Do we live in a super bubble?

Daniel and Kelly explore the structure of the space between solar systems and reveal our cosmic context. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Daniel Whiteson?

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• 3:09Daniel Whiteson?

  Think about where you are right now. Maybe you're in your car commuting, or you're out walking the dogs, or at home doing the dishes. But where are you relative to everyone else? How do you define it? Probably you zoom out and put yourself in a wider context, your town, your region, your whole country. Listeners to this pod will likely be inclined to zoom out even further, to all of Earth, and then the whole solar system. From there, it's very tempting to skip forward and put our solar system into its place in the galaxy, then the galaxy into its cluster, and then that cluster within the supercluster foam.

• 3:50Daniel Whiteson?

  But have we been too hasty? Is our galaxy so simple, just a collection of stars floating around a central black hole? Or is there more nuance to our galactic context? We think of the galaxies mostly stars, but they're formed, they burn and they die. There's a constant sloshing of energy back and forth between those stars and vast clouds of raw material. Where is our solar system relative to those clouds? Are we in the midst of a galactic fog? Or are we floating through an open bubble? Today on the pod, we'll zoom out to this fascinating scene, understand the dynamics of star formation and destruction, and let you know exactly where you, we, all of us,

• 4:33Daniel Whiteson?

  stand, galactically speaking. Welcome to Daniel and Kelly's Extraordinary Galaxy.

• 4:52Kelly Weinersmith?

  Hello, I'm Kelly Wiener-Smith. I study parasites and space, and I love bubbles.

• 5:06Daniel Whiteson?

  Well, hi, I'm Daniel. I'm a particle physicist. I love bubbles and I have all of my organs.

• 5:12Kelly Weinersmith?

  Okay, yeah, right. So that's, I was gonna ask you about the most painful thing you ever experienced in your life as a lead into why my laugh is going to be a little off today.

• 5:24Daniel Whiteson?

  Folks, I'm going to try to restrain myself from being funny today because if I make a joke that makes Kelly crack up, she could literally crack up.

• 5:33Kelly Weinersmith?

  Oh, I could, yes, I could split a stitch. So, so what, aside from labor, the most painful thing that ever happens to me was the gallstones that tried to pass their way out of my body on Friday, but ended up getting removed surgically. The whole thing, the whole gallbladder was removed. And so I am still working on resolving my stitches. And so it hurts to laugh. So I think I've come up with a way to laugh that hurts less. And so my, my laugh will sound a little, I don't know, maybe even less annoying. I have seen, there's a couple complaints about the way I laugh. Y'all can just kiss my rear ends. But anyway, Daniel, what is the most painful thing that's ever happened to you?

• 6:15Daniel Whiteson?

  Ooh, most painful thing that's ever happened to me. I've broken my wrist, but weirdly that didn't hurt that much. I think maybe the most painful thing is when I broke my little toe. I like massively stubbed it on a little concrete outpost and cracked it. And wow, yeah, I thought I was gonna die.

• 6:35Kelly Weinersmith?

  Ouch. Yeah. How long did it take to heal? What do you do? Just put like a?

• 6:38Daniel Whiteson?

  Nothing. You just tape it to the next toe and like whatever happens inside of it happens. Like, there's no surgery to correct like the little bones inside your little toe. Nobody needs it. I was like, just cut that bugger off. But I was like, no, you just tape it to the next one. That's it.

• 6:56Kelly Weinersmith?

  Well, you know, Daniel, it sounds like if you lived in a bubble, you would have been safe. But if I lived in a bubble, that wouldn't have saved me. And so, well, you know, maybe if the whole universe was in a bubble, that would save us from some of the bumps and bruises of existence. But do we live in a bubble, Daniel?

• 7:15Daniel Whiteson?

  Well, we are trying to pop the bubble of ignorance out there and help everybody understand their cosmic context, where we are situated in our solar system, where our solar system is situated in the galaxy, and the whole context for our existence. I love filling in all those details and helping people understand really where they live. And today, we're gonna dive into that topic, essentially where our sun is in the galaxy. Is there structure out there? How does it all work?

• 7:41Kelly Weinersmith?

  And I think all of our inner kids are excited today to find out whether or not we live in a super bubble.

• 7:49Daniel Whiteson?

  So before we dive into it, I was wondering what people out there knew on the topic. So I went to our group of volunteers and I asked them, do we live in a super bubble? Here's what folks had to say.

• 8:01Speaker 07

  Well, I don't even really know what a super bubble is. So maybe? What do you think, Glenn?

• 8:07Speaker 08

  I have no idea what a super bubble is.

• 8:09Speaker 09

  I have not heard the term super bubble. It makes me think about some conceptualizations of fourth dimensionality.

• 8:16Speaker 08

  Considering I couldn't tell you what a super bubble is, I can't really say if we live in one, but it sounds like one of these really exciting astrophysical, maybe who knows, cool universe things. So I'm eager to learn about it. I have never heard of the term super bubble, but we probably live in one.

• 8:37Kelly Weinersmith?

  I like the, I've never heard of it, so yes.

• 8:42Daniel Whiteson?

  This is really exciting to me that nobody had heard of it because it means we get to share this really cool thing about the universe with everybody. Yay.

• 8:50Kelly Weinersmith?

  We love doing that.

• 8:52Daniel Whiteson?

  You know that great X K C D comic? I mean, they're all great, but the one about the moment you discover somebody doesn't know something really cool about the universe and how you should treat that with reverence and respect because it's a wonderful teaching moment. That's where we are today about Super Bubbles.

• 9:07Kelly Weinersmith?

  Yeah, I mean, I think we can all agree that Randall Monroe is the greatest web cartoonist to have ever lived, right?

• 9:12Daniel Whiteson?

  Wow.

• 9:13Kelly Weinersmith?

  Ow, it hurts. Ah, no, I'm getting punished in the moment.

• 9:17Daniel Whiteson?

  That's instant karma right there.

• 9:20Kelly Weinersmith?

  Ouch.

• 9:27Daniel Whiteson?

  I hope Zach is listening and laughing at that.

• 9:33Kelly Weinersmith?

  All right, all right, bubbles.

• 9:34Daniel Whiteson?

  All right, bubbles. So we're talking about the sun and the solar system's context in the galaxy. But before we get there, let's do like an overview of like, what's out there in space? How empty is it anyway? So to get oriented, remember that our situation here on Earth is very unusual. Like, the density of stuff around you is very rare in the universe. The universe is mostly very, very low density, and we live in a very, very high-density situation. The Earth, of course, is wonderful and cozy. And in like a normal atmosphere, you'll find a huge number of molecules per cubic meter, like 10 to the 25 molecules per cubic meter. It's a big number. And that's just because like Avogadro's number is pretty big and

• 10:16Daniel Whiteson?

  gravity has gathered the Earth and the stars and all that stuff together into little dense clumps.

• 10:22Kelly Weinersmith?

  I thought Avogadro's number was 10 to the 23.

• 10:24Daniel Whiteson?

  It is. Yeah, exactly.

• 10:26Kelly Weinersmith?

  Oh, but this is 10 to the 25th.

• 10:28Daniel Whiteson?

  Yeah, because a mole is less than a cubic meter.

• 10:30Kelly Weinersmith?

  Got it. Got it. I'm with you now. Okay, these are big numbers, Daniel. Big numbers.

• 10:34Daniel Whiteson?

  These are big numbers and you know, sometimes on Earth, we want to do experiments in low density situations. Like when we collide particles together, we don't want a whole bunch of other particles around. Or when you build a plasma inside eater to do fusion, you want mostly a vacuum before you start. And so we have all these vacuums in labs. We've created these situations. You pump out all the air. And you might be surprised to learn that even in those highly specialized vacuums on Earth, we still have something like 10 to the 12 or 10 to the 15 molecules per cubic meter. Like that's down 10 or 13 orders of magnitude from your general atmosphere. So like really good work. This is not easy, but still it's a lot of molecules.

• 11:17Kelly Weinersmith?

  When somebody says the vacuum of space, are we doing better than the vacuum of space? When we make a vacuum in the lab, or are we doing about the same as the quote unquote vacuum of space?

• 11:29Daniel Whiteson?

  Neither. We're not doing anywhere close to the vacuum of space. The density of vacuums in labs here on Earth is much higher, is much, much more stuff in our lab quote unquote vacuums than there is out there in space.

• 11:43Kelly Weinersmith?

  Okay.

• 11:43Daniel Whiteson?

  But let's dig into that and zoom our mental picture out from the surface of the Earth. Obviously the atmosphere drops gradually, right? There's no like huge wall or gate or anything. This is not space balls where atmosphere is like contained by a huge bubble, right? Where the code is one, two, three, four. But in a funny way, there is because space officially starts at like 100 kilometers above the surface of the Earth, which I always thought was weird.

• 12:08Kelly Weinersmith?

  There's some debate about that though, isn't there?

• 12:10Daniel Whiteson?

  Yes, absolutely. There is. And like you can be an astronaut. If you've gone 80 kilometers above, there's definitely a debate. It's something about astronomy. So it's not just, it depends. It's a bunch of nerds arguing about it.

• 12:21Kelly Weinersmith?

  I thought 100 kilometers was something like, uh, at that point, there aren't enough molecules to hold up your airplane anymore. And so that's where space starts.

• 12:30Daniel Whiteson?

  Oh, that's cool.

• 12:32Kelly Weinersmith?

  Isn't it the Von Karman line?

• 12:33Daniel Whiteson?

  That's the Von Karman line. Yeah. And the ISS is like 300 or 400 kilometers above the surface of the earth. So definitely in space. And so if you're inside our solar system, but outside Earth and its atmosphere, then there's still stuff out there. The vacuum of space inside the solar system is not a perfect vacuum. There's a bunch of particles. Most of it comes from the sun because the sun, as we say often, is producing a lot of stuff, not just photons, but also matter. The solar wind is electrons and protons and alpha particles. These things are zooming around with a lot of energy. And so there's something like 10 million protons per cubic meter between planets. So remember on Earth is 10 to the 25 molecules per cubic meter.

• 13:17Daniel Whiteson?

  Labs on Earth is 10 to the 12. Now we're down to like 10 to the seven protons per cubic meter. So that's down like five or six orders of magnitude compared to vacuums on Earth.

• 13:27Kelly Weinersmith?

  Wow. You wouldn't want to go out there, people.

• 13:31Daniel Whiteson?

  But it would be a good place to do vacuum based experiments.

• 13:34Kelly Weinersmith?

  Sure, yeah.

• 13:35Daniel Whiteson?

  But it's still not that empty, right? Like millions and millions of protons are out there. It's not something you could say is empty. When we talk about vacuum in physics and theoretical physics, we mean space with no extra energy and at the minimum energy state of space. And so this interplanetary space is not approaching the theoretical physics vacuum at all.

• 13:56Kelly Weinersmith?

  Okay.

• 13:56Daniel Whiteson?

  And then take your mental picture and zoom out from the solar system. And now we're between stars, right? Somewhere between our star and Alpha Centauri, for example. Now we're in what we call the interstellar medium, the stuff between the stars. And even this is not that empty, right? It turns out there's a big range of densities of stuff from 10 to the six molecule per cubic meter, which is like the density in our solar system, all the way down to 10 to the four molecules per cubic meter.

• 14:23Kelly Weinersmith?

  Isn't this where we thought the Oort cloud would be? Or this is like when we get past the Oort cloud?

• 14:27Daniel Whiteson?

  Yeah, so there's a couple of different boundaries here. You go out past the planets, you get to like the dwarf planets and then the Kuiper belt objects. Beyond that, there's the boundary of the heliosphere where the sun dominates and it meets the interstellar wind, the galactic wind. That's a region where you're exiting the solar system. But you're right, beyond that is more stuff. So the Oort cloud technically is part of the interstellar medium because it's past that boundary. It's in a region where the interstellar wind dominates over the sun's wind, but it is gravitationally bound to the sun. So it's sort of like playing both sides of the equation, if that makes sense.

• 15:07Kelly Weinersmith?

  It does, but but it's still so overwhelmed by the gas in that area that the interstellar medium is 99% gas and all the rocks out there are just make up 1% dust. Would that be fair to say?

• 15:20Daniel Whiteson?

  Yeah, that's right. And we're going to come back to the interstellar medium and do a deep dive into it. It has a lot of really interesting structure. But yeah, most of it is gas. It's 99% gas. And then a little bit of it is rocks or comets like in the Oort cloud and dust. There's a few cosmic rays out there, but mostly it's just gas. And it has all sorts of interesting turbulence and structure that we're going to talk about. And don't forget, out there also in the galaxy is dark matter. We're only talking right now about the luminous matter, the kind of stuff that's made up of atoms. But we know that the universe is dominated by dark matter. We know that it's spread out throughout the galaxy, but there's five times as much of it as there is luminous matter. So you can't interact with it, but it is there.

• 16:04Daniel Whiteson?

  And so if you're just thinking about like the density of matter overall, you have to also account for that dark matter.

• 16:10Kelly Weinersmith?

  Okay, so in the interstellar space, mostly gas, little bit of dust, you're not likely to stub your toe. But so that now we're talking about what's between the stars. But what about what's between the galaxies and the intergalactic space?

• 16:24Daniel Whiteson?

  Yeah, so now we're leaving the region we've ever probed directly. Like, we've sent probes that have left the solar system just barely. Like, Voyager 1 reached the interstellar medium in 2012, Voyager 2 in 2018. But now, we're going well beyond where we've ever probed directly, and we're relying just on observations and theory and simulation. So now, between the galaxies, what is there? Well, this is something we call, very creatively, the intergalactic medium. And this is mostly rarefied plasma. So that means essentially ionized hydrogen, it's just protons. And out there, on average, it's like 1 to 10 atoms per cubic meter. So, a pretty small number, but still not that small. I mean, intergalactic space is unimaginably vast.

• 17:08Daniel Whiteson?

  There's just so much of it. Even if you think about space between the planets as being big, and most of the space in the solar system is the space between the planets, right? Planets and the sun are rare. Like, if you randomly sampled inside the solar system, you would very rarely touch a planet or a star. And inside the galaxy, right, structure is also very rare. But between the galaxies, it's even hard to comprehend, because we have like millions of light years between these galaxies. And yet, it's still filled with stuff. There's 1 to 10 atoms out there. And actually, these tendrils of plasma, there's also structure to it. It's not just randomly distributed. All the galaxies are connected by these tendrils of plasma, these filaments.

• 17:49Daniel Whiteson?

  And something like half of all the atoms in the universe are outside of galaxies and are inside these tendrils between galaxies. It's incredible.

• 17:58Kelly Weinersmith?

  Wow, and how do they get there? Do they get like shot out by suns? Or they were just there from the beginning of the creation of the universe, and they just stayed there?

• 18:05Daniel Whiteson?

  Both and all of those.

• 18:07Kelly Weinersmith?

  Okay.

• 18:07Daniel Whiteson?

  Like, there's a feedback loop where galaxies are emitting particles, right? Supernovas and shock waves and all sorts of stuff. But mostly, this structure represents the dark matter structure of the universe. Galaxies form in the deepest wells, and then there are tendrils of dark matter between these wells. And that's where you find these tendrils of plasma. So the plasma is like a tracer that tells you where the dark matter is. And also, between these galaxies, of course, is a significant amount of dark energy. Dark energy is everywhere in space. It's just a really, really tiny amount. But because it doesn't get diluted as the universe expands over vast spaces, it really starts to add up. Between the galaxies, you have to start accounting for the dark energy. And then as you zoom out, galaxies form clusters.

• 18:52Daniel Whiteson?

  And between those clusters, most of the energy density between those clusters is not matter at all. It's dark energy.

• 19:00Kelly Weinersmith?

  All right, so at the big scale, we're mostly thinking about dark energy. And as we zoom in, we're starting to think about gas. Let's take a break. And when we come back, I want to go back to the interstellar medium and go into more detail about what's there.

• 19:16Daniel Whiteson?

  Yeah.

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• 23:15Kelly Weinersmith?

  And we're back and Daniel is going to be telling us more about what is in the interstellar medium.

• 23:20Daniel Whiteson?

  That's right. Today, we're talking about the context of our solar system, where it is and how much stuff there is around us and whether our solar system is jumping into a huge bubble in the galaxy.

• 23:31Kelly Weinersmith?

  Sounds fun.

• 23:32Daniel Whiteson?

  And so the important thing is to understand this bit between the stars. And something I love doing is going back in history and understanding when we understood something and what our like early silly ideas were, you know, because it's easy to just like download all of human knowledge into your brain right now. But remember that people like struggled and puzzled to figure this out. And there were many wrong paths and silly ideas entertained for hundreds of years along the way.

• 23:57Kelly Weinersmith?

  Well, it's kind of cute to imagine that nobody in like 50 years is going to look back at us and be like, oh, remember that cute thing that the physicist thought in 2026 about, anyway, we they could be laughing at us eventually, but but let's go ahead and laugh at them.

• 24:12Daniel Whiteson?

  So first, remember in the broader context, we didn't know that our galaxy was one of many galaxies until like the early 1900s, right? We thought we had a galaxy. It was a bunch of stars and that was it. It was just our galaxy. And that was everything. The galaxy was basically the universe and everything that was out there that wasn't actually in our galaxy, but we didn't know it yet. We called like a nebula because it was like a smeary blob in the sky. We couldn't resolve. So that's sort of like the bigger picture. But then what about inside the galaxy? Well, people tried to make like maps of the galaxy early on to try to understand the structure. But we're interested in like the bit between the stars. And it was like the late 1700s, people started to wonder like,

• 24:54Daniel Whiteson?

  hmm, is the space between the stars really a vacuum? Or is there something there?

• 25:01Daniel Whiteson?

  You know, and how would we know? And this coincides a little bit later with Maxwell's development of electromagnetism, the idea that light is a balance between electricity and magnetism. The energy sloshes back and forth between the two fields, but then it moves as a wave in these fields at the speed of light. And this is Maxwell's great discovery that light is a wave, which of course begged the question of a wave in what, right? And so we saw the rise of the theory of the luminiferous ether, right? This is a famous theory which was invented to explain what light is wiggling, right? Maybe there's something out there like if waves are wiggles in water and sound is wiggles

• 25:42Daniel Whiteson?

  in air, maybe light is a wiggle in this luminiferous ether that we hadn't ever discovered before. It fills the universe. So this would be an early theory of the interstellar medium. What is out there between the stars? What allows the light to go from Alpha Centauri to us here?

• 25:58Kelly Weinersmith?

  And so the luminiferous ether, I thought I was going to get it on the first shot.

• 26:03Daniel Whiteson?

  Totally did.

• 26:04Kelly Weinersmith?

  I stumbled a little. Would you find that between like Earth and Mars? Or that was just between the end of our solar system and the start of the next solar system?

• 26:14Daniel Whiteson?

  Great question. No, it should be everywhere. If it exists, it should be everywhere. And that was key because people looked for it and they said, well, let's try to measure our velocity through this ether. If it's out there and it's a medium for light, light is wiggling through this ether, then Earth moving around the sun should be moving zigzagging basically back and forth through this ether. And if light has a constant velocity relative to the ether, then we should be able to measure light moving at different velocities as we move at different velocities through the ether.

• 26:44Kelly Weinersmith?

  Okay.

• 26:44Daniel Whiteson?

  Right? The same way that like you can catch up to sound waves in air because they have a constant velocity relative to the air. So you could catch up to them and even pass them, right? Just like when you're on a boat in a lake, you're making ripples. If you go fast enough, you can catch up to them, which it gives you a wake, right? And so if light is moving through this ether, and then as the Earth moves around the sun, we should see a change in the velocity of light. Famous Michelson-Morley experiment, of course, proved that light is the same speed year round regardless of the direction. And so apparently there is no luminiferous ether. So then people are like, oh, wow, maybe there really is a vacuum in between the stars.

• 27:22Kelly Weinersmith?

  And you all didn't find another use for the word luminiferous ether? That's a total bummer because it just sounds so nice.

• 27:30Daniel Whiteson?

  Well, these days is actually a proliferation of ether theories, which is very confusing because ether most generally just refers to like a theory of the substrate of space. And our current theory, that quantum field theory, where space is filled with these fields, and these fields exist in space, then that's sort of the modern ether. And that's totally cool, and nobody objects to that. But that's an ether where you can't measure your velocity relative to it. Like the luminiferous ether has a frame, it's at rest in some frame, and light moves at a constant speed only in that frame. The modern ether, quantum field theory vacuum, has no frame. And so there's a big distinction between the

• 28:11Daniel Whiteson?

  luminiferous ether of two centuries ago and today's concepts of the ether. And in lots of pop side descriptions, you see this important distinction sort of ignored or fuzzed over.

• 28:22Kelly Weinersmith?

  Okay, all right. And I could use a quick step back and a reminder about where we are. So we've been talking about what is in space between things.

• 28:30Daniel Whiteson?

  Yeah.

• 28:31Kelly Weinersmith?

  And we've just established that we used to think there was ether between stuff, but there's not.

• 28:36Daniel Whiteson?

  But there's not.

• 28:37Kelly Weinersmith?

  But there's not. And so now, okay, so we're going through the history of our understanding of what was in the interstellar medium.

• 28:44Daniel Whiteson?

  That's right.

• 28:45Kelly Weinersmith?

  Okay.

• 28:45Daniel Whiteson?

  And then in 1899, people discovered, oh, there is some actual stuff out there between the stars. Not ether, but like actual matter. There were these things called dark nebulae, essentially these dusty clouds you could see out there that passed between the stars, right? Between us and other stars, there were like these clouds of stuff, which were sometimes silhouetted against this like background star field. This is essentially the first discovery of interstellar gas. It's just blobs of gas that are out there between the stars. And so as you make more and more accurate observations of these stars, and understand like why they dim, or what you can see and what's between you and those stars, you start to build a map of the galaxy and understand like where there's dust and

• 29:28Daniel Whiteson?

  where there's gas. And this is our first like understanding, really, of the context of our solar system.

• 29:34Kelly Weinersmith?

  Understanding that there's gas out there, that gas isn't... Like the Oort Cloud, that's just understanding where other galaxies are out there and you're just kind of seeing them as gas? Or what is that gas exactly?

• 29:47Daniel Whiteson?

  Yeah, this is gas within our galaxy, right? Not between galaxies. This is just understanding like what's between the stars. Are the stars dots in a true vacuum? Or is there like a soup out there? And it turns out it's pretty soupy. We discovered these dark clouds of gas between the stars. And then in the early 1900s, we discovered cosmic rays, which are just particles from space that are zipping towards the earth. And if there are particles zipping towards the earth, they're zipping from somewhere, right? Which means like space out there is filled with all of these particles. So we had this growing awareness that the space between the stars is filled with gas and with ions and with electrons, just like in the solar system. And so instead of thinking of it like, here's a pocket of stuff, the star and the planets,

• 30:31Daniel Whiteson?

  you know how now we think about like a spectrum of stuff inside the solar system? Little bits of dust all the way up to Jupiter and then the sun? In the same way, you should think about a smooth transition between the solar system and the interstellar medium, that between the stars, there is still stuff. Yes, there is a boundary where the solar wind stops dominating and the interstellar wind starts dominating. But in terms of the distribution of stuff, it's pretty smooth. And so you should expect there to be stuff out there in the interstellar medium. And it actually makes up like between 10 and 15% of the mass of the galaxy is not inside the stars. Yeah, it's between the stars. And it's going to play an important role in how those stars are formed and the health of our galaxy in forming stars, whether our galaxy keeps making stars or whether it

• 31:16Daniel Whiteson?

  gets quenched, as they say.

• 31:18Kelly Weinersmith?

  Don't get quenched.

• 31:22Daniel Whiteson?

  So let's dig a little bit more deeply into what's in the interstellar medium. We said before the break that it's mostly gas, which means that it's mostly hydrogen. It's like 90% hydrogen, 10% helium. And the universe is mostly hydrogen, so that makes sense. Something that's interesting, though, is that even though there's a lot of space out there and it's not super dense, this stuff still behaves like a gas, like it interacts with itself. It's not like non-interacting, like the way The exosphere of the moon is just a bunch of particles that ignore each other. Out there, these particles are moving so fast that the mean free path is pretty short. Like, they don't go very far before they run into another one or interact with another one. They don't have to actually touch because they have charges. They can interact without touching each other.

• 32:06Daniel Whiteson?

  So that's really fascinating. That was a little bit of a surprise to me when I learned about that.

• 32:09Kelly Weinersmith?

  So if somebody were living out there, could they be getting power from that?

• 32:15Daniel Whiteson?

  They could be getting killed from that. I mean, this is radiation, right? These particles are moving at high speeds. Even though it's fairly low density, these things are moving pretty fast. Like, inside the solar system, the solar wind is moving at 400 kilometers per second. Sometimes these particles have energy up to 10 keV. And in the interstellar medium, these things are even higher speeds. So technically, the temperature can sometimes be like up in the millions of Kelvin, even though the density is very, very low. So you would like freeze because there's not a lot of heat being deposited, but also you'd be riddled with all of these tiny little high-speed bullets.

• 32:49Kelly Weinersmith?

  That's definitely worse than New Jersey.

• 32:53Daniel Whiteson?

  It's a close call, but I agree with you.

• 32:54Kelly Weinersmith?

  Oh, I do like New Jersey, though. I was born there.

• 32:57Daniel Whiteson?

  The other fascinating bit about the interstellar medium is what's not gas, which is the dust. And dust sounds like, you know, stuff you'd sweep up and throw away. But dust is actually super fascinating from a cosmic sense. Because dust comes from dead stars, right? Like, how do you make dust? You need heavier elements. You need those elements to stick together. You need them to form these little crystals and to gather more bits to themselves. And dust is like the seed for new solar systems and new planets, right? Our Earth formed by a bunch of dust, which bounced into itself and stuck together and formed a bigger piece of dust and gathered more together. So these are really the seeds of future solar systems, as well as the ashes of previous ones.

• 33:38Kelly Weinersmith?

  Oh, man, at first I thought you were talking about gallstone formation, and then it sounded much more beautiful. And I'm glad my gallstones, you know, stopped getting bigger before they formed planets and universes.

• 33:50Daniel Whiteson?

  But there's huge amounts of like iron and silicon, magnesium, a bunch of oxygen and carbon. There's even like nano diamonds and fullerenes out there. They're really fascinating. And some of these things are pre-solar grains, which are preserved from the original star. So things formed like in the atmosphere of the star and then there's a supernova that blasts out and spreads all these dust grains and some of them survive, right? They're not shattered. A lot of this stuff is smashed into itself and shattered and reprocessed. But some of these pre-solar grains survive and we actually find them like on the ocean floor. We can find little grains of dust that survived a supernova and zoomed across the universe and gathered together to help form the Earth.

• 34:34Kelly Weinersmith?

  How could you know? How could you find a grain of sand and be like, I know where you came from?

• 34:41Daniel Whiteson?

  Yeah, they have really unusual isotope mixtures, which you only find like in those conditions that are formed like in the atmospheres of supernovas. It's really incredible.

• 34:50Kelly Weinersmith?

  Wow, that is incredible.

• 34:52Daniel Whiteson?

  And so the interstellar medium, it's mostly gas. There's a little bit of dust out there. It's like 1%, but I think that's the most interesting part. And then of course, some of it is very, very high energy particles zooming through space, cosmic rays. And so you mix all this together and that's what the interstellar medium is made out of. Mostly gas, which is actually mostly molecular, not ionized, and then 1% dust and then a few very high energy ions.

• 35:16Kelly Weinersmith?

  All right, we are getting closer to talking about the amazing and exciting super bubble. So we're going to take a break. And when we get back, we're going to talk about how the gas and the dust are structured inside of the interstellar medium.

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• 37:20Speaker 15

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• 38:02Speaker 12

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• 39:38Kelly Weinersmith?

  We are back and we're talking about the interstellar medium, the space between the stars. And we've established that it's mostly gas, some dust in there. How is all of that stuff structured in this space, Daniel?

• 39:51Daniel Whiteson?

  Yeah, so you might imagine, oh, it's just smooth. It's just spread out out there. But it's not. It's like a vast fluid, which means it has all sorts of stuff going on. It's like our atmosphere, right, which we model and has currents and winds and high-density regions and low-density regions. And so you can think about the interstellar medium as a vast fluid because it is interacting, right? The mean free path of these particles is pretty short, but it's also more complex than a simple fluid because often these particles are moving really, really fast or whole regions of particles are moving super duper fast. So you can have supersonic regions of this interstellar medium, right? Places where the gas is moving faster than the speed of

• 40:31Daniel Whiteson?

  sound in that medium. Remember, you can define the speed of sound in anything, in water, in air, in steel, and it depends like on the density of the thing. Things that are denser, the bonds are tighter. And if you push on one, then the pressure wave propagates more quickly than it would if something was really low density because then the particles have to drift longer before they bump into each other.

• 40:53Kelly Weinersmith?

  Okay.

• 40:53Daniel Whiteson?

  And so what happens in the interstellar medium is you have these shock waves, these supersonic shock waves, which smash into something else. And so you get this supersonic shock wave, but that compresses the gas, makes it more dense, and that actually raises the speed of sound. So then the local flow becomes subsonic. So you get supersonic shock waves, which then actually becomes subsonic. And so it creates this really complex structure and these really interesting patterns of density. It's really hard to model because it's so chaotic.

• 41:25Kelly Weinersmith?

  So in order to hear sound, you need like molecules to transmit it. So like if you were out there, would you be hearing these particles or not really?

• 41:36Daniel Whiteson?

  You can technically scream and be heard in the interstellar medium. It would be very faint and very, very slow, right?

• 41:43Kelly Weinersmith?

  Okay.

• 41:44Daniel Whiteson?

  You could hear these things. I can tell you what they would sound like, you know, probably like

• 41:50Kelly Weinersmith?

  Cool, okay.

• 41:50Daniel Whiteson?

  Very interesting.

• 41:52Kelly Weinersmith?

  I don't know why, but in my mind, I had imagined the interstellar space as being like a quiet, calm, still place, which is probably silly because you know, like solar winds keep things moving. There's probably nowhere in space that's actually super still.

• 42:03Daniel Whiteson?

  No.

• 42:03Kelly Weinersmith?

  But I hadn't imagined it being super turbulent like this. So

• 42:06Daniel Whiteson?

  Yeah, it's really turbulent and you know, on time scales that we don't usually think about. But if you played a movie, you know, where a million years passes in a second, it would look like bubbling frothing tea or something, you know, it's a lot of stuff going on. And the most interesting stuff that goes on is that when you get dense regions, you get star formation, right? If you have a clump of gas and it cools down enough, then one of those grains of dust can start to gather other stuff near it. It's important that it's cool, remember? Because if things are really, really hot, that means they're moving fast and then gravity has no chance to pull them together. But if things are cool, that means they're slow moving and gravity can gather dust grains together, little bits at a time.

• 42:49Daniel Whiteson?

  They stick together, they start forming bigger and bigger stuff. And that's when you get gravitational collapse, leads to a runaway effect, you get more gravity, and then you get a star forming and there are stuff around it, starts to swirl around it into a protoplanetary disk. And that's how a star is born. It's born from denser regions in the interstellar medium and those regions come from the turbulence. So the structure in the interstellar medium is the reason we have stars.

• 43:14Kelly Weinersmith?

  The structure in the interstellar medium, but don't you only have interstellar medium because it's the region between pre-existing stars?

• 43:22Daniel Whiteson?

  Yeah.

• 43:24Daniel Whiteson?

  There's sort of a circular definition there, but yeah, the interstellar medium can make new stars and then it's no longer the interstellar medium.

• 43:30Kelly Weinersmith?

  Okay. And so you could, you could imagine this process going on infinitely many times and then you just have an infinite number of suns. So how, how often does this process happen?

• 43:39Daniel Whiteson?

  So it depends a lot on the details of the temperature of the cloud and the composition. But when it happens, it happens a lot. Like you get a lot of stars all born in one neighborhood together. And this is why, for example, stars are very often found in binary systems, like pairs of stars, gravitationally bound to each other, or even briefly trinary systems, or more long-lived systems, where you have like a binary star and another one that's more distant, so that's like effectively a binary system. So you have these regions of star formation where a huge cloud meets these requirements, where you have like little dense grains and the whole thing is cold enough. And how often it happens depends a lot on the history of your galaxy. There's actually an interplay between the stars and the interstellar medium, which

• 44:24Daniel Whiteson?

  we're going to talk about, because the stars push back and form these bubbles. And so it's a whole complex evolving thing. And the rate at which stars form changes a lot where you are in the galaxy and over time. And it's not something we fully understand. Like some galaxies out there have stopped forming stars at all and some of them are still making stars. And this is called quenching and it's not something that's fully understood.

• 44:48Kelly Weinersmith?

  All right. Well, we still haven't gotten to the bubbles. How do we get closer to bubbles?

• 44:51Daniel Whiteson?

  Yeah. So you have the interstellar medium. You have these clouds of gas. Sometimes they collapse and form a star. What happens when you make a star is that it pushes back against the interstellar medium. Right? Like our sun has a solar wind and that's pushing back. The reason we call it a solar system is it's the region where our solar wind dominates. So now you have this cavity inside the interstellar medium. Right? And so the sun basically makes like a little bubble, which we call the heliosphere, by pushing back against the interstellar medium. So now imagine you have a bunch of stars, each with their little bubbles, and one of them goes supernova, right? So this blows out a huge region inside the ISM, and this can create a super bubble that's like hundreds of light years across.

• 45:34Daniel Whiteson?

  Because remember, a supernova is the collapse of a star gravitationally, which then bounces back out and dumps an enormous amount of energy. A single supernova can be brighter than the entire galaxy. And it doesn't just put out photons, it also shoots out a huge number of particles.

• 45:50Kelly Weinersmith?

  How many light years across is our solar system?

• 45:54Daniel Whiteson?

  So our solar system is like one or two light years in radius.

• 45:58Kelly Weinersmith?

  Okay.

• 45:59Daniel Whiteson?

  Right? So it's pretty small, but then a supernova can make a bubble that's like hundreds of light years across.

• 46:04Kelly Weinersmith?

  Wow.

• 46:04Daniel Whiteson?

  And if there's multiple supernova, because remember, stars are formed in bunches, and that means they can also go supernova in bunches. They have like their cycles all synchronized.

• 46:13Kelly Weinersmith?

  Oh, boy.

• 46:13Daniel Whiteson?

  Then you can get a really big bubble that lasts for like tens of millions of years.

• 46:18Kelly Weinersmith?

  Okay, so the the bubbles that we're talking about then are solar winds of some sort blowing interstellar medium out, and that is the bubble. These are not the fun bubbles that I was imagining, Daniel.

• 46:32Daniel Whiteson?

  So you get little bubbles from solar winds, but you really get the super bubbles from a bunch of supernovas all going off at once and making these really vast bubbles. It's super cool. Also, if one happens near the edge of the galaxy, like the galaxy is a big disk, right? It's like much broader than it is thick. So if it occurs like near the top of the disk, then essentially it creates like a hole in the galaxy, and they call it a galactic chimney. Because then hot gas from inside the galaxy basically vents out from the galaxy. So you have like a little spurt.

• 47:06Kelly Weinersmith?

  So the question that we're answering today is, do we live in a super bubble? But timing's got to matter too, right? If you get a bunch of stars going supernova all at once to make your super bubble, you would be dead if you were inside of the super bubble region, right? So it's got to go super bubble, make a bubble.

• 47:22Daniel Whiteson?

  Exactly.

• 47:22Kelly Weinersmith?

  And then a planet needs to like wander in or get captured and then become habitable. And then you find yourself inside the super bubble. That's how that would have to work, right? Okay.

• 47:31Daniel Whiteson?

  Right. And remember, our solar system has existed for like almost 5 billion years. But these super bubbles last for tens of millions of years. They're very transient. And our star is moving through the galaxy, right? And its orbit takes hundreds of millions of years. And so they're created by supernova, but you don't want to be there when they're made. You're right. And so coincidentally, there is a bubble. It's called the local bubble. It's 300 to 1,000 light year wide. It has a much lower gas density than the rest of the interstellar medium, but like down by 10. And it's surrounded by denser walls of neutral gas and dust. Probably was formed by like maybe 10 to 20 supernovas that happened over the last 10 to 15 million years.

• 48:15Daniel Whiteson?

  So sort of recently on cosmic history scales. We can't pinpoint exactly which stars they are. Probably they're stars related to the Scorpius Centaurus Association of Stars. And the shape of it is kind of weird. It's not spherical and there's other like nearby bubbles that it connects to called Loop 1. And so fortunately, it formed before we got here, right? The sun orbits the center of the galaxy at like 220 kilometers per second and moves at like 25 kilometers per second relative to the interstellar medium. And so that means that we go like 20 to 30 parsecs every million years or so. This bubble is a few hundred parsecs across.

• 48:59Daniel Whiteson?

  So it seems like about 15 million years ago, the first supernova started inflating this bubble. Five or 10 million years ago, we entered the cavity. And now we're in this super bubble. We're drifting through a little warm cloud of gas inside this bubble. And in a couple million years, we're probably going to exit it back into the interstellar medium, which is a little bit denser.

• 49:22Kelly Weinersmith?

  So when we enter the interstellar medium, where there's all of that turbulence and it's crazy, is that going to be a bad time for us?

• 49:30Daniel Whiteson?

  No, probably not, because we have the solar wind which protects us. We have

• 49:34Kelly Weinersmith?

  Okay.

• 49:35Daniel Whiteson?

  We're like moving within our own little bubble anyway.

• 49:37Kelly Weinersmith?

  Okay.

• 49:37Daniel Whiteson?

  So it doesn't really matter that much. It's just a way to highlight that there's a lot of stuff happening out there, right? There's turbulence and chaos and all that stuff and star formation and star dying and all that happens because of this structure. Like you get stars because you have denser regions and then those stars blow out, make super bubbles when they die to make less dense regions, but then those super bubbles collapse because the interior is lower density but higher temperature and that hot gas leaks out and ruptures the shell. And so you get this cycle of stuff. And so, you know, you shouldn't think of the galaxy out there between the stars as empty. In fact, it's filled with stuff and it's really dynamic.

• 50:17Daniel Whiteson?

  And the structure out there is what determines like where stars are made and then stars explode and replenish the interstellar medium. And so the interplay between the stars and the interstellar medium really determines like, hey, is your galaxy making stars or not? Is it depleasing all of its gas? All of this kind of stuff is determined by this interaction between the stars and the interstellar medium.

• 50:41Kelly Weinersmith?

  So, so we live inside of a bubble made by our sun, which is currently inside of the local super bubble.

• 50:48Daniel Whiteson?

  That's right. That was made by supernovas of other stars, fortunately, before we entered this region.

• 50:54Kelly Weinersmith?

  Amazing.

• 50:57Daniel Whiteson?

  Exactly. And so that's the broader context of your life. We happen to be living inside this bubble and now you know all about it.

• 51:04Kelly Weinersmith?

  That's right. And we hope that you don't stub your toes on any galactic dust or happen to get dust of any sort inside of your gallbladder or kidneys or anything else. We wish you health and this is not a great transition, but man, it hurt to laugh at Daniel's jokes today.

• 51:20Daniel Whiteson?

  I tried to hold back. I really, really did.

• 51:22Kelly Weinersmith?

  I appreciate it, Daniel. You can't help yourself. You're just a funny guy.

• 51:27Daniel Whiteson?

  Looks aren't everything.

• 51:30Daniel Whiteson?

  Thanks, everybody.

• 51:39Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 51:44Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 51:50Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 51:57Daniel Whiteson?

  We really mean it. We answer every message. Email us at [email protected].

• 52:03Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.

• 52:13Daniel Whiteson?

  Don't be shy. Write to us.

• 52:17Speaker 16

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