that is truly beyond our imagination. But then how do you go about trying to describe it, or measure it, or understand it, if it’s outside of our realm of reality?

SARAFINA: Yeah. I mean, that’s like, one of the things that I think is so fascinating about astronomy: We have to measure things that we can’t touch, we can’t really see a lot of the time. We just have to be clever about the ways that we go about it, and then use physics to do that.

LEAH: Yeah, I mean, I think a lot about how we discovered the shape of helical DNA, right? It wasn’t it wasn’t about zooming in on a picture of a cell and, and looking at what it looked like. It was about shining light at it and looking at, you know, a shadow. And just the thinking outside of the box required to say, ‘Okay, we just can’t look at this thing directly. It’s not possible.’ So we’ve got to, you know, interact with it with some tools, poke it, shine light at it, and see if there is any kind of, anything we can extrapolate from that. And then trust. Trust that what we’ve seen is real.

SARAFINA: I think there’s a stereotype that scientists aren’t creative or there’s not a lot of room for, like, unique thought, I guess, in science or math. And I actually think we’re, I don’t know, there are some really incredibly creative and like, out-of-the-box thinkers in the scientific field. And I mean, that’s where, again, you come up with this really cool conclusion, but it’s not just one and done. The scientific method relies upon many different people doing that experiment over and over again, or doing slightly different versions and seeing what comes out of it. 

LEAH: We talked about supergiants, we talked about supernovae, we talked about dark energy, we talked about dark matter… What does all of this have to do with the fate of the universe?

SARAFINA: So the composition of the universe, meaning all the things that you just sort of mentioned, determine the shape of the universe. So is the universe flat? Is it round? Is it concave? What is that shape? But in tandem with that, will dark energy cause the universe to continue to expand, bigger, faster, whatever, with time until the universe sort of rips apart at the seams? Or does dark energy decrease with time? Does dark energy change with time? And therefore does gravity take over and cause everything to collapse into this, like, sort of inverse Big Bang type idea? So the behavior of some of these things that compose the universe impact the fate of the universe.

LEAH: Are there leading theories? I mean, you just named a couple of different options for the fate of the universe. Are scientists as a whole leaning one way or another? Or is it really completely unknown?

SARAFINA: Well, so, right now, we think that the universe is flat.


SARAFINA: And that the universe will continue to expand with time because we think that dark energy is intrinsic to spacetime. It’s like a factor of the universe, and so it’ll just kind of continue to expand forever. The space between galaxies will be so far apart that everything will die, and the universe will be plunged into darkness. 

LEAH: Just like a slow dimming of the lights?

SARAFINA: Yeah, basically.

LEAH: So, ok, I didn’t know that we think that the universe is flat, so my brain is processing that.

SARAFINA: Yeah, no, it’s very hard to picture. 

LEAH: I’m imagining like… My immediate thought was just like, like when you pour pancake batter onto a griddle, and it just, like, spreads out. And I’m just imagining that continuing on forever until there’s just basically, all the little particles of batter are so far apart that you have no pancake.

SARAFINA: Yeah. I mean that’s, I think, a really good way to picture it. I try to think about the flatness of the universe in terms of… So triangles—that’s basically how it’s explained in like a math-y way. So like, I think most people know or think that a triangle’s angles add up to 180 degrees. However, triangles don’t always add up to 180 degrees. So if you drew a triangle on the earth, because the Earth is round, a sphere, your triangle would be more than 180 degrees. And basically, a flat universe is a universe in which the triangles that are drawn are 180 degrees. But if we lived in a spherical universe, they would be more than that. If we lived in a concave universe, they would be less than that. 

LEAH: Just a quick interruption to say, if your brain hurts trying to understand what exactly these triangles have to do with the shape of the universe, you’re not alone. After watching quite a few YouTube videos, I’ve come to the conclusion that it’s a pretty esoteric mathematical concept with no easy explanation. But the basic idea is that our measurements of temperature and microwave variations in the universe indicate that the universe is expanding out in every direction without curving up or down, with only a 0.4% margin of error. In other words, although we can’t say for sure just yet, it’s probably flat. That said, take Sarafina’s advice and try not to overthink it. 

LEAH: Given what we know about the composition of the universe right now, would it be very surprising to you to learn that it wasn’t flat, or that it was going—that the end of the universe would be different from just like a slow spreading out?

SARAFINA: Our understanding of the composition of the universe implies something about the fate of the universe. If that understanding were to change, then our result would change. But right now, there wouldn’t be any evidence as to the fate of the universe changing unless one of these other things changed.

LEAH: I see. Something major, like…

SARAFINA: Like modified gravity, for example. Or some, you know, like, dark energy that changes with time. Or, you know, like, our amount of dark matter is actually double what we expected. Something like that—that’s like the standard cosmological model, the standard model of physics that we use when we model our universe is dramatically different.

LEAH: Okay, so, to my question that I had as a little kid, I’ve always wanted to ask an astrophysicist: If the world, if the universe is a discrete thing, or you know, a flat—I can’t think of a better word than thing—what’s outside of it? Does it have edges? Does it end?

SARAFINA: That’s a great question. The universe… There is an observable universe and an unobservable universe. So the observable universe has an edge—it is the edge beyond which light has not had time to travel to us. But does the unobservable universe have an edge? I don’t know. I don’t think anybody knows. But when we talk about the universe, in a meaningful way, meaning we’re talking about the composition, the fate, all of these things about it, we’re really talking about the observable universe. Because we can’t speak to this thing beyond which we can’t see.

LEAH: It just, like, twists my brain. You know, like, in my conception of the existence of things, every thing has an end. Therefore, if that’s a thing, it has to have an end. But my conception of things doesn’t necessarily have any bearing on the unobservable, far reaches of the universe.

SARAFINA: You get really comfortable as a physicist with, like, saying what you do understand and then also what you don’t understand and not like making… I don’t know, being okay with not being okay with it. It’s like this, like, existential, just sort of acknowledgment of like, well, I can understand up until this point, and then beyond that, like, my assumptions break down, the models break down. Like we… yeah, you have to be okay with your mind kind of being blown.

LEAH: What is exciting to you in the field of cosmology and your work right now, like recent discoveries, or just, you know, a new cool machine that’s changing the game?

SARAFINA: JWST is huge. 

LEAH: JWST is the James Webb Space Telescope?  

SARAFINA: Yes. Yeah. Yeah.

LEAH: What have we learned recently, that is, like, from JWST that we didn’t know before, that is game-changing, or enlightening?

SARAFINA: It’s big, and it sees in the infrared, so it can pierce, like, basically parts of the universe that we haven’t seen yet. And it can go back in time far enough to be basically imaging the earliest galaxies that were formed. And there’s this question of whether these early galaxies had been forming earlier or later than we expected. I’m interested in sort of the early supernovae. Can we see the first stars? And can we see sort of when they exploded? And those would be the biggest stars we’ve ever seen. But it’s early. I mean, it’s only… Data has only been public for what, nine months? So we’re in the very earliest stages, I think, of JWST.

LEAH: Do you think now is an exciting time, a particularly exciting, hot, innovative time for our understanding of the universe?

SARAFINA: When you say time, do you mean, like, where we are in time with respect to the universe, so being like, looking at our local universe, which is 13.7 billion years after the Big Bang? Or do you mean like, time in terms of when we’re researching stuff? Like, what does time mean?

LEAH: I’ve never had to explain to someone what I mean by time.

SARAFINA: It’s like a Friday afternoon. My brain is kind of fried. 

LEAH: No, I was gonna say, your astrophysics colors are showing in full force. 

SARAFINA: Mortifying.

LEAH: I meant like right now in our academic pursuit of the study of the universe.

SARAFINA: Yeah, I think it’s a really exciting time. I think we’re constantly on the cusp of discovery. Like, we didn’t expect the discovery of dark energy to come from the data. I mean, it was a surprising thing. I think that’s like the enthralling part of discovery. You sign up for this, like, it’s not there until it’s there. And then all of a sudden, it’s there. And it’s really cool.

LEAH: I wanted to ask you just a little bit about yourself. I can’t imagine that being a woman, being a woman of color in astrophysics is necessarily easy. I can’t imagine being an astrophysicist is easy, in general. I’m curious if there was ever a time when you almost gave up, and you know, what’s kept you going?

SARAFINA: There were a lot of times when I felt like I couldn’t continue. I wasn’t good enough. I wasn’t smart enough. I didn’t belong. For all the reasons that you kind of listed. And also, I wasn’t a person who was inherently good at physics or math. I think that’s always been something that was really challenging for me, and kind of what kept me going was this, like, deep existential need to understand the universe and to, like, bear witness to these really like mind-blowing things that we’re talking about. I wanted to be in this space, no pun intended, but, I mean, there was a lot of struggle in that. I’m glad you asked me about that because that’s why I wrote the book. That’s why I wrote Starstruck. I think it’s really important to share kind of the challenges and the heartbreak of what it’s like to carve out a place for oneself in a field that is not, has not been historically for someone like you or I.

LEAH: It really is surprisingly rare to hear from someone saying, ‘Yeah, like, I didn’t just walk out of the womb writing physics equations, but that doesn’t mean that I can’t contribute something valuable to this, because I care about it.

SARAFINA: I think there are people who feel this way, the same way that I feel. But I think it’s a scary thing to talk about. I think it’s a scary thing to admit. And I also think, you know, there’s social conditioning and stereotypes that we all have about what being a scientist is and what that looks like. And I think a lot of me carving out of place for myself in science has been restructuring that idea and replacing who I am now and, you know, the woman I want to be and that picture with the picture of what I thought a scientist looks like, which is this, you know, unsurprisingly, white dude who is inherently good at physics, and can just do all of this. 

LEAH: How do you feel about your memoir coming out soon and your story being launched out into the universe?

SARAFINA: I’m definitely really nervous about how it will be received. I’m very honest in it. I think, you know, as you said, I’m sort of this public figure online and have done, I don’t know, some things have sort of put me in front of people. But still people see what they want to see. I think there’s a flattening of oneself when they are on social media or even on TV or whatever it is. And my book is sort of, I think, an exercise in building that picture out into 3D and really trying to be as authentically myself as possible, and I don’t know how people are going to receive that. I hope well. But I don’t know. I think it’s just kind of trusting in the process. And I hope that this book will resonate with the people that it needs to resonate with, you know, the people who need it.

LEAH: People who maybe are trying to carve out their own space.

SARAFINA: That is my hope.

LEAH: It has been such a pleasure to talk to you.

SARAFINA: You too. Thank you for the great questions. I feel so energized now. 

LEAH: Oh, I’m glad. Energized on a Friday afternoon is unheard of.

SARAFINA: I know, I know. This is all due to you. Thank you. 


LEAH: This is The Edge, brought to you by California magazine and the Cal Alumni Association. I’m Leah Worthington. This episode was produced by Coby McDonald, with support from Pat Joseph and Margie Cullen. Special thanks to Sarafina Nance. Original music by Mogli Maureal.


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