Do we live in a super bubble?

25:32It 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? And so the luminiferous ether, I thought I was going to get it on the first shot. You totally did. 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? Great question. No, it should be everywhere. If it exists, it should be everywhere. And that was key because people looked for it and said, well, let's try to measure our velocity through this ether.

26:10If 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. Okay. 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 can catch up to them and even pass them, right? Just like when you're on a boat in a lake, you're making ripples.

26:42If you go fast enough, you can catch up to them, which gives you a wake. And so if light is moving through this ether, 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. And you all didn't find another use for the word luminiferous ether? That's a total bummer, because it just sounds so nice. Well, these days, there's 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.

27:25And our current theory, the 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, the quantum field theory vacuum, has no frame. And so there's a big distinction between the 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.

28:08Okay, 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. Yeah. And we've just established that we used to think there was ether between stuff, but there's not. But there's not. 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. That's right. Okay. 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.

28:46Between us and other stars, there were like these clouds of stuff, which were sometimes silhouetted against this 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, 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 where there's gas. And this is our first understanding really of the context of our solar system. Understanding that there's gas out there, that gas isn't like the Oort cloud.

29:24That'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? 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?

30:00Which means that 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, 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.

30:32Yes, there is a boundary where the solar wind stops dominating and the interstellar wind starts dominating. 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 gets quenched, as they say. Don't get quenched. So let's dig a little bit more deeply into what's in the interstellar medium.

31:11We 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.

31:46They don't have to actually touch because they have charges. They can interact without touching each other. So that's really fascinating. That was a little bit of a surprise to me when I learned about that. So if somebody were living out there, could they be getting power from that? 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.

32:18And 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. That's definitely worse than New Jersey. It's a close call, but I agree with you. Oh, I do like New Jersey, though. I was born there. 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.

32:51But 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. I mean, at first I thought you were talking about gallstone formation, and then it sounded much more beautiful.

33:28And I'm glad my gallstones stopped getting bigger before they formed planets and universes. But there's huge amounts of iron and silicon, magnesium, a bunch of oxygen and carbon. There's even nanodiamonds 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 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.

34:06But 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. How could you know? How could you find a grain of sand and be like, I know where you came from? Yeah, they have really unusual isotope mixtures, which you only find in those conditions that are formed in the atmospheres of supernovas. It's really incredible. Wow, that is incredible. And so the interstellar medium, it's mostly gas. There's a little bit of dust out there.

34:41It'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 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. 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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37:37It's a sci-fi fantasy epic about two protagonists on opposite sides of a war and a prophecy neither of them wanted. My first book was Divergent. And when that came out, like, because it was so popular, I think it attracted, like, mostly positivity, but the negativity I sucked in like a sponge. And I think it was, like, critiques of things I liked when I was, like, you know, I was 23 and I wrote this book and it had all my, like, dorky little cheesy or maybe unrealistic loves in it. And I started to feel a lot of shame about those things. And so for the rest of my career, I steered away from those little things that like make you feel pleasure when you read.

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39:16We're 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? 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?

39:51The 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 sound in that medium. Remember, you can define the speed of sound in anything, in water, in air, in steel, and it depends 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.

40:31And 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 shockwave, 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 shockwaves, which then actually become 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. So in order to hear sound, you need like molecules to transmit it.

41:08So like if you were out there, would you be hearing these particles or not really? You can technically scream and be heard in the interstellar medium. It would be very faint and very, very slow, right? Okay. You could hear these things. I can tell you what they would sound like. You know, probably like whoosh. Cool. Okay. Nothing very interesting. I don't know why, but in my mind, I'd imagine 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.

41:41But I hadn't imagined it being super turbulent like this. 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.

42:21But if things are cool, that means they're slow moving and gravity can gather dust grains together, little bits at a time. 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's 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.

42:52The structure in the interstellar medium. But don't you only have interstellar medium because it's the region between pre-existing stars? Yeah. 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. Okay. And so you could imagine this process going on infinitely many times, and then you just have an infinite number of suns. So how often does this process happen? 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 little neighborhood together.

43:29And 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 a binary star and another one that's more distant so that it's effectively a binary system. So you have these regions of star formation where a huge cloud meets these requirements where you have 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 we're going to talk about because the stars push back and form these bubbles.

44:06And 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. This is called quenching. And it's not something that's fully understood. All right. Well, we still haven't gotten to the bubbles. How do we get closer to bubbles? 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?

44:40Like 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. Because remember, a supernova is the collapse of a star gravitationally, which then bounces back out and dumps an enormous amount of energy.

45:21A 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. How many light years across is our solar system? So our solar system is like one or two light years in radius. Okay. Right. So it's pretty small, but then a supernova can make a bubble that's like hundreds of light years across. Wow. And if there's multiple supernova, because remember stars are formed in bunches, and that means they can also go supernova in bunches. that have like their cycles all synchronized. Oh boy. Then you can get a really big bubble that lasts for like tens of millions of years.

45:57Okay, so 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. 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.

46:35They 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. 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 exactly 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 right and remember our solar system has existed for like almost five billion years but these super bubbles last for tens of millions of years they're very transient.

47:18And 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 by 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 so sort of recently on cosmic history scales we can't pinpoint exactly which stars they are probably their stars related to the scorpius centaurus association of stars and the shape of it is kind of weird it's it's not spherical and there's other like nearby bubbles that it connects to called loop one 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.

48:27And so that means that we go like 20 to 30 parsecs every million years or so. This bubble is a few hundred parsecs across. 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. 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?

49:08No, probably not, because we have the solar wind, which protects us. We're moving within our own little bubble anyway, 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.

49:42And that hot gas leaks out and ruptures the shell. And so you get this cycle of stuff. And so 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. And the structure out there is what determines 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 this kind of stuff is determined by this interaction between the stars and the interstellar medium.

50:19So we live inside of a bubble made by our sun, which is currently inside of the local super bubble. That's right. That was made by supernovas of other stars, fortunately, before we entered this region. Amazing. 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. 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.

50:58I try to hold back. I really, really did. I appreciate it, Daniel. You can't help yourself. You're just a funny guy. Looks aren't everything. Alright, thanks everybody.

51:17Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you. We really would. We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us at questions at danielandkelly.org. 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 &K Universe. Don't be shy. Write to us.

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53:09They recommended a Frigidaire Gallery French door with crispers that block dry air and excess moisture to extend the life of fresh food. And wow, it really works. No more wasted food in my house. Expert appliance recommendations for the win. Shop GrandAppliance.com. Hey everyone, it's Cal Penn. I'm inviting you to join the best sounding book club you've ever heard with my podcast, Earsay, the Audible and iHeart Audiobook Club. Every episode, I nerd out with amazing guests and dive into the best new audiobooks available on Audible. It's the book club for your ears. Listen to Earsay, the Audible and iHeart Audiobook Club.

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