Why haven't we seen a Milky Way Supernova in 400 years?

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Speaker 1: Katie, if you want to learn more about some creature you're planning to feature, what's a good way to get started? Well, I use this obscure technology called Google, or I could find one and observe it. That's it. You just read about it or observe it. You don't like poke it or anything. I mean, in general, I think in the field of evolutionary biology, they discourage poking wild animals. Yes. Does that mean therefore that you also, like, never smashed two of them together at high speeds? Pretty strongly discouraged. Interesting? I mean, in my experience, that's a pretty good way to learn what something's made out of. I think that someone needs to find some advocacy organization or pro bono lawyers to represent the protons you guys have been smashing. Oh, we are facing the biggest class action lawsuit in history. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I am not prepared for a lawsuit from ten to the thirty protons. Hi. I am Katie Golden. I host the podcast Creature Feature, and I am boning up on my lawyerism to try to sue the pants and particles off of Daniel, how are you going to manage all of those clients? I mean, what if they disagree about how much money to ask for or you know, whether to send me to prison or something. I mean, tended the thirty clients is a lot to wrangle. I mean, I think I'll just send out a general mailer or do an ad on TV. That's like, if you're a particle, and you are a loved particle were involved in a smashing well, I hope that all the protons out there are not mad at us for all the times we've been smashing them together to try to learn about the nature of the universe. And Welcome to the podcast Daniel and Jorge Explain the Universe, a production of I Heart Radio in which we do encourage people to smash stuff together in order to learn about how the universe is put together. We try to ask and answer some of the deepest questions about the very nature of the universe, the fundamental fabric of reality. What in the end is the universe made out of our space and time, fundamental our particles, the basic building blocks of the universe, Or is it something even weirder, even deeper, even stranger than our little minds can suppose, and we don't just smash particles together. We look out into space to see other things that Mother Nature or Grandpa Universe has smashed together on our behalf, so that we can ask huge questions about how those things are put together. The moral of the story is smash stuff, learn a lot, but of course, watch out for people's rights and be careful about applying this to wild animals. My co host and friend Jorge can't be here today. He is traveling through Spain and Portugal, so we are delighted to have our regular co host, Katie Golden. Katie, thanks very much for joining us. Yeah, of course, I think it is a little bit suspicious that Jorge is missing when we are talking about smashing things together to learn more about them. But okay, and you know, astronomers are always delighted when stuff smashes into other stuff in outer space. They're not capable of building a collider like we can. You know, we can smash protons together to learn about what's inside of them, but astronomers can't build a black hole collider for example. Yes, they got dreams, they got plans, but sometimes they get lucky anyway, and the stuff just sort of happens out there in the universe for them to watch, and they don't have to worry about the black holes rights or it's legal fees or or whether he gets mad because they're just taking notes. And I wonder, Katie, if the same thing happens for you. I mean, sometimes I watch these nature shows about like giraffe battling by slamming their next together, or you know, big horn rams being in themselves together. You guys also learn a lot, don't you when animals smash themselves together. Yeah, I mean, to be fair, I am not currently doing research, but when I was a little undergrad, I was definitely involved in some of these research projects. And one of them was just going out and watching squirrels and taking notes, and it made us very popular among the school body, just sitting there being the squirrel people, watching the squirrels, writing down what the squirrels were doing. But yes, observing wildlife, observing animals is extremely instructive for revolutionary biologists. It can be very difficult because, of course, when you're observing things in nature, it is not a laboratory environment, so you know, eliminating the variables that are inevitable when you have sort of this chaotic natural environment can pose a kind of tricky thing for research, But yeah, it is. It is a hugely important side of evolutionary biology is just looking at what an animal is doing and hoping it doesn't notice you looking at it. All right, But I want to clear answer here. Do squirrels smash themselves together or not? They smash acorns into their little faces, but they don't smash acorns against each other's no, like squirrel acorn Collision research program, not exactly. They may smash it into the ground a little bit to get that buried in there, but they don't have a squirrel acorn hadron collider quite yet. Well, you know, there's just a limitation to how much you can learn by observing stuff. We were just looking at protons, for example, we never would have understood what was inside them and seeing this incredible dance of the corks and the gluons that are all tied together to make this crazy thing we call a proton. And so it's these dramatic events when protons smashed together or when larger objects smashed together. Squirrels black holes for example, a list nobody has ever made before in the history of time that consists only of squirrels and black holes seems pretty complete to me. What else could there be onside a list? But when these things do happen, you have an incredible opportunity to learn something about the nature of what's inside these objects. I mean, we know something about what's inside squirrels, but we have deep questions about what's at the heart of neutron stars, what's inside black holes? Hey, what's going on even inside normal stars? Stars like our sun have incredible convection zones of plasma slurping and slashing and making magnetic fields that flip every eleven years, we still don't even understand what's going on inside a normal star. To be fair, if you want to know what's going inside of a squirrel, you can't really do that just by watching the squirrel. Uh. It involves the process that I may not want to describe to people right now, but I imagine it is quite difficult to know in a similar way what is going on inside something like a star when you can only look at it and not dissect it on a table. Yeah, and even star collisions are pretty rare, But astronomers are lucky that there's another way to see inside of stars. You don't have to smash them together. Sometimes they just blow up on their own. They erupt and deposit all of their innerds now on their outers, so that we can study them and see what was inside that star, just like squirrels, just like got to one too many acorns, right exactly, I'm stuffed. And when stars do go boom, it's an incredible opportunity to learn about the processes that are going on inside the star, why they suddenly got imbalanced. Plus it's a pretty dramatic light show. Now you say that, but I have never gotten to see a star explode, and I would really like to. So where can I sign up? Well, one problem is that we don't even really understand why it happens and when it happens. We don't have a scientific model that can predict when a given star is going to go supernova. So all we can do is scan the night sky. And you know, to me, that's fascinating because the night sky seems pretty stable. If you like looking at the stars, you probably are looking at the same stars that looked pretty much the same as your parents did, and your grandparents and their ancestors did. The night sky does not change very much, but when it does, when that happens, when a supernova goes boom, it's very dramatic. These things can be as bright as an entire galaxy, so it's a pretty exciting event when it does happen. But you know, you're right, it's not something that we've been able to see very recently, and even though supernovas are expected to be rare, something of a mystery about why we haven't seen more supernovas. In fact, it's been over four hundred years since we have seen a supernova in our galaxy. It seems strange to me because even if it's rare, it seems like there's so much stuff in the galaxy that you would at you know, you are increasing your odds. Like when they say an infinite number of monkeys on an infinite number of typewriters. It's like an infinite maybe not infinite, but many many exploding monkeys who once in a while explode. You would think you'd catch a monkey explode. I'm so sorry to everybody out there who just had the mental image of an exploding monkey put into their head. I hope that's a positive addition to your day. Whatever else is going on, which we don't condone. We do not condone it. We simply describe it exactly scientifically. We're just observing super monkey nova's. You know, it's not our fault. We're just you know, doing our best to learn something from it. So that's exactly what we're gonna be talking about today. The title of today's AT episode is why haven't we seen a Milky Way supernova in more than four hundred years? Have we just not looked up? Like, has anyone you know, just kind of checked it out? You mean, have we been so self involved for the last four hundred years that we haven't noticed incredible explosions in our sky? Right? It's these kids with their noses and their smartphones all the time. They just don't notice them. Well, I think that the last person to see a supernova in the Milky Way was Kepler, himself, famous man of astronomy, of course, and I'm pretty sure that he didn't have a smartphone, and that in most of the intervening four hundred years, astronomers have not been distracted by their smartphones. So that's a good idea. But I think we're gonna have to look for other explanations for why we haven't seen any supernovas from the Milky Way in our sky in four hundred years. It's something of a cosmic mystery that we're going to dig into today, but before we do, we were wondering what our listeners thought about this question. People understand why there hasn't been a supernova in our galaxy that we've spotted in several hundred years. So thanks very much to everybody who volunteered to answer random questions. They heard this question without any chance to prepare, and we asked them to just speak from the top of their head, so we get a sense for what you, dear listener, are thinking about as you listen to this podcast episode. What you know, what you don't know? What the ideas are out there. If you'd like to hear your voice speculating on the podcast for a future episode, please write to us. We'd love to have you on the show. Just drop us an email to questions at Daniel and Jorge dot com. Here's what our listeners had to say. If I remember rightly, there are a couple hundred billion stars in the Milky Way, and supernovas are made from stars that last much shorter than ours. So if they were all eligible, there the right kind to be a super nova and being produced at a constant right throughout the life of the universe, we'd be seeing about one every two and a half years, if I've done my math right. Since we haven't seen one in at least four hundred years, which is about a hundred and sixty times as long as expected, my guess is that only about one in a hundred and sixty stars is the right kind to end up as a supernova. I've actually thought about this question a couple of times, because, like most people, I would love to see a supernova in broad daily. I keep asking beetle juice, but it doesn't listen. So I think that in individual galaxies they are just a rare event. Um, there's only so many stars in a galaxy, so I think they're pretty rare on a galaxy scale, but on a universal scale, because there's countless galaxies, they become common events. So that's my guess. Years is like a blink of an eye in the galactic time scales. So I think we just haven't seen a supernova in our Milky Way yet. We haven't seen a Milky Way supernova in more than four hundred years, because supernova are not just that frequent. Quite a bit of the Milky Way galaxy is hi didn't behind various gas clouds and the central budge, so we don't see what's happening on the other side of the lisk. But what is the rate of supernova occurrence? I have no idea. I think the answer to this is that there's probably a statistical anomally that if the Sun has been around for five billion years and there's like a billion stars in our galaxy or a couple of billion stars in the galaxy, and that not every star turns into a supernova, you might just seduce the statistics suggests that one might not have during this time, that one might happen in a reasonable period of time. But four under views is not a reasonable period of time in terms of the galaxy. This question reminds me of the question that I asked my grandmother when she was trying to tell me about God and angels and all that good stuff. And I would ask her, but Grandma, why can't we see angels? Why can't we see these things that you're talking about that used to be a long time ago? And she was telling me that the reason is that people are bad, not the people used to be good before then, and that's why we could see all that stuff. Now we are bad and they won't. We cannot see them anymore. They won't appear before our eyes. So with the milk, keep with supernova. Could say, also, it's a combination of dus distance and dumb luck that we cannot see them, but also might be because we are bad. I love the idea that we've just been too naughty to see a supernova. This is our punishment. It's like some kind of galactic council has like, oh, humans destroying your environment, naughty, naughty, no supernovas for you. I know, like we don't deserve a supernova. You know, we haven't earned it. It's for like the good aliens, not for us. Right, we got to clean our rooms and rainforests before we get a supernova. Yeah, exactly. I like the idea that the universe is judging us. But I think that the rest of the listener has really put their finger on sort of the spectrum of ideas here. You know, it's true that we haven't seen one in four hundred years, but is that unexpected? Do we think we should have seen one in four hundred years, because it's true that four years feels like a long time to us humans, but it's just a blink on cosmic iron scales or processes play out over millions and billions of years. So it's a fair question whether or not we should have expected to see one, right, because like if you're you know, if you live in England, you expect to see rain all the time, but if you live in Southern California you never see rain ever, So the different environments between Southern California and England, you will have kind of a different expectation for these phenomenon. So, you know, when we don't see stuff in the Milky Way, I guess I'm asking is the Milky Way like, should it be exploding all the time? Is it more of a southern California? What is the weather of the Milky Way? Well, you know, meteorologists are famously bad at predicting supernovas. You know, they say they're going to come on Tuesday afternoon and then boom Wednesday afternoon. When you make your picnic, that's when all the supernovas come. And I'm always dressed the wrong way. What is your supernova outfit look like Katie. It's very shiny, lots of lots of frills and shoulder pads. Of course, well, I think you put your finger on the question, and so we are going to talk about that today. How often we expect to see supernovas in our galaxy and then ideas for why we are not seeing as many as we expect. But first maybe we should talk about the basics, for example, like what is a supernova, how does it work, what do we know about it? And why is it even possible to see one from so far away? I mean from the name, it sounds like it's a nova, but really big and super they are in fact the nova and nova is a Latin word that means new, and so the name supernova actually comes from another famous man of astronomy, Tico bra Hey, who observed one in the fifteen hundreds, and he wrote a book about them, whose title is in Latin but includes the words nova and stella as in new star. And the word nova was later used to describe new things in the sky, including supernova super new things in the sky. That's really interesting because I think of a star exploding is kind of like the death of a star, the violent death of a star. But yeah, nova being like it's something new, some kind of new thing, is I guess a nicer way to think about it. Yeah. Well, often these things that are blowing up are in other galaxies, really far away things that we couldn't see. The star that exploded was individually way too dim for us to see with the naked eye. But once it becomes a supernova, it can outshine the entire galaxy that is in becoming visible and bright in the sky. So you're right that when a star goes supernova, it's at the end of its life, so it's not really new. It's new to us because it went from totally invisible in the sky, one star out of billions in a distant galaxy, to something that is now visible, so it's new in the sky. That's interesting to me that it's so bright, because when I think of something dying, I think of it's sort of fading, you know, like when a light bulb dies, it kind of fades, it flickers out, or when a candle dies, it flickers out. But if it's really bright, if after it dies it explodes and creates something really bright, it sounds like it's releasing a huge amount of energy. It seems counterintuitive to it being a dying star. That's a good point, and it shines a light right on what's going on in the supernova, because supernovas are not like fires that burn gently for a long time and then eventually just sort of flicker out and fade. They're more like a bomb, right where the fuse is going for a long time and then most of the energy is released at the very very end. Right, it's very dramatic sort of ending of the life of an object. And you know, interestingly, the same thing is sort of true of black holes. Black holes evaporate, you leave them out in the middle of space, and they do so by giving off hawking radiation. And the hawking radiation they give off is brighter as the black hole gets small. So as the black hole gives off radiation gets smaller and smaller, it starts to give off more and more radiation. So the last gasp of a black hole would actually be very bright. You would like go off in a bang of glory. That's I love that, that's I'm proud of them, that's good for them. Is that how you want to go out, Katie, You don't want to just podcast to the end, dribbling out a few last words. You want to have like one dramatic podcast at the very end of your career. Exactly. Yeah, if I could go the way a black hole goes, and you know, just like shootout podcast rays everywhere all at once, that'd be great, right, the super podcast nova. Well, in order to understand why supernovas are so bright and so dramatic, we have to think a little bit about what's going on inside them. Now, like everything out there in the universe, it's not something that we understand very well, but we do have something of a reasonable cartoon description of what's going on why supernovas are o dramatic. And remember, the context is how a star works at all, Like why are there stars anyway? You know? Which you have is a huge blob of gas and dust in the universe which gravity has gathered together into a compact, dense object, and squeezing it together makes it hot. And then because it's so hot and so dense, it triggers fusion in the heart of the star. So you squeeze hydrogen together, for example, and you get helium. You get those two protons to overcome their positive charges and to pop together into a new kind of thing. That helium gets fused together into something even heavier. And the cool thing about fusion is that it doesn't just make heavier stuff. It also releases a lot of energy. So gravity pulls this stuff together and then creates the conditions necessary for fusion, which pushes back out. So the energy from fusion, the particles, the photons, pushes back out on the star. And that's why the star can burn for billions of years. That's why you get like a stable, say, situation, like why do stars even happen? Because there's this incredible balance between gravity pushing in on the star, trying to make it into a black hole, and fusion pushing out on the star, basically a bomb blowing the star up. And these two things can stay in balance for billions of years, depending on how massive the star is. But one thing I know about stars, since we have one, and it's called Mr. Sun or Mrs Son, it gives off heat. So this to me seems to indicate that, like, you know, this is not like a self contained system. If I can feel the Sun, feel the warmth from it, and it doesn't that mean it is losing energy at some kind of rate, Like, is it in the same way as like a fire burns fuel, is it running out of fuel as it burns? The star definitely is giving off energy. It's not self contained, so it is using up fuel, and the fuel four fusion are light elements, so mostly hydrogen. Hydrogen is the most plentiful thing in the universe. Most of the universe that's made out of baryonic matter like protons and neutrons and our kind of stuff is hydrogen. So there's no shortage of hydrogen around. And most of the life of a star is burning that hydrogen and turning it into something heavier helium, and then if it's big enough and heavy enough, it can create the conditions to burn that helium into something heavier, and if it's then hot enough to burn the results of that fusion, it can just keep going heavier and heavier and heavier until it gets up about too iron. In each case, though, You're right, it's using up some of the energy stored in those light elements to turn into heavier elements and give up some of that energy, right, So, like where does that energy come from? It comes from the original energy of that hydrogen. You know, you have that hydrogen, it's floating around in the universe. You now squeezed it down to a very compact object a star. It's buzzing around, it's very high temperature. Now you've captured those protons into helium and in doing so, they give up some of that energy, which then gets radiated away into space to make you have a nice summer southern California day. I don't know about nice, but yes, uh, given how hot it has been, thanks Son. So I guess like I'm curious, like when a star dies, is it running out of fuel or is it like collapse and getting too heavy, because it seems like that one of those things would quote unquote kill the star right exactly. It's definitely not running out of fuel. When a star dies and go supernova, there's still an enormous amount of hydrogen left over, and that's why the star doesn't die in a quiet fizzle. It's not just like burning through all of its fuel. But what's happening is that this balance between gravity and fusion is getting upset. And there's really two different kinds of supernova is that we should talk about. One of them is called core collapse, and what happens there is that the star is making this ash, this product of fusion, making heavier and heavier elements which then settle at the heart of the star. And if the star is not heavy enough to create the high temperatures needed to burn that into something even heavier than it's like inert material at the heart of the star. So now at the heart of the star you are no longer producing a lot of energy, and so this balance between gravity and fusion tips towards gravity, and gravity rushes in and collapses the star. The star collapses, but it doesn't just like suddenly go out. Collapse makes it super dense and so super hot inside the star and quickly burns through a huge amount of fuel. So first gravity is the upper hand to cause this collapse, but then fusion surges back and blows it outwards again, and it leaves behind a very dense core which is gonna be a black hole or a neutron star. So you mentioned earlier that it is it's this push and shove kind of where gravity is trying to push it inwards like almost to create like a black hole, and then there's the push out. And so once that outward pushing is like outweighed by the inward pushing, the heaviness of the inward pushing. Why doesn't it just become a black hole all the time? Right? So gravity wants to make everything a black hole. Right. Gravity can only do one thing, which is pulls stuff together, and if there was only gravity in the universe, eventually it would just make everything into a black hole. The reason that things aren't a black holes that there's some way to resist it. Like why isn't the Earth a black hole? It's a big blob of mass. Why doesn't gravity squeeze it down into a peanut and make it a black hole? The answer is that the Earth is structurally strong enough to resist gravity. Gravity actually kind of a weakling, not really a very powerful force. It's like ten to the thirty times weaker than all the other forces. So you just need something to be able to resist it. So fusion can resist it for a long time, but eventually, as the star gets more and more massive at its core, gravity is winning and fusion is losing. So what happens when a supernova collapse? Sometimes you do get a black hole at its heart. It depends on the mass of the original star. Sometimes you get a black hole. Sometimes, however, there's another stage, like a neutron star can be formed, So the incredible dense core that's left over after the supernova happens can be another kind of matter which resists collapse into a black hole. But again, neutron stars are not something that we understand very well. We did an episode recently about what's inside a neutron star. It's something that scientists are still exploring. So it's sort of like a ladder of ways that you can avoid black hole collapse if you can make the right kind of matter, or you can sort of like hold up against the trash compactor of gravity trying to squeeze you down into a black hole, So it could become a black hole, could become something like a neutron star. Do we know like why it becomes a supernova sometimes, like what would cause that explosion rather than it just collapsing. So supernova is when it starts to collapse. It's like an implosion and you get this shock wave that's traveling towards the center of the star and then it hits the core and it bounces back and you get the supersonic shock wave that comes out and blows out all of this material, huge amounts of light are released because you have a lot of fusion happening all at once. You know, a star is like a very slow burn, using a very tiny fraction of its fuel every year, and the supernovit it can use of a big fraction of its fuel all at once, which is how it can become brighter than the entire galaxy around it. It also spews a lot of that material out into space. So you have this implosion that creates very high temperatures, very briefly, a lot of really really rapid fusion, and then it blows up and sends a lot of that energy in terms of photons and protons and just like raw star stuff out into space. So just a hypothetical, it'd be like a kid throwing a ball against a wall really hard, and again hypothetically the ball like shooting back out because you threw it against the wall really hard and hitting you in the face. But that times like a billion with a billion children and a billion balls. Yeah, and it's not something that we can under stand because there's a lot of really strong forces involved and it's all of very fast physics. So you know, we can't look at a star and say this one's gonna go supernova in forty two point two billion years, or this one's gonna go supernova tomorrow. It's the kind of thing we just sort of like see happening, and we're like, oh, everybody, watch that one. Watch that one, and we're trying to understand what goes on inside it. But modeling the process of a supernova is not something we're capable of yet. You know, when we want to understand something in physics, either we find like a set of equations that describe it very simply, like F equals m A that ignores a lot of the details of the particles that's going on inside, or we do like really complicated calculations using a computer to model all of those details and see if we can describe the sort of big picture that comes out. So far, we haven't found a simple set of equations to describe supernova, and we don't have the computing power necessary yet to like come up with a basic model of the innards and understand how that describes a supernova. So we're still really learning about how this stuff works. So it sounds like if we want to catch a supernova, we have to kind of stay frosty and keep watching the sky, which I mean, we haven't done that in like half an hour, so we should probably take a break and check just to make sure one hasn't happened while we've been talking, right, absolutely, all right, I looked outside, bad news, not yet, and you might wonder maybe Katie missed her supernova? Right. The amazing thing about a supernova is that they're not just a flash like they last for sometimes weeks or months. I mean, there's sort of a flash on the cosmic time scale, but for a human they can really last for quite a long time. So if there's a supernova in the sky, you could miss it today, you could miss it tomorrow. You might even be able to ignore for a week and then eventually look up and catch it up there in the sky shining its photons at you. So it seems like if we have like billions of observers, we should catch it if it happens. And have we ever caught one? So we have seen supernova and in fact, they are so dramatic that you don't even have to just look at Like the recent period of modern astronomy, when we have space telescopes and incredible ground based telescopes and all these new digital technologies to see supernova. People have been seeing supernova in the sky for thousands of years because sometimes they're just obvious. You know, if all of a sudden there's an incredible bright new star in the sky outshining everything else, then people are gonna see that, and they're gonna say something to each other, and it's gonna show up in historical records that people have done a deep dive into the history of astronomy and found times when locals have looked up in the sky and seen something they didn't understand and wrote it down, and we can now reinterpret those writings as evidence for supernova. I can't imagine what I would be thinking like before. I mean, I mean, if I saw a supernova today, let's be honest, I would also be surprised and scared. But back then even more so. If I didn't have physicists like you telling me what was going on, I'd probably think this guy was like really angry at me. Yeah, it's hard to put yourself in the minds of these folks, which is why it's really interesting to read these things, like what does it mean for them? How do they describe it what questions were they asking about this kind of thing. It gives you not just a picture into what supernovas are doing and how often they happened, but also you know what people are doing, what they are thinking, their relationship with the night sky itself super fun. So how do we know? Because, like we we have a history of things. We have written and drawn histories and also oral histories. But there's a lot I think lost in translation. And if someone described something, they're not going to say, oh, saw a supernova last week it was pretty cool. You know, They're going to describe it in terms that they understand at the time. So how do we know what they're describing is something like a supernova? So we can't always know, but it depends on sort of the quality of the notes that we're looking at. And some of these things are a little speculative, and in other cases people took really good notes and they described something that we really can't otherwise explain. You know. It's like, let's get into the details of the earliest record we have that might be a supernova comes from forty five hundred b C. This is like sixty hundred years ago. Is a rock carving found in Kashmir in India that people think depicts what is a hunting scene with two very bright objects in the sky. The idea is that it looks like a sky with two sons. I mean, if you were out hunting in the old days and saw a second son in the sky, you might also be inspired to make a drawing of it. So it's not exactly a hundred percent, but this might be the least record of a supernova observation. And then note taking now I am looking at this drawing and it does look like two really bright suns in the sky or or hear me out giant eyeball like eyeball aliens with tentacles. Yeah, exactly. It's far from something you would accept is like figure one in a scientific paper, you know, But it's fun to think about what these folks were imagining and what it was like for them, Like was this so bright that they could see it in the daytime? Itself? Like really a second son in this guy, It's possible, you know. The most reliable ancient recording of a supernova comes from Chinese astronomers. They noted it in one five the appearance of a bright star in the sky, and they observed that it took about eight months to fade from the sky, and it sparkled just like a star, and it didn't move like a comet. And so this scene really matches what we expect from a supernova. It's very bright, it doesn't move like a comet, it sparkles like a star. It fades on the timescale of weeks or months. So this very likely was a supernova captured by those Chinese astronomers. So when they're looking at this doesn't look like just kind of an extra bright star or is it just astoundingly bright, like almost having a little sun in the sky at night or something. It depends exactly on where the supernova is, and so if it's in another galaxy, remember that our galaxy is like a hundred thousand light years across, but other galaxies are millions of light years away. So if a supernova happens in another galaxy, then that star is going to go from invisible to visible at night. If it happens in our galaxy, then the supernova is going to go from like a star you can see at night to a star you might be able to see in the daytime, depending exactly on how close it is so yeah, this could appear like a second son if a nearby star goes supernova, because remember us, supernova can be as bright as the entire galaxy. It can be a hundred billion times brighter than our sun. Would we be in trouble? Are there any stars that would cause us a little bit of trouble here on Earth if it went supernova? Oh yeah, if Proximus Centauri went supernova, we would be quite literally toasted because the amount of energy and radiation would really fry at least one half of the Earth, you know, the half the Earth that was in the direction of that star at the time. The rest of us, having like an entire Earth shielding us from the supernova, would probably be fine until, of course, the Earth's one around and roasted the other half. So we're sort of like rotisserie Earth. We just have to kind of like all move to that side and keep moving, you know, like hamsters and a hamster wheel of death. While I'll be adding that to my long list of things that I cannot directly change but cause me existential dreads so great. It also sounds like a great pitch for a Netflix show. You know, entire cities on wheels rolling around the planet to avoid the frying radiation. Yeah, like snow piercer, but except it's like to escape the heat. Supernova piercer sounds good. And so if we look back in the historical record, there's like five of these things observed in like the last thousand years. There was a time in a thousand and six when people all over the world noticed one, from China to Japan. Astronomers in Iraq and Egypt and also in Europe noticed something they called a guest star which appeared sorry they called it a guest star. Yeah, like we have a sudden guest somebody said, another place for dinner, welcome or not. I do like the idea of like just on one of these night shows, you know, Jimmy Fallon or whoever, it's like, oh, we got a guest star, and then it's a supernova and everyone gets fried exactly. Sometimes they just blow it all up. And then as we enter in sort of like the more modern historical period, we have folks whose names we know well writing about these things. So Tico Bray noted one in fifteen seventy two in the sky and took a lot of great data. He's sort of famous for taking meticulous notes about what he saw. What's interesting also about this one is, like we said earlier, it helps us think about what people thought about supernova at the time and Europe around this time. Most people thought that the cosmos beyond the moon and the planets couldn't change. They thought it was just like out there static. You know, the whole universe was a bunch of stars hung out into space. They weren't even aware of the idea of other galaxies, right. They thought the whole universe was infinitely old, with stars not moving or changing. That was the idea at the time, just kind of painted on a big globe or something. Yeah, so this didn't really fit well with the idea of supernova because this is like a change in something. So they thought that probably of something happening in the atmosphere. They thought supernovas were like weird bright lights created like, you know, fifty miles above the surface, instead of super duper far away swamp gas. Well ali exactly. But you know, Bray took a lot of notes, and he realized that this thing remains stationary from night tonight, doesn't change. It's parallax, and so it has to be really really far away, and so he wrote a book with all of these details, and it's that book which gave the name nova to things that appear in the night sky. But then the last one that we've ever seen coming from our galaxy was Kepler. He saw one in sixteen o four, And so you know, here we have to make a distinction. We are seeing supernova from all over the universe because they are so bright that we can also see them from other galaxies, but the one we saw in sixteen o four was the last one we've ever seen from the Milky Way itself. So I haven't necessarily been keeping good track of the dates, but had we been seeing them at a rate greater than once every four hundred years before this point or no. So we have some estimates for how likely it is to create supernovas from watching other galaxies and from think about the ones we've seen in our galaxy, and so we estimate that in the Milky Way we should get about one supernova every twenty years or so. The galaxy of about a hundred billion stars should give us a supernova every twenty years. So in the four hundred years since Kepler's observation, we should have seen about twenty supernova in the Milky Way, right, and you mentioned that we saw like potentially saw like five in the past one thousand years, So that'd be one every two hundred years, right, not one every four hundred years. That's right. But we don't think that the historical record is complete, right. We don't think that we have found every written record of supernovas, and also we don't think that people have seen all the supernovas that were out there. But something we'll dig into in a minute is like how often we expect these things to happen, and how likely we are to see one if it does happen in our galaxy. Right, So we have this mystery of these missing supernovas, right that if like we should be seeing one every maybe twenty years, or at least more frequently than every four years. But before we waste time trying to figure it out, just a case it popped up while in the past, like you know, fifteen minutes while we were discussing, I'm gonna go out and check again, just make sure, because then that that would settle it. Right. I'm so impressed with how dutiful you are as a researcher. Yeah, yeah, I'm gonna get up on the roof and stare right into the sun for a little while. I will be right back. Well, I am back. I stared right into the sky directly at the Sun just so I wouldn't miss the supernova if it happened, and I gotta tell you was not one of my best ideas. I'm glad that you're still back and you have everything you need to continue podcasting, because you know, we don't really need your eyeballs for podcast. Yes, the uh. I basically just see a big glowing orb all the time, which is great. So we were talking about there is this mystery because it seems like we should be seeing these supernovas more often. Not only in historical records, does it seem like there were more supernovas, you know, not maybe one every two hundred years or so, not one every four hundred years, but also there was a calculation done to see how often should we be seeing it, and that resulted in about twenty years, right exactly. And remember we don't understand supernovas very well, so it's hard to predict, sort of theoretically, how often they should happen. But we can learn by looking at all the other galaxies to try to get a sense for the rate of supernovas. So remember, we can see supernovas in our galaxy even just using the naked eye, going back thousands of years. But in the more recent era, because we have telescopes, we could now study very thoroughly supernovas in other galaxies. So, starting in the nine or so, people began these campaigns to scan the sky for new stars appearing in other galaxies using telescopes, and they saw dozens and so we have definitely seen supernovas recently. For example, we saw one in seven that came from a large Magellanic cloud. This was one you could actually see with the naked eye, even though it wasn't in the Milky Way, because a large Magellanic cloud is a satellite of the Milky Way, so it wasn't that far away. So we're seeing supernovas in other galaxies, and that lets us estimate what we think is the rate of supernovas we should expect in the Milky Way, being about one every twenty years. You know, there's still a lot of uncertainty in that number, and so then the question is, you know, are they not happening in the Milky Way, or are we just not seeing them, are they hiding it from us? Or maybe the Milky Way is super weird and doesn't make supernovas as often as these other neighboring galaxies. Right these are like just the basic questions we have about supernove is because we're so clueless about the thing that triggers them, the moment that the star decides to collapse, and exactly what's going on inside. I mean, it seems odd though, for like an entire galaxy to have different rules from another galaxy, or is that not so odd? It would be odd, But there are odd galaxies out there. There are galaxies that have lots of dark matter and galaxies that have very little dark matter. They're huge galaxies and little galaxies. There are galaxies that are still making stars and galaxies that are quenched that will no longer be producing stars. And we just don't know if the Milky Way is typical or not, the same way we don't know if Earth is typical or if it's weird and rare in the universe. These are the kind of questions we get to ask as we look out deep into the universe to try to figure out whether our context is normal or really really weird. But we can do more than just look for supernovas as they happen. We can actually also look for supernovas that have happened that we might have missed because we can see supernova remnants. When a supernova happens, it's very bright, it's very dramatic, but also it creates this shock wave that shoots out into the universe, leaving a sort of interesting fingerprint that you can continue to see four hundreds or thousands of years later. So it's kind of like looking at a star's kind of fossil record exactly. Because when a supernova happens, remember, it shoots out like several solar masses worth of stuff, right, Like, think about what that means. A solar mass is an entire sun's worth of stuff. And so now you have a supernova shooting out like several times the mass of the Sun, just in plasma and hot gas out into the universe and incredibly high speeds, like significant fractions of the speed of light. So what happens is that it's going to smash into all the stuff around it. Space is not totally empty, it's filled with the interstellar medium. Which is like a very dilute gas, but when the supernova remnants smash into it, you get this expanding shell of a shock wave. So that's what we call a supernova remnant. It's very distinctive. And scientists have looked out into the night sky and for example, in our galaxy in Cassiopeia, they see a remnant which looks to be about three hundred and twenty five years old, which means they think there was a supernova there three hundred years ago that everybody missed. Nobody saw it, but we can see evidence that had happened, and it happened fairly recently, so it's a It's an interesting thing because like as kind of postmortem detectives of this star explosion, you can't necessarily look just at the sight of where the star was. You're looking at where the remnants go, right, So it's this expanding force. It's not just like looking at where the star died. You have to look far afield of where the star has exploded to yeah, precisely. And this stuff moves out, but it moves out much slower than the speed of light. You know, it's pretty fast and pretty energetic, but it sort of in the vicinity of where it happened. It's not like looking at black hole collisions, and we're getting evidence of that from gravitational waves that travels at the speed of light directly to us. Now we're looking at like stuff spewing out sideways from the supernova ramming into something else and then sending us light from that collision. So we're seeing this like shock wave emanate from the supernova, and we're seeing light from that shock wave as it bounces into gas and heats it up. So it's pretty cool. You can tell that as supernova was there. It's sort of like you missed an explosion, but now you're looking at the burn marks on the ground or something like that. Yeah, yeah, that's really interesting. Yeah, and people have done scans for these kind of remnants in our galaxies and they found a few. They found this one in Cassiopeia. That found another one that they're pretty sure happened at the end of the nineteenth century. But you know, there's no records that anybody saw the actual supernova. So all this kind of stuff lends credence to the idea that maybe supernova's actually are happening in our galaxy. We're just not seeing them right that they're out there, they're blowing up, but we are missing the party. But wait just a minute, because you said that they're super bright. There's a lot of energy released, and our galaxy isn't you know, too far away from us. So how could we miss that? Did we podcast over it? This? This is what I was worried about. It's because you were napping. You know, you took a break, a nap in a coffee, and you missed all the important datah man I got like fomo on a galactic scale. Now, no, no, don't feel bad. It's not because we're lazy. The issue, like everything else, is probably location. You know, the galaxy is not that big on a universal link scale, but it's also not that transparent. You know, the galaxy is a lot of gas and dust in it, and if something happens on the other side of the galaxy, then the center of the galaxy, which is a swirling maelstrom of intensity and gas and dust and it's pretty opaque, then we might not be able to see it. And so some of these things might just be happening behind big dust clouds or big gas clouds, and this dust is really good at blocking the light. And unfortunately, the places that have the most dust are the densest places, which are also the places that supernova are most likely to happen. So this really was a punishment for us not cleaning our rooms. I knew it it might be. You know, this is still really speculative. Is the kind of explanation people are trying to understand if it works. And I read a paper last week exploring this in detail, trying to say, like, can we explain not having seen supernova based on where the gas and dust are in the galaxy? And they did a really interesting, very thorough study. They have a model for where they think supernova should happen based on where are the stars in the galaxy. From that they get a sense for how much light should have arrived on Earth from each of these supernova, you know, how bright it was, how far away it was. Then we have maps actually of where or the dust is in the galaxy, right like where these clouds of choking gas and dust that would obscure our view are sitting. And then we can do a calculation and say where do we expect to see supernova in our galaxy? And where do we expect not to see them because the gas and dust are blocking them, and drum roll, do we have those results. We do have those results. I'm looking at them right now from this paper, and interestingly, the results are kind of a surprise. Most of the supernova that we have seen in our galaxy actually land in places where we didn't expect to see them because either there actually was a lot of gas and dust, which somehow they are mysteriously overpowering, or it's a place where you don't expect to see many supernovas. So we like saw a super rare supernova. It's not something that we understand, and so it's the kind of moment in science where we go, well, that didn't work. You know, we have this data, have a basic model that doesn't explain it. What's wrong? Probably something simple is wrong with our model, our estimate of how often the supernova happen, or our guests for where the dust is or how transparent that dust is to the light from the supernova. Something is going wrong with these models. But the upshot is that we can't explain it. We do not understand why we are seeing the supernova that we are seeing and why we are not seeing supernova in some areas of the galaxy. So the model was saying not necessarily that the supernova was in the location where it couldn't happen, but that it was unlikely for us to be able to see it at this location. Some of the ones that we have seen are happening in places where they should be very very rare. Not impossible, but you know, you should expect to see more supernova where there are more stars. And if you look at the historical distribution of supernova that we have seen in our galaxy, there's a lot of them in places where there aren't very many stars, which is kind of confusing. Are supernova more likely to happen there, or maybe there's some structure to the galaxy that we haven't described in our model, you know, some like clusters of gas and dust that are creating supernovas or blocking supernovas. Something else is missing from our model to add to this recipe to give us a depiction that matches what we actually see out there in the night sky. Yeah, that's really interesting. Like I wonder if it could even be something like, you know, if a star that's more likely to go supernova is more likely to be a loner. It could be because we know also that there's a lot of difference in sort of the wrong ingredients in the galaxy. You know, in the center of the galaxy there are more heavy metals, so the stars there are more metallic, and so the galactic conditions are very different in the center of the galaxy and the spiral arms, and then above the galactic plane. So this is the kind of next round of research is understanding, like what are the conditions to make supernova? What are the conditions in the galaxy? Can we make a model that explains why you might be getting more supernova or maybe just why we're not seeing them? You know, maybe this more structure to this gas and dust than we thought we understood. But it's a great opportunity. Every time you have something you don't understand is a chance to refine your models and learn something new about the universe. Maybe it's just something about the map of the gas and dust, or you know, maybe like the discoveries of dark matter, this is not something that we can resolve with like a little tweak to our models. Maybe it's gonna require something really big and new in our understanding of the universe. Right, so you gotta double and maybe even triple. Check your mouth first, make sure someone didn't just forget to carry a one. And then maybe it's a sign of some really interesting discovery or mystery about the universe. And now we are lucky enough to have other ways to see supernova, not just through light. Supernova also produce a lot of radiation in new trinos for example, In fact, most of the energy produced in a supernova comes from new trinos something like that. The radiation from the supernova is in the form of neutrinos, which are these weird little particles that mostly pass through matter, ignoring us. And in the supernova, we actually saw the new trinos from the supernova first because they're produced at the heart of the supernova. But then they fly right out through the craziness and get to Earth before the photons do, because the photons have to make their way sort of through the star before they get to the surface and can get emitted. So neutrinos are a new way to see supernova. So now we have like new kinds of supernova eyeballs, so that even when you're nappying, Katie, we are watching the sky for supernovas. Thank you. That does make me feel a little more relaxed. So I guess you use some kind of like neutrino glasses so that you can see these If these are not actual visible light particles, Yeah, you cannot see new grina's very easily. But particle physicists have neutrino experiments deep underground to try to understand how neutrinos changed from one kind due to another, or to do other kinds of neutrino experiments. We recently did a whole episode about how neutrinos can teach us about supernova, so go check that out. But one of my favorite facts about neutrinos is that they used it to take a picture of the Sun. Remember, neutrinos interact very very rarely with us, but there's huge numbers of them, Like a hundred billion neutrinos from our sun passed through your fingernail every second. But you're lucky if you can even detect one in a huge specialized device. So they pointed this thing at the Sun for like months and they got a picture of the Sun in neutrinos, which is is kind of cool because it's like another way to see the Sun. I just get really excited every time humans build another kind of eyeball to look out into the universe. I'm guessing though, like neutrino glasses that I can wear and look and spot neutrinos myself are a little bit far from the market, maybe another ten years or so. Yeah. Currently these things have like thousands of tons of heavy water in them, And so unless you're very strong or willing to put on extremely large glasses and you're not going to be seeing neutrinos with your eyeballs. I'll work on my neck exercises. But in the meantime, the universe continues to churn and burn, and supernovas are happening out there, blowing out these incredible cosmic engines of the night sky. You know, we are grateful that stars last for as long as they do, that they balance on this nice edge between gravity and fusion for so many billion years to light up the night sky and to provide life here on Earth, and just to provide us with something nice to look at as we shiver in front of the fire and sip our hot coco on our camping trips. But eventually those stars do give up their life and they do go out in incredible cosmic explosions, which then give us another opportunity to learn what's going on inside the heart of those stars. And so the more things blow up, the more we learn about them. A bunch of drama queens. So if you're out there interested in supernova, to keep an eye on the night sky. You might see one with the naked eye. Or if you're excited about squirrels, keep watching to see how many acorns they eat. Maybe you'll see you one of those blow up. I do not endorse that message. Well, thanks Katie very much for joining us on this tour of cosmic catastrophes. Thanks for having me, and thank you all for listening. Tune in next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of I Heart Radio. Or more podcast from my Heart Radio, visit the I Heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.