Daniel and Kelly’s Extraordinary Universe - Why do some galaxies have bars?

Episode Date: February 6, 2024

Daniel and Jorge explain what we know about the bars at the center of galaxies, and what mysteries they might hide.See omnystudio.com/listener for privacy information.

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Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System On the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot.

He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want or gone. Hold up. Isn't that against school policy? That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast and the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. I'm Dr. Scott Barry Kaufman, host of the Psychology Podcast.

Here's a clip from an upcoming conversation about how to be a better you. When you think about emotion regulation, we're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome. Avoidance is easier, ignoring is easier, denial is easier, complex problem solving, takes effort. Listen to the psychology podcast on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts. Hey, Daniel, remember a few weeks ago when you insulted America's favorite chocolate bar? Hershey's. How could I forget? I got some strong emails about that comment. Oh yeah? Did you get us barred from the airwaves? No, but I think I might have to raise the bar in terms of my

comments. You mean we're not already at the bottom? We're scraping the bottom of the bar roll. Yeah. Sounds like you need to spend less time in bars drinking away your chocolate sorrows. I'll have a chocolateini, please. Sure, give it a shot. Hi, I'm Horry and my cartoonist and the author of Oliver's Great Big Universe. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I do have strong opinions about chocolate. Do you have opinions about strong chocolate or do you have strong opinions about chocolate? Yes and yes. Is it because you're really passionate about chocolate?

It gives you a lot of comfort. I feel like if you're going to eat chocolate, then you should eat chocolate that you like. But, you know, people prefer lots of different kinds of chocolate. I don't mean to judge people who like Hershey's, you know, all the power to them. If that's what they want, it's everywhere. I feel like you're maybe being a bit of a chocolate snob. I mean, are you being a snob to know what you like and know what you don't like? I don't want to insult anybody else's preferences to anybody who is offended by me calling Hershey's chocolate garbage.

I do apologize. That sounds like a non-apology, Daniel. It did sound like a non-apology. But it sounded like I was trying, didn't it? I don't think that counts these days. I think you could raise your bar in terms of apologies to the masses. Well, I'll say this. I enjoy chocolate.

I hope everybody else out there enjoys chocolate, whatever flavor or variety they prefer. Yeah, there you go. But anyways, welcome to our podcast, Daniel and Jorge explained the universe, a production of Our Heart Radio. In which we whip up a delicious concoction of everything that's out there in the universe, all the dark energy, all the dark matter. all the dark ideas that describe how the universe actually works, what it's made out of and how it came to be. We melt it all down, form it into squares, and ship it to you over the airwaves. That's right.

We try to lower the bar of science and make it accessible to everyone out there. We try to make it silky and smooth and delicious and easy to go down. Because wondering about how the universe works is something everybody does and everybody should have access to what we know and what we don't know about the universe. Thinking like a physicist is not something only professional physicists can do. It's something everybody can do and should do about how the world works. And we're here to walk you through that. That's right, because we all live in the universe.

We're all members of the universe and we can all wonder about how it all works, how it's all put together and why things are the way they are. Do you have like a membership card for the universe? I don't have one. Oh, oh, awkward. Awkward. Maybe she's got lost in the mail. Daniel. I'm sure that's what happened.

I'm sure you were invited. Maybe it's part of the Hershey's Cabal that's secretly organizing the whole universe. Oh, you've been blacklisted or dark chocolate listed. Yeah, milk chocolate. That's what you get for insulting. The powers that be. Accessible chocolate to the masses, yeah. But you can still come to the universe, I guess.

All right, thank you. You can be somebody's plus one. Wow. Maybe they just don't invite physicists. Maybe they don't. They all prefer to be. be on the outside of the universe looking in. No, I love being the part of the universe that's looking at itself, right? That's what makes the question so important that we're not outside the

universe studying it like some weird object. It's part of our context, our lives. It's our existence that we're trying to understand. Physics is sometimes sold as like bigger than humanity, but to me, it's the fundamental question of humanity, answering the deepest questions about our own existence. Is it sold as bigger as humanities? Where is that listed? That's how it was sold to me, I guess. So maybe I shouldn't generalize from n equals one. But one reason I got into physics originally was that it seemed like the questions were universal. They weren't limited to things happening on Earth.

You know, you study biology and you know it's fascinating and important, but it might be very different on other planets. Maybe that's why you're not invited, Daniel. You think you're bigger than the rest of us. I think some of the appeal of physics is that it does seem to be universal, though. of course, we don't know, and that's a question we can only answer in the future when we meet aliens. I guess it does sort of tap into that sense of the cosmos, the grand picture that we all kind of have inside of us.

And it is pretty amazing what we've been able to understand and study and come to terms with in terms of where we are in the universe within our solar system, within our galaxy, within our super cluster of galaxies, within the observable universe. It's pretty amazing that we can see so much structure out there just by looking out to the night sky. Yeah. It's amazing what we know about the universe, never having left our tiny little neighborhood. Everything we know about the structure of galaxies and super clusters and all that stuff

is based just on the few photons that happen to hit Earth from those distant locations. It would be incredible to get in a warp ship and actually visit some of these places and learn more about what's actually going on there right now. But we can do a pretty impressive job even from here. But of course, there are still lots of unanswered questions about how solar systems form what planets there are in our solar system, what planets there have been in our solar system, and larger questions about galaxies, why they exist, how they form, and why they take their weird shapes. That's right, because we're orbiting around our sun, and the sun is orbiting in a galaxy,

which is a huge cluster of stars. There are 100 billion stars in the Milky Way galaxy, but how these galaxies get formed is kind of still up in the air, right? It is, and it's an important question, because galaxies are kind of the basic building block. of the universe, they're like the atoms of the universe from which you can put together structures and superstructures and all sorts of other stuff. It's incredible that like galaxies actually exist and they tend to be a certain size. So it's an important thing to understand like why are universe features galaxies, how they come to be, how long they will continue to be the basic building block of the universe. Yeah, and why do they have some strange features?

And so today on the podcast, we'll be asking the question. Why do some galaxies have bars? Are these like drinking bars? Like our galaxy has a bar, you can go and get a drink? There's definitely bars in our galaxy, right? I've been to some of them. So that's a question we can definitively answer today on the podcast. They seem to be clustered around Earth.

At least all the bars we know are on Earth. Is that kind of a physics question or is that a biology question? Well, I think there's some history of drinking alcohol in space, right? I think Zach and Kelly's new book has some stories about astronauts who have smuggled things up to the ISS. So perhaps there is an unofficial bar off Earth as well. I wonder if anyone has tried to like brew beer on the space station. Space moonshine. Either on purpose or by accident.

The moon does shine more brightly in space. So maybe it's easier to make moonshine. There you go, yeah. And so of course we don't know if aliens enjoy relaxing in bars or whether they like Hershey's chocolate in their chocolatinis. But today we're not talking about places to drink. We're talking about the structures at the centers of galaxies. I see.

So when you say a galaxy bar, it's not like a buffet bar or a beer bar. It's more like a structure is a structure in a galaxy that looks like a bar. Or structures, multiple ones. Yeah, there's a kind of galaxy out there called a barred galaxy. But not every galaxy has this kind of bar at the center. And so it's an open question about why some galaxies. have bars. Well, as usual, we were wondering how many people out there had thought about galaxy bars

and why some galaxies have them, why some don't. Are they over 21? Can you get your favorite drinks in them? And so Daniel went out there into the internet to find out what people think about this question. And whether you like salad bars or chocolate bars or any other kind of bar, you are welcome to participate in this segment of the podcast. Just write to me to questions at Danielanhorpe.com and I'll set you up. So think about it for a second.

And why do you think some galaxies have bars? Here's what people had to say. So I didn't know galaxies had bars in the center. So I'm going to say maybe it has it because gravity is alighting them in a special way but can't be made into a sphere because of the black holes gravity. I think planets don't share orbits because the center of gravity would have to be absolutely,

stationary or it oscillate and shake itself out of sync. I think that the bars in the center of galaxies are there to give people somewhere to go. A bit like if women's clothes shops have bars, they'd have somewhere for men to go instead of trying to look interested. I expect that galaxies have bars at their centers because you're going to need a place to kill some time and have a beer if your interstellar flight is delayed. I have no idea what does a bar in the center of a galaxy mean. All right, some fun ideas here. Some of them a little bit inappropriate, perhaps.

I think this might be the highest fraction of joke answers we've ever gotten, which means people really just don't know anything about bars and galaxies. Maybe they're just barring not knowing anything about them. Or maybe they'd already spent too much time in a bar when they got these questions. and they were in a silly mood. Oh, there you go. Yeah, maybe you shouldn't send these questions out during happy hour. Monday morning, 9 a.m.

Is that when happy hour is? Depends on how much you like your job, I guess. And where you are in the world, right? We have listeners all over the world. Yeah, we are a global podcast. All right. Well, Daniel, maybe step us through this. What exactly do you mean by a Galaxy Bar?

So Galaxy Bars are features. of one particular kind of galaxy and galaxies it turns out can have all sorts of different shapes in them galaxies of course are clusters of stars and they can be very very small from dwarf galaxies that have thousands of stars to super mega galaxies that have billions and billions of stars but we tend to notice some features in them and you know the beginning of any science is basically just botany it just like look at it categorize it sort it see what patterns emerge and very early on when people were studying galaxies, they noticed that they could have different structures, even things we could see here from Earth with limited telescopes.

Right. And we've talked a little bit about before the different shapes that galaxies can have, right? They can be sort of like a blob, like a football, or maybe like a spiral or maybe just like a pancake, right? Yeah, exactly. And there are regular galaxies that are probably in the process of merging between two galaxies. There's a galaxy out there that looks like a question mark, for example. but the most common galaxy out there is a spiral galaxy.

How common is it? Is it like 90%, 50%? It's somewhere around two thirds of all the galaxies that we have studied are spiral galaxies. And there's a reason for that. We went into all of that in our recent podcast about the various shapes that galaxies can have. But basically galaxies form from huge blobs of gas, which spin and collapse and then form a disk. And that dense disk of gas tends to form those stars. spinning of the galaxy turns that disc into a spiral.

That's kind of like the default shape of a galaxy, right? But then you can get other shapes, as we talked about, when they merge with each other, other galaxies, or they crash into other galaxies, right? Exactly. So a spiral galaxy is like the basic building block of the universe. In that sense, it's the most important one to understand because the other ones, ellipticals and irregulars are all made out of spiral galaxies. And also, our galaxy is a spiral galaxy.

So if we're going to understand like the context of our lives, let's begin at home. Right. And I guess maybe just to paint the picture for folks, by a spiral, it sort of looks like a toilet flushing, right? Like it's not like one, like you're tracing out a spiral with a pen on a paper. It's more like a swirl that's kind of converging in the middle. Yeah, and a spiral galaxy usually has several arms. It can have two. Sometimes they have three. There are galaxies out there with four arms. Each one is like a spiral that usually goes like one time around the galaxy. It starts from the center and then comes out. So it's sort of more like a pinwheel. Right. And so that gives you the image of the spiral galaxy.

And so that's the basic shape of most galaxies out there. But you're saying some of them have a bar through them or on them. At the center, some galaxies don't have bars. The spiral arms just start at the very center. But a good fraction of galaxies, the center of them isn't part of the spiral arm. It's a separate thing. It's like a bar. Like the Milky Way galaxy has this big blob of stars and gas and dust at the center of it,

which forms this big bar. and a spiral arm that comes out of each end of the bar. So the spirals don't go to the very center of the galaxy. They start at the edges of this bar. Whoa. Wait, wait. So you're saying our Milky Way galaxy is a bar. Yes, we live in a barred galaxy.

Well, is it a milk chocolate bar? Since we're the Milky Waying. Now I'm afraid to answer because I don't want to anger the Hershey universe cabal that's in charge of everything. Yeah. What if the Milky Way is a her cheese product? This is why it's so important for science to be free of this kind of oppression. People should feel free to speak their minds on chocolate, right? Well, people shouldn't disparage other people's favorite chocolate.

I think that is maybe the most important part here. Yes, yes, point taken. But our milk away has a bar, you're saying, so like our spirals, the arms of our galaxy, don't spiral into a single blob in the middle. They spiral into a bar. That's right. Our galaxy is two major arms, and then there's some spurs that come off of them.

And each arm comes from a different side of the bar. So the two major spirals that make the Milky Way galaxy, they don't actually meet at the center. They come to different sides of this bar that's at the core of the galaxy. Interesting. The picture I'm getting is sort of like, I don't know if you seen those like Hawaiian fire dancers where they have a stick. And at each end of the stick, it's on fire. And so then they spin it. That's kind of sort of what's the picture that we get of the Milky Way Galaxy, isn't it?

Like there's a bar, and from each end of the bar, you get these swirls that spiral out. Well, I'm not fancy enough to take vacations to Hawaii. I tend to spend my money on chocolate instead, but it sounds accurate. You know that you can get chocolate in Hawaii. In fact, it's kind of known for having good chocolate. Macadamia and that chocolate, here I come. There you go. But yeah, that sounds accurate.

Wow. Or if you imagine like a baton twirler at a football game and ribbons at the end of the baton, if he or she spins the baton, then the ribbons spiral around it. But the ribbons, of course, don't meet at the very center. They come from the edges of the baton. Now, this is an interesting idea just because to me and I wonder to a lot of our listeners, this is kind of news, right? Like most of the time that you see a picture or a drawing of the Milky Galaxy,

it doesn't have the bar in the middle, right? Like the famous, isn't the famous image of like you are here where it points to a point in the Milky Way of a drawing of the Milky Way that one doesn't have a bar, I don't think. Yeah, that's a good point. I'm not sure if that image is scientifically vetted.

It also really doesn't have big distinct arms the way you might expect. But a more scientifically accurate picture of the Milky Way shows the specific arms and the bar. The bar is not like a very rigid rod. It's a bit of a blob also. To me, that is stars.

Like a just a, elongated cluster of stars. Yeah, it's stars and it's gas and it's dust like the rest of the galaxy. But, you know, this is us imposing order on a huge swirling mass of stuff. So part of this, of course, is just the impression we make, how our brains filter the full details of the buzzing chaos of reality. All right. Well, so then the Milky has a bar.

Lots of galaxies maybe have bars in the middle of them. So the question is, why do they have bars? What's going on there in the middle of the galaxy? And can you play galactic limbo with that bar? And so let's get to these deep and profound questions. But first, let's take a quick break. Ah, come on. Why is this taking so long? This thing is ancient.

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There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal glass. The injured were being loaded into ambulances. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and order criminal justice system is back. In season two, we're turning our focus to a threat that hides in plain sight.

That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam, maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone.

Now, hold up. Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet.

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and then as we try the whole thing out for real wait what oh that's the run right i'm looking at this thing listen to no such thing on the iheart radio app apple podcasts or wherever you get your podcasts all right we're talking about the milky way galaxy and the fact that it has a bar of stars in the middle the swirls of the galaxy don't converge to a round blob in the middle they converge to kind of like a rod of stars right yeah it might be more accurate to call it like a football because it's a little wider in the center than it is at the edges but yeah there's sort of a long blob it's not a bar why don't we just call them footballed galaxy that sounds great I am done defending the naming schemes of astronomy because it's indefensible because you just can't do it sorry astronomers you have no champion in me anymore oh boy you're throwing the astronomers under the bus no i'm just throwing in the towel you win

naming uh peculiarities oh it certainly does and nobody's ever asked me my opinion when they make up these names okay so the milky way has a bar or a football kind of in the middle which is weird right because you would expect um a galaxy to be sort of you know round and symmetrical trick, right? You would kind of expect it. And that's exactly why we are really interested in these questions. Like, what structures form in the universe? The answer to that question reveals the forces at play. Why are galaxies created in the first place? Well, there's gravity. Why don't they just collapse into a central ball? Well, there's angular momentum that turns them into disks. Why do the arms form at all? Well, maybe a question we can ask here is like, what percentage of galaxies are barred

galaxies. Like, is the Milky Way super weird that it has a bar? Or is it like most spiral galaxies have bars or some of them or 50-50? That's a really interesting question. And the answer depends on the mass of the galaxy in a super weird way. For galaxies about the mass of the Milky Way, like billions and billions of solar masses, it's around two-thirds of spiral galaxies that have bars. The more mass of the galaxy, the smaller the fraction of the spirals have bars. And also, the less mass. of the galaxy, the smaller fraction have bars. So there's this like sweet spot in the middle where galaxies are

more likely to have bars. And if you move away from that lower mass or higher mass, then the fraction that have bars decreases. Interesting. So it depends on the size and mass of the galaxy. But maybe let's take a step back to the basics because it sounds like there's a lot of physics

going on here, as you might expect from our podcast. But so what's going on at the center? Or like, how can this bar still be there after all these billions of years? It all comes from asymmetries, like little over densities. If you start from a galaxy that was perfectly smooth, like all the matter was evenly distributed or like you had a cluster of stars that were perfectly distributed in circles on perfectly circular orbits, then it wouldn't form a bar.

But if you tweak it a little bit, like one little spot has more density than another spot, then that spot's going to have more gravity and it's going to attract more stars to it. And so you get these density waves forming inside the galaxy. So the same process that forms the arms, the density waves of the arms that we describe as like traffic passing through a freeway, can also form density structures in the center of the galaxy. In this case, they form bars. Whoa.

Yeah, I remember we spent the whole episode talking about this idea of density waves in a galaxy. It's not like the arm of a galaxy. What you see is the swirl, like it doesn't rotate around the galaxy. It's more like a ripple through a whole bunch of stars. Exactly. It's more like a ripple. It's like people doing the wave in a football stadium. They're not moving sideways, but the wave itself is moving.

Right. And so the galaxy arms are waves in the density of stars. And the bar is the same way. The bar is like a density wave. So it's a structure that forms, but it moves through the stars. It doesn't move at the same speed. The stars are moving around the galaxy. You can move faster.

or it can move slower. Wait, what do you mean? The bar, does the bar rotate? And like what you say it's moving, like which, which direction is the bar moving? The bar rotates the same direction as the galaxy. And it definitely rotates. Oh, kind of like the arms and a clock. Kind of like the arms and a clock.

And also like the arms of the galaxy, right? Because the arms are density waves, they move relative to the stars. So one star might be in the density wave and then they'll get passed by the density wave, which moves on. Oh, are there like, videos online about this? That would be cool to see. You mean like actual footage of bars rotating? No, like a simulation or something. Yeah, I would love to see that, but the timescales are ridiculous. Like the Milky Way takes 250 million years to rotate. So there's no chance for like watch

a galaxy rotate unless you're going to set up your camera for tens of millions of years. So how do you know it's rotating? Well, we build up a model of how the galaxy works. That includes angular momentum and you need those components that rotation to explain what we see. If they weren't rotating. There's no angular momentum and these things would collapse a lot faster. A lot of galaxies and a lot of these structures are supported by angular momentum. Like where else would the spiral arms come from if it wasn't spinning? I don't know. Yeah. So we do a lot of simulations to try to explain the galaxies we see. We say, well, we think we understand all the forces at play. We put it in the computer. We press the button. It goes and we see what comes out of the simulation. Then we compare that

to what we see in the sky. Interesting. That's what we do to try to explain what's happening or try to gain some understanding of the forces at work. Wow. So nobody's ever actually seen a galaxy swirling. We just think it does. I mean, it's the most likely or obvious explanation, but still it's a fact that we haven't, like, seen a galaxy swirl. Like, you've only seen a photograph of it.

It's like you've only seen a photograph of a merry-go-round, ever. Like, you've never seen a mirror-go-round move. Well, that's true. And we've only even known about other galaxies for like 100 years. He was Hubble who saw these little smudges in the sky and deduced their distance from analyzing special stars in them. The sephids whose brightness, of course, is famously connected to their pulsing frequency. And he's the one that understood that these things were outside our galaxy. So we've only known that other galaxies exist for like 100 years and had good enough telescopes to, like, resolve their structure for a few decades,

which is basically no time at all relative to their actual motion. So yeah, we're studying galaxies basically frozen in time. That's interesting. Now, you're saying that about two-thirds of galaxies have a bar in the middle of them, and this bar is rotating and rippling through the center of the galaxy. Like, what's the mechanism for making these bars? Like, why a bar shape, why not a star shape or a banana shape? I would love to see a banana shape at the center of galaxy.

And probably there is some galaxy out there that has that because there's so many galaxies out there and so many of them are doing weird things. I guess if you think about a galaxy is banana shape. Each arm is like a banana. So it's like if you take several bananas and arrange them in a star shape, you would get a spiral galaxy

right? Yeah, that's true. That's a message from the universe that bananas are important and you should cover them in chocolate. Yes, they're galactic. And that's why we sell frozen chocolate covered bananas here in Orange County. That is the purpose of the universe, isn't it? Is it deep fried too?

I don't think so. I don't think so. They're fried butter. Why not fried chocolate covered bananas? I think I've had that at a restaurant, actually. But you asked a great question that wasn't about dessert, which is, why do some galaxies have bars? Why do they form bar shapes and not other things?

And the answer is that we're not sure. Like, we don't understand why some galaxies form bars and some don't. We don't totally understand the mechanism for the bar formation itself. like we've seen this happen in simulation sometimes, and it comes out of instabilities. You put a bunch of stars a little bit closer to each other. They tend to form these density waves. And what's happening here at the center

is that the stars are no longer totally in a circular orbit. The bar like twists the orbits so that some stars are moving more on radial orbits, passing close through the center of the galaxy. What do you mean radar, like more oval-shaped orbits? Yeah, more oval-shaped. So they're not equally distributed around the center of the galaxy, they're like passing back and forth.

Oh, interesting. I guess maybe it's sort of rare or sort of a coincidence is that you would have circular orbits, right? Like if I threw a rock at the sun right now, it would not necessarily go in a round orbit. It might maybe more likely go in an oval shape orbit. Yeah, there's lots of possible solutions and a circle is like a special case of it.

And also to maintain a circular orbit requires resisting the tugs and the poles of everything else around you. Remember that you're not orbiting by yourself. The earth is being pulled on by juice. and Jupiter is being pulled on by everything else. That's one of the reasons why our orbits are eccentric. So the more complicated dynamics you have, the more lack of symmetry,

the more of these things are getting pulled by other stuff, the more they're going to fall out of perfectly circular orbits and clump together in other kinds of orbits. So you're saying the stuff at the center of our Milky Way galaxy is not necessarily going around in a circle. It's maybe going in an oval shape around the center. But then how does that explain the ripple that is the bar? that is the bar?

Are all of these things going in an oval in sync? Yes, the bar itself is like a denser region, which passes through the center of the galaxy, and it influences other stuff. So gas near the center of the galaxy, for example, moves at faster speeds than gas further out. So the gas can catch up to the bar and pass through it, but then the bar stronger gravity slows it down

so the material tends to lose some energy. This actually ends up funneling a bunch more gas towards the center of the galaxy, and can create new stars right there in the bar. But then I guess if like making a bar so common, you're saying about two-thirds of galaxies, spiral galaxies have a bar in them, why don't we see that effect like in other places?

Like when I flush the toilet or, you know, in our solar system even, we don't have a bar in our solar system. I guess we don't have spiral arms in our solar system either. Like, you know, things coalesce into the planets. Although we do have an asteroid belt. I think it comes down to the balance of all the things. things at play. We have gravity in the solar system and of course in the galaxy, but the galaxy is a lot more friction. You know, there's gas and there's dust. This stuff is interacting much more than things

in the solar system. So there's a lot more transfer of energy between the stuff in the galaxy than there is in the solar system. Solar system is like a cleaned up little galaxy. It's not nearly as much stuff between the planets as there is between the stars in the galaxy. So there's a lot more of this exchange of energy and friction and stuff bumping into other stuff in the galaxy, which can give you more interesting, complex, basically turbulent structure. And we don't know how long these bars last. There's some theories that say these bars will last forever. And we see some galaxies that have had their bars for like 10 billion years. There are other theories that the bars could be oscillatory, like maybe the bar forms, but then the formation of the bar tends to destroy the bar.

And then another bar forms a billion years later. So it could be that every galaxy goes through phases where it has a bar and doesn't have a bar. These are really open questions in the study of galaxy formation. It's doing like pull-ups or something on a bar going back and forth. But this is an interesting idea that it maybe depends on time, like you're saying, they can come in and out. What do you see when you look at the galaxies out there in the universe? Like, can't we see them across different ages and times?

Yeah, do we see that older galaxies have more bars or younger galaxies have more bars? What do we see? Yeah, so we're not sure. We have a lot of different clues that sort of. point us in different directions. Like on one hand, we see some galaxies with really, really old bars. I was reading a paper about a galaxy. We're looking at 10 billion years in the past, and we can see the formation of a bar back then. So that suggests like some bars formed really early in the universe and may have lasted a really long time. Of course, you can't see the same galaxy over time.

Yeah, how do we know? We don't know necessarily how long a bar lasts when it's formed, But in this one particular galaxy, they could see evidence of the bar over about 10 billion years because they could see how the bar had disrupted the structure of the galaxy. So it's a little speculative, but they think that that bar lasted for about 10 billion years in that galaxy because of how it like shepherded the stars and influenced the gas to create new stars. So it's not everybody that believes that paper, but that one paper argues that there are galaxies with really long-lived bars. And then I guess how do you know that they can go away?

Yeah, another great question. That's again from simulation. We see in some simulations that a bar can drive gas inwards towards the center of the galaxy. It's an over density and so it pulls stuff in. And that can disrupt the motion of stars through the galactic core. So like the formation of the bar could create gravity which pulls in more gas, which ends up disrupting the bar itself. So we don't know. And this is the kind of thing we study in simulations.

And these simulations are never perfect, right? You can't include everything in the galaxy and your simulation, every tiny particle we take forever. So whenever they do these simulations, they're always making approximations. Oh, let's leave out the dark matter, or let's ignore this effect, or something to make it tractable.

And so you got lots of different simulations with sort of different answers. And people are still exploring this like right now. Nobody really knows the answers to these questions. So when you simulate it, you add a little asymmetry or like an offset to the distribution of the stars. And then you get a bar. and then what you see in the simulation is that sometimes the bar disappears after a while.

And then maybe comes back later. And there's some other hints from the simulations. Some suggest that it depends on the thickness of the disk. Like our galaxy is mostly flat, but it's not a totally thin disk. It's not like it's a paper thin, right? There's a width to the galaxy. And some galaxies have thinner disks and some galaxies have thicker discs. And some of these simulations suggest that galaxies with thinner disks are more.

more likely to have bars than galaxies with thicker disks in them. What would that be? I think it has to do with basically the gravitational environment in which a disruption forms. Like if there's a lot of other gravity happening anyway, then the small over density is less likely to cause a big pile up in the galactic spin. Like if, for example, you had a huge amount of dark matter in your galaxy, and then the gravity of the galaxy is mostly dominated by that dark matter. In the same way, if you have lots of other stars like above and below where the over, density forms, then those stars are going to tend to smooth stuff out because of their gravity.

A thicker disk might prevent a little over density from turning into a big bar. Interesting. It's like a 2D effect. Like it only happens in thin sheets. Yeah, it could be. Another study suggested that mergers can destroy bars. Remember, spiral galaxies merge and form sometimes new spiral galaxies, sometimes elliptical galaxies. And it could be that bars are destroyed in these mergers, and then the new galaxy doesn't necessarily have a bar. It depends on the instability and the formation of the stars in that new galaxy. Wow. But there's another really interesting clue, and to answer a question you asked a few minutes ago,

about whether bars form more recently or not, we find that it also depends on the mass of the galaxy. So a lot of the bars tend to form more recently in lower mass galaxies. So it might be that lower mass galaxies take longer to form stars and then take longer to form bars. So most of the bars that are forming in the universe today, we think are in the lower mass galaxies. They might not just have had time to make a bar until now. So you need like time and mass and flatness to make a bar. And some sort of seed, right, some gravitational over density. I think it's really fascinating that basically all the structure in the universe comes from one place being heavier than another.

When the universe began, if it was totally smooth, nothing would have happened because all the gravity would have been balanced. Basically, everything that happened in the universe is because of gravitational imbalances. Yeah, I definitely happen, and I definitely have a mass imbalance in certain parts of my body. Blame gravity. Yeah, it's all good. I blame chocolate bars. So then, but then how does the mass of the galaxy affect these bars? Like, why would having more mass or less mass affect the bar formation?

Because structure formation takes time, right? Gravity is very, very weak, and it takes time for gravity to overwhelm these other things. It takes time for gravity to pull stars together. It takes time for those stars to then influence each other. So all these things take time and lower mass galaxies have less gravity. So it just takes them longer to sort of come together and form these things. And structure is sort of hierarchical, right? It's bottom up.

You build the stars. The stars build a galaxy. You build a structure on that. And then you build structures on those structures. the barred spiral galaxy is sort of like the pinnacle of universal structure. It's like the most complex organized thing the universe has made so far. Well, it definitely sounds like we've raised the bar here in terms of galactic mysteries. It seems like this is still a question for many scientists.

And in fact, it might even have interesting consequences for our conception of dark matter and how they influence the formation of these galaxies. So let's dig into those dark matters. But first, let's take another quick break. Ah, come on, why is this taking so long? This thing is ancient. Still using yesterday's tech, upgrade to the ThinkPad X1 Carbon, ultra-light, ultra-powerful, and built for serious productivity,

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iHeartRadio app, Apple Podcasts, or wherever you get your podcasts. We're talking about galactic bars. Now, Daniel, is this where single stars go to meet other single stars? This is where molecular clouds hang out, hoping to one day make stars. They're like celebrity bars. You see a lot of stars in them. this is where stars are born baby all right well we talked about how we're not quite 100% sure why some galaxies are bars it seems like it's just a matter of the physics like the weirdness

of how stars and the gravity interact and form ripples in a cluster of stars but there is sort of another component to it which is dark matter right dark matter plays a big role in how galaxies form how fast they form what shapes they have and so what do these bars tell us about dark matter Yeah, you're exactly right. And remember that the stars themselves are just like tracers that tell you where most of the mass is in the universe. So most of the dynamics, the gravitational forces are hidden from us. We live in a dark universe where we only see a tiny little fraction of what's going on out there. So by studying the motion of the gas and the dust and the stars, we could try to get a glimpse for what's going on sort of behind the curtain where the fuller picture of the universe is. And you're absolutely right, that dark matter control. the formation of the galaxies. We wouldn't have galaxies this early in the universe if it wasn't for dark matter. 80% of the matter in the universe

is dark matter, but more like 90% of the matter in galaxies is dark matter. So galaxies are basically mostly dark matter. And because dark matter feels gravity, it has a huge influence on gravitational turbulence like the formation of bars, absolutely.

So all this time that we've been talking about the mass of a galaxy, I thought we were talking about the stars, but Are we also talking about the mass due to dark matter? Yeah, we're talking about the total mass, not just the visible mass of the galaxy. Or are they pretty correlated? Like, the more stars you see in a galaxy, the more dark matter it has. Yeah, great question.

We did a podcast recently about the variation of dark matter in galaxies. Most galaxies have around 90% of their mass is dark matter. But there is a variation. There's some that are like 99% dark matter and some that have almost none or maybe zero dark matter. It tells a really fun story about the history of galaxies and how they form and merge and might even strip each other of dark matter. On the whole, there's a reasonable correlation and like 90% of a galaxy's mass is invisible. 90%? 90%. But isn't like the proportion of the universe, something like 5 to 1, dark matter to normal matter? Yeah, so galaxies tend to have

more dark matter than most of the universe. Why is that? Is it just because there's a lot of like stars and does kind of in between galaxies out there? It's because dark matter is really responsible for the formation of galaxies. And once a galaxy forms, it's hard for it to lose its dark matter, but it's possible for it to lose its gas and its dust and even its stars. There's all sorts of processes that tend to blow out gas and dust from a galaxy. You know, supernova or radiation from the central black hole, all these things can push normal matter out of a galaxy, but they don't affect its dark matter.

So the dark matter is sort of there in the background forming the gravitational structure on which the rest of the universe lives and it's much less influenced by everything else that happens to the normal matter. Okay, so if our galaxy is 90% dark matter, I would think that was like the most powerful thing that determines the shape of the galaxy.

So does that mean that the dark matter in our galaxy is also in a spiral shape and maybe also has a bar of dark matter in the middle? Yeah, great question. We don't know the answer. sort of that because we don't know the distribution of dark matter in the galaxy with a lot of precision. We know roughly that it's denser in the core and then it's spread out like a big halo. But we also know something about the physics of dark matter. Like we know that dark matter

doesn't feel friction with other dark matter the way normal matter does. Like two clouds of gas that pass through each other will exchange energy. One of them will tend to fall towards the center, for example. These kinds of energy exchanges just don't happen for dark matter because dark matter passes right through itself. It only feels gravity, and gravity is not strong enough for those kinds of exchanges. So the complex structures you see forming in normal matter don't always happen in dark matter. That's why we think dark matter is sort of a bigger, fluffier halo than the more compact visible matter in the galaxy.

But the shape of the regular stars is mostly due to gravity, right? And dark matter feels gravity with itself, doesn't it? It's mostly due to gravity, but it also crucially relies on losing energy. Like if stuff is just swirling around gravitationally, it won't collapse nearly as easily if it's not sticky, if it just passes right through itself. When things are orbiting a black hole, for example, they could orbit there forever, but they're much more likely to fall in if they exchange some energy. So one of them gives up energy to another one rather than passing right through. You mean like if the stuff swirling around crashes with itself or like bumps into itself, then it's going to lose some energy? Yeah, so things are just much more stable if there's no interactions other than gravity.

The other interactions, they tend to exchange energy and then stuff can fall into the center to make stars or to make galaxies or to make arms, these kind of stuff. The truth is we don't really know what structure there is to the dark matter. We don't have a lot of great probes for exactly where the dark matter is throughout the galaxy. It could be a dark chocolate bar in the middle. It could be just like a moose spread out throughout the whole galaxy. It could be. We're getting better and better techniques to map where the dark matter is in the galaxy.

but we don't have a very accurate picture. But, you know, simple models of the physics of dark matter tell us how it should be distributed, though we don't know if those models are accurate, of course. But then in all these simulations you're talking about where we are finding out all these things about the bars, do they include dark matter in those simulations? And what kinds of assumptions do you have to make there?

They do include dark matter, and they treat dark matter as like a collisionless fluid, just sort of like there in the backdrop providing more gravity. And the dark matter does definitely influence the formation of bars. In these simulations, the more dark matter you have, the less often you get a bar, even when you have some sort of like gravitational instability or over density in the visible matter. So does dark matter help the bars or prevent the bars? It prevents the bars from forming.

So the more dark matter you have, the more like tends to smooth out the gravity of the disk. Just like having a thicker disc, right? As you were saying earlier, sort of like a 2D effect, the more 3D your galaxy becomes, the less susceptible it is to these effects. In the same way, dark matter basically reduces the effect of gravity of the visible matter. It provides this like background gravity pulling on everything. I see. Like it evens out the strength of gravity in the galaxy, right?

Because you think dark matter is distributed, spread all over the place. The more significant that is, the more generally speaking gravity is in that galaxy. Exactly. And so if you have no dark matter, do you always get a bar? Nothing happens every time in simulation, but the proportion definitely goes up if you have no dark matter. But we don't expect that to happen in the universe very often. As we talked about recently on the podcast, we don't think galaxies can form without dark matter, though they might later be stripped of their dark matter in some collisions.

But then collisions are also going to destroy a bar. So is the bar kind of a sign of how much dark matter there is in a galaxy then? Yes, exactly. And people have used bars to try to measure the dark matter fraction in, galaxies. Like if you look up at the night sky and you measure the fraction of galaxies that have bars in them, that tells you something about how much dark matter there can be in those galaxies. Because if there was a lot of dark matter in all those galaxies, they wouldn't have bars. And if there was less dark matter in those galaxies, more of them would have bars.

So we can actually use the bars as a way to like indirectly estimate the dark matter fraction of the universe in a way that's separate from like the galactic rotation speed or any of the other probes. You mean like the bar sets the bar. For dark matter. Yeah, it raises the bar on our understanding. Our precision to measure dark matter. Yeah, this is not a very precise measurement because we don't have that many galaxies

where we can measure this kind of thing, but it's consistent with everything else we know about dark matter. So the fraction of galaxies with bars is consistent with our understanding that 80% of the matter in the universe is dark matter and something like 90% of the matter in galaxies is dark matter. And then what? What percentage is Hershey's chocolate versus snob chocolate?

I see. Now you're disparaging people's chocolate choices, right? It goes both ways. I'm just disparaging yours. All right. I can take it. If I can dish it out, I can take it. That's fair.

I'm sure that Hershey's dominates the chocolate market, but I'm glad that they're now a variety of chocolate offerings for people to enjoy. All right. Well, another interesting feature of our galaxy and of our universe, the way physics works, create these weird characteristics of the things we see out there, which then kind of reveal what things are made of and how things work in the grand cosmic scale things.

Absolutely. And it's just part of the game we're playing, looking out at the night sky and out deep into the universe to try to get a handle on what's going on out there. What structures form? Why do they form? And what do they reveal about the clash of Titanic forces that's happening underneath it all?

Sounds like these bars are giving us a good handle on galaxy formation. dark matter content of the universe. I'm going to go hit the lunch bar after this. Yeah. Just don't hit the dessert bar too hard. Depends on the kind of chocolate thing. Yeah, that's right.

The brand, apparently. Love all you and love all your chocolate choices. Keep chomping on bars. That sounds really credible, Daniel. I'm working on it, man. I'm working on it. All right. Well, we hope you enjoyed that. Thanks for joining us.

See you next time. For more science and curiosity, come find us on social media where we answer questions and post videos. We're on Twitter, Discord, Insta, and now TikTok. 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. In the heat of battle, your squad relies on you. Don't let them down. Unlock elite gaming tech at Lenovo.com.

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Then, everything changed. There's been a bombing at the TWA terminal. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order. criminal justice system on the iHeart radio app apple podcasts or wherever you get your podcasts my boyfriend's professor is way too friendly and now i'm seriously suspicious wait a minute sam maybe her boyfriend's just looking for extra credit well dakota luckily it's back to school

week on the okay story time podcast so we'll find out soon this person writes my boyfriend's been hanging out with his young professor a lot he doesn't think it's a problem but i don't trust her now he's insisting we get to know each other but i just want her gone oh hold up isn't that against school policy that seems inappropriate maybe find out how it ends by listening to the okay storytime podcast and the iHeart radio app apple podcast or wherever you get your podcasts this is an iHeart podcast