Are there stars made of dark matter?

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Speaker 1: Hey, Daniel, I've got a really hard physics question for you. Oh yeah, bring it on. All right, it's gonna sound simple, but I think it's a tricky question. All right, now you're scaring me. Here's a question. How do you define a star? I guess i'd say a fusion burning ball of plasma in outer space. But what about a neutron star that's not burning? Um? Well, then I guess i'd say a star is a glowing ball of stuff in space. See, totally different. What about a cold white dwarf that's not technically glowing? Right? You? And there is no good definition for a star? All right? Do I get a star? Then a gold star? Hi am poor handmake cartoonists and the creator of PhD comics. I'm Daniel. I'm a particle physicist, and I never give out gold stars in my class. What you're one of those tough professors that are always marking things up with red ink. No. The reward for doing well in my classes you get to come do research with me more. I see, Oh, I see. It sounds like a great class. But you know that reminds me of what happened to me. This is a real story in undergrad I turned into physics assignment, and then the day they were to be returned, the professor gives a lengthy apology. He says, to the person whose assignment this was, I apologize, I misunderstood your answer and that's why I corrected it this way. And then he hands it to me and the page is covering in red ink all these insulting comments. What are you doing? This doesn't make any sense. And then at the very end of my solution, he says, oh, now I see where you were going. Good work, full credit. That sounds really familiar. That's how I feel every time we were recorded podcast Dane, I'm like, where are you going with this? What's oh? Now? I get it? Yeah, And that's why you don't use red inc the first time you read someone's designment. But we are the stars of the podcast. Daniel and Jorge explain the universe production of I Heart Radio, in which we mark up the whole universe and say, this doesn't make sense, how does that work? Where are you going with this anyway? Why do you have black holes? What's the plan? We try to unpack the entire universe, from the tiniest little particles to the spinning hurricanes on Earth, to the craziness inside neutron stars, to everything in between, and explain all of it to you. Yeah, the universe definitely gets stars for being awesome and mysterious and big and full of interesting ideas and things to discover. It gets a lot of stars, right, Daniel. I mean there are a lot of stars in the universe. There are a lot of stars in the universe, and there are a lot of different kinds of stars, and this is one of those really cool fields because the naming conventions for stars, what we call them pre dates, are like actual understand ending of stars. We started to call them this, We started to call them that, and then later we realized, oh, that's not actually a star, or this is really two stars or something. So it's fun when like the naming conventions pre date your actual understanding. Oh I see, is it like being a Twitter star or a YouTube star? Like that's not quite the same as being a movie star. You know, it turns out that's not exactly as bankable. Yeah. Yeah, but stars are something we've been studying for, you know, literally forever since we looked up at the sky and wondered, like, what are those things out there in the sky, and then later realizing they were like our sun, and then realizing there are lots of different kinds of stars, very very different from our son. Yeah, I guess you know, the definition probably started when we looked up at the night sky and saw you know, bright shiny things, and you know, that's what we thought were stars, like you know, the pinpoints in the night sky. Yeah, and it takes a real leap to understand that those are like actually super duper massive and far away, right, that they appear to be so tiny only because they are so distant, and the fact that they're so distant, yeah, we can still see them means they must be almost impossibly bright, right, And then we figured out that some of them are glowing in the nice guy are actually planet So those are definitely not stars. But there is sort of a wide variety of types of stars out there in the universe, right. There is a kind that are burning, and there's the kind that's just simmering there, and then there are the kinds that they're just sitting there right and glowing out of their own kind of restlessness. Right. Yeah, Well, we had a whole fun podcast episode about the weirdest kinds of stars, And in that episode, I define stars to be things that have fusion inside them that prevent them from collapsing. And you pointed out, well, a lot of the things we were calling stars, like neutron stars and white doors, don't have fusion going on inside. They're still just really hot because they are leftovers from something that used to be burning. But they're not actually burning anymore, even if they're still glowing right right, So then would you say maybe a better definition is just you know, things that glow out there in space above a certain you know, glowiness. I don't know, wouldn't you need a minimum size? Also, like would a rocket ship be a star because it's glowing out the back? Yeah? Why not? I mean to an alien seeing you, you know, burning in the sky, you'd be like, oh look there's another star and it's coming towards us. Yeah, I suppose. I mean you might wonder like, why do we even have this concept of a star. It's because we see this pattern. We see this thing out there in the universe. There's a lot of this sort of similar type of thing, and we do this categorization where we try to say, well, this and that are kind of related can we fit them into a larger scheme only because we're trying to simplify, because we're trying to get down to the root cause and understand why do we have stars at all? You know, why is this a thing that seemed to happen in the universe more than other shapes of matter. But there is sort of one thing that all of those stars have in common, you know, like nutron stars, regular star, fusion stars, white dwarves, brown dwarfs, they all have one thing in common, and that is that they're all made out of regular matter. They are, right, They're made out of quirks and leptons, just like you and I are, And just like all the ice cream in the world is, and just like all the planets out there and all the kind of normal stuff that we're familiar with are made out of the same fundamentutil building blocks, mostly up corks and down corks and electrons, sometimes a few strange quirks or whatever else thrown in. But yeah, it's the same basic building blocks that make all of those different kinds of stars. Yeah, they're all made out of the same kind of matter. But then it turns out that that kind of matter is not the only type of matter in the universe. In fact, there's a lot more of another kind of matter that makes up all the stuff in the universe. Yeah, the kind of matter we're made out of is actually unusual. We call it normal matter, but it's really kind of abnormal. It's only fifteen percent or so of all the matter in the universe is our kind of matter. Yet you always struck me as a little abnormal for a physicist. That's dark, Ben, No, that's a compliment, trying to compliment you're an abnormal physicist, Adnuel. Well, then your whole joke is kind of dark. Yeah, there is a whole slice of the pie of the universe's stuff that is not regular matter, and in fact it's there's still a big mystery. It is still a big mystery, and one mystery is what is it made out of and what is it doing? In fact, I was having dinner with my family the other day and my son was asking me about dark matter, and then he says to me, does dark matter form things the way normal matter does? Like? Can it make planets and stars and all sorts of crazy stuff? M interesting? So light dinner conversation in the whites and household is either sewage, which my wife studies, or dark matter, which I study. But they're both kind of dark. I wonder what you were eating to inspire these conversations. But yeah, there's a whole bunch of other matter in the universe that is not regular matter. It's dark matter. And so a pretty interesting and big question is what can you make with this dark matter? So to the on the program, we'll be asking the question, are there stars made out of dark matter? Now, Daniel, would those be called dark stars? Because that sounds like a pretty cool, like fantasy sci fi name. It does. Yeah, it sounds like a wizard with some special skills. Dark star. What's the matter with dark star? Yeah? Well, if we ever do discover one, I will definitely call you up and ask you to name it. I want bet on it. Neither the part where you discover something or the part where you can actually call me if you do discover something. Yeah, that's unlikely above them. Yeah, so this is a pretty interesting question. Can you make a star out of dark matter? I mean, you can make a star in many different ways at a regular batter, quirks and electrons and other kinds of particles, like that, but can you make it out of dark matter? And I guess the big question is would it glow like would a dark matter star glow or would it anti glow. Yeah, it's a super fun question because we see all these weird things that normal matter can do. Right, if you just were given like quarks and electrons, it would be really hard to predict, like all the kinds of things that those particles can do. They can make protons and neutrons, and those protons and neutrons can make all sorts of crazy stuff like hurricanes and water droplets and hamsters and bananas and ice cream and stars, and that's really complicated. We don't even know what dark matter is made out of, so it's hard to get your mind around, like what can those bits of dark matter do when they get together and dance? Can they make really complex emergent phenomena like our matter can or not? And so, as usually, we were wondering how many people out there had thought about this question, whether you can make a start out of dark matter. So Daniel went out there into the wilds of the Internet and asked a couple of people, well, Daniel, did you go into the internet for this one, or did you just walk down the hall? This is on the internet. Thanks for the volunteers. I have not recently asked my children these questions. They tend to run away when I show up with my phone on record. I guess the internet has halls sort of. And so you ask people if they thought that dark matter could make a start, and so think about it for a second. What would you answer. Here's what people had to say. Well, dark matter feels the force of gravity, so it definitely can clump together. I don't know if it can get dance enough, but even if it did, it doesn't feel the strong nuclear force. And in order to have a start you need to have a nuclear fusion reaction inside of it. So if it can't feel the strong nuclear force, I don't think it can make stars. About the definition between no stars today, As far as I understand its stores are bodies of burning gas um involving nuclear reactions. Since they are made of elements, and through nuclear actions they produce new elements, I don't see how that could happen through doc matters. So no, I don't think so all right. I like how they avoided the answer, like they said a lot of words, but not yes or no. It's complicated. You can hear that they were sort of thinking about it for the first time as they answer the question. They were like doing physics on the fly. I love hearing that. Hearing people like use the models they have in their minds to try to fit them together and think about how they can come up with an answer. That's the process they're doing physics. It's wonderful to hearing you don't always get to the answer right away. I see, but these are pretty cool answer. I mean, some people allude to the idea that maybe dark men it doesn't feel the strong force or therefore there's no fusion in a dark matter star, and also talking about what kind of reactions you would need to make a star. Yeah, it's super fascinating to think about, Like could there be dark strong force, like a different kind of strong force just for dark matter. That would be super cool. What a dark strong force? It sounds like another superhero villain name. It's the power that the dark Star is going to use to battle our star when it comes into our solicits to destroy us, right right right. It sounds like some muce. You make a movie about the dark side of the force. Or something, and how it has two sides and how there should be balanced. That sounds like a billion dollar idea, maybe a fifty billion dollar idea. He had three ego dark star Wars. We'll call it all right, Well, let's recap for maybe those listeners that don't know Daniel, what can we say about dark matter? Like how do you define it? And what do we know about what it could be made out of? So there's a lot we don't know about dark matter, but there is also a lot that we do know. Dark matter is just our word for this invisible source of gravity that we haven't been able to explain. Like when we look out at the night sky and we see things tugging on stars, we know that there's something they're providing gravity to tug on those stars. And so, for example, when we look at galaxies and how they spin, we see that galaxies are spinning really really fast and something is holding them together. But if you add up the mass of all the stars in the galaxy, there's not enough gravity to hold them together. And so we say there must be something else there, something weird and new, something invisible that's providing the gravity to hold the galaxy together. So that's the basic hypothesis that there must be something else out there providing gravity but being invisible. And we have more evidence than just the rotation of galaxies. We can see the effect of dark matter all over the place, from the very early universe, how it affected the ripples in the plasma that gave us the cosmic microwave background, to then the evolution of the universe. The whole structure of the universe, from galaxies to clusters to superclusters, would look totally different if there wasn't dark matter. And we can also see blobs of dark matter in the sky bending the light from background galaxies. So we know that it's a thing. It's matter. It's out there, but it's invisible and we don't know what it's made out of. Right, it's the name we give something that we know it's there. You can see it in many different ways, or you can know that it's there in many different ways. You just can't like see it or touch it or somehow like shine a light on it. Right, because it's dark, it doesn't feel or interact with the magnetic force, that's right. That's the key thing is that it doesn't seem to have any interactions other than gravity, Like if you shoot photons at it, they passed right through. It doesn't feel photons because it has no electric charge, and only things with electric charge feel photons. We know that it doesn't feel the weak force, and it doesn't feel the strong nuclear force. None of the forces that we are familiar with affect these particles except for gravity, and that's the only way we know about its existence and the only way we have so far to interact with it. And that makes it really challenging because gravity is so weak. So it's really hard to use gravity to study, for example, a particle, because the particle has almost no gravity it's so tiny. Instead, we have to use it to study like enormous blobs of dark matter like at the center of the galaxy or huge halos that's surround the galaxy. Makes it hard to figure out exactly where the dark matter is and what it's doing because we have such a weak way to probe it, just gravity. Right, It's like the only way you can know it's there is if you have a lot of it, like you can like grab a little bit of it and look at it closely, because it's invisible. Because it's invisible and you can't grab it, you know, it's like grabbing something that your hand passes right through. And in fact, dark matter is here. It's all around us. It's everywhere. It's not just like out there in space. It's here on Earth. We think we don't know what it's made out of. It if it's made out of a particle or something else, or many particles, but it is all around us. We just have to figure out a way to interact with it other than gravity. If we're going to figure out what it's made out of. Right, that's a huge question. And it's not just like a little bit of stuff out there in space. It's a huge percentage of the mass and energy of the universe. Right, It's not like a rare thing out there. It's like most a thing. Yeah, if you just look at like the fraction of the mass in the universe, it's like eight percent. So the universe, the stuff in the universe, is mostly dark matter. So you can't pretend to like understand the universe at all if you don't understand anything about eighty percent of it for sure. And then if you look at the whole energy budget, mass and energy of the universe, dark matter makes up about twenty of all the energy in the universe, whereas our kind of matter is only like five percent. But that doesn't mean that we know like nothing about dark matter, because, as you say, we can study really big blobs of it. We actually do have some clues about dark matter. Like we can tell how quickly dark matter is moving around, because if it moves around really really quickly, if it's hot, then it changes how it's distributed. And if it's not moving around really really quickly, if it's cold, then it tends to like move more slowly and clump a little bit more. That changes how it's distributed, and that changes how it tugs on other matter. So we've been able to figure out, for example, that dark matter is around. It hangs out and need really big fluffy diffuse clouds that surround the whole galaxy. Yeah, I guess you could say it's pretty cool, totally chill, it's cool and dark exactly just likes to hang around. Yeah, we know it's all around this. It basically forms like a big blob in our galaxy, right, Yeah, and most of it is concentrated at the center of the galaxy. That's where the strongest gravitational pull is that it gets sucked in like other things. But because it's cold and slow moving and it doesn't feel a lot of other forces, it tends to mostly stay in a big fluffy cloud and swirl around the galaxy. So the galaxy is actually much bigger than you can see. The dark matter halo extends out like twice as far as the visible stars, So we're hanging out in a huge cloud of dark matter, right, and it it sort of makes up most of the galaxy in a way. Right, Like the stars are just like the sprinkles on the ice cream exactly. If you want to visualize the whole universe as a cupcake, then like the cake part is dark energy, the frosting is dark matter, and then the stars are the little sprinkles on top, right, and then we are the cherry on top. Obviously we are a piece of dust on one of those sprinkles. So that's dark matter. And so let's get into the big unknown question, which is can you make a star out of dark matter? So we'll get into that, and also you would go around doing that. But first let's take a quick break. All right, we're talking about dark matter and whether or not you can make a star out of it, or as I might call it, a dark star, which, Daniel, I just looked it up on the internet. It is a Marvel superhero or villain. It's a Marvel character for sure, called dark Star. You know, I really appreciate the deep research you do for this podcast. Thank you. You're right, I should have looked it up on Wikipedia, like like you do for physics. Oh, so what are the powers that dark Star and the superhero has. I don't know. You know, if it was scientifically accurate, I guess she would be invisible, but still the way something right, Yes, she could gradually influence motion of stars in the universe. Like well, I guess we all do that. That's not that special. But she is a superhero, and I guess. But that's what we're talking about today, is that if you made a star out of dark matter, if you could, we don't know that. What kind of powers would it have, what would it be like? And is it even possible? So I guess let's talk first about how to make a regular star and then we'll see if that applies to dark matter or if it can apply to dark matter. So, Daniel, how do you make a star besides putting it up on the Internet and hoping it goes viral. A star has a few ingredients. One is the raw materials, right. You start with like a big cloud of gas or dust left over from the explosion of a previous generation, or left over from the Big Bang, and you have it hanging out there in space, So that's ingredient Number one is the stuff you need to make the star. But of course a star is much more compact than a big cloud of gas and dust, so you need something to pull it together, and that, of course is gravity. So eventually, if you have a big cloud of of out there, gravity will tug on all the little particles, the little atoms hanging out there in space and pull them together. And as that happens, it'll get faster and faster, because you'll get a clump of stuff near the middle that has stronger gravity, and it'll pull on stuff more and more, and it's gravity will get stronger as it gets more massive, and then it'll get more massive as it gets dur under gravity, etcetera, etcetera. But there's one more critical thing you have to do to make that gas actually collapse into something that's really dense that can actually start to burn. What do you mean, Well, not everything that feels gravity's tug eventually falls into the center. For example, the Earth is feeling the tug of our star, but we're not plummeting into the Sun helping to contribute to its mass, right, and the same with Jupiter. For example, Jupiter is being tugged by the Sun, but it doesn't fall into the Sun. And the same is true of everything. If you have a big swiling cloud of gas and dust, stuff can start to fall in, but it'll just move faster and faster and end up in an orbit. It won't necessarily fall into the center. And that's because of angular momentum. If it's spinning at all around that point, it's going to keep spinning, and it's got to keep spinning. And that angular momentum is what, for example, keeps the Earth from falling into the Sun or Jupiter from falling into the Sun. So in order to form a really dense object, you have to find a way to get rid of that angular momentum, right. You needed to compress, otherwise it'll just be like a swirling cloud forever. It'll just be a swirling cloud exactly. The way. For example, stuff around a black hole doesn't instantaneously and automatically fall into the black hole. Right. The reason a black hole has an accretion disk around it is that that stuff is swirling around really fast, too fast to fall into the black hole. What makes it fall into the black hole, It's got to lose some of its angular momentum. The way it does that is by bumping into other stuff in the disc. Right, it's a big, jostling, hot mess. That's why these things are glowing, and they bump into each other and then one of them falls towards the center. Another way you can do it is you can radiate off some energy, like shoot off a photon, and then lose some energy and fall towards the center. The same thing is true if you're trying to make a star. You know, this big swart in the cloud of gas and dust. It's got to somehow lose some of that energy so it falls together. But that only works if you can bump into stuff, or if you can radiate away energy, and that requires using electromagnetic forces. Right, it needs some kind of like I guess, stickiness. It's kind of what I think what you're saying, like It can't just be like out there swirling around. It has to somehow have a little bit of like a molasses to it so that it groups together in the middle. Yeah, you need more than just gravity. You need another force to make really compact stuff. You know. Another way you can think about it is as you just describe goopiness, Like imagine stuff out there in the cloud banging together and sticking together, right, holding themselves together with electromagnetic forces, chemical bonds, so that when they touch, they like hang out together, and then they touch another one and another one, and it gradually accumulates into something that likes to hang out together. You know. The thing that's holding the Earth together, for example, is not gravity, right, it's the bonds of in the rocks for example, that are holding those pieces together. The Earth wouldn't have come to other if the bits of the Earth didn't have those forces, those ways to hang onto each other. Yeah. I guess that's one sort of like definition maybe or requirement of a star is that things have to be really compact, right, Like do you have to like they have to be squeezed in together a lot and not just like flying around like a cloud of dust. Yeah, a huge cloud of gas is not a star, right. We don't have a great definition of the star, as we talked about earlier, but we know that it has to be something which collapses so that you can start fusion, so you can get it to somehow glow, because a big cloud of gas and dust and space doesn't glow, doesn't give off any light. It just sits there. It can reflect light, it can hang out, but it doesn't glow. So in order to have glowing happening, you have to have some sort of release of energy from fusion at the heart of it. And fusion doesn't start unless you make that star compact enough. You need to squeeze the stuff down, and that's what gravity and then some sort of inelastic force that can clump the stuff together can do together. Right, And so then that's the problem, kind of like trying to make a dark matter star. Is a dark matter doesn't feel the electromagnetic forces and so it can bump into it itself and so therefore it can't really you know, stick together. Yeah, exactly. That's why we think that dark matter stays in these really big fluffy clouds. We don't know what dark matter is made out of, but we've never seen it have any kind of interaction other than gravity. So if it doesn't feel anything else, it's really hard for it to slow down. Like it just passes through normal matter without clumping. It passes through itself, right dark matter bouncing into dark matter, it just passes right through. It's like invisible even to itself, and so that makes it really hard to slow down. So it's just gonna like swirl around the galaxy forever. It's not sticky, I guess, right, because it's transparent to itself. To like, if we have a particle of dark matter and another particle of dark matter flying towards each other, they'll just pass right through each other and keep going. Yeah, it's transparent and also intangible. Right, if you're talking about superheroes, like somebody who's invisible, you can still push against them and feel them there. But dark matter is not just invisible. It's also intangible. If you have like an enormous lab of dark matter, somebody had like a cube of dark matter they put right in front of you, you could push your hand right through it and not even feel it, just the same way. New trinos are almost intangible. They can pass right through you. Without noticing you, because they don't have an interaction that's strong enough for them to feel you. That's why newtrinos can pass through the entire Earth without even noticing. And so dark matter can do that to itself, right, it just passes right through itself. It's a dark matter that's swirling around has a hard time collapsing into any kind of compact object a star, a planet, even a dark rock or anything like that. So that's one big problem with trying to make a dark matter star. There's also the other problem, which is that stars also have another ingredient that's important, which is some sort of something that has to make it glow as well. Yes, stars are an amazing balance actually between gravity and something fighting back. Right, if we just had gravity and then these other forces to get things to clump, then everything would go straight to a black hole. Right. You have a huge cloud of gas and dust and it would start to collapse and it would get stickier and stickier and eventually boom, black hole. You wouldn't have a star at all. A star only exists and burns for so many billions of years because it overcomes gravity. The fusion at the core that gives us the light that makes the stars twinkle also pushes back against the stuff inside the star so that it doesn't collapse. So the star is this incredible balance of gravity squeezing in and then fusion pushing out, right, Because without the fusion, like you said, it would just turn into a black hole. And so even if dark matter was sticky and could collapse and what could compress if it did that, it would just become a dark matter black hole, right, yeah, exactly the darkest of black holes right now. Dark black That sounds like an oxymron definition. And there are some other ways to avoid collapsing to do a black hole, Like many stars use fusion to avoid gravity. But some of the things we talked about earlier, which people argue about whether or not they're a star, like a neutron star or a white dwarf, those don't have fusion inside of them, but they're also not collapsing into a black hole. That's because there are other ways to prevent a collapse into a black hole, like a white dwarf does it by electrons and not wanting to overlap with each other. You know, Electrons are these weird particles that don't like to be in the same state as any other electron, so they resist being in the same place, and that's enough to prevent a white dwarf from collapsing into a black hole. And similar things are going on inside neutron stars. They don't have enough mass to overcome those barriers, so they don't turn into a black hole. Mostly it's fusion for stars, but for some of these other objects, there are other ways to overcome gravity and prevent collapsing into a black hole. And some people would say a neutron star is a star, and some people would say it's just a bunch of neutrons in space. It's just a neutron ball, all right. Well, it's just sort of sounds like maybe you define star as something that would go into a black hole, or like something on the edge of being a black hole. But something's keeping it alive, something's keeping it from collapsing, and somehow in that sort of tugger war, it generates a lot of light. Yes, like balance there on a nice edge, two totally different forces tugging in each other. And it's always been incredible to me that it's so stable. You know that this thing can last for millions or billions of years. In this incredible tug of war between these powerful forces. Whenever you have these two things battling against each other, I feel like any instability is going to slide off one way or the other. So I'm amazed that stars last for longer than, like, you know, a minute. That would be a pretty quick lifespander for the star. All right, So then dark matter would need to have something sticky about It's something that makes it collapse and not just fly or swirl around. And it also would need something to prevent it from becoming a black hole, because I guess you could make a black hole out of dark matter, Like if you put enough dark matter in one place, you would get a black hole. Right in theory, you could make a black hole out of dark matter. But for the same reason, it's hard to imagine that any of those exist because dark matter doesn't have a way to slow down. Like we have a huge amount of dark matter in our galaxy and we have a huge black hole at the center of it. How much of the stuff in that supermassive black hole came from dark matter, Probably not very much. It's probably mostly normal matter because normal matter is better at falling into black holes and losing its angular momentum. So probably there's very little dark matter in those black holes. But in theory, if you had some way to get dark matter to lose its angular momentum, where some other way to manipulate it, in theory, it could form black holes. It's just gravity, and dark matter definitely feels gravity. And then by the same talking, it could have something that prevents it from collapsing to right, Like, we just don't know because dark matter doesn't is not to thinky, so it never collapses that much, and so we don't know if it would become a black hole or if it would become some sort of stark. Yeah, let's be clear about what we don't know about dark matter. Like we know it feels gravity. We know it doesn't feel electromagnetism or the weak force or the strong force, and we've never seen it have another kind of interaction. That doesn't mean it doesn't, Right, It's very hard to study dark matter because you can't like take one particle in the other particle. We've never even seen a particle of dark matter, and so we don't know if there aren't other weird forces that dark matter does feel with itself. We just have a note. It's totally possible. Yeah, it needs a better publicist maybe. All right, let's get into the question of can you make a star at a dark matter and what would that take and would we ever find them? But first let's take another quick break. All right, Daniel, we're talking about dark matter stars dark stars, and we talked about all the things dark matter would need to have in order for them to become stars. But we know that dark matter at least doesn't have one of those things. It's not sticky. We don't know that it has any way to be sticky. But we have some guesses, and you know, the best theory of dark matter we have right now the sort of leading candidate something called the WHIMP, the Weekly Interacting Massive Particle, And this says, well, maybe dark matter is made out of one kind of particle, and maybe it does have another interaction. It's just really really weak. And when they say weakly interacting, they don't mean the weak force with a capital W, the one we know and love the neutrinos interact with. They mean a not very powerful force, like a feeble force. So maybe there is some other kind of force out there we've never discovered before. That lets dark matter interact with normal matter in some very very gentle way we just haven't seen yet, all right, So then the question I guess is could you make a star out of dark matter? Or what kinds of stars would you be able to make? Well, there's sort of a variety of answers, you know, there's like maybe plausible stuff to less plausible to totally bonkers and crazy my favorite kind go on. So the most plausible, like best worked out scenario for dark stars is something which is a bit or a cop out because it's not just dark matter stars. It's like a combination. It's an object that's made of both normal matter and dark matter together acting in a weird way that you might call a star. But it's an interesting idea, like interacting with each other or just hanging out in the same place. Because I met like even our sun probably has dark matter in it, or it has dark matter floating around inside of it. It definitely has dark matter floating around inside of it, but that dark matter isn't very dense, like dark matter is everywhere and there's a lot of it, but it's sort of spread out evenly through the universe we think, And so in any particular volume, there's not that much dark matter, so the dark matter in the Sun doesn't really affect it. Like inside the volume of the Earth, there's about one squirrel's worth of dark matter, so it doesn't really have much gravitational effect on anything on Earth. Right, But it's a really big squirrel the side of the Earth. It's got some superpowers. I'm sure that's the side of dark star. It's dark squirrel. To the idea for these combination stars is that maybe they were the first kind of stars in the universe. So take your mind back to the very very early universe when there's gas and dust and all sorts of stuff left over from the Big Bang, but we haven't had a chance yet to form those into the first generation of stars and what we call population three stars. But there's also a bunch of dark matter, and so this idea is as well, maybe dark matter does have some kind of interaction, some way to get sticky that we just haven't seen yet. We haven't noticed yet because we haven't been studying it for long enough. And if that happens, then maybe sometimes dark matter bumps into itself and explodes and like turns into radiation, like giving off light or other kinds of particles, And this would be a rare thing to happen, because if it happened a lot, we would have seen it already. But according to their calculations, it's not something we can rule out. So then the ideas you have this like blob of dark matter sitting inside a cloud of normal matter, and the blob of dark matter is bumping into itself, annihilating, producing this radiation not from fusion, but from this other kind of interaction, and that radiation prevents the cloud from collapsing. So this cloud, which might eventually otherwise turn into a normal star, is now this big, huge blob of stuff gently warmed by the radiation from this annihilating dark matter. I see now, what makes us think that dark matter would annihilate with itself? Like, is there anti dark matter as well? There could be. There are various theories of dark matter. Remember that there are two kinds of particles. There are direct particles like the ones we're made out of, that have an antiparticle, and then there are myrona particles that are their own antiparticle. And we don't know if dark matter is direct or myrna. We don't know but in a lot of theories there is anti dark matter, and so it could bump into itself and it could annihilate. Right until the idea then is that you have a normal star with like a dark matter turbo. They're generating extra radiation. Yeah, but it's not even a normal star. Is just a cloud of stuff. It's not compact enough to do fusion, and the dark matter is preventing it from collapsing. It's got this like little glow of dark matter radiation that's keeping it from actually getting compact enough to become a real star. These things are huge. They could be like one or two thousand a U in radius, like really really enormous clouds, but not very dense. Right, au is the width of our solar system right A use the distance between the Sun and the Earth. So two thousand a U would be much much bigger than even our solar system. So we're talking about a really huge cloud and it would be gently warmed by this dark matter annihilation, not very very warm, not very hot. In fact, the temperature of this thing would be so low that it would basically be invisible. Right. But then maybe the question is what would the dark matter be doing there? Why would it annihilate there and not elsewhere, or are you saying it's annihilating everywhere. This is assuming that dark matter somehow clumps, right, If it's going to be able to annihilate, that means it has some kind of interaction, which means it's a little bit sticky, and so it could clump. And so dark matter could annihilate anywhere. Anywhere the dark matter bumps into other dark matter, it could annihilate. It would be more likely to happen in places where there is more of it. In fact, we have experiments looking for exactly this. It's called the Firmy Space telescope. It points to the center of the galaxy where we think there's a lot of dark matter, to see if it can see special flashes of light that come from annihilating dark matter. So far nothing, but the idea behind this theory is that maybe that happens, and may You've got clumps of dark matter in the early universe which then annihilate, and if they happen to also be in the center of a big cloud of gas and dust, it gently warms it and you can call that a dark star. So what would do this look like if we look out into the nice guy. You would would look like a cloud of dust that was glowing a little extra more than usual. Yeah, it would look like a big cloud of dust, which would be mostly invisible unless you use the right frequencies to see it. And it would be a little bit warm. So it would be a cloud of gas and dust that would be a little bit warm and would give off some weird radiation. You might be able to see that dark matter radiation, like really high energy photons or a bunch of extra neutrinos from that dark matter annihilation at the heart of this dark star. Wow, And that's the boring possibility here for a dark star. That's like most detailed, worked out possibility. The rest of it is just sort of speculation. You know. If you're going to go that way, then you can say, well, dark matter might be more complicated. Why do we think dark matter is just one kind of particle? Right? Our matter is many, man the kinds of particles. We have different kinds of quirks and leptons that do all sorts of crazy stuff. Why would we imagine that dark matter would just be one particle. That's boring, and the universe seems to be complex. So let's imagine like a whole different spectrum of dark matter with lots of different kinds of dark particles and maybe even new dark forces that they can use to interact with each other. M I see. The idea is that maybe dark matter is not this like homogeneous cloud of one stuff, but rather it's like complex like we are, like like our matter, Like there's maybe different kinds of it, and they're all interacting with new, different kinds of forces. Although they're all invisible to us, they have like a rich life in their little clouds of dark matter. Yeah, exactly, because there's two separate ideas. There can dark matter interact with us, and that might be only through gravity, But even if it can only interact with us through gravity, it might have really complex self interactions and might be able to do all sorts of crazy things to itself with itself that we can't see and we can't do. And so then maybe you could use some of these interesting forces, dark forces and different kinds of dark particles to make a dark matter sun. Exactly now, to be consistent with what we see in the universe that mostly dark matter is cold and slow and fluffy, you can't have all the dark matter doing this crazy stuff. But you know, we can't probe all of the dark matter, even if like only of the dark matter is this cold, slow, fluffy gas of particles. That still leaves you with ten percent of the dark matter to do crazy interesting stuff with, and we wouldn't notice. It wouldn't change the distribution of dark matter that we look at. And ten percent of the dark matter is about the same mass as all the normal matter in the universe. And so if you'd like allocate ten percent of the dark matter to have complicated interactions and do complicated things, then it might form really weird interesting emergent phenomena like we're talking about before. It would be really hard to predict what quirks and leptons look like when you get ten to the forty of them into a star. Right, what happens in the universe when you get these things together and wait a billion years, and now you have new kinds of particles with new kinds of interactions that might form dark stars, but they might also form something else totally crazy we never even imagined because we've never seen that kind of matter before. Right, it would have his own interactions, and then what would this dark star emit would it emit like a different kind of light, like, can we call it dark light? Yeah, dark photons exactly. There could be a copy of electromagnetism, which only works on dark particles, particles with a dark charge, and they could emit a special photon called a dark photon, and it could be very very similar to the kinds of things we see over here. It could be like a copy. You know, we see that a lot also in particle physics, that structures we see in one place are replicated elsewhere because they refer to deeper patterns in the universe we haven't understood yet. So it could be that all the forces we see over here in normal matter are copied over in dark matter. There's just a special dark version of them, in which case you could very well get dark stars having dark fusion with a dark strong force and emitting dark photons and dark neutrinos, and to have dark planets around them. Yeah, and that the whole universe of light and matter would be existing right on top of us right basically and just be kind of invisible to us, intangible to us, intangible and invisible. Yeah, it can be literally here with us and we wouldn't even know it existed. The only way we could communicate with it would be through gravity. Like imagined that on top of us is another solar system with a dark star at its center and dark planets in orbit, and we don't know about it. The only way they could, for example, here about it is if you, like, we create a black hole the Large Hadron collider. They would notice that. Yeah, I think everyone would notice that. Unfortunately, we would have noticed that we're very long. Yeah, and there could even be like you know, dark podcasts host talking in a dark podcast episode about what we're made out of? Right, Yeah, because to them we are dark. Right if we can't see them, if they are intangible to us, then the reverse is also true. Right. We call ourselves normal matter, but they might be calling us dark matter because they can't see us, or touch us, or interact with us. I mean, they would call us light matter. Maybe that would be cool, maybe, yeah, I suppose, But they wouldn't even know what's going on with us. They wouldn't understand how we interact because they wouldn't be able to probe us other than through gravity. So in this picture where dark matter only interacts with normal matter through gravity, which is the only thing we've ever seen but has really complex, rich self interactions, which is possible as long as it's not all of the dark matter. We could have it be fairly simple copy of our matter and our particles and our interactions. But it could also be something totally wacky and different. And that's the scenario that I'm hoping for, you know, something totally weird and new, because then it would form weird structures that we wouldn't even have names for. You could come up with the names for them when we discover them. Do you have that sign above your door, like hoping for wackiness, looking for waking. I'm rooting for the bonkersness in the universe for sure. And then also these dark stars, could they form dark matter galaxies and more absolutely, you know, we don't know what kind of structures they form. We know that our galaxy started with dark matter. The reason that we have a galaxy today is because there was a big blob of dark matter around that pulled together the normal matter and let it form these stars and form these galaxies. Right, so you could imagine also just a pure dark matter blob, Like what if you had a huge blob of dark matter out there in space with no normal matter in it. So the dark matter is not just there to help the normal matter turn into a galaxy. It's just going to turn into its own galaxy, and it makes its own dark matter stars that certainly could be out there, all right, Yeah, well then I guess maybe to answer the question of the episode, can you make a star out of dark matter? It sounds like the answer is maybe, but not likely. Maybe, but we're totally clueless on the critical question of whether dark matter can interact with itself. If dark matter has no self interactions, then to be pretty tough to see it making a star. If some subset of dark matter has really complicated self interactions, then yeah, absolutely you can make stars and all sorts of other crazy stuff. Right, But from what we know of dark matter, you know, it seems like what we know of it is that it's kind of diffused and blobby and cloudy out there. Could it be that there are stars out there, we just see them as the aggregate of them, you know what I mean? Like, maybe the universe is full of dark matter stars, but we don't see them as pinpoints. We just see like a fuzzy version of them. Yeah, that's really the cool idea. It's possible, but that would also change the whole dynamics of how they interact with themselves and the overall speed of them. So I think it would give us sort of a different distribution of stuff. So we can tell a little bit about how slow moving and how diffuse the dark matter is just by like the overall distribution, like the velocity and where it is. So it's much more likely that it's big fluffy clouds than that it's a smaller number of stars these pin pricks. Oh, I see you're saying that maybe dark matters can't exist, but maybe they're rare. They would be rare if they could exist. Yeah, if they do exist, then there would be just a portion of the dark matter, not all of it. But there's plenty of dark matter out there, So if you want to slice off a ten percent of it to make dark matter stars, that would be totally acceptable. Or if you want to make a Marvel superhero called dark Star. It's too late that one already exists, but you can option it for your media. Yeah, I'm sure Marvel was eager to sell out their IP and you know, we have looked out there in the universe, and we have seen some really weird galaxies, like some galaxies have extra dark matter. We found one that has a hundred times more dark matter than our galaxy. So there could certainly blobs of stuff out there that are pure dark matter and we just haven't seen them. Right, Yeah, it just has an enormous amount of icing on that cupcake. That's like my kind of bakery exactly, all right, Well, as usual, the answer is stay tuned. As scientists learn more about dark matter and what it can and cannot do to itself and how sticky it is, we'll know whether or not these stars can exist and how often you find them in the universe, if you can find it. And what we do know is that we are still at the very beginning of understanding even what the universe is made out of, and so we haven't even really begun to think about what that stuff can do. Can stars and camels and all sorts of crazy stuff or something else completely bonkers that nobody has even thought of. All right, Daniel, I think I'll give you a star for the podcast episode, but maybe a dark star so you can't see it or touch it or feel it, but it's there. Trust me. You's gotta you know, close your eyes and feel it's pool well, it's intangible and invisible, and yet it means so much to me. All Right, thanks for joining us, See you next time. Thanks for listening, and remember that Daniel and Jorge explained. The Universe is a production of I Heart Radio. For more podcast from my heart Radio, visit the I heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows. Ye