Speaker 1: Think about where you are right now. Maybe you're in
Speaker 1: your car commuting, or you're out walking the dogs or
Speaker 1: at home doing the dishes. But where are you relative
Speaker 1: to everyone else? How do you define it? Probably you
Speaker 1: zoom out and put yourself in a wider context, your town,
Speaker 1: your region, your whole country. Listeners to this pod will
Speaker 1: likely be inclined to zoom out even further to all
Speaker 1: of Earth and then the whole Solar System. From there,
Speaker 1: it's very tempting to skip forward and put our solar
Speaker 1: system into its place in the galaxy, than the galaxy
Speaker 1: into its cluster, and then that cluster within the supercluster foam.
Speaker 1: But have we been too hasty? Is our galaxy so simple,
Speaker 1: just a collection of stars floating around a central black hole?
Speaker 1: Or is there more nuance to our galactic context? We
Speaker 1: think of the galaxy as mostly stars, but they're formed,
Speaker 1: they burn, and they die. There's a constant slashing of
Speaker 1: energy back and forth between those stars and vast clouds
Speaker 1: of raw material. Where is our Solar system relative to
Speaker 1: those clouds? Are we in the midst of a galactic fog?
Speaker 1: Or are we floating through an open bubble? Today on
Speaker 1: the pod will zoom out to this fascinating scene, understand
Speaker 1: the dynamics of star formation and destruction, and let you
Speaker 1: know exactly where you we all of us stand galactically speaking.
Speaker 1: Welcome to Daniel and Kelly's Extraordinary Galaxy.
Speaker 2: Hello, I'm Kelly Winer Smith. I study parasites and space
Speaker 2: and I love bubbles.
Speaker 1: Well, Hi, I'm Daniel. I'm a particle physicist. I love bubbles,
Speaker 1: and I have all of my organs.
Speaker 2: Okay, yeah, right, So that I was gonna ask you
Speaker 2: about the most painful thing you ever experienced in your
Speaker 2: life as a lead into why my laugh is going
Speaker 2: to be a little off today, folks.
Speaker 1: I'm going to try to restrain myself from being funny
Speaker 1: today because if I make a joke that makes Kelly
Speaker 1: crack up, she could literally crack up.
Speaker 2: Oh I could, Yes, I could split a stitch. So
Speaker 2: what aside from labor, The most painful thing that ever
Speaker 2: happens to me was the gallstones that tried to pass
Speaker 2: their way out of my body on Friday, but ended
Speaker 2: up getting removed surgically. The whole thing, the whole gallbladder
Speaker 2: was removed, and so I am still working on resolving
Speaker 2: my stitches. And so it hurts to laugh. So I
Speaker 2: think I've come up with the way to laugh that
Speaker 2: it hurts less, and so my my laugh will sound
Speaker 2: a little I don't know, maybe even less annoying. I
Speaker 2: have seen there's a couple complaints about the way I laugh.
Speaker 2: You'all can just kiss my rear ends. But anyway, Daniel,
Speaker 2: what is the most painful thing that's ever happened to you?
Speaker 1: Oh? Most painful thing that's ever happened to me? I
Speaker 1: broken my wrist, but weirdly that didn't hurt that much.
Speaker 1: I think maybe the most painful thing is when I
Speaker 1: broke my little toe. I like massively stubbed it on
Speaker 1: a little concrete outpost and cracked it and wow, yeah,
Speaker 1: I thought I was gonna die.
Speaker 2: Ouch. Yeah, And how long did it take to heal?
Speaker 2: What do you do? Just put like a nothing.
Speaker 1: You just tape it to the next toe, and like
Speaker 1: whatever happens inside of it happens. Like there's no surgery
Speaker 1: to correct, like the little bones inside your little toe,
Speaker 1: nobody needs it. I was like, just cut that bugger off,
Speaker 1: but I was like, no, you just tape it to
Speaker 1: the next one. That's it.
Speaker 2: Well, you know, Daniel, it sounds like if you lived
Speaker 2: in a bubble, you would have been safe. But if
Speaker 2: I lived in a bubble, well, that wouldn't have saved me.
Speaker 2: And so well, you know, maybe if the whole universe
Speaker 2: was in a bubble, that would save us from some
Speaker 2: of the bumps and bruises of existence. But do we
Speaker 2: live in a bubble? Daniel?
Speaker 1: Well, we are trying to pop the bubble of ignorance
Speaker 1: out there and help everybody understand their cosmic context, where
Speaker 1: we are situated in our solar system, where our solar
Speaker 1: system is situated in the galaxy, and the whole context
Speaker 1: for our existence. I love filling in all those details
Speaker 1: and helping people understand really where they live. And today
Speaker 1: we're going to dive into that topic essentially, where our
Speaker 1: sun is in the galaxy? Is there structure out there?
Speaker 1: How does it all work?
Speaker 2: And I think all of our inner kids are excited
Speaker 2: today to find out whether or not we live in
Speaker 2: a super bubble.
Speaker 1: So before we dive into it, I was wondering what
Speaker 1: people out there knew on the topic. So I went
Speaker 1: to our group of volunteers, I asked them, do we
Speaker 1: live in a super bubble? Here's what folks had to say,
Speaker 1: I don't even really know what a super bubble is,
Speaker 1: so maybe what do you think, Glen. I have no
Speaker 1: idea what a super bubble is. I have not heard
Speaker 1: the term super bubble. It makes me think about some
Speaker 1: conceptualizations of fourth dimensionality.
Speaker 3: Considering I couldn't tell you what a super bubble is.
Speaker 3: I can't really say if we live in one, but
Speaker 3: it sounds like one of these really exciting astrophysical maybe
Speaker 3: who knows cool universe things, so I'm eager to learn
Speaker 3: about it. I have never heard of the term super bubble,
Speaker 3: but we probably live in one.
Speaker 2: I like the I've never heard of it, so yes,
Speaker 2: this is.
Speaker 1: Really exciting to me that nobody had heard of it,
Speaker 1: because it means we get to share this really cool
Speaker 1: thing about the universe with everybody. Yay.
Speaker 2: We love doing that.
Speaker 1: You know that great XKCD comic. I mean they're all great,
Speaker 1: but the one about the moment you discover somebody doesn't
Speaker 1: know something really cool about the universe and how you
Speaker 1: should treat that with reverence and respect because it's a
Speaker 1: wonderful teaching moment. That's where we are today about super bubbles.
Speaker 2: I mean, I think we can all agree that Randall
Speaker 2: Monroe is the greatest web cartoonist to have ever lived.
Speaker 1: Right, Wow, that's instant karma right there. Wow. I hope
Speaker 1: Zach is listening and laughing at that.
Speaker 2: All right, all right, bubbles, all.
Speaker 1: Right, bubbles. So we're talking about the Sun and the
Speaker 1: Solar System's context in the galaxy. But before we get there,
Speaker 1: let's do like an overview of like what's out there
Speaker 1: in space? How empty is it? Anyway? So to get oriented,
Speaker 1: remember that our situation here on Earth is very unusual.
Speaker 1: Like the density of stuff around you is very rare
Speaker 1: in the universe. The universe is mostly very very low density,
Speaker 1: and we live in a very very high density situation.
Speaker 1: The Earth, of course, is wonderful and cozy, and in
Speaker 1: like a normal atmosphere, you'll find a huge number of
Speaker 1: molecules per cubic meter, like ten to the twenty five
Speaker 1: molecules per cubic meter. It's a big number. And that's
Speaker 1: just because like Avagadro's number is pretty big, and gravity
Speaker 1: has gathered the Earth and the stars and all that
Speaker 1: stuff together into little dense clumps.
Speaker 2: I thought Avagadro's number was ten to the twenty three.
Speaker 1: It is, yeah, exactly.
Speaker 2: Oh but this is ten to the twenty fifth.
Speaker 1: Yeah, because a mole is less than a cubic meter.
Speaker 2: Got it, got it. I'm with you now, Okay, these
Speaker 2: are big numbers, Daniel, big numbers.
Speaker 1: These are big numbers. And you know, sometimes on Earth
Speaker 1: we want to do experiments in low density situations, like
Speaker 1: when we collide particles together, we don't want a whole
Speaker 1: bunch of other particles around, Or when you build a
Speaker 1: plasma inside eater to diffusion, you want mostly a vacuum
Speaker 1: before you start. And so we have all these vacuums
Speaker 1: in labs. We've created these situations you pump out all
Speaker 1: the air, and you might be surprised to learn that
Speaker 1: even in those highly specialized vacuums on Earth, we still
Speaker 1: have something like ten to the twelve or ten to
Speaker 1: the fifteen molecules per cubic meter. Like that's down ten
Speaker 1: or thirteen orders of magnitude from your general atmosphere. So like,
Speaker 1: really good work, and this is not easy, but still
Speaker 1: it's a lot of molecules.
Speaker 2: When somebody says the vacuum of space, are we doing
Speaker 2: better than the vacuum of space? When we make a
Speaker 2: vacuum in the lab or are we doing about the
Speaker 2: same as the quote unquote vacuum of space.
Speaker 1: Neither. We're not doing anywhere close to the vacuum of space.
Speaker 1: The density of vacuums in labs here on Earth is
Speaker 1: much higher, is much much more stuff in our lab
Speaker 1: quote unquote vacuums than there is out there in space. Okay,
Speaker 1: but let's stick into that and zoom our mental picture
Speaker 1: out from the surface of the Earth. Obviously, the atmosphere
Speaker 1: drops gradually, right, There's no like huge wall or gait
Speaker 1: or anything. This is not space balls, where atmosphere is
Speaker 1: that contained by a huge bubble right where the code
Speaker 1: is one two three four. But in a funny way,
Speaker 1: there is because space officially starts at like one hundred
Speaker 1: kilometers above the surface of the Earth, which I always
Speaker 1: thought was weird.
Speaker 2: There's some debate about that though, isn't there.
Speaker 1: Yes, absolutely there is, and like you can be an
Speaker 1: astronaut if you've gone eighty kilometers above. There's definitely a debate.
Speaker 1: It's something about astronomy, so it's not just it depends.
Speaker 1: It's a bunch of nerds arguing about it.
Speaker 2: I thought one hundred kilometers was something like at that point,
Speaker 2: there aren't enough molecules to hold up your airplane anymore,
Speaker 2: and so that's where space starts.
Speaker 1: Oh that's cool, isn't it.
Speaker 2: The Von Carmon line.
Speaker 1: That's the Von Carmen line. Yeah, And the ISS is
Speaker 1: like three hundred or four hundred kilometers above the surface
Speaker 1: of the Earth, so definitely in space. And so if
Speaker 1: you're inside our Solar system but outside Earth and its atmosphere,
Speaker 1: then there's still stuff out there. The vacuum of space
Speaker 1: inside the Solar System is not a perfect vacuum. There's
Speaker 1: a bunch of particles. Most of it comes from the
Speaker 1: Sun because the Sun, as we say often is producing
Speaker 1: a lot of stuff, not just photons but also matter.
Speaker 1: The solar wind is electronic and protons and alpha particles.
Speaker 1: These things are zooming around with a lot of energy.
Speaker 1: And so there's something like ten million protons per cubic
Speaker 1: meter between planets. So remember on Earth it's ten to
Speaker 1: the twenty five molecules per cubic meter, labs on Earth
Speaker 1: is ten to the twelve. Now we're down to like
Speaker 1: ten to the seven protons per cubic meter, so that's
Speaker 1: down like five or six orders of magnitude compared to
Speaker 1: vacuums on Earth.
Speaker 2: Wow, you wouldn't want to go out there, people, but.
Speaker 1: It would be a good place to do vacuum based experiments.
Speaker 2: Sure.
Speaker 1: Yeah, but it's still not that empty, right, Like millions
Speaker 1: and millions of protons are out there. It's not something
Speaker 1: you could say is empty. And when we talk about
Speaker 1: vacuum in physics and theoretical physics, we mean space with
Speaker 1: no extra energy, and it's a minimum energy state of space.
Speaker 1: And so this interplanetary space is not approaching the theoretical
Speaker 1: physics vacuum at all. Okay, And then take your mental
Speaker 1: picture and zoom out from the Solar System and now
Speaker 1: we're between stars. Were between our star and Alpha Centauri
Speaker 1: for example. Now we're in what we call the interstellar medium,
Speaker 1: the stuff between the stars. And even this is not
Speaker 1: that empty, right. It turns out there's a big range
Speaker 1: of densities of stuff from ten to the six molecule
Speaker 1: percubic meter, which is like the density in our Solar system,
Speaker 1: all the way down to ten to the four molecules
Speaker 1: per cubic meter.
Speaker 2: Isn't this where we thought the Ork cloud would be?
Speaker 2: Or this is like when we get past the Ork Cloud.
Speaker 1: Yeah, so there's a couple of different boundaries here. You
Speaker 1: go out past the planets, you get to like the
Speaker 1: dwarf planets, and then the Kuiper Belt objects. Beyond that,
Speaker 1: there's the boundary of the heliosphere where the Sun dominates
Speaker 1: and it meets the interstellar wind, the galactic wind. That's
Speaker 1: a region where you're exiting the Solar System. But you're
Speaker 1: right beyond that is more stuff. So the Orc Cloud
Speaker 1: technically is part of the interstellar medium because it's past
Speaker 1: that boundary. It's in a region where the interstellar wind
Speaker 1: dominates over the Sun's wind, but it is gravitationally bound
Speaker 1: to the Sun, so it's sort of like playing both
Speaker 1: sides of the equation off.
Speaker 2: That makes sense, it does, but it's still so overwhelmed
Speaker 2: by the gas in that area that the interstellar medium
Speaker 2: is ninety nine percent gas and all the rocks out
Speaker 2: there are just make up one percent dust. Would that
Speaker 2: be fair to say?
Speaker 1: Yes, that's right, and we're going to come back to
Speaker 1: the interstellar medium and do a deep dive into it.
Speaker 1: Has a lot of really interesting structure, but yeah, most
Speaker 1: of it is gas. It's ninety nine percent gas, and
Speaker 1: then a little bit of it is rocks or comets,
Speaker 1: like in the oricloud and dust. There's a few cosmic
Speaker 1: rays out there, but mostly it's just gas and has
Speaker 1: all sorts of interesting turbulence and structure that we're going
Speaker 1: to talk about and don't forget out there. Also in
Speaker 1: the galaxy is dark matter. We're only talking right now
Speaker 1: about the luminous matter, the kind of stuff that's made
Speaker 1: up of atoms, but we know that the universe is
Speaker 1: dominated by dark matter. We know that it's spread out
Speaker 1: throughout the galaxy, but it there's five times as much
Speaker 1: of it as there is luminous matter, so you can't
Speaker 1: interact with it, but it is there. And so if
Speaker 1: you're just thinking about like the density of matter overall,
Speaker 1: you have to also account for that dark matter.
Speaker 2: Okay, so in the interstellar space mostly gas, little bit
Speaker 2: of dust, you're not likely to stub your toe. But
Speaker 2: so now we're talking about what's between the stars, But
Speaker 2: what about what's between the galaxies and the intergalactic space.
Speaker 1: Yeah, so now we're leaving the region we've ever probed directly,
Speaker 1: Like we've sent probes that have left the Solar System
Speaker 1: just barely like Voyager one reached the interstellar medium in
Speaker 1: twenty twelve, Voyager two and twenty eighteen. But now we're
Speaker 1: going well beyond where we've ever probed directly, and we're
Speaker 1: relying just on observations and theory and stimulation. So now
Speaker 1: between the galaxies, what is there, Well, this is something
Speaker 1: we call very creatively the intergalactic medium, and this is
Speaker 1: mostly rarefied plasma, so that means essentially ionized hydrogen is
Speaker 1: just protons and out there. On average it's like one
Speaker 1: to ten atoms per cubic meter, so a pretty small number,
Speaker 1: but still not that small. I mean, intergalactic space is
Speaker 1: unimaginably vast. There's just so much of it. Even if
Speaker 1: you think about space between the planets as being big,
Speaker 1: and most of the space and the Solar System is
Speaker 1: the space between the planets, right, planets and the Sun
Speaker 1: are rare, like you're randomly sampled inside the Solar System.
Speaker 1: You would very rarely touch a planet or a star
Speaker 1: inside the galaxy. Right structure is also very rare. But
Speaker 1: between the galaxies, it's even hard to comprehend because we
Speaker 1: have like millions of light years between these galaxies, and
Speaker 1: yet it's still filled with stuff. There's one to ten
Speaker 1: atoms out there, and actually these tendrils of plasma there's
Speaker 1: also structure to it. It's not just randomly distributed. All
Speaker 1: the galaxies are connected by these tendrils of plasma, these filaments,
Speaker 1: and something like half of all the atoms in the
Speaker 1: universe are outside of galaxies and are inside these tendrils
Speaker 1: between galaxies.
Speaker 2: It's incredible, wow, And how do they get there? Do
Speaker 2: they get like shot out by suns? They were just
Speaker 2: there from the beginning of the creation of the universe
Speaker 2: and they just stayed.
Speaker 1: There both and all of those like there's a feedback
Speaker 1: loop where galaxies are emitting particles right, supernovas and shockwaves
Speaker 1: and all sorts of stuff. But mostly this structure represents
Speaker 1: the dark matter structure of the universe. Galaxies form in
Speaker 1: the deepest wells and then there are tendrils of dark
Speaker 1: matter between these wells, and that's where you find these
Speaker 1: tendrils of plasma. So the plasma is like a tracer
Speaker 1: that tells you where the dark matter is. And also
Speaker 1: between these galaxies, of course, is a significant amount of
Speaker 1: dark energy. Dark energy is everywhere in space. It's just
Speaker 1: a really, really tiny amount, but because it doesn't get diluted,
Speaker 1: as the universe expands over vast spaces, it really starts
Speaker 1: to add up. Between the galaxies, you have to start
Speaker 1: accounting for the dark energy. And then as you zoom out,
Speaker 1: galaxies form clusters, and between those clusters most of the
Speaker 1: energy density between those clusters. It's not matter at all.
Speaker 1: It's dark energy, all right.
Speaker 2: So at the big scale, we're mostly thinking about dark energy,
Speaker 2: and as we zoom in, we're starting to think about gas.
Speaker 2: Let's take a break, and when we come back, I
Speaker 2: want to go back to the interstellar medium and go
Speaker 2: into more detail about what's there. Yeah, and we're back,
Speaker 2: and Daniel is going to be telling us more about
Speaker 2: what is in the interstellar medium.
Speaker 1: That's right. Today we're talking about the context of our
Speaker 1: solar system where it is and how much stuff there
Speaker 1: is around us, and whether our solar system is jumping
Speaker 1: into a huge bubble in the galaxy. Sounds fun, and
Speaker 1: so the important thing is to understand this bit between
Speaker 1: the stars. And the thing I love doing is going
Speaker 1: back in history and understanding when we understood something and
Speaker 1: what our early silly ideas were, you know, because it's
Speaker 1: easy to just like, download all of human knowledge into
Speaker 1: your brain right now. But remember that people like struggled
Speaker 1: and puzzled to figure this out, and there were many
Speaker 1: wrong paths and silly ideas entertained for hundreds of years
Speaker 1: along the way.
Speaker 2: It's kind of cute to imagine that nobody in like
Speaker 2: fifty years is going to look back at us and
Speaker 2: be like, oh, remember that cute thing that the physicists
Speaker 2: thought in twenty twenty six about anyway, they could be
Speaker 2: laughing at us eventually, but let's go ahead and laugh
Speaker 2: at them.
Speaker 1: So first, remember, in the broader context, we didn't know
Speaker 1: that our galaxy was one of many galaxies until like
Speaker 1: the early nineteen hundreds, right, we thought we had a galaxy.
Speaker 1: It was a bunch of stars and that was it.
Speaker 1: It was just our galaxy, and that was everything. The
Speaker 1: galaxy was basically the universe and everything that was out
Speaker 1: there that wasn't actually in our galaxy, but we didn't
Speaker 1: know it yet. We called like a nebula because it
Speaker 1: was like a smeary blob in the sky we couldn't resolve.
Speaker 1: So that's sort of like the bigger picture. But then
Speaker 1: what about inside the galaxy, Well, people try to make
Speaker 1: like maps of the galaxy early on to try to
Speaker 1: understand the structure. But we're interested in like the bit
Speaker 1: between the stars. And it was like the late seventeen
Speaker 1: hundreds people started to wonder, like, hmm, is the space
Speaker 1: between the stars really a vacuum or is there something there?
Speaker 1: You know? And how would we know? And this coincides
Speaker 1: a little bit later with Maxwell's development of electromagnetism, the
Speaker 1: idea that light is a balance between electricity and magnetism.
Speaker 1: The energy slashes back and forth between the two fields,
Speaker 1: but then it moves as a wave in these fields
Speaker 1: at the speed of light. And this is Maxwell's great
Speaker 1: discovery that light is a wave, which of course begged
Speaker 1: the question of a wave in what right, And so
Speaker 1: we saw the rise of the theory of the luminiferous ether. Right.
Speaker 1: This is a famous theory which was invented to explain
Speaker 1: what light is wiggling? Right, Maybe there's something out there,
Speaker 1: like if waves are wiggles and water, and sounds wiggles
Speaker 1: and air, maybe light is a wiggle in this luminiferous
Speaker 1: ether that we hadn't ever discovered before it fills the universe.
Speaker 1: So this would be an early theory of the interstellar medium.
Speaker 1: What is out there between the stars, what allows the
Speaker 1: light to go from Alpha Centauri to us here?
Speaker 2: And so the luminiferous ether. I thought I was going
Speaker 2: to get it on the first shot. Totally did I
Speaker 2: stumbled a little. Would you find that between like Earth
Speaker 2: and Mars or that was just between the end of
Speaker 2: our solar system and the start of the next solar system?
Speaker 1: Great question. No, is should be everywhere. If it exists,
Speaker 1: it should be everywhere. And that was key because people
Speaker 1: looked for it and they said, well, let's try to
Speaker 1: measure our velocity through this ether. If it's out there
Speaker 1: and it's a medium for light, light is wiggling through
Speaker 1: this ether, then Earth moving around the Sun should be
Speaker 1: moving zigzagging basically back and forth through this ether. And
Speaker 1: if light has a constant velocity relative to the ether,
Speaker 1: then we should be able to measure light moving in
Speaker 1: different velocities as we move in different velocities through the ether. Okay, right,
Speaker 1: the same way that like you can catch up to
Speaker 1: sound waves and air because they have a constant velocity
Speaker 1: relative to the air, so you can catch up to
Speaker 1: them and even pass them. Right, just like when you're
Speaker 1: on a boat in a lake, you're making ripples. If
Speaker 1: you go fast enough, you can catch up to them,
Speaker 1: which it gives you awake, right, And so if light
Speaker 1: is moving through this ether, and then as the Earth
Speaker 1: moves around the Sun, we should see a change in
Speaker 1: the velocity of light famous Michaelson. More the experiment of
Speaker 1: course proved that light is the same speed year round,
Speaker 1: regardless of the direction, and so apparently there is no
Speaker 1: luminiferous ether. So then people are like, oh, wow, maybe
Speaker 1: there really is a vacuum in between the stars.
Speaker 2: And you all didn't find another use for the word
Speaker 2: luminiferous ether. That's a total bummer because it just sounds
Speaker 2: so nice.
Speaker 1: Well, these days is actually a proliferation of ether theories,
Speaker 1: which is very confusing because ether most generally just refers
Speaker 1: to like a theory of the substrate of space, and
Speaker 1: our current theory that quantum field theory, where space is
Speaker 1: filled with these fields and these fields exist in space.
Speaker 1: Then that's sort of the modern ether, and that's totally
Speaker 1: cool and nobody objects to that. But that's an ether
Speaker 1: where you can't measure your velocity relative to it, like
Speaker 1: the luminiferous ether has a frame it's at rest in
Speaker 1: some frame, and light moves at a constant speed only
Speaker 1: in that frame. The modern ether quantum field theory vacuum
Speaker 1: has no frame. And so there's a big distinction between
Speaker 1: the luminiferous ether of two centuries ago and today's concepts
Speaker 1: of the ether. And in lots of pop side descriptions
Speaker 1: you see this important distinction sort of ignored or fuzzed over.
Speaker 2: Okay, all right, and I could use a quick step
Speaker 2: back and a reminder about where we are. So we've
Speaker 2: been talking about what is in space between things. Yeah,
Speaker 2: and we've just established that we used to think there
Speaker 2: was ether between stuff, but there's not. But there's not,
Speaker 2: but there's not. And so now, okay, so we're going
Speaker 2: through the history of our understanding of what was in
Speaker 2: the interstellar medium.
Speaker 1: That's right. And then in eighteen ninety nine people discovered, oh,
Speaker 1: there is some actual stuff out there between the stars,
Speaker 1: not ether, but like actual matter. There were these things
Speaker 1: called dark nebulae, essentially these dusty clouds you could see
Speaker 1: out there that passed between the stars, right between us
Speaker 1: and other stars. There were like these clouds of stuff
Speaker 1: which were sometimes silhouetted against this background starfield. This is
Speaker 1: essentially the first discovery of interstellar gas. It's just blobs
Speaker 1: of gas that are out there between the stars. And
Speaker 1: so as you make more and more accurate observations of
Speaker 1: these stars and understand like why they dim or what
Speaker 1: you can see and what's between you and those stars,
Speaker 1: you start to build a map of the galaxy and
Speaker 1: understand like where there's dust and where there's gas. And
Speaker 1: this is our first like understanding really of the context
Speaker 1: of our Solar.
Speaker 2: System, understanding that there's gas out there. That gas isn't
Speaker 2: like the ort cloud. That's just under standing where other
Speaker 2: galaxies are out there and you're just kind of seeing
Speaker 2: them as gas or what is that gas exactly?
Speaker 1: Yeah, this is gas within our galaxy, okay, right, not
Speaker 1: between galaxies. This is just understanding like what's between the stars.
Speaker 1: Are the stars dots in a true vacuum or is
Speaker 1: there like a soup out there? And it turns out
Speaker 1: it's pretty soupy. We discovered these dark clouds of gas
Speaker 1: between the stars, and then in the early nineteen hundreds
Speaker 1: we discovered cosmic rays, which are just particles from space
Speaker 1: that are zipping towards the Earth. And if there are
Speaker 1: particles zpping towards the Earth. There's zipping from somewhere right,
Speaker 1: which means that like space out there is filled with
Speaker 1: all of these particles. So we had this growing awareness
Speaker 1: that the space between the stars is filled with gas
Speaker 1: and with ions and with electrons, just like in the
Speaker 1: Solar System. And so instead of thinking of it like
Speaker 1: here's a pocket of stuff the star and the planets,
Speaker 1: you know how, now we think about like a spectrum
Speaker 1: of stuff inside the Solar System, little bits of dust
Speaker 1: all the way up to Jupiter and then the Sun.
Speaker 1: In the same way, you should think about a smooth
Speaker 1: transition between the Solar System and the interstellar medium that
Speaker 1: between the stars there is still stuff. Yes, there is
Speaker 1: a boundary where the solar wind stops dominating and the
Speaker 1: interstellar wind starts dominating, But in terms of the distribution
Speaker 1: of stuff, it's pretty smooth, and so you should expect
Speaker 1: there to be stuff out there in the interstellar medium,
Speaker 1: and it actually makes up like between ten and fifteen
Speaker 1: percent of the mass of the galaxy is not inside
Speaker 1: the stars. Yeah, it's between the stars, and it's going
Speaker 1: to play an important role in how those stars are
Speaker 1: formed and the health of our galaxy in forming stars,
Speaker 1: whether our galaxy keeps making stars or whether it gets
Speaker 1: quenched as they say, don't get quenched. So let's dig
Speaker 1: a little bit more deeply into what's in the interstellar medium.
Speaker 1: We said before the break that it's mostly gas, which
Speaker 1: means that it's mostly hydrogen. It's like ninety percent hydrogen
Speaker 1: ten percent helium, and the universe is mostly hydrogen, so
Speaker 1: that makes sense. Something that's interesting, though, is that even
Speaker 1: though there's a lot of space out there, it's super dense,
Speaker 1: this stuff still behaves like a gas, like it interacts
Speaker 1: with itself. It's not like non interacting, like the way
Speaker 1: the exosphere of the Moon is just a bunch of
Speaker 1: particles that ignore each other. Out there, these particles are
Speaker 1: moving so fast the mean free path is pretty short,
Speaker 1: Like they don't go very far before they run into
Speaker 1: another one or interact with another one. They don't have
Speaker 1: to actually touch because they have charges. They can interact
Speaker 1: without touching each other. So that's really fast and that
Speaker 1: was a little bit of a surprise to me when
Speaker 1: I learned about that.
Speaker 2: So if somebody were living out there, could they be
Speaker 2: getting power from that, They.
Speaker 1: Could be getting killed from that. I mean, this is radiation, right,
Speaker 1: These particles are moving at high speeds even though it's
Speaker 1: fairly low density. These things are moving pretty fast, like
Speaker 1: inside the solar system, the solar wind is moving at
Speaker 1: four hundred kilometers per second. Sometimes these particles of energy
Speaker 1: up to ten keV, And in the interstellar medium, these
Speaker 1: things are even higher speeds. So technically the temperature can
Speaker 1: sometimes be like up in the millions of even though
Speaker 1: the density is very very low, So you would like
Speaker 1: freeze because there's not a lot of heat being deposited,
Speaker 1: but also you'd be riddled with all of these tiny,
Speaker 1: little high speed bullets.
Speaker 2: That's definitely worse than New Jersey.
Speaker 1: It's a close call, but I agree with you.
Speaker 2: Ah, I do like New Jersey though I was born there.
Speaker 1: The others fascinating bit about the interstellar medium is what's
Speaker 1: not gas, which is the dust. And dust sounds like,
Speaker 1: you know, stuff you'd sweep up and throw away, but
Speaker 1: dust is actually super fascinating from a cosmic sense because
Speaker 1: dust comes from dead stars. Right, Like, how do you
Speaker 1: make dust? You need heavier elements. You need those elements
Speaker 1: to stick together, You need them to form these little
Speaker 1: crystals and to gather more bits to themselves, and dust
Speaker 1: is like the seed for new solar systems and new planets. Right,
Speaker 1: our Earth formed by a bunch of dust which bounced
Speaker 1: into itself and stuck together and formed a bigger piece
Speaker 1: of dust and gathered more together. So these are really
Speaker 1: the seeds of future solar systems as well as the
Speaker 1: ashes of previous ones.
Speaker 2: Oh man, at first I thought you were talking about
Speaker 2: gallstone formation, and then it sounded much more beautiful, and
Speaker 2: I'm glad my gallstones, you know, stopped getting bigger before
Speaker 2: they formed planets and universes.
Speaker 1: But these huge amounts of like iron in silicon, magnesium,
Speaker 1: a bunch of oxygen and carbon. This even like nano
Speaker 1: diamonds and fullerines out there. They're really fascinating. And some
Speaker 1: of these things are presolar grains, which are preserved from
Speaker 1: the original star. So things formed like in the atmosphere
Speaker 1: of the star, and then there's a supernova that blasts
Speaker 1: out and spreads all these dust grains, and some of
Speaker 1: them survive, right, They're not shattered. A lot of this
Speaker 1: stuff is smashed into itself and shattered and reprocessed, but
Speaker 1: some of these presolar grains survive and we actually find them,
Speaker 1: like on the ocean floor, we can find little grains
Speaker 1: of dust that survived a supernova and zoomed across the
Speaker 1: universe and gathered together to help form the Earth.
Speaker 2: How could you know? How could you find a grain
Speaker 2: of sand and be like, I know where you came from.
Speaker 1: Yeah, they have really unusual isotope mixtures which you only
Speaker 1: find like in those conditions that are formed, like in
Speaker 1: the atmospheres of supernovas. It's really incredible.
Speaker 2: Wow, that is incredible.
Speaker 1: And so the interstellar medium, it's mostly gas. There's a
Speaker 1: little bit of dust out there, it's like one percent,
Speaker 1: but I think that's the most interesting part. And then
Speaker 1: of course some of it is very very high energy
Speaker 1: particles zooming through space cosmic rays, and so you mix
Speaker 1: all this together and that's what interstellar medium is made
Speaker 1: out of. Mostly gas, which is actually mostly molecular not ionized,
Speaker 1: and then one percent dust and then a few very
Speaker 1: high energy ions.
Speaker 2: All Right, we are getting closer to talking about the
Speaker 2: amazing and exciting superbubble. So we're going to take a break,
Speaker 2: and when we get back, we're going to talk about
Speaker 2: how the gas and the dust are structured inside of
Speaker 2: the interstellar medium. We are back and we're talking about
Speaker 2: the interstellar medium, the space between the stars, and we've
Speaker 2: established that it's mostly gas some dust in there. How
Speaker 2: is all of that stuff structured in this space?
Speaker 1: Daniel, Yeah, So you might imagine, oh, it's just smooth,
Speaker 1: it's just spread out out there. But it's like a
Speaker 1: vast fluid, which means that it has all sorts of stuff
Speaker 1: going on. It's like our atmosphere, right, which we model
Speaker 1: and has currents and winds and high density regions and
Speaker 1: low density regions, and so you can think about the
Speaker 1: interstellar medium as a vast fluid because it is interacting. Right,
Speaker 1: The meanfree path of these particles is pretty short, but
Speaker 1: it's also more complex than a simple fluid because often
Speaker 1: these particles are moving really really fast. Our whole regions
Speaker 1: of particles are moving super duper fast. So you can
Speaker 1: have supersonic regions of this interstellar medium right places where
Speaker 1: the gas is moved being faster than the speed of
Speaker 1: sound in that medium. Remember, you can define the speed
Speaker 1: of sound in anything, in water and air and steel,
Speaker 1: and it depends like on the density of the thing.
Speaker 1: Things that are denser, the bonds are tighter, and if
Speaker 1: you push on one, then the pressure wave propagates more quickly,
Speaker 1: and then it would if something was really low density,
Speaker 1: because then the particles have to drift longer before they
Speaker 1: bump into each other. Okay, And so what happens in
Speaker 1: the interstellar medium is you have these shockwaves, these supersonic
Speaker 1: shockwaves which smash into something else, and so you get
Speaker 1: this supersonic shockwave, but that compresses the gas, makes it
Speaker 1: more dense, and that actually raises the speed of sound.
Speaker 1: So then the local flow becomes subsonic. So you get
Speaker 1: supersonic shockwaves, which then actually becomes subsonic, and so it
Speaker 1: creates this really complex structure and these really interesting patterns
Speaker 1: of density. It's really hard to model because it's so chaotic.
Speaker 2: So in order to hear sound, you need like molecules
Speaker 2: to transmit it. So like if you were out there,
Speaker 2: would you be hearing these particles or or not?
Speaker 1: Really you can technically scream and be heard in the
Speaker 1: interstellar medium, it would be very faint and very very slow. Right, Okay,
Speaker 1: you could hear these things, I can tell you what
Speaker 1: they would sound like. You know, probably like shh cool.
Speaker 2: Okay, interesting, I don't know why, But in my mind,
Speaker 2: I'd imagine the interstellar space is being like a quiet, calm,
Speaker 2: still place, which is probably silly because you know, like
Speaker 2: solar winds keep things moving. There's probably nowhere that in
Speaker 2: space that's actually super still, But I hadn't imagined it
Speaker 2: being super turbulent like this.
Speaker 1: So yeah, it's really turbulent. And you know, on time
Speaker 1: scales that we don't usually think about, but if you
Speaker 1: played a movie, you know, where a million years passes
Speaker 1: in a second, it would look like bubbling, frothing tea
Speaker 1: or something. You know, it's a lot of stuff going on.
Speaker 1: And the most interesting stuff that goes on is that
Speaker 1: when you get dense regions, you get star formation. Right,
Speaker 1: if you have a clump of gas and it cools
Speaker 1: down and then one of those grains of dust can
Speaker 1: start to gather other stuff near it. It's important that's cool, remember,
Speaker 1: because if things are really really hot, and that means
Speaker 1: they're moving fast and then gravity has no chance to
Speaker 1: pull them together. But if things are cool, that means
Speaker 1: they're slow moving and gravity can gather dust grains together
Speaker 1: little bits at a time. They stick together, they start
Speaker 1: forming bigger and bigger stuff and that's when you get
Speaker 1: gravitational collapse leads to a runaway effect. You get more gravity,
Speaker 1: and then you get a star forming and there stuff
Speaker 1: around it starts to swirl around it to a protoplanetary disc.
Speaker 1: And that's how a star is born. It's born from
Speaker 1: denser regions in the interstellar medium, and those regions come
Speaker 1: from the turbulence. So the structure in the interstellar medium
Speaker 1: is the reason we have stars.
Speaker 2: The structure in the interstellar medium. But don't you only
Speaker 2: have interstellar medium because it's the region between pre existing stars.
Speaker 1: Yeah, there's sort of a circular definition there, but yeah,
Speaker 1: the interstellar medium can make new stars and then it's
Speaker 1: no longer the interstellar medium.
Speaker 2: Okay, So you can imagine this process going on infinitely
Speaker 2: many times, and then you just have an infinite number
Speaker 2: of suns. So how often does this process happen?
Speaker 1: So it depends a lot on the details of the
Speaker 1: temperature of the cloud and the composition. But when it happens,
Speaker 1: it happens a lot. Like you get a lot of
Speaker 1: stars all born in one of the neighborhood together. And
Speaker 1: this is why, for example, stars are very often found
Speaker 1: in binary systems like pairs of stars gravitationally bound to
Speaker 1: each other, or even briefly trinary systems or more long
Speaker 1: lived systems where you have like a binary star and
Speaker 1: another one that's more distant, so that it's like effectively
Speaker 1: a binary system. So you have these regions of star
Speaker 1: formation where a huge cloud meets these requirements where you
Speaker 1: have like little dense grains and the whole thing is
Speaker 1: cold enough, and how often it happens depends a lot
Speaker 1: on the history of your galaxy. There's actually an interplay
Speaker 1: between the stars and the interstelling medium, which we're going
Speaker 1: to talk about, because the stars push back and form
Speaker 1: these bubbles, and so it's a whole complex of all thing.
Speaker 1: And the rate at which stars form changes a lot
Speaker 1: where you are in the galaxy and over time, and
Speaker 1: it's not something we fully understand. Like some galaxies out
Speaker 1: there have stopped forming stars at all, and some of
Speaker 1: them are still making stars and this is called quenching,
Speaker 1: and it's not something that's fully understood.
Speaker 2: All right, well, we still haven't gotten to the bubbles.
Speaker 2: How do we get closer to bubbles?
Speaker 1: Yeah, so you have the interstellar medium. You got these
Speaker 1: clouds of gas. Sometimes they collapse and form a star.
Speaker 1: What happens when you make a star is that it
Speaker 1: pushes back against the interstellar medium, right, Like our Sun
Speaker 1: has a solar wind, and that's pushing back. The reason
Speaker 1: we call it a solar system is it's the region
Speaker 1: where our solar wind dominates. So now you have this
Speaker 1: cavity inside the interstellar medium, right, And so the Sun
Speaker 1: basically makes like a little bubble, which we call the
Speaker 1: heliosphere by pushing back against the interstellar medium. So now
Speaker 1: imagine you have a bunch of stars, each with their
Speaker 1: little bubbles, and one of them goes supernova, right, So
Speaker 1: this blows out a huge region inside the ism, and
Speaker 1: this can create a superbubble that's like hundreds of light
Speaker 1: years across. Because remember a supernova is the collapse of
Speaker 1: a star of gravitationally, which then bounces back out and
Speaker 1: dumps an enormous amount of energy. A single supernova can
Speaker 1: be brighter than the entire galaxy and it doesn't just
Speaker 1: put out photons. That also shoots out a huge number
Speaker 1: of particles.
Speaker 2: How many light years across is our solar system?
Speaker 1: So our solar system is like one or two light
Speaker 1: years in radius, okay, right, So it's pretty small. But
Speaker 1: then a supernova can make a bubble that's like hundreds
Speaker 1: of light years across. And if there's multiple supernova, because
Speaker 1: remember stars are formed in bunches, and that means they
Speaker 1: can also go supernova in bunches they have like their
Speaker 1: cycles all synchronized. Oh boy, then you can get a
Speaker 1: really big bubble that lasts for like tens of millions
Speaker 1: of years.
Speaker 2: Okay, so the bubbles that we're talking about then are
Speaker 2: solar winds of some sort blowing interstellar medium out, and
Speaker 2: that is the bubble. These are not the fun bubbles
Speaker 2: that I was imagining, Daniel.
Speaker 1: So you get little bubbles from solar winds, but you
Speaker 1: really get the super bubbles from a bunch of supernova's
Speaker 1: all going off at once and making these really vast bubbles.
Speaker 1: It's super cool. Also, if one happens near the edge
Speaker 1: of the galaxy, Like the galaxy is a big disc, right,
Speaker 1: it's like much broader than it is thick. So if
Speaker 1: it occurs like near the top of the disk, then
Speaker 1: essentially it creates like a hole in the galaxy. And
Speaker 1: they call it a galactic chimney because in hot gas
Speaker 1: from inside the galaxy basically vents out from the galaxy,
Speaker 1: so you have like a little spurt.
Speaker 2: So the question that we're answering today is do we
Speaker 2: live in a super bubble? But timing's got to matter too,
Speaker 2: right now, if you get a bunch of stars going
Speaker 2: supernova all at once to make your super bubble, you
Speaker 2: would be dead if you were inside of the super
Speaker 2: bubble region, right, So it's got to go super bubble,
Speaker 2: make a bubble exact, and then a planet needs to
Speaker 2: like wander in or get captured and then become habitable,
Speaker 2: and then you find yourself inside the super bubble. That
Speaker 2: would have to work, right, okay.
Speaker 1: Right, And remember, our Solar system has existed for like
Speaker 1: almost five billion years, but these super bubbles last for
Speaker 1: tens of millions of years. They're very transient, and our
Speaker 1: star is moving through the galaxy, right, and its orbit
Speaker 1: takes hundreds of millions of years, and so they're created
Speaker 1: by supernova. But you don't want to be there when
Speaker 1: they're made, you're right, And so coincidentally, there is a bubble.
Speaker 1: It's called the local bubble. It's three hundred to one
Speaker 1: thousand light year wide. It has a much lower gas
Speaker 1: density than the rest of the interstellar medium by like
Speaker 1: down by ten, and it's surrounded by denser walls of
Speaker 1: neutral gas and dust. Probably it was formed by like
Speaker 1: maybe ten to twenty supernovas that happened over the last
Speaker 1: ten to fifteen million years, so sort of recently on
Speaker 1: cosmic history scales. We can't pinpoint exactly which stars they are.
Speaker 1: Probably their stars related to the Scorpius Centaurus association of stars.
Speaker 1: And the shape of it is kind of weird. It's
Speaker 1: not spherical, and there's other like nearby bubbles that it
Speaker 1: connects to called loop one, and so fortunately it formed
Speaker 1: before we got here. Right, the Sun orbits the center
Speaker 1: of the galaxy at like two hundred and twenty kilometers
Speaker 1: per second and moves it like twenty five kilometers per
Speaker 1: second relative to the interstellar medium, and so that means
Speaker 1: that we go like twenty to thirty parsecs every million
Speaker 1: years or so. This bubble is a few hundred parsecs across,
Speaker 1: so seems like about fifteen million years ago the first
Speaker 1: supernova started inflating this bubble. Five or ten million years
Speaker 1: ago we entered the cavity, and now we're in this superbubble.
Speaker 1: We're drifting through a little warm cloud of gas inside
Speaker 1: this bubble, and in a couple million years we're probably
Speaker 1: going to exit it back into the interstellar medium, which
Speaker 1: is a little bit denser.
Speaker 2: So when we enter the interstellar medium where there's all
Speaker 2: of that turbulence and it's crazy, is that going to
Speaker 2: be a bad time for us?
Speaker 1: No, probably not, because we have the solar wind which
Speaker 1: protects us. Were moving within our own little bubble anyway,
Speaker 1: So it doesn't really matter that much. It's just a
Speaker 1: way to highlight that there's a lot of stuff happening
Speaker 1: out there, right there's turbulence and chaos and all that stuff,
Speaker 1: and star formation and start dying and all that happens
Speaker 1: because of this structure. Like you get stars because you
Speaker 1: have denser regions, and then those stars blow out make
Speaker 1: superbubbles when they die to make less dense regions. But
Speaker 1: then those superbubbles collapse because the interior is lower density
Speaker 1: but higher temperature, and that hot gas leaks out and
Speaker 1: ruptures the shell, and so you get this cycle of stuff.
Speaker 1: And so you know, you shouldn't think of the galaxy
Speaker 1: out there between the stars is empty. In fact, it's
Speaker 1: filled with stuff, and it's really dynamic, and the structure
Speaker 1: out there is what determines like where stars are made
Speaker 1: and then stars explode and replenish the interstellar medium, and
Speaker 1: so the interplay between the stars and the interstellar medium
Speaker 1: really determined, like, hey, is your galaxy making stars or not?
Speaker 1: Is it to pleasing all of its gas? All this
Speaker 1: kind of stuff is determined by this interaction between the
Speaker 1: stars and the interstellar medium.
Speaker 2: Huh. So we live inside of a bubble made by
Speaker 2: our son, which is currently inside of the local super bubble.
Speaker 1: That's right, that was made by supernovas of other stars,
Speaker 1: fortunately before we entered this region.
Speaker 2: Amazing exactly, And.
Speaker 1: So that's the broader context of your life. We happen
Speaker 1: to be living inside this bubble, and now you.
Speaker 2: Know all about it, that's right. And we hope that
Speaker 2: you don't stub your toes on any galactic dust or
Speaker 2: happen to get dust of any sort inside of your
Speaker 2: gallbladder or kidneys or anything else. We wish you health.
Speaker 2: And this is not a great transition. But man, it
Speaker 2: hurt to laugh at Daniel's jokes today.
Speaker 1: I try to hold back. I really really did.
Speaker 2: I appreciate it. Daniel. You can't help yourself. You're just
Speaker 2: a funny guy.
Speaker 1: Looks, Aren't everything right ready?
Speaker 2: Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We
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