Daniel and Kelly’s Extraordinary Universe - How do galaxies die?

Episode Date: July 12, 2022

Daniel and Jorge talk about the mysteries of why galaxies stop making stars.See omnystudio.com/listener for privacy information.

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founding partner of IHeart Women's Sports Network. Hey, Daniel. Are you? Well, I'm turning 47 pretty soon. Does that feel old to you? Well, whenever I'm feeling old, I just decide to count my age differently. Oh, yeah? Like a different base number?

No, like, how old would I be if I lived in a different place? For example, because Mars is years longer, I'd be in my early 20s if I was a Martian. Oh, but you would die in like a few hours, wouldn't you? Or a few minutes if you lived on Mars right now? Depending if Elon Musk has money left over from his Twitter purchase, to fund our oxygen on Mars. But I guess even if you're a Martian, you would still live the same amount of time,

so you'd be just as old, it'd just be a different year unit. That's right, but it's all about how the number feels, man. Yeah, but also you have to take into account your life expectancy. That also gets changed on Mars, right? Yep. On Mars, we all die young anyway. Or maybe not me, the era there is so much better that people would live longer, long enough to tweet forever. Hi, I'm Horham, a cartoonist, and the co-author of Frequently Asked Questions About the Universe.

Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I plan to teach until my old, old age. Do you think you'll still have something to teach when you're that old? I often teach freshman physics, which hasn't changed since Newton, so I'm planning to teach it until I grade away. What if you start forgetting things, even Newtonian physics? I'm sorry, what did you ask? I forgot the first part of your question. But anyways, welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeard radio. In which we explore this incredibly old and incredibly young universe, which has lived for billions of years and may survive for trillions and trillions. more. We explore all of the fast-moving and slow-paced processes that grow and shape and define our

universe and make it the one that we love. Our goal is not to be intimidated by the age of the universe or the billions and trillions of years that are to come, but to try to understand the processes that shape it and to explain all of them to you. That's right, because we live in an amazing universe. It's been around for 14 billion years or maybe more, which sounds like a long time, but who knows, we could be maybe in the universe's infancy right now. And the universe might live on for a long time, maybe even forever. Imagine if you had lived in the universe in the first few hundred thousand years, you might think, oh, this is what the universe is like,

not realizing that it was but the first flash of the universe and that it would live for billions of years with glowing stars. And now we live in the starfield era of the universe, but it might just be the first flash of a universe that lasts for trillions of years and is mostly filled with black holes. So this era of the universe could be very short-lived and very weird. Weird, but also pretty cool, I think, you know, right? Technically, we are living in the cool years of the universe,

not the hot, crazy, insane years, and also not the, you know, super freezing years. Well, because the universe is expanding, it's cooling. And so technically it gets cooler every year. And not just because they keep releasing more Marvel movies about more multiverses. Yeah, but at some point, I think it is possible to be too cool, you know? Like if the entropy of the universe is at a maximum and the temperature is like zero out there, Kelvin, that's not cool. Dr. Strange and the heat death of the universe coming soon. I hear the last one is pretty dark, right?

Everything in the Marvel universe is getting darker. Everything in the Marvel universe is getting darker. But in our universe, things continue to glow and brighten and to fill the universe with photons that allow us to see the incredible views across billions and billions of miles. and allow our tiny human brains to grapple with questions about how everything came to look the way that it does and how long it will continue to shine. Yeah, because unless you're Elon Musk, I guess, the universe will be bright and shiny and beautiful and amazing

and an open mystery pretty much until your old age, right? Is Elon Musk going to figure it out? Is that what you're saying? I think he's going to figure out how to beat old age, probably. He's going to download himself into a Tesla and launch himself in a space and live there forever. That's right. with that, you know, special module for Twittering. Maybe he should combine all of his companies, you know.

Twistler. Put Twitter onto a Tesla in a boring tube and launch that whole thing into space. That's right. Yeah, it'd be a Twistler X. Would be the new company name. The boring Twistler X. But we are not funded by Elon Musk, although, you know, you know where to send your donations, Elon, if you're a listener.

But we are motivated by the desire to understand. and everything that's around us and figure out exactly how long this flash in the pan will last. Yeah, because as much as we know about how the universe works right now, there are still open questions about how the universe will keep on working and how it's going to work up until the end of the universe. Remember that when you look up at the sky, it seems sort of static and frozen. You don't notice from day-to-day stars appearing or disappearing or changing their location. But that's just because we live in a tiny little slice and the universe operates on much longer times.

scales. If you could see a movie of the universe sped up to see all 14 billion years in just a minute, it would seem like a crazy, chaotic, dynamic place. Even our solar system has had planets shift around in orbit. Our galaxy is spinning and swirling and colliding. All sorts of stuff is happening. And as we look out into the universe on those cosmic scales, we ask questions about how everything came to be and how long it will continue to swirl and glow. Yeah. Yeah. So you plan it to be a professor until you grow old or even die apparently oh they're going to have to drag me out of this office i'm going to be here until the milky way stops spinning i have a similar plan i plan is to be a cartoonist until i'm rich i see so another hundred billion years yeah so basically the same thing

as growing old till i die yeah but anyways it is an interesting and growing universe which makes you sort of wonder how it's all going to end if not the universe at least maybe our galaxy That's right. On the podcast, we have talked about the future of the Earth and our solar system, how the Earth will get subsumed eventually into the outer layers of the sun. We've even talked about the lifetime of stars, how long they will burn, and how long our universe will keep making stars. But the universe is not made just of planets or solar systems or stars. One way to look at the universe is that the basic unit are galaxies, these big swirling swarms of billions of stars that float through the universe. How do galaxies form and how long do they live? Do they just keep spinning until they become rich cartoonists?

So today we have an amazing and galactic question. Today we'll be tackling. How do galaxies die? Or how do they fade away, I guess, maybe. Maybe that's a nicer way to say it. How do they become emeritus galaxies? How do they move on? How do they join the farm?

How do we make room for new galaxies because they're just taken up? of space, these old folks. Oh, right. Well, there's plenty of space in space, right? That's true. There is plenty of room for galaxies. But, you know, they are using the raw materials. They're occupying a whole lot of carbon and iron and all sorts of other stuff that could go to seed new stars and new galaxies and new forms of life and intelligence. Well, the first thing I thought when I saw this title was like, what?

Galaxies can die? I didn't know that. Do they have like a beating heart? It depends a little bit how you define death, right? galaxies don't live, of course, in the same sense that organic entities do the way you and I do, but they do have a really interesting life cycle. And at some point, they stop making stars. And eventually those stars will burn out. And so the fate of galaxies is, in fact, to fade away into darkness, which some could describe as the death of a galaxy.

I see. You're defining the death of a galaxy as when it, I guess when it goes dark. Is that what you're saying? Like when it stops shining or you can't see it? Or do you define it as like no activity? or when it breaks apart or when it turns into a black hole. There are many possible ways to kill a galaxy, I guess. There are lots of milestones in galaxy retirement. But scientists call this galaxy quenching when it stops making stars. Those stars burn for a while and then eventually they will all fade out. And you will indeed get just black holes with lumps of stuff swirling around it.

I see. So you're really talking more about asking how do galaxies retire? Like when they start producing light, kind of. Yeah, it's not a funeral. home we're pushing these galaxies into. It's a nice senior living facility with all sorts of other active galaxies to hang out with. Interesting. There are cosmic retirement homes. Do you have early bird specials too? Well, once you're no longer making stars, you're not really part of the game anymore. Yeah, yeah. I guess you're off the rat race kind of to see who's the shiniest galaxy.

You're definitely onto a new era in your life because if you don't have stars, then you can't make. make new elements. You're stuck with the composition that you have. Remember that new elements are only born in the heart of stars and supernovas and then the collisions of neutron stars. But once you've stopped making stars, you're no longer initiating that process that ends up with new elements. And so eventually you're going to get stuck with whatever you have. You'd be sort of frozen with your mixture of stuff. All right. Well, we'll get into the details of a galaxy's retirement, both from its job and maybe from its life. But as usual, we were wondering how many people had these dark thoughts about galaxies

and what happens to them in the future. So as usual, Daniel went out there and asked people the question, how do galaxies die? So thanks everybody who participates in these fun questions. If you'd like to hear your speculations here on the podcast for everybody else to enjoy, please don't be shy. Write to us to questions at danielanhorpe.com. Think about it for a second.

How do you think galaxies retired? Here's what people had to say. I think I have a decent guess for this. There are supermassive black holes at the center of every galaxy. And at some point, black holes do die or evaporate rather. It is possible. So maybe when that happens, when the black hole kind of doesn't exist anymore, then the galaxies just disintegrates.

As far as I know, galaxies die when their stars start dying. stop making more stars and it just becomes a cold cloud of sadness and you can't see it. So I guess that's a dead galaxy. I don't know how galaxies die. I would guess that either if it's a black hole in the middle of the galaxy, that it as opposed swallows large parts of the galaxy or that stars either should of die by not admitting as much light as they previously would have been, and so the galaxy becomes sort of darker and darker. So I think galaxies are so large and have so much angular momentum that all the stars within the

galaxy will reach the end stages of their life, and we will no longer have the starting conditions necessary to get new stars created before the entire galaxy will be eaten up by the supermassive black hole. Galaxies die in three ways. First, when they crash in and murder, with a larger galaxy. Second, when the stars fall into the black hole at the center of the galaxy, and third, when the galaxy creates no new stars, and the stars in the galaxy die from old age.

The galaxies die if eventually they lose the ability to make new stars, so this can, can come from a different reasons, but one of them that I know it might be that the black hole from the center of the galaxy starts this process by emitting probably a powerful cluster. Hmm. Does supermassive black holes in the center suck them up or something? All right. Dark answers for a dark question, I guess.

A lot of people went for the black hole scenario. Yeah, that's really interesting. People are aware that eventually everything will fall into the black hole. And they imagine that that's what's going to kill the galaxy. But little do they know. There's something worse around the corner or what? Yeah, there's sort of two competing timelines there. Like, will the stars burn out?

Will it stop making stars before it falls into the black hole? Or were the black hole like eat up a bunch of otherwise lively elements of the galaxy, which could have made new stars if only they hadn't fallen into the black hole? gravitational well. All right, well, before we start to consider a galaxy's death, let's, you know, let's do a little retrospective here and let's consider the whole life of the galaxy. So let's start at the beginning, Daniel. How do galaxies even get born? Yeah, I like that we begin with a nostalgic sort of like... Like a montage? Musical montage. All the good moments. Remember those?

With an all-timey, like, film feel or something. Yeah, soft focus, you know, fuzzy pictures of the galaxy playing in a kiddie pool. like the projector sounds going the galaxy being mean to the cat you know that kind of stuff but looking so cute no pictures of the galaxy crying or throwing a fit i think of galaxies is one of the most fundamental units of the universe because even though the stars are within them it's the galaxies that are formed by like the initial fluctuations so you know you begin at the very very early stages of the universe everything is smooth and filled with matter there are no wiggles, there's no extra lumpy bits.

Then you get quantum mechanics that fluctuates and gives you like a little bit more stuff here, maybe fewer particles over here, but that's really tiny, that's super microscopic. That kind of quantum fluctuation is happening all the time even today. But in the very early universe, those fluctuations were blown up to a huge scale by early universe inflation. The way space itself was stretched by like a factor of 10 to the 30 blew up these little tiny lumps, these little bits where something was heavier and something was lighter into actual macroscopic sizes and then gravity took over and it said all right this spot is a little bit denser than that spot so all the particles are going to

get attracted over there so you begin with these very shallow gravitational wells that slowly roll more particles into them and then they get stronger and stronger and stronger and what we're talking about it's mostly dark matter it builds these gravitational wells that then normal matter falls into and where you have these wells you have big blobs of matter and that's what forms galaxies. Yeah, I think we sometimes talk about it as the seeds of the galaxies, right? Like, the seeds, like where the galaxies came from was all sort of laid out in those first moments of the Big Bang. So kind of in a way, like galaxies were maybe the first things that were defined in the universe, the first kind of organization, right? Yeah, you can think about it like that. We think that

probably stars formed and then they formed into galaxies, but they all formed out of these big clouds of gas that were themselves formed by these dark matter halos. which were themselves formed by those initial seeds that we talked about. Something I think is really cool is that the pattern of dark matter, which controls the whole structure of the universe, is like these filaments, you know, these like lines through a cosmic web. And where those filaments overlap with each other are the dark matter halos. You could think of it's sort of like these filaments are funneling matter into these lakes,

which are where they intersect. And so gas is falling from these filaments into these halos and then swirling together to form stars, which form galaxies. Interesting. I guess what I mean is like if you had been around at that time that early in the universe, you could have been predicted maybe like, oh, there's going to be a galaxy and there's going to be a galaxy. Do you know what I mean?

Like the definition of a galaxy was sort of right there at the beginning of the universe. Yeah, you definitely would know that this is where complex structure is going to form. I think it's super fascinating what you end up getting. I mean, you start with just clouds of hydrogen and then like you get stars and you get galaxies, it's really fascinating, sort of the size and the scale and the shape of what forms out of that. It's one of the, I think, the deepest questions in modern physics, why these objects emerge at these scales?

You know, why do we get stars? They're about this size. Why do we get galaxies about this size? Why don't we get just one humongous galaxy in the whole universe? Or why do galaxies form at all? Why don't we just have stars spread out everywhere? So I think it would have been pretty hard to predict computationally. I think it's really interesting to sort of see what emerges.

from structure in the universe. And that's just what we're doing now. We're still seeing structure emerge. Now we have clusters of galaxies and super clusters of galaxies. And that process is still happening because we're still fairly young in the universe. Yeah. Are we still in the like the teenage years, you think? The awkward teenage years full of pimples.

Well, gravity is really slow. So it takes a long time to form structure. And so it's formed galaxies and galaxy clusters. But those galaxy clusters themselves are like very loosely grouped into superclusters. Gravity hasn't had time really. to gather that together into something greater. In the meantime, dark energy is doing the opposite job. It's pushing everything apart, preventing gravity from making superstructures.

So we might be living in the moment when the universe is the most organized. So I don't know if those are really the teenage years or that's like it's mid-thirties. You know, it's really sort of on top of its stuff before it starts to get old. Before it has its midlife crisis, as we all do. Exactly. All right. Well, I guess that's the birth of a galaxy. And so a surprising idea is that galaxies change.

and they might even die one day. So let's dive right into that. But first, let's take a quick break. I'm Dr. Joy Harden Bradford. And in session 421 of Therapy for Black Girls, I sit down with Dr. Athea and Billie Shaka to explore how our hair connects to our identity, mental health, and the ways we heal.

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special game show week of the Puzzler podcast. The Puzzler is the best place to get your daily word puzzle fix. Listen on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. All right, we're talking about the Death of Galaxies, Daniel. Does that mean our galaxy is going to die? Scientists think that the Milky Way itself is beginning to quench. We're entering a period where we're making fewer and fewer stars, and that's going to happen faster and faster.

So sort of the Milky Way's peak might be behind us. Oh, boy. Well, for a second, I thought you were going to give us a super plot to us and say like, oh, our galaxy is already dead. It's been dead the whole time. We're just ghosts. That's right. This whole galaxy is nothing but an M. Knight-Shummelan film.

Yeah. Spoiler alert. I talk to dead galaxies. I see dead galaxies with my telescopes. You kind of do maybe, right? Like if you look out into the far universe, you might be seeing galaxies that have died already, right? We do see a lot of galaxies that are dead.

And one of the surprising things and something we struggle to understand is what's killing all these galaxies. Why are there so many galaxies out there that are already quenched and have stopped making new stars? Wow. It just went from a thriller to a mystery.

We just switched genres here. It's a combination because somebody's still out there killing galaxies. It's not over yet. We still have a chance to catch the killer and maybe save future galaxies. And maybe do nothing about it because it's just physics. Are you saying we can't learn things about the universe and intervene in its fate that we have no control, no power to change the future of humanity? That doesn't sound like an engineer. You tell me, I guess.

you know, the physicist has to tell us if it's possible. Well, the first thing to do is to understand what it is that's killing these galaxies. And then we can figure out if it's possible to intervene. Then we have to start training some galaxy engineers. That might be the sequel, I guess. All right, well, I think one interesting bit of news is that, you know, galaxies seem to change. And I guess they quench and they maybe eventually die. And so is that something that we've always believed in that galaxies can change?

Or did we think that we would just, you know, swirl around forever? We've always thought that eventually galaxies would use up their raw materials in terms of making stars. You know, you have a certain amount of hydrogen in the universe, for example. That's primarily what's burned to make stars. Eventually, you will run out of them. We have a podcast episode about how many generations of stars will there be. Because, you know, stars don't live forever. And eventually they explode and spew their materials back out in the universe, which gets gathered back together.

But we don't think that can happen forever. So we've always known that eventually waves of star formation will end. But we were surprised when we looked at the galaxies and discovered how many are already dead. How many have stopped making stars well before we expected them to? Interesting. Yeah. I guess, you know, there's a limited supply of fuel in the universe, right? And at some point we run out.

Like we have a gas tank kind of. And, you know, hydrogen gets used up, gets turned into heavier element. And then to break it apart, you need extra energy. Yeah, there's two things that drive this. One is you need to have the fuel. And the other is that the fuel has to have the right conditions. Like, it's not just enough to have hydrogen. You have to have hydrogen in the right situation in order to get a star to form.

And the key thing to making stars form is that you need huge blobs of cold hydrogen. If you have hydrogen, but it's really hot, it's not going to form a star. Because remember that gravity is super duper weak. In order to tug things together to make a star, the particles have to be very slow moving. If your gas is heated up too much, then gravity's not going to be able to pull it together to make a star. All right. Well, that's the birth of a galaxy. I guess that's how they start, right? They start from fluctuations and the universe and the dark matter. Seeds, which pull in stuff and then galaxies form. That's just kind of the beginning of the story, right? Eventually, galaxies also merge. And our thinking there has changed also. Originally, we looked down to the universe and we saw really big galaxies, mostly because that's all we could see because they were brighter. And we thought, oh, wow, maybe these really big galaxies sort of form all at once, like monolithic collapse. of a huge cloud of gas into a swirling disk.

But then as we developed the technology to look further and further into the universe and earlier and earlier back in time, we discovered a lot more galaxies that were much smaller. And we realized that the story of galaxy formation is more complicated. Instead of having really big galaxies form at once, it turns out it's much more likely to form

a bunch of small galaxies than have those merged together. So galaxies are doing a lot of merging. Most of the galaxies we see out there are actually the products of lots of little baby that came together to make a bigger galaxy. I see it's like we looked at it into the universe and saw more baby galaxies than we expected. Is that kind of the idea, which I guess would then logically mean the bigger galaxies don't, didn't form. They're the result of two baby galaxies merging.

Exactly. Or multiple baby galaxies merging. It also helps explain the shape of these galaxies. Like when an individual galaxy forms, it tends to form as a disk. You have a big cloud of gas and it's spinning up. a certain way and that spin prevents it from collapsing in one direction, but gravity can collapse it in the other direction, which is why you get a disc. So disc galaxies tend to have formed from a single cloud of gas and dust with some spin. But then when two of these things merge, they come in

with different shapes and different sizes and different disks. And that's how you get like elliptical galaxies and things that are more rounded because you're combining like the spin of two disc galaxies together. So the idea is that the bigger elliptical galaxies are formed from a bunch of smaller disc galaxies that came together. For example, the Milky Way and Andromeda are going to collide, but they're not spinning in the same direction. So you're not going to end up with one big disc galaxy. You're going to end up with something more elliptical that has like two spin axes. Interesting. So a lot more of the galaxies formed from merging than we thought before. I guess why do we have that wrong impression? Well, we had the wrong impression just because we didn't see a lot of these

baby galaxies early on. You know, we didn't have the technology before Hubble and those kinds of telescopes, we just couldn't see them. But when Hubble looked really deep into the universe and discovered how many galaxies are out there, that's when we got the clue. It's from seeing these baby galaxies in the early universe. You know, the number of galaxies out there is sort of amazing. If you hold up your pinky at arm's length, then the part of your sky blocked by your fingernail on your pinky contains about a million galaxies in the observable universe, you know, layered further and further and back in space and time. And so now like scan your pinky around the whole sky, each of those is a million galaxies. So it's an incredible number out there. And now we have a huge

population to study. So we get a better sense of how these things have evolved. Wow, that's wild. What if I let my pinky fingernail and grow? Then you are containing more and more galaxies. You're becoming master of even more of space and time. A pinky master, I guess. All right. So I guess that's the basic of the life of a galaxy, right? They're born as little baby galaxies. They grow up to be a teenager.

They start hooking up doing inappropriate things. And then they merge into bigger galaxies. And then they keep burning for a while, I guess. Like, what's the average age of a galaxy? Galaxies are really old. Like, the Milky Way is almost as old as the universe. Galaxies formed very early on, a lot of them in the first billion years of the universe. Many of them are more than 10 billion years old.

But at some point, as you say, they die. They stop shining. They stop making stars. They quench. So I guess, Daniel, what's the process for a galaxy to die? So for a galaxy to die, for it to stop being able to make stars, something has to heat up its gas. Something has to prevent its gas from staying cold. Because in order to form stars, you need that cold gas.

And some of the star formation comes from the original gas that the galaxy started with, whatever was in that dark matter halo. Remember that galaxies are not really alone in the universe. They're sort of like at the intersections of these dark matter filaments. So there's also sometimes new gas falling into these galaxies along those filaments like tributaries to a lake. And so galaxies can also continue to form stars as they suck in more gas from these filaments. So the way a galaxy dies is somehow losing access to that new gas or blowing out the cold gas or heating up the cold gas that's inside it. because remember, you need cold gas to make star.

I see. All right. So you're defining the death of a galaxy is when it stops forming new stars. Is that, I guess, the definition? But even if it stops forming stars, it's still shining, though, right? Astronomers don't talk about galaxies dying. They use the technical term quenching, which means it stopped forming new stars. But you're right, even if it stopped forming any stars at all, those stars themselves can burn for billions and billions for sometimes even trillions of years.

Remember that stars, there are. lifetime depends on their size. So really big stars don't burn very long and really small stars will burn for a very, very long time. So even a galaxy that's totally quenched will make any new stars, but those stars will continue to burn for a very long time. Oh, I see. I see what's going on. I think really what you mean to ask is how do galaxies peak or like how do galaxies stop getting brighter maybe, right? Yeah, exactly. How do they stop having any babies? I see. I think I see what's going on here, Daniel, I think you associate not working with dying. That's why I'm never going to retire.

Yeah, that's why you're never going to retire. Like if you retire, you die. Like if a professor stops making papers, that's like death to them. Who even are you anymore in that situation? But yeah, just to be clear, that's kind of what we're saying, right? Like galaxies don't maybe, I guess they will at some point grow dark. But here today, what we're talking about is like how to galaxies peak or when do they stop being active?

Exactly. And as we look out into the universe, we can tell whether galaxies are still making stars or not. And when we do that, we see some pretty surprising things about what's going on with all the galaxies in the universe. Wait, that's kind of weird because I guess we're defining death as stopping production of stars. You need to like know how many new stars are being made. But how do you tell that on a short human lifespan? So it's not that we look at the galaxies and we count the stars and we say, oh, look, it's still making more because these galaxies, a lot of them are really far away.

So all we can see them is like a pixel or two on the Hubble. But what we can do is look at the color of the light that comes from these galaxies. And that tells us about whether or not there are new young stars in the galaxy. And the reason is that new young stars means hot stars. When stars are formed, you get big ones and you get small ones. But the big ones, the ones that burn hot, that burn blue, they don't last for very long. So if you're looking at a galaxy and you're seeing blue light from it, that means recently formed stars.

If you're looking at a galaxy and you only see red light, that means cooler stars, only older stars, no recently formed stars. So by looking at the color of the light from the galaxy, you can tell if there are recently formed stars in it because the recently formed ones burn hotter and burn bluer. Interesting. You're saying that galaxies kind of peak, they stop producing new stars when they sort of turn from blue to red. Exactly.

Blue means you've recently produced some stars because only young stars are ever blue because blue stars don't last very long. And eventually, all your blue stars burn out and all you're left with are red stars. So if you're in all red galaxy, it means you haven't made anything recently. I feel like that's like people these days, you know? Everyone starts out as Democrat and liberal, but then they get more conservative. red in the old age. You know, maybe we should avoid the political analogy and think about like musicians.

You know, are you a musician that keeps putting out new albums or are you just like the Beatles? You're just making money on your old catalog. Well, the Beatles came out with the white album. So I don't know how you fit that into the color spectrum. Well, the fascinating thing is when you look at this color spectrum, you see some that are still making new stars. You see a whole huge number that are already quenched, many more than we expect.

And you see very, very few in between. Very few in this region they call it the green valley between the blue and the red, the ones that are like quenching. So there's a bunch that are not quenched. There's a huge number that have been quenched and very few in the process of quenching. Interesting. Yeah, as you said, it's a big mystery. And so let's dive into that mystery of what is killing or I guess retiring all the stars in the universe. But first, let's take another quick break.

I'm Dr. Joy Harden-Brand-Bradford, and in session 421 of therapy for black girls, I sit down with Dr. Afea and Billy Shaka to explore how our hair connects to our identity, mental health, and the ways we heal. Because I think hair is a complex language system, right, in terms of it can tell how old you are, your marital status, where you're from, you're a spiritual belief. But I think with social media, there's like a hyperfixation and observation. of our hair, right? That this is sometimes the first thing someone sees when we make a post or a reel is how our hair is styled. We talk about the important role hairstylists play in our community, the pressure to always look put together, and how breaking up with perfection can actually free us.

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To start the conversation, visit talk about vaping.org. Brought to you by the American Lung Association and the Ad Council. All right, we're talking about, I guess, when galaxies change their political views, right? When galaxies stop recording albums. When they stop publishing papers, which according to professors is the same as death. Like, give me research or give me death. What if galaxies get a hobby, then they won't see retirement as death anymore? I do see a lot of emeritus professors still around the department.

And I wonder, like, why do you still come in every day? And I think, you know, maybe their partners at home don't really want them hanging out doing their hobbies at home, you know? They probably thought the same thing. You start thinking right now. You're like, they're going to have to drag me out, drag me out, or call the Grim Reaper, I guess. But apparently galaxies do retire at some point. They peak. They stop making new stars.

And I guess it shows on a galaxy, you know? Like, it's pretty, it sounds like it's pretty obvious. Like if a galaxy is pretty blue, then it's young. If it's pretty red, it's old. Exactly. And there are very few galaxies in between, which suggests that this process of quenching a galaxy happens rapidly. Whether you're not in the green valley for very long on your trip from blue to red. Well, it's sort of like when you run out of gas in your car, it's not a gradual process.

Like, once you run out of gas, your car stops. Yeah, but the galaxies are huge, right? You might imagine that one part of it might quench, another part of it might still be active. But this seems to happen on sort of a galaxy wide scale all at once. Well, let's recap a little bit here. You said the galaxy's peak, they stop producing new stars when they run out of gas. And you said that it's not because you run out of hydrogen gas, but because you run out of cold hydrogen gas. In order to form stars, you need clouds of cold gas that gravity can work on to pull together to make those stars.

And so it might be that there's plenty of hydrogen left in those galaxies. We suspect that there might be. It might just be that it's too hot. So either you lose the gas or the gas gets too hot. Those are the ideas for how to quench a galaxy. Wait, what happens if it gets too hot? Like, why is it hard to make a star from hot gas? Well, hot gas, the particles are just flying around too much.

You know, gravity is not very strong. And so if you have a gas that's really hot where the particles are flying around with high speed, gravity just doesn't have the power to gather them together into a star. Counterintuitive because gravity makes them hot, right? But once it's already trapped them in a gravitational well. Think of it like escape velocity. The Earth is boiling off molecules right now at the edge of our atmosphere because they're too hot for the Earth's gravity to contain them. And so it's certainly possible to have a whole gas where there's not enough gravity to contain it,

where it just disperses and just keeps flying around instead of clumping it together to make a dense well at the heart of it. But I guess, you know, the temperature is just one part of it. What if you have hot gas but really dense or like enough of it or a lot of it? Wouldn't that also sort of create the intensity you need for a start to form? Yeah, if you have a seed, if you have a dense seed, but how does that form? So you need gravity to do that work. The only way to get density is to have gravity pull things together. So primarily it's from having cold clouds of gas. There are other rarer ways for it to happen. You can have supernova shock waves, for example, that compress things and then make over densities, but primarily it's just from having clouds of cold gas.

Interesting. It's like a chill process, star formation. You know, like it needs a certain amount of calm and zen for things to kind of build up to a star, right? Yeah, you can't rush it, man. It's like the slow food movement. You want a hot star. It takes a time. Yeah, yeah. No fast food galaxies in this universe. It's all organic. You can't microwave a star. That's right. It's all natural and organic. All right. Well, you said there's a bit of a mystery, right, in the sense that there are more stars sort of quenching or dying or retiring than maybe we thought was possible or think is likely. Yeah, we sort of can't explain all the quenching that's happening. Or at least, you know, 20 years ago when we discovered that this was

happening. We were very surprised. We thought that very gradually galaxies would start to lose the ability to form stars. And we'd see this slow drop-off with a lot of galaxies in the Green Valley and a few that are totally quenched. But it happens faster than we thought. And so there's been a lot of research in the last 10, 15, 20 years into why galaxies quench. And now we have a few ideas. All of course stimulated by this observation. We dug deep into the physics of and thought, how would you quench galaxies? What are the processes we might have overlooked that could be doing this. I see. So what do you mean? I guess it happens faster than you thought or earlier than you thought. Do you know what I mean? Like are you saying that, you know, the process of retirement

is faster than you thought or that it's happening earlier than you thought or that it happens, you know, more than you thought? All of that, right? It definitely happens earlier than we thought because there are more galaxies that are quenched that are in that red category than we expected. And it also happens faster than we thought because they're very few in the green valley. There's like two peaks. If you look at a distribution of the color of galaxies, you don't have a broad spectrum from red to blue. You have a peak in the blue and a peak in the red and almost nothing in between. We suggest that galaxies don't live in the green valley very long. When they start to quench, they quench quickly and then it's over. It's a fast process, you know?

You can't vote for like an independent third party. It's like you're either blue or you're red. I mean, I guess that's good news. If you like a quick death, you know, it's not a long tortuous struggle against the eventual demise. Man, I feel like a lot of the competing here today is that to a professor, retirement, is that? They're like, wait a minute, why are so many galaxies retiring? Why would anyone retire? That's crazy. Yeah, well, maybe they like it.

You know, maybe they're tired of making new stars. And they just want to enjoy the stars that they have and hang out for a long time, trillions of years until they eventually all fade. Maybe it's a happy retirement. Yeah, there you go. You don't have to call it death. It could be their new life. Their second life. So that was the mystery, I guess, and you said that physicists have looked into this.

And so they have theories, I guess, about what's making all of these galaxies retire so quickly. They have theories. None of them are really perfect or comprehensive, but they have a few cool ideas. One is related actually to the black hole at the heart of the galaxies. And it's not that the black hole is gobbling up stars. Not that stars are falling into it. But black holes actually emit a lot of radiation. The black hole itself is not hot.

Gas in the dust that's swirling around the black hole before it falls in, that gets really hot and emits a lot of radiation. And so that blows out a lot of the gas and dust. Once we talked about like how supermassive black holes form and there's a maximum rate at which they can grow because the bigger they get, the more they push away the gas that's feeding them. And so this could also be spewing out radiation into the galaxy, which heats up a lot of the gas in the galaxy or also helps blow it out of the galaxy. And so that can stifle star formation. I see. You're saying like, you know, because star formation depends on very chill, calm conditions, anything that kind of excites the galaxy is not good for star formation.

So one idea is that maybe it's the black hole in the middle of galaxies that's, I guess, agitating everything. Yeah. And we see that these black holes grow at the same rate of galaxies. It's like a ratio of the black hole mass to the galaxy mass, which seems roughly constant. And so that might help explain it because as these galaxies get big and get old, the black hole gets big and powerful and not just heats up the gas, but also helps prevent more gas from falling into the galaxy from these filaments

from the intergalactic medium or the circumgalactic medium. All right. So that's one idea, but it's not perfect. It's not perfect because, you know, the black hole is at the center and they don't really understand how it could like heat up the entire galaxy. You should still see star formation sort of at the edges in that case. So we can't explain everything. Okay.

So then what's another possible star retireer? Another possibility is that stars themselves are killing the galaxy. Remember that stars also generate radiation. Our sun doesn't just send out light. It sends out protons and electrons and all sorts of stuff, the solar wind. And so if you have a lot of these stellar winds, these particles, they can be blowing the gas out as well or heating it up. It's sort of the same process as happens at the galactic center from the black hole, but instead

it's happening everywhere all at once. So in some senses, it's a better candidate for what might be stopping star formation because it's spread out across the whole galaxy. I see. It's sort of like stars form, but then they kind of spoil star formation for the area around them, right? Because it like it forms a star, but then it starts to heat up this gas around, and so nobody can make new stars. It's really nimbism.

You know, it's all these old professors. They've got these nice houses they bought in the 60s, and they don't want anybody building apartments in their backyard. Yeah, yeah. Or like, you know, like a, you know, the old professor is taking up office space. Exactly. Got to make room for those young bloods. All right.

But this one is also maybe not a perfect idea or cause. Yeah, because remember that the galaxy is surrounded by gas and that gas should be falling in. The dark matter halo should be pulling that gas in and supplying us with fresh, cold gas that's been out there between galaxies. It hasn't been heated up from the stellar winds. So another area of research is understanding why that might stop happening. And one suggestion there is that the dark matter halo, the thing that forms our galaxy and shapes it and provides the gravitational well to suck in this new gas, that might get a little too big. If that gets really big, then its gravitational power is really strong. And then what happens when the gas falls in from outside the galaxy into the galaxy is that it gets heated up.

This is called shock heating. if there's like a gradient in the density outside the galaxy. And when this gas falls in, it basically collides with the other gas. And then it gets too hot. And so you get hot gas falling in instead of cold gas. And then you can't use that to make new stars. Oh, interesting. But what do you mean the dark matter halo gets too big?

Like dark matter is growing in galaxies? The dark matter halo is definitely growing because, you know, we have these filaments and it's sucking in not just gas, but also dark matter. Gravity in the universe, remember its goal is to gather. everything together. So the reason you have these halos and then these filaments is that the filaments are feeding the halos and not just barionic matter, not just normal matter like gas and dust, but also dark matter is flowing along these filaments into our halo.

So our halo is getting bigger and bigger. Oh, I see. So as the galaxies get older, they pull in more of the stuff around them. They get bigger and heavier, which makes it more gravitationally powerful, which sucks in the gas maybe too fast. And so when it sucks it in, it's too hot and it can't form any more stars. And all of this just comes together to give you a picture of star formation in galaxies as sort of fragile and special. Like you need these special conditions.

Things have to be just right. And it's not that hard to perturb it. And to make it to the galaxies, while they're still big swarling blobs of stuff, they're no longer have the conditions to make these stars. And, you know, without stars, wow, the universe just wouldn't be the same place. It makes you wonder of like, if the universe had been a little bit different, would we have not gotten stars at all, just like big swirls of hot gas that never formed stars. Yeah, it would be a much darker universe, right, without stars. You couldn't see anything. Exactly. We certainly wouldn't be here, right? You could have a literally dead universe without the capacity to form life if it had been

a little hotter. If all the gas in the universe had been too hot to form stars in the very beginning. Well, you would still have, I guess, quasars, right? Like black holes, maybe shining because of the gas swirling around them. You still have some bright spots in the universe. right oh yeah absolutely quasars you're right and there were some of the brightest things in the universe i wonder if life could form in the vicinity of a quasar yeah that'd be pretty cool right i mean to that to that civilization it would be indistinguishable right that would be their son that would be their son yeah and in podcasts around that quasar they would wonder you know our quasars still growing they would wonder why quasars are no longer forming because you know also quasars formed in

the early universe and we're not really making quasars at the same rate as we used to that's the episode, I guess. How do Quasars retire? Who's killing off the Quasar? I'm just trying to tap into the paranormal true crime podcast, which seem to be so popular. I see, I see, I see. You're going for murder. Is Bigfoot killing galaxies? Next time on Daniel and Jorge. That's right. Although we still want the science category. Amazingly, Bigfoot podcasts are in the science category. Don't get me started on that. Oh, really? Wow. Well, maybe not the natural sciences category. yes, in the natural sciences. Well, I guess who killed Bigfoot would be another topic.

Why is Bigfoot retiring? Did his foot or her foot stop growing? Maybe he stepped on a galaxy and got quenched. All right. Well, so those are three reasons why a galaxy might stop making new stars. Are there any other reasons? There are lots of other theories out there. Some of them involve how galaxies tug on each other.

Like we've noticed that sometimes galaxies that are more by themselves that aren't part of a cluster tend to get quenched less often. One hypothesis is that galaxies are like harassing each other, that gravity from other nearby galaxies might be preventing gas from falling into those galaxies to help spur new star formation. So if you're in the middle of a cluster rather than by yourselves, it might be that your friends in the cluster are interfering with a gas infall that you need to make those new stars. Wouldn't other galaxies though help you in a way slow down or cool off your own gas, making it better to make stars?

Yeah, they do slow it down, but they might slow it down to the point where it doesn't fall into your galaxy anymore. So you still need that gas. It's a really fine balance that. You need the gas to fall in, but not too fast. You need it to be like in the Goldilocks zone, kind of. Yeah.

You need a very slow delivery of frozen gas. All right. Well, I guess that's the mystery. Although, wouldn't you just think that, you know, a galaxy is born, it has a certain amount of a cold gas and at some point it runs out. And so the whole thing just kind of end all at the same time. Like, wouldn't that also be an explanation why it seems to be so binary? So red or blue, like it just runs a lot of gas at the same time because all of it was formed at the same time.

Yeah, it's a good question. Why it's a surprise that galaxies quench all at once. You're imagining that they start with some supply of gas and then the star forming that uses that gas is like used up all at once all over the galaxy. But remember that it's not really about having the raw materials, the gas to make stars. You could keep making stars for zillions of years with the gas supply that we have. Stars burn and die and you can just keep making new stars for a long time. The issue isn't the supply. It's the conditions to make new stars. Remember, you need cold conditions. So it's a surprise if something is spoiling those conditions and doing it quickly

and in a sort of coordinated banner across the galaxy all at once. That's not what you expect to have from just the sort of random distribution of conditions that you expect in galaxies. I see. It doesn't match a random distribution is kind of the mystery.

All right. Well, I hope you find a culprit, Daniel. You know, maybe before it kills our galaxy. Or no, wait, our galaxy's already dead. Well, I thought you were pro-galactic. retirement anyway. So I thought you'd want to find the culprit and then encourage it. Like, hey, man, why don't you let all these galaxies off the hook? They've done enough. I am pro-retirement in any situation. All right, you teenagers out there, Jorge is suggesting

that you retire tomorrow. Yeah, if you can, you know. Go for it, you know. I feel like I retired in my early 20s. So I definitely recommend. You retired and then started three or four new careers. Yeah. I retired about three times. That's how pro-retirement I am, Danube. Exactly. So I guess we shouldn't think of these galaxies as quenched. We should think of them as beginning their next adventure. Yeah, there you go.

And we hope you also find your next adventure pretty soon in the near future. Hopefully in this galaxy before it retires. And remember that we are still learning about the process of galaxy formation and galaxy quenching and galaxy retirement. And hopefully we will live long enough to learn much, much more. Yeah. And I guess the main lesson is that the universe is still a work in progress. It's changing. It's growing.

it's changing its color and showing its age. It's getting redder and redder. Well, we hope you enjoyed that. Thanks for joining us. See you next time. Thanks for listening. And remember that Daniel and Jorge Explain the Universe is a production of IHeart Radio.

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