Could black holes help form stars?

00:00:08
Speaker 1: Hey, hoorra, I have a visual challenge for you.

00:00:12
Speaker 2: Ooh, I'm qualified for that.

00:00:13
Speaker 1: So I'm thinking about how babies always seem cute. You know, kittens, puppies, even little baby plants are cute.

00:00:20
Speaker 2: Yeah, Candy and I, babies are cute.

00:00:22
Speaker 1: So I'm wondering if we can apply to some of that visual arts logic to physics. Can you think of a way to make like a baby star look cute?

00:00:31
Speaker 2: Oh? I thought you were gonna ask me if I can make physicists look cute. The answer is no, that's impossible. But what do you mean like the stars of babies.

00:00:38
Speaker 1: Well, we say that they're born in stellar nurseries, and we have you know, lullabys like Twinkle Twinkle, Little Star. So how would we draw a baby star to make it look cute?

00:00:49
Speaker 2: Well, you know, maybe you can give it like a plasma ribbon or have a burp little cute solar wind burps. I wonder if that would make people go, oh, that would be the plan. Or we should come up with like an internet meme like baby star, baby Star Star Star Sistar Star baby.

00:01:09
Speaker 1: Star sounds like a viral TikTok video.

00:01:12
Speaker 2: Bam, We're billionaires. Hi, I'm more ham Mad, cartoonist and the creator of PhD comics.

00:01:31
Speaker 1: Hi I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I think the universe is beautiful, but I'm not sure I would call it cute. Hmmm.

00:01:41
Speaker 2: It's pretty cute, right, the way it all works together, that's pretty like cute, but maybe like in an ironic sense, like generalativity.

00:01:48
Speaker 3: Oh that's cute. Oh like clever, yeah, clever. Yeah, yeah, it's clever. I think it's more grand though, and dramatic than cute. Cute makes me feel like we're minimizing it somehow.

00:01:59
Speaker 2: M see what you mean. Yeah, but still it Maybe it can be grand and cute. It can be cute in a grand way.

00:02:04
Speaker 1: Does anybody ever look at the Grand Canyon and say, ooh, what a cute canyon.

00:02:09
Speaker 2: Well, it maybe depends on your size, Like if you're a giant, maybe the Grand Canyon looks cute to you. Yeah, there you go. If you're the size of a galaxy, maybe the universe would be cuter. Hmm.

00:02:19
Speaker 1: So maybe astronomy and cosmology is grand and particle physics is cute.

00:02:23
Speaker 2: Yeah, Or maybe you're just too small.

00:02:26
Speaker 1: I'm just trying to get you to call me cute. That's the whole game here. I see it's not going to happen though.

00:02:31
Speaker 2: Oh that's cute, Daniel, right, success, But anyways, Welcome to our podcast Daniel and Jra Explain the Universe, a production of iHeartRadio.

00:02:41
Speaker 1: In which we try to avoid getting too cute and do dig deep into the mysteries of the universe. We want to understand all of the grand questions of reality. How does it all work, where does it all come from, how big is it? And fundamentally, what are the rules that decides what happens and what does not happen. We cast our minds out into the universe to look at all the crazy things that we observe black holes and stars and galaxies and tiny little quirks and photons, to try to understand how all of it comes together to make our universe.

00:03:13
Speaker 2: That's right, because it is a pretty adorable and cuggly universe. And every episode we try to have a little meat cute here between the universe and your mind.

00:03:21
Speaker 1: We want you to look at the universe and understand something about how it works and go aw, that's so cute, or at least that's so clever.

00:03:29
Speaker 2: It's a fine line between awe and awe, and you, well, then you got to change one of the valves.

00:03:35
Speaker 1: But it's true that we often look at the processes in the universe or discover something that's happening out there, and even if we don't say awe, we might say wow, that's awesome, that's cute some at least, I mean the sheer power and the magnitude of what's going on, like at the heart of our star, or understanding the forces involved in the energy scales of the formation of the Solar system, or the black holes at the centers of galaxies. It really makes you feel tiny. It makes you understand that the universe operates at these grand scales that are almost beyond our ability to understand, and yet here we are on this tiny little rock trying desperately to understand it.

00:04:14
Speaker 2: It's a pretty amazing universe, and one thing I think a lot of people forget is that it's still happening right now. I think to think of the universe as super duper old and establish and fix out there in the cosmos, but actually there's a lot going on right now, including new stars being born.

00:04:29
Speaker 1: Yeah, when you look up at the night sky, you think, oh, this is the night sky. It's always been the night sky. My great grandparents and my great Kirkirkirk grekork great grandparents saw the same stars, and my grandchildren will see the same stars. It feels immutable, feels unchanging. But we know that the cosmos are dynamic. We know that things are changing up there. It just happens sort of over deep time instead of over the sort of minutes and days and months that we are used to. But if you look in the right places at the right times, you can still see stuff happening and learn something about how the universe works.

00:05:01
Speaker 2: Yep, it happens over long stretches of times. But it's also happening right now as we speak. There are probably countless stars being born right now. There's a bunch of baby stars out there in the night sky.

00:05:11
Speaker 1: That's right. Stars are being born in stellar nurseries, and stars are dying, exploding in supernovas, or just fizzling out. Stars do not last forever, And what I think is interesting is that we give them a life cycle as if stars were alive. You know, they have a process of formation and then dissipation or ending. But they're never like life in the sense of the bile and just think about it. But we talk about them being born.

00:05:35
Speaker 2: And how do you know, Daniel, have you asked the star?

00:05:38
Speaker 1: I have actually shouted at the Sun many times and never heard a response.

00:05:41
Speaker 2: Well, that could just be you, not the star.

00:05:44
Speaker 1: Maybe I just don't speak sun ease or stella.

00:05:48
Speaker 2: You need to get the right email address or a toll freak number to call.

00:05:52
Speaker 1: Maybe my accent is just terrible and I can't understand what I'm saying.

00:05:55
Speaker 2: Oh right, maybe you need to speak star ease.

00:05:57
Speaker 1: In order to spark a conversation.

00:05:59
Speaker 2: Yeah, and illuminating discussion about the life of stars.

00:06:04
Speaker 1: But the formation of stars, how they operate internally, how they die, These are still big questions in science. We look out there in the universe and we see regions where stars are being formed rapidly, in regions where galaxies have quenched, where they are no longer making stars. This is a really important thing to understand in the universe because formation of stars is pretty basic to creating the conditions for life. If we'd like to understand our very very long term future in the universe, we need to understand if more stars are going to be made for us to eventually move to when ours fizzles out.

00:06:37
Speaker 2: Yeah, there are a lot of amazing ways that stars are being born out there, and every day we're also discovering new ways that stars might be born out there. And so today on the podcast, we'll be tackling the question can black holes form stars?

00:06:55
Speaker 1: I think this is a little bit of a rehabilitation of the reputation of black holes.

00:07:00
Speaker 2: Oh whoa, whoa, whoa, you just went deep there? What what's wrong with black holes? Do they get canceled in culture?

00:07:06
Speaker 1: I mean they're seen as destructive, right. They're like the vacuum cleaners of the universe. They slop stuff up and disappear it. They're not usually talked about. It's like part of a process of rebuilding. They're like the end point of the universe. Everything will finally end up inside some vast black hole, separated by infinitely empty space. But instead it might turn out that black holes have some positive sides to it. Well.

00:07:28
Speaker 2: As a person who cleans up the dishes and washes them every night in my house, I would say that, you know, props of the whoever is out there in the universe cleaning things up and vacuuming all the all the dirty dishes.

00:07:39
Speaker 1: I mean, the universe is a bit of a mess, but I don't want to see the whole thing just like sucked up into a giant black hole. That would seem kind of dull. You know. Sometimes you got to embrace the mess.

00:07:48
Speaker 2: That black holes were exciting, but you don't know what's going on. Maybe there's a big party inside the black hole.

00:07:53
Speaker 1: Maybe they're actually just a big mess. You know, we thought they're cleaning things up, but it's sort of like that closet you shove stuff in and if you went inside, you'd be like, oh, oh no, this is a disaster.

00:08:01
Speaker 2: But full of amazing things.

00:08:02
Speaker 1: I'm sure that's right. You could learn so much about the universe or that family by opening their closet slash black hole.

00:08:09
Speaker 2: But this is an interesting question, I think because we don't often associate black holes with the formation of stars, like usually maybe think of stars forming out there in space and the emptiness of space in a big dust cloud. But to think that it could maybe be born from a black hole, or buy a black hole, or through a black hole, it's pretty interesting to think.

00:08:28
Speaker 1: About Yeah, it's a really fun counterintuitor topic, and I recently read a paper exploring this and I thought this would be fun to talk about.

00:08:34
Speaker 2: Well, as usual, we were wondering how many people out there had thought about the possibility of stars being born from a black hole or buy a black hole, So as usual, Daniel went out there into the wilds of the Internet to ask people do you think black holes can form from stars?

00:08:49
Speaker 1: So thanks very much to the denizens of the wild Internet who answer these questions for us. Everybody is welcome to do so, don't be shy. Write to me two questions at Daniel and Jorgeedon and I'll set you up.

00:09:01
Speaker 2: Here's what people had to say.

00:09:02
Speaker 4: I feel like they could definitely form some stars. You know, they're stripping some material off of some stars, probably move some material from that star over to this spot and maybe becomes enough material to make a star.

00:09:16
Speaker 2: Well maybe if they only if only hydrogen falls in and they radiate away enough, most become a stag in.

00:09:23
Speaker 5: I don't think that black holes can form stars directly, but maybe they can form them indirectly, you know, since they have such a strong pull of gravity, I'm sure they have influences on the molecular clouds around them, and maybe they can help stars form that way.

00:09:43
Speaker 2: All right, some interesting possibilities here. Two out of three people in the Internet think it's popple.

00:09:50
Speaker 1: So boom, that's it. Question settled.

00:09:52
Speaker 2: That's right. We all know how well polls work on the Internet.

00:09:55
Speaker 1: Hey, this was done by a scientist, so therefore it's scientific. Right.

00:09:59
Speaker 2: Sure, that's cute.

00:10:01
Speaker 1: It's not pretty, but it might be cute.

00:10:03
Speaker 2: Yeah. So some people here thought that it wasn't possible for stars to be born from black holes, and some people thought maybe, like enough hydrogen falls in and or it gets shut out, it maybe can become a star again.

00:10:14
Speaker 1: Yeah, there's some really creative problem solving here. I love hearing people use the physics in their minds to try to tackle a new question. I mean, that's really what physics is all about. It's a little set of tools to try to answer questions about the universe. And everybody who's doing that out there in their heads is doing physics. You're all physicists deep down. I mean that as a compliment, of course.

00:10:34
Speaker 2: You mean, you're all cute on the outside, but physicists on the inside. Is that what you're saying, which means you're clever all the way through. All right, Well, let's dig into this possibility of black holes forming stars, and let's start with the basics. Let's talk about what a black hole actually is.

00:10:48
Speaker 1: So, boy, do I wish I knew what a black hole actually is. You know, we tend to think of it as like the endpoint of the life of a star. You start with a big blob of stuff and gravity pulls it together and you form a star and it burns for a long time, and in burning, it helps prevent further gravitational collapse. You know, gravity is always tugging on this stuff. It's always pulling one bit of hydrogen closer to another bit of hydrogen. But gravity is pretty weak, so it has to sort of wait until all the other forces have stopped working in the opposite direction. When a star is burning, it's pushing energy out. It's like blowing radiation out, and that prevents the star from collapsing further. But that fire can't last forever, and eventually it burns out and then gravity takes over and the whole thing collapses. If you have enough mass and that initial clump of stuff, you know, maybe more than forty times the mass of our Sun, then it can collapse and form a black hole, which is essentially the gravitational endpoint. When gravity's collapsed, everything to be so dense that space is twisted and, as you'd like to say, forms a hole, a place where anything that goes in can never come out.

00:11:57
Speaker 2: Now, just to tear, Not all stars end up. Dye is a black hole, right, It has to be a certain kind of star, and even if it does collapse, they don't all form black holes, right. In fact, it's kind of rare, right, Most stars just keep aching and lead a long and pretty unexciting demise.

00:12:14
Speaker 1: Yeah, that's right. And the endpoint of a star is determined almost entirely by its initial amount of mass. So if you don't have enough mass to even burn and start fusion, then we call you like a brown dwarf for a failed star. If you're a low mass star, like less than eight times the mass of our Sun, you don't form a black hole. It'll become a red giant, and then maybe like a planetary nebula, at the core will be something like a white dwarf, which will last for a long time and eventually cool into a black dwarf, but will not form a black hole. Only the larger stars that have the capacity to become a black hole. So bigger than like eight or ten times the mass of our Sun, you end up as a red super giant. And then you get these core collapse supernova And if it has enough mass, if it started out with more than forty times the mass of the Sun, it's almost certainly going to end up being a black hole. But you're right, those are rare because the most common type of star in the universe are not the massive stars. They're the little red dwarfs, stars that are cooler and smaller than our star. Because the smaller the star, the colder it is at it's core, and so the dimmer it burns. So very big stars end up as black holes, but they also don't last very long. Very small stars are cooler but last longer. The smallest ones can burn for billions and billions, maybe even trillions of years.

00:13:30
Speaker 2: Yeah, like you said, it's rare, and only if it has enough mass. But even if it has a lot of mass and it collapses, well, I guess we're not super duper sure that a black hole will form, right, Like they could also form, like you said before, a neutron star maybe, or we actually don't know. We've never seen a black hole born out of a star.

00:13:46
Speaker 1: So if you have like more than ten times the mass of the Sun but less than forty times, you're likely to make a neutron star, which is a very very dense object filled with neutrons. But those neutrons are resisting gravitational collapse. They're preventing it from becoming a black hole. If it happens to eat some more mass and then get over the mass threshold, then it can collapse into a black hole. But if you have enough mass, it's almost inevitable to become a black hole, at least if black holes even exist. As you said, we're not even really sure that black holes are black holes. The way we think about them, we have a description of them in general relativity, which uses a pure gravitational description and ignores all the quantum mechanics and says that gravity will pull everything together to this tiny dot a singularity at the heart of these black holes and form an event horizon around them. That's the general relativity view, and for a long time that was basically the only idea out there that could explain what we were seeing in the universe. We saw these very dark, very dense blobs of matter that we could only explain with black holes. Now though we have other ideas. Maybe these are fuzzballs, maybe these are dark stars, maybe these are something else entirely. Last week we talked about balls of vacuum energy that could look like black holes, So there's been a real flourishing of ideas for what these things actually are.

00:15:02
Speaker 2: Yeah, and also not all black holes come from stars, right like, there are some black holes that may be formed at the beginning of the universe, right from in the Big Bang. And there's also kind of a mystery how some black holes out there form, right like some of the big ones in the middle of galaxies. Were not quite sure how those started.

00:15:19
Speaker 1: It's definitely a mystery how black holes the hearts of galaxies started. We tend to have two categories of black holes. The ones that came from like an individual star like we just talked about, and then these mammoth black holes we call them super massive black holes that the hearts of galaxies that can have masses of millions or billions of times the mass of the Sun. And these we think are black holes in the same category as other things, although some theories say there's something totally different. But you're right, we don't understand how they got so big. If you look back into the deep history of the universe, we can see galaxies forming in like the first billion or two billion years already having huge black holes at their centers, and we don't understand how they got to be so big. One theory, as you said, is prime ordial black holes, that maybe black holes were made during the Big Bang, before there actually even was matter, black holes were made, and they may have served as like seeds for these supermassive black holes. That's just one idea, but it's definitely a big question mark how these black holes at the hearts of galaxies formed. But you know, we have seen black holes like in the act of feeding. Black holes don't just have to form from like their initial star. They can later on gobble up other stars and get bigger.

00:16:27
Speaker 2: Well, So it's kind of like a weird cycle of stellar life out there in the cosmus, right, Like some black holes are born from stars, and then those black holes and start eating up other stars.

00:16:38
Speaker 1: Yeah, you might have a star that burns and would eventually become a white dwarf, but it gets gobbled up by a black hole, so it eventually ends up as a black hole. It might not have had enough mass to make a black hole on its own, but it gets vacuumed up into a neighboring black hole. And this is really dramatic and sort of amazing to watch.

00:16:55
Speaker 2: Yeah, because we've actually seen this happen, right, Like we have pictures of it.

00:16:59
Speaker 1: Yeah, I've actually seen this happen. There's a great example using X ray telescopes that watch this happen to a black hole and another galaxy. Two hundred and fifty million light years from Earth. There's a black hole in the center of another galaxy. It has like ten million solar masses, and they watched the star get too close and the star was pulled apart by the gravity of the black hole before it even went in. It like pulled it apart into a long river of hot gas. Remember that gravity does more than just tug on things. It can also tear them apart because of the tidal forces.

00:17:32
Speaker 2: Right, Because I guess the part of the star that's closer to the black hole gets pulled faster or stronger, and so then it starts to get stretched out, sort of like if you're sucking up with a vacuum a big pile of legos or something, it's going to break up the cluster of legos.

00:17:47
Speaker 1: Yeah, gravity pulls more strongly on things that are closer to the source. And if gravity is really really powerful and you have something big enough, then the force on one side of it is pretty different from the force and the other and that's effectively like gravity pulling that thing apart. I mean, the Earth right now is pulling on your body with a different force than it's pulling on your head, and effectively, that's like the Earth trying to pull the head off of your body. It's not very powerful because Earth's gravity is pretty gentle and your head is pretty close to your body, but if you were a big enough object and close enough to a very strong source of gravity, you'd be pulled apart. And they saw this happen to a star. We've seen this happen also to like comets that have entered the Solar System. Common shoemaker levee got pulled apart into like twenty six pieces by tidal forces. Anyway, this hot river of gas was formed and then it fell into the black hole and they could observe it due to this increase in X ray emission from the accretion disk of the black hole. The whole thing took like weeks to months to happen.

00:18:42
Speaker 2: M that's pretty interesting. Do we have pictures of that? Like if can you google that and find the pictures of this star getting spaghetified?

00:18:50
Speaker 1: We don't have cool images of that, unfortunately. We just have like X ray telescope data that showed like rises in X ray emissions as this hot gas enters the accretion disk. We know that it happened. We could sort of like watch it conceptionally, but we don't have like pictures of it, and that would be awesome.

00:19:05
Speaker 2: So then how do you know it's stretched down into a long river of hot gas? You just think it did well.

00:19:10
Speaker 1: I mean, we have X ray data, and we have X ray telescopes, but X ray telescopes don't have the same kind of optics as like optical telescopes and the same kind of resolution. They're more like particle detectors. You're looking at a single stream of photons more than you're like taking an image with high resolution.

00:19:26
Speaker 2: So you're looking at like a pinpoint of an X ray source and suddenly that X ray stars changed and you're inferring that it means that the star got stretched out.

00:19:34
Speaker 1: Yeah, exactly. And you used to see a star which then disappeared, and you see this emission of X rays very close to this black hole, and so you know, there's a few steps of inference there. But people are pretty convinced that this was a star getting eaten by a black hole, or at least by something right, by something, something very dense, and something very dark ended this star.

00:19:53
Speaker 2: Something out there had star pasta for dinner. All right, Well, that's a pretty quick primer on black holes, and now let's get into how a black hole can form a star, whether it can, and what would it mean. But first let's take a quick break. Or we're answering the question can black holes form stars? Now? Are these black holes forming the stars like out of clay?

00:20:30
Speaker 4: Like?

00:20:30
Speaker 2: Are they molding the stars? Are they giving birth to the stars? Are they making, you know, pulling a magic trick making them appear out of nowhere?

00:20:38
Speaker 1: Maybe they're just ordering them on Amazon like everybody else around here?

00:20:41
Speaker 2: Does that count as forming a star?

00:20:42
Speaker 4: Though?

00:20:43
Speaker 2: If I order order a toothpaste from Amazon, am i making the toothpaste.

00:20:48
Speaker 1: I don't know. But if I order a Christmas present on Amazon and give it to my kids, you know, they feel like it was from me, even if I didn't make it myself.

00:20:55
Speaker 2: Oh it's from you, but you didn't form the gift.

00:20:58
Speaker 1: No, that's true. I did not build those earbuds myself.

00:21:01
Speaker 2: There you go. Well, here the question is can a black hole form a star? And I'm guessing maybe it's like putting together some of the gas around or is it, like, you know, is it happening when the the stuff is falling into the black hole? What's going on here?

00:21:14
Speaker 1: So so far we mostly think about black holes is like the death of stars or eating other stars. To understand how black holes can contribute to star formation, we have to think a little bit and understand a little bit about how stars are formed in the first place. Like if you went out to build a star, You're like, oh, Christmas is coming, I got to get my spouse something. What should I do? I'm gonna build them a star? How would you do it?

00:21:36
Speaker 3: Right?

00:21:36
Speaker 1: How do you make a star? Anyway? And it turns out that star formation is a little bit more delicate and difficult than you might imagine and I think all you need is a big block of gas and eventually gravity's going to pull it together. So you know, just get a scoop a hydrogen, put it in space and wait a few million years and there's your star. But it's not so simple. You need sort of special conditions for that gas in order to get it to collapse and form a star.

00:22:02
Speaker 2: Yeah, I know, we talked about this before. It's kind of related to the temperature of the gas, right, and also how much gas there is you need, like the right amount of the right temperature.

00:22:12
Speaker 1: Yeah, it depends a lot on the temperature and also the metallicity. Remember, at the beginning of the universe, we had basically just clouds of hydrogen. That's what was made during the Big Bang, a little bit of helium and a tiny little bit of heavier stuff, but almost entirely hydrogen. So the first stars were all just hydrogen. Then the hearts of those stars fusion makes heavier elements helium, lithium, carbon, oxygen, nitrogen, et cetera, all that good stuff that makes up us and ice cream and hamsters. So to form a star, you need gas that's cold. As you said, it can't be too hot because hot gas means the particles are flying around at really high speeds. And you have to remember that gravity is really really weak, so it can only really pull together stuff if it's almost at rest. If the gas is cold enough, you have a big blob of hot gas, it'll just sit there forever without forming a star. So you need that gas to be cold. And it also depends on how much metal you have and like how it's distributed.

00:23:06
Speaker 2: Maybe not forever, Like eventually wouldn't hot the cloud of gas cool down or eventually some of that energy might you know, get canceled out or something.

00:23:15
Speaker 1: Well, energy is concerned, right, so the energy can't just go away and left literally by itself. I suppose it would expand because there's empty space around it. So hot gas will expand into empty space and that will cool it down.

00:23:28
Speaker 2: Yeah, but I guess I mean, like, wouldn't eventually the gas particles hit each other and lose some of that energy? Right?

00:23:35
Speaker 1: These clouds of gas are pretty dilute, so you can model them as almost entirely collisionless, I'm pretty sure. So what you really need to form a star is you need this gas to be cold. We also need some sort of seed, and that can come in the form of like having a dense little blob of some metal, you know, formed in the heart of another star and then left out in the cosmos to seed another star, or some sort of like shock wave sometimes like a nearby supernova will give a little push to some of these things and collapse them in a way that gets gravity started. But it's got to be gentle. It can't be too hard. If you shock these things too much, then you're just heating up the gas. So you got to like cool it down so it can coalesce and give it like the right kind of nudge or give it like the right seed to get that collapse started.

00:24:16
Speaker 2: You also need the right amount of gas, right, Like, if you have a small cloud of gas, it's going to clump and maybe not clump and form the kind of pressure you need to form a star. Right, You need like a lot of gas so that you get a big clump and then so that the clumps kind of squeezes the stuff in the middle enough to be ready for fusion.

00:24:36
Speaker 1: Yeah, if your clumps are too small, you just get a bunch of Jupiter's. You don't get real stars. And the size of the clumps that tend to form does depend on the metallicity. So in the very early universe, we had big stars form huge clouds of hydrogen. But later on in the universe, we had like more seeds, more like little dots of metal in those clouds. So you've got more stars, but they were smaller, and then they tend to burn longer, and so they burn from bi millions of years rather than just like a few million years or tens of millions of years, like the first generation of stars. So like the composition, so it's sort of like cooking, you know, it's really dependent on the temperature and the exact mixture of what you put in there. You might get a bunch of really big stars, or smaller stars, or just a bunch of jupiters, or almost no stars at all, depending on the temperature of your ingredients.

00:25:22
Speaker 2: M All right, then that's generally how stars form. You need a cloud of gas and then it has to be the right conditions, has to be cold, and there has to be a lot of it. And this happens a lot in certain situations out there in the universe, right like in dwarf galaxies.

00:25:36
Speaker 1: Yeah, so lots of galaxies out there are forming stars, and lots of galaxies out there are not forming stars. We have some bigger galaxies and we have some smaller galaxies. And when galaxies stop forming stars, we say that they are quenched. And actually our galaxy, like the Milky Way and Andromeda, we think are both starting to quench. They're forming like maybe seven stars per year. The rate of star formation in galaxies like ours is a lot less than it was a billion years ago, for example. So it's really interesting to look, as you say, at little galaxies, like what we call dwarf galaxies. These are like little mini galaxies that have not yet combined with other galaxies in order to form mega galaxies. All the galaxies that you see in the Milky Way, the Androma, all the big ones are all just combinations of these little galaxies that came together to form a big galaxy. So looking at a dwarf galaxy is sort of like looking at a baby galaxy.

00:26:29
Speaker 2: All exactly thin, cute. You're kind of cute and fuzzy, aren't they.

00:26:33
Speaker 1: They are kind of cute and fuzzy. In fact, one of the things that's fuzzy about them is what constitutes a dwarf galaxy. Like some astronomers say, you know, a few thousands to a few billions of stars can be a dwarf galaxy. Others have a different definition. There's a big argument about whether the large magallinic cloud, which is a big blob of stuff orbiting the Milky Way, counts as a dwarf galaxy or not. But there's a lot of interesting physics happening in these dwarf galaxies. They're a great place to learn about star formation and about black holes and about how they might be connected.

00:27:05
Speaker 2: Well, I guess, first of all, why is it the dwarf galaxies have more stars being born than regular stars? Are they denser? Are they just like a newer gas.

00:27:14
Speaker 1: Maybe we're not exactly sure because we don't really understand this quenching process. Some theories of why galaxies stop making stars, and so why dwarf galaxies might still be making more of them could be due to the black holes at their center. These black holes don't just suck stuff up, they also emit radiation. As they get really massive, they generate a lot of heat in their accretion disk, which shoots out a lot of X rays, which ends up heating up the galaxy and preventing it from making stars. The other thing that can stop these big galaxies from making stars are the stars they have stars emit radiation. Also, there's stellar wind right or super novas that from the end of stars life cycles. These can also heat up the stellar nurseries too much, so they're not really prime for making stars anymore. Are too hot, and so these dwarf galaxies don't have that as many stars and don't have super big black holes at their hearts, so they might be better places to make stars.

00:28:09
Speaker 2: So the next time somebody asked me like, hey, are you gonna have more kids, I can just say we're quench we got too.

00:28:14
Speaker 1: Hot, exactly. But this is sort of like the astronomical equivalent of teenagers having kids, because now you're having like dwarf galaxies making lots of stars in their stellar nurseries. And there's one in particular that people are really interested in with there's a black hole doing something very strange and weird. It's a dwarf galaxy called Henise h e Nize two Dash ten. It's about thirty million light years away in the direction of the Pike Sis constellation, and it's not that tiny. I mean, it's like a tenth of the size of the Milky Way, but it has a huge star formation rate. People think it's making stars about ten times the rate of the large madell in a cloud, even though it's about the same size.

00:28:56
Speaker 2: Mmm. Interesting, So what explains why this galaxies making so many stars?

00:29:01
Speaker 1: It's not something that we understand all.

00:29:04
Speaker 2: Right, then, how do black holes come into the picture? How are we studying whether black holes can form stars?

00:29:09
Speaker 1: So this dwarf galaxy is super interesting for a bunch of reasons. One is just like to see the black hole being formed, right, Dwarf galaxies are little galaxies, and one question we have is like how do black holes get formed? Do they start really big? Do they grow with the galaxy? Are they coupled to cosmic acceleration? So looking at the black hole, this dwarf galaxy is interesting just from that point of view.

00:29:28
Speaker 2: Oh wait, so this dwarf galaxy has a black hole in the middle.

00:29:31
Speaker 1: We know that we do think this dwarf galaxy has a black hole. We didn't know if all dwarf galaxies have black holes, but this one seems to have it. The sort of distinctive radio and X ray emissions from the heart of this dwarf galaxy that are consistent with a black hole and like not consistent with other explanations like X ray binaries or a supernova or something like.

00:29:53
Speaker 2: At the center of this dwarf galaxy, you can tell there's like you know, gas being maybe stretch out, and as it falls into the black hole, you can see signature X ray signals from it.

00:30:04
Speaker 1: Yeah, exactly. You could see the radio and the X ray emissions you would expect from the gas around the black hole. You can also see something else. This gas accretion disk is swirling around the black hole, and then it's slamming into this other blob of gas basically into a stellar nursery. So stellar nursery is just a big blob of cold gas right where stars might be formed if you get like the right seed to form, where you get like the right supernova shockwave. And this black hole is emitting a bunch of radiation and pushing gas out also right as black holes sometimes do, and it's slamming into this stellar nursery, but it's doing it kind of gently.

00:30:41
Speaker 2: Right.

00:30:41
Speaker 1: Earlier, we talked about how super massive black holes might stop stars from being formed by emitting really powerful radiation that heats it up. This one is emitting radiation and pushing the gas into the stellar nursery, but it's doing it kind of gently. It's moving it like only a million miles per.

00:30:57
Speaker 2: Hour, a million miles per hour.

00:31:00
Speaker 1: A million miles per hour, which is gentle compared to the radiation from massive black holes, which can approach almost the speed of light.

00:31:07
Speaker 2: I'll try that excuse the next time I'm pulled over for a speeding ticket. I was like, great, you know, according to this, I was going really gently, more than gently.

00:31:17
Speaker 1: You know, these super massive black holes have incredible jets of gas. Some of them you can see from almost across the galaxy. These jets that go up and down from the poles of the black hole. They form these quasars, and they're super bright emissions, and these enormous structures spew out of these black holes. But this is a little baby galaxy with a little baby black hole that's spewing gas out, but only at a million miles per hour. So it's slamming into this dense gas, and they think that's what's actually sparking the birth of all of these stars.

00:31:47
Speaker 2: All right, let me see if I got this straight, there's a black hole in the center of this dwarf galaxy. It's sucking in stuff. That's how we know it's there because it's sucking in stuff and as the stuff falls in, it creates a lot of energy. So now some of that energy that it's releasing when it's sucking that stuff in is being shut out and that's pushing another cloud of gas into a third cloud of gas. Is that's kind of what's happening, Like it's actually shooting radiation, not gas, but that radiation is pushing one cloud of gas into another cloud of gas around the black hole at a million miles per second forever.

00:32:22
Speaker 1: A million mile per hour. Burps of gas into the stellar nursery are sparking the birth of new stars in this dwarf galaxy.

00:32:30
Speaker 2: But the gas is not coming from the black hole, right The black hole is just pushing the gas that's already there. Or is it like accelerating, you know, you know what I mean, like slinging it around and then pushing into a cloud of gas. Or is it just emitting radiation and that's pushing one cloud into another cloud.

00:32:46
Speaker 1: It's a little bit of both. The gas is heated up from the radiation from the black hole, and that helps it swirl around really really fast, and some of it slams into this stellar nursery, this cocoon of gas that exists already in the galaxy. And so it's just like depositing a bunch of energy there, but doing it gently enough not to overheat this nursery. It seems to be like really right on the edge there.

00:33:08
Speaker 2: But pushing it just enough so that it creates better conditions for star formation. Is that the idea, like you're like compressing the gas making it denser, or is it that it's causing the ripples that you know you need to like jump start a start.

00:33:22
Speaker 1: Yeah, it could be either. We have lots of different models of star formation. You know, imagine like taking something and needing to compress it, but just enough, and if you squeeze too hard, it's going to blow up essentially, And so this is a really delicate operation. You know, star formation is surprisingly fragile. It's incredible how many stars have been born in the universe given how difficult it is to arrange the conditions for stars to come together.

00:33:45
Speaker 2: Mmmm. Interesting. All right, Well, let's dig into the details of what's going on here and maybe what it means about what we know about star formations and black holes. But first, let's take another quick break. All right, we're talking about black holes forming stars, and specifically there's a dwarf galaxy nearby sort of nearby us called Henis, where there seems to be a black hole in the middle of that dwarf galaxy, but it's also got a lot of gas around it, and somehow the black hole is helping the cloud of gas form new stars, or at least that's the idea, right mm hmm.

00:34:34
Speaker 1: And one thing that's really fascinating about this black hole is that it's not really at the center necessarily. These dwarf galaxies are not as like organized as big old galaxies that form really nice spirals. They're more like fuzzy blobs. And the black hole here is not associated with like any bulge or any other like well defined nucleus. You know. The whole theory of like black hole formation is that they're made from stars, and they live at the very center, the dense place, and so they gobble up a bunch of stars and get bigger and bigger and bigger. This one's a bit of a puzzle because it's not sitting at the center or like associated with the densest place in this dwarf galaxy.

00:35:12
Speaker 2: And you can actually look up a picture of this dwarf galaxy, right, you spell it h E and I ze two dash ten and there are pictures of it online, right.

00:35:22
Speaker 1: Yeah, they're pictures of it, and you can see red stars and blue stars and stripes of gas and all sorts of stuff. It's a bit of a blob. I mean, would you say this is a cute galaxy for Yeah?

00:35:32
Speaker 2: Yeah, all babies are blobs. That's kind of what makes them cute. You're blobby.

00:35:36
Speaker 1: Some of them look sort of like Winston Churchill. I don't know how cute that is, though. I mean, I'm sure your babies are all cute, dear.

00:35:41
Speaker 2: Listeners, and I'm sure all of you Winston Churchill's out there are also cute.

00:35:48
Speaker 1: How many Winston Churchills are listening to the podcast right now?

00:35:52
Speaker 2: You never know. It's a big universe. You never know, or you might be married to Winston Churchill, or maybe you were married to Winston Churchill.

00:36:00
Speaker 1: This is getting pretty surreal.

00:36:02
Speaker 2: We just still want to insult any Winston Churchills or any Winstons at.

00:36:05
Speaker 1: All, or really anybody. We're not out to insult.

00:36:08
Speaker 6: Anybody, especially not this galaxy, which is a cute little dwarf galaxy, but doesn't sort of fit your mental image of a galaxy because it's a little dwarf galaxy, right, it hasn't it formed its big boy structure or it's big girl structure.

00:36:21
Speaker 1: And so it's a bit of a blob, and its massive black hole is not like right at the heart of it.

00:36:26
Speaker 2: Now, is this a galaxy, this dwarf galaxy unusual or is this just like one where we found that we know for sure it has a black hole in it and where we seeing some interesting star formation, or is Henny's pretty typical of dwarf galaxies and we just happened to pick this one because it's convenient to study.

00:36:43
Speaker 1: I think it's the latter. It's one that we have studied, people have identified the black hole in it. It's taken a while to figure that out. There was like papers for years about whether this is a black hole or something else. It's not always easy to identify a black hole in a distant galaxy, especially a dwarf galaxy. So dwarf galaxies having always got as much attention as normal big Mama and Papa galaxies have because they're harder to see, you know, they're fainter. But recently we've understood that they're really great laboratory for understanding the formation of galaxies and dark matter and black holes. We had a whole episode recently about the physics of dwarf galaxies, so it's really just sort of like taking off. It's like a whole new field within astronomy, the study of dwarf galaxies, and so this is one of the first ones. We don't really know the general properties of dwarf galaxies very well because a lot of them are missing. You know, we expect in our simulations to see lots and lots of dwarf galaxies sometimes around big galaxies, and we don't see very many. And we don't always know if that's because our simulations are wrong, or we just can't see them because they're so faint, or they're mostly dark matter or what. So. Lots of big open questions about dwarf galaxies. This is just one that we happen to study recently.

00:37:51
Speaker 2: Okay, now, as you were saying, this black hole in the middle of Hennis that the dwarf galaxy is spouting out a jet of gas or a jet of radiation. And sometimes black holes don't spout out radiation like that, They just suck stuff in, and sometimes they shoot stuff out so violently that it pushes everything away and disrupts any possibility of stars forming. But this one seems to be like just in the right Goldilogs situation here, where it's shooting out radiation, but it's not doing it in such a way that it disrupts things. In fact, it's sort of doing it gently enough that it's maybe helping more stars form in this dwarf galaxy. Do we know why, We don't know why.

00:38:30
Speaker 1: And in general, we'd like to understand more about how black holes form in these galaxies and whether they are linked to the mass of the galaxies. In lots of cases we see a connection between the mass of the black hole and the mass of the galaxy, where bigger galaxies have bigger black holes, but also sometimes we don't see a connection. And we've recently talked about a paper that saw that black holes the hearts of galaxies grow faster than the galaxies do, suggesting that they're maybe not just eating mass that there's some other weird thing connected to dark energy. Right, it's a big question of like the relationship between the mass of a black hole and the mass of the galaxy that it's in. It's not something that we understand, especially for these super massive black holes, these ones that are bigger than just like the endpoint of a single star. So it's a great way to study not just star formation, but also to understand how black holes get their start, right, because again, this is not a super massive galaxy with a super massive black hole, and this is like looking back to its childhood to understand how it got so big so fast.

00:39:28
Speaker 2: We're looking at this baby galaxy as a way to study baby galaxy development kind.

00:39:33
Speaker 1: Of yeah, exactly. And this one is exciting because it sort of hints towards this primordial black hole theory. You know, this idea that maybe black holes, the ones become monsters, that the hearts of galaxies didn't form the way we described earlier, just like a single star that collapses and then gradually eats stuff. Maybe they got a boost, they got like a head start by being formed very early on, before even hydrogen was formed. These primordial black holes which then just grew rapidly. The idea here or is that maybe that's the reason it's not like right at the heart of the galaxy, where like there's the densest blob of matter. Maybe that's why it's sort of like offset a little bit, because it was seated by a primordial black hole. Not just like the formation of a black hole at the densest spot in the galaxy.

00:40:16
Speaker 2: Already just moved to the suburbs, you know, for quieter light. That's what makes it so gentle.

00:40:22
Speaker 1: But this is a young black hole, right It's going to want to be at the heart of the action. It's going to move into the city, right where all the clubs stay out.

00:40:29
Speaker 2: I say that all young black holes are hipster black hole. I thought Brooklyn was where it's at, not Manhattan.

00:40:35
Speaker 1: I mean, maybe this is a black hole with an old soul. I don't know.

00:40:38
Speaker 2: Yeah, you go, But I guess the point is that this black hole in this dwarf galaxy is actually helping make stars, right Like, without this black hole, this dwarf galaxy wouldn't be making as many stars. Now, is the idea that this is happening all over the galaxy or this is like a weird situation like, could it be that our star was somehow aided by a black hole or that black hole holes are helping stars form all over the our galaxy and in other galaxies.

00:41:06
Speaker 1: It's certainly possible, and what it does is paint a picture of star formation is more complex than we previously imagined. You know, we had a sort of simple model before these clouds of gas which then collapse, and now we understand that creating the conditions for a star formation is tricky and lots of different processes out there in the universe can play a role, probably more than just black hole formation. Other things we haven't even thought about might trigger star collapse or end star collapse. So this is something that astronomers are really digging into in detail now, finding regions of the universe where stars are being formed, rapidly finding regions of the universe where stars are not being formed, trying to understand all the effects at play. And it's a very complex situation. It's not like the kind of thing you can study in isolation and say I'm just going to take a blob of gas and think about how it works. It's something that really depends on everything that's going on in the neighborhood. You know, you have to understand not just nearby black holes emitting radiation that might coax your stars to forming or might quash their ability to form stars. You also have to understand, like the dark matter halo for this galaxy, is the dark matter halo getting too big so that it's pulling in too much gas from the intergalactic medium, which then like heats up all the gas in the galaxy and make it stop forming stars. To understand how this works, we're going to have to get almost everything right. And that's the kind of problem that's really hard. You can't like break it into pieces and isolate and think about it all by itself. You really got to get lots of different elements in place all at once. So it's sort of like a big grand challenge for astronomy over the next couple of decades.

00:42:37
Speaker 2: I guess there's a lot going on here out there in the universe, and there's a lot that could be disrupting or influencing the formation of stars.

00:42:45
Speaker 1: Yeah, exactly. And we're working on this sort of in two directions. On one hand, we're like looking out there to see what's going on, and on the other hand, we're building models, we're doing calculations to predict what we should see. Right, Scientias progresses along these two fronts, the experimental or observational and the theoretical. Can we explain what we are seeing? So people working on the theoretical side have these models, these simulations where they build little galaxies and watch them form and measure the star formation rate and see if they get it right. But you can never simulate an entire universe or even entire galaxy. These simulations are always simplified. Sometimes they for example, like only have one kind of matter, or they ignore the dark matter, or they ignore the non dark matter, or they ignore the supernovas. To add all those details, all those bells and whistles into your simulation is really complicated. So at first we were able to do it with sort of simplified versions, But now what we're learning is we really got to break out all the bells and whistles to understand star formation. It's not something where you're going to understand it with a simple model using only one component of the universe. It's like a whole symphony of chaos that sometimes come together to form just the right conditions.

00:43:48
Speaker 2: And sometimes there's a lot of stuff out there that you can't see, right, like these black holes. Maybe you didn't know it was there before, or it could confirm that it was there. And also things like dark matter could be influenced things that you can get in a way that you can't see, or maybe also dark energy.

00:44:03
Speaker 1: Right, Yeah, And that's why these are such fun puzzles because you work really hard to explain them using the pieces that you do understand, but sometimes you just can't and you're like, well, something else is going on. Maybe we're missing a piece of the puzzle. There's something else going on out there we haven't figured out yet that we need to add to our description in order to explain what we're seeing out there in the universe. So that's why we chase these discrepancies so hard, because sometimes the explanation is boring. It's like, oh, you're making this approximation, but it's actually really important, so you can't make that approximation. You got to really do it right. But sometimes the answer is really exciting. It's like, oops, you missed something fundamental. There's something really big, and this is a little thread that unravels the whole story. So science is all about chasing down these details and getting them right, not knowing whether they're clues to a bigger mystery or just dotting the I and crossing the T.

00:44:50
Speaker 2: Yeah, and I guess it's important to sort of understand how stars form out there in the whole universe, to kind of maybe understand our own context, right, Like, was our story formed in a really special or unlikely way? Or are we just, you know, one of many many stars out there that form in the regular way. That would kind of tell us a little bit about how likely it is that we're here and here to study this kind of stuff.

00:45:13
Speaker 1: Yeah, it's sort of incredible that stars form at all in this universe. You know, so much has to go right for stars to form, Otherwise the universe would have just been dark forever. It's sort of incredible. And you're right that our star is kind of unusual. Remember, most of the stars out there in the galaxy are not yellow stars like ours, they're cooler stars, they're red dwarf stars. So our star already is a little bit unusual. And of course, behind all of these questions, we're always wondering, like, is our whole situation unusual? Is it really rare for us to exist, or are we pretty common in the universe. So understanding like how stars get formed in the first place, and how you get bigger stars and little stars, that's a whole other mystery. We don't even really understand this initial mass function of the stars. How you get bigger clumps and smaller clumps is part of the deeper question to understand how we got here in the first place.

00:46:03
Speaker 2: M All right, Well, at least if you don't figure it out, it's not so bad at Lisia end up with a lot of cute baby pictures to put up on your fridge and your galactic fridge.

00:46:14
Speaker 1: That's right. I wonder if there's a certain kind of astronomer who likes babies, who tends to work on star formation because they want to look inside the windows of stellar nurseries.

00:46:23
Speaker 2: You mean there are physicists who don't like babies.

00:46:25
Speaker 1: There definitely are people out there who don't like babies.

00:46:28
Speaker 2: Man, not everybody has the anti baby.

00:46:32
Speaker 1: I don't know if anybody's anti baby. They might just be like mutual on babies.

00:46:37
Speaker 2: I see you're cool on babies exactly.

00:46:40
Speaker 1: People. You know, I could take a baby or leave a baby I'm not sure. Personally, I love babies, but you know, not everybody out there feels that way anyway. Star babies and galaxy babies are also super fun because they teach us about where' staharson galaxies come from.

00:46:54
Speaker 2: All right, Well, hopefully the next time you look up at the night sky you get to see maybe the birth of any star. It's possible to be looking other and into space and then suddenly a new star is.

00:47:03
Speaker 1: Born, because this guy and the cosmos in the universe are constantly changing, and we are keeping our eyeballs on it to learn as much as we can from everything it does.

00:47:12
Speaker 2: And if you remember anything from this conversation, just remember Baby Star Stars, Stars and Stars Star Baby.

00:47:19
Speaker 1: Start for his new viral hit.

00:47:22
Speaker 2: We hope you enjoyed that. Thanks for joining us, See you next time.

00:47:33
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.