Where do we live within the galaxy? What shapes the local bubble? How long will we be inside it? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

One of my favorite aspects of astronomy is how close it is to a seemingly unrelated field of science, archaeology. And I don't mean Indiana Jones archaeology where the hero runs around stealing precious cultural artifacts, but the real modern archaeology that is slow, careful, deliberate, and is able to tell some of the most dramatic stories in human history. Take, for example, the city of Troy. Long fabled Greek legend, etcetera, etcetera. And it turns out it's a real city.

It was discovered by Heinrich Schliemann in 18/70, which is a long complicated story, but that's for a different show entirely. And there, literally sitting in the dirt, is thousands of years of history of a magnificent city that was built, destroyed by earthquake, rebuilt, destroyed by fire, rebuilt, destroyed by by war, rebuilt, suffered famine, enjoyed centuries of prosperity. The cycles continue and you can read these stories. We see the evidence for trade, for wealth, for power struggles, for armies, for palaces, for families, for humanity. And how archaeologists are able to tell these stories is worthy of an Ask A Dirt Man podcast in his own right.

It's from the tiniest of clues. You know, evidence of a fire here, a fracture that can only come from an earthquake over there, a pile of discarded pottery, an older style of stone reused in a newer building, a seal with an inscription used by a scribe. It's from these tiny clues that archaeologists are able to reconstruct the past giving us a window. A small, hazy, imperfect window, but a window nonetheless into the drama of humanity stretching back 1000 of years. And astronomy is like that.

Yeah. Our tools are different. Shovels and brushes aren't so handy in deep space. Our clues are different. Fewer pottery shards, more hydrogen atoms.

Our space and time scales are different. We're talking 1,000,000 of years, not 1,000 of years. But the basic philosophy, basic approach, the basic mindset is the same. When we look out at the wider universe, we are surrounded by clues, and it's up to us to put together the clues and tell a story that fits all the evidence. And today, I'm talking about the local bubble.

The region of the Milky Way galaxy that surrounds the immediate vicinity of the sun. Now, before I tell the story of the local bubble and define exactly what it is, I want to talk about some of the clues that we've put together. When we come up on the excavation site of the local neighborhood of the Milky Way, what do we see? Well, let's start with January 12, 2003 when NASA launched the CHIPS satellite. That's short for Cosmic Hot Interstellar Plasma Spectrometer, and it is probably, let's be honest, a backronym where they came up with the name first and then they filled in all the gaps later.

Anyway, its goal was to study something called the interstellar medium. This is the the stuff between the stars. It's a super hot plasma, but it's also incredibly thin. It would it's so thin it would register as a vacuum on a laboratory on the Earth. But there's a lot of volume between the stars, and so it it adds up.

You can't really see it. You can't really detect it. The closest thing you can get to to seeing something like the interstellar medium is something called zodiacal glow that, just after sunset, if you're looking in that direction and the sky looks nice and dark, you might see a triangular shaped, like, fuzz of light that's from sunlight scattering off of all the little bits of dust grains in our solar system. Most of the interstellar medium is hydrogen on and helium. There's also some heavier elements, and then there's also dust like we have in the solar system.

And it's super hot, super thin, and just kinda there. It's like it's like the fog that sits between all the stars, and all the stars swim through this fog. And CHIPS was launched to study this fog because this fog, this interstellar medium fog, is super duper hot. It's glowing in the ultraviolet. Now it's not glowing a lot because there isn't a lot of it.

Its density is super super low. But like I said, there's a lot of it once you add up all of its volume, and so you should be able to see this glow from the interstellar medium. And chips launched, it looked around, and it didn't find anything. A huge null result. A negative result.

It couldn't find any ultraviolet glow from the interstellar medium. That told us that there wasn't nearly as much of the interstellar medium around us as we initially thought. We, we can look in, like, distant galaxies. We can average over large volumes. We can make estimates of how much interstellar medium stuff should be there.

But then when chips launched about 2 decades ago to look for the interstellar medium around us, near close to us, it didn't find anything. It's like digging into the dirt where all the sources are telling us there should be a city and not finding anything. Another clue, and this one is uses a technique that goes back to the 19 eighties, is to map what's called the column density of the interstellar medium. I know this is just, like, yet another jargon term, but the idea is, if you look around in this fog, presumably, it's going to be thicker in some directions than the other. And if you've ever stood in fog, you can tell.

You look around, like, oh, when I look up, the fog is relatively thin. Maybe I can see some stars hanging out above the fog, you know, flickering in and out. But if I look in that direction, it's super thick. I can't even see, like, a meter past me. But if I look behind me, I can see a little bit more distant.

What you're sampling here with as you survey the fog around you is the column density of the fog. Just it can be thicker or thin along certain lines of sight in a certain directions. And we can measure this column density, the thickness along various lines of sight, by again looking for the imprint of hot gas. Instead of, in this case, looking for ultraviolet radiation, we can look for the specific fingerprints of hot elements. So it's mostly hydrogen and helium, but there are some other things.

And so those other things are also very hot in this fog, in this interstellar medium. They're glowing on their own. They are giving off their own light. And so we can measure how much total light we get from those specific elements in that direction, in this direction, in that direction, in this direction, And we can map out the thickness of the interstellar medium, or its density, or its cloudiness in all these different directions. And over the decades, a picture has emerged that the interstellar medium is relatively thick in the direction of the galactic plane, but very thin and elongated within the disk of the galaxy.

So, already we know this fog that surrounds us is thicker in one direction than it is the other. And we're finding when we look really close to us, in our solar neighborhood, we we don't see anything at all. Another clue that helps us map the interstellar medium is our favorite, magnetic fields. Magnetic fields, we can use magnetic fields to map something called Faraday rotation. So what the deal is is there's a bunch of dust in the interstellar medium.

Well, I say there's a lot of dust, but very, very low densities but large volumes. There's a decent amount of dust once you add up all the volume. And the dust, these dust grains, these tiny tiny little microscopic things have little magnetic fields associated with them. They're spinning a little bit, and this can affect background light. So if there's light passing from a distance source, if there's light from a distance source passing through this fog, it can alter the polarization of that light.

We call it Faraday rotation because we like fancy names for this kind of stuff. And so it's like looking at distant sources of light in a fog and using that to get a sense of the the depth of the fog, how dense it is, how thick it is, how thin it is in various directions. And we get the same picture. That it's thicker in some directions, thinner in the others, and then there doesn't seem to be a lot of the interstellar medium immediately surrounding us. So it looks like there's a cavity.

There's some sort of cavity, an opening, a hole in the interstellar medium surrounding the solar system. What's happening here? Why is the interstellar medium so different around the solar system? What could possibly cause this? Well, here's another clue.

That cavity, that open space that's been carved out of the interstellar medium is surrounded by a denser than average shell. And within that shell of slightly denser than average gas, remember, we're all talking about everything that would register as a vacuum. It's just way less dense around the solar system, and then it gets higher density than average, and then it smooths out to median density, average density past that. And in this shell, there are many associations of large stars, like Gould's belt, this association of super bright stars that look like a ring of stars discovered in the mid 1800. There are a lot of molecular clouds.

There seems to be a lot of recent star formation activity in the shell surrounding this cavity. And then here's another clue, the earth itself. There are radioactive elements naturally occurring everywhere in the universe. We know plutonium just exists. It's radioactive.

Radioactive potassium is in your banana. It's just natural. There's another radioactive element, iron 60. Iron 60 has a half life of a couple 1000000 years. And, presumably, when our Earth was formed 1000000000 of years ago, it naturally had some percentage of iron 60.

It was just along for the ride. It's produced naturally. But that iron 60 should have decayed away a long time ago. There should be no iron 60 left on the Earth at all. But when we dig up ancient ocean seabeds or dig through Antarctic ice, we see a bunch of iron 60.

And as we correlate the the the levels of iron 60 with various geologic ages, we can put some time stamps on this, we see that the levels of iron 60 have changed with time. Like, there'll be nothing nothing nothing and then a big spike and then that big spike will decay away over the next few 1000000 years, and then there'll be nothing nothing nothing, and then a big spike of iron 60, and then that decays away. What is producing these radioactive elements? Right here on the earth, we are seeing, big spikes in radioactive elements. What's going on?

And the last clue the last clue is a little star called Zeta Ophiuchi. Okay. It's a big star. It's about 20 times the mass of the sun, and it's about 440 light years away. It's only 3000000 years old.

It's a young star, but it is booking. It is absolutely moving. It has a velocity of 30 kilometers per second, which is just fast. And it's known as a runaway star. It is moving so quickly.

It's, like, disconnected from everything. It's just like, I am out of here. I've had enough of this galaxy. I am getting out of here. And it is just moving.

It is moving so fast. It's creating a bow shock. It's like ramming into the gas in front of it, and it's making like a like a boat moving through a water creates a shock front and its front, like, where the water piles up. The gas in front of the star is visibly piling up. It's crazy.

What is going on? In our neighborhood of the universe, we we see a region surrounding the solar system that is far less dense than average. But that space, that cavity is a very irregular elongated shape. What little material is left inside of this cavity is insanely hot as a it has a temperature of around a 1000000 Kelvin. We have extra radioactive elements on the earth.

We have young stars that are flying away like they're fleeing the scene of a crime. We have a buildup of material on the edge of the cavity where we're seeing new stars appear. What are these clues telling us? These clues are telling us that the solar system is located in a region of extreme violence. More extreme than an earthquake.

More extreme than an invading army or a devastating fire. Here's what we have. We call this region surrounding the solar system the local bubble, which is a bit unimaginative, but it gets the job done. This bubble is carved out of the interstellar medium and is about 1,000 light years wide. Has a density about 1 tenth the average density of the Milky Way.

At the edge of the bubble is a shell of gas with slightly higher density than the surroundings. And in that shell, there are a bunch of young hot star forming regions. We know based on the sun's current position, our velocity, the size of the bubble, our location in the bubble that we were not born here. We entered one end of the local bubble somewhere between 5,010,000,000 years ago, and we will continue traveling through the bubble for tens of 1000000 of years. We are visitors at this scene of violence.

What could have the energy to create this enormous cavity, this giant bubble? A supernova? I don't know how a supernova isn't powerful enough. An AGN, active galactic nuclei. I know those are at the centers of galaxies.

A Patreon? Yeah. Patreon could do it. That's patreon.com/pmsutter. It's how you support this show and just quite possibly how you can create cavities in the interstellar medium.

Isn't that fascinating? Patreondot com/pm Sutter. I'm I'm genuinely grateful for all the contributions. No. Supernova is not gonna do it.

An active galactic nucleus, a, you know, blazar, those those are in the cores of galaxies. We're out here in the in the neighborhoods, like, walking around your peaceful suburb, and then all of a sudden you come across, you know, a crater where a house is supposed to be. What we think needs to happen to create a giant bubble like this, a cavity in the interstellar medium, is a lot of supernova going off simultaneously. Like, one bomb can't do the trick, but maybe a 100 can kind of situation. Is that even possible?

Oh, yeah. Oh, yeah. It's possible. Here's the thing. Stars aren't born alone.

When you have a giant molecular cloud, a big cloud of gas and dust, it's big. A single giant molecular cloud has enough material to form 1,000, tens of thousands of stars at once. Most of that gas does not get to participate in star formation for various complicated physics reasons. But once the process of star formation gets going, you will see an entire cluster of stars appearing out of that gas cloud because it will compress, it will fragment, and then all of a sudden, you'll get a whole population of stars. Now, when these gas clouds make populations of stars, they make all different kinds of sizes of stars.

They make small ones. They make medium ones, and they make big ones. And then because physics is physics, and it's easier to make small stuff than it is to make big stuff, you get a whole bunch of small ones, a few medium ones, and then, like, a couple giant ones. But a single star forming region will produce a large number of large stars. This is how star formation works.

And all of these stars will have very similar lifetimes because the lifetime of a star is dictated by its mass. That's that its mass dictates how powerful its gravitational pull is. That sets the rate of fusion reactions. That rate of fusion reactions determines how quickly it runs out of material to to continue fusing. That's it.

A a star's entire trajectory in life is determined by its mass that it forms with. So we have a whole bunch of stars. Some of those are big ones. Those big ones, their clocks all start ticking at the same time because they're all born from the same molecular cloud. They're all born relatively the same age.

Yeah. I mean, it's within a few 100000 years, but close enough for astronomy. You get a whole bunch of stars. They all live their lives. They all turn into red giants, and then they all go supernova within a relatively short amount of time.

We see these associations of supernova, these clusters of supernova all the time. We see their remnants. Giant stars. A single supernova is capable of blowing a cavity. We see it.

Like, look at the Crab Nebula, where you have this expanding shock wave that's been expanding for, like, 100 of years, maybe a 1000 years if I remember right. And then inside, it it has evacuated this cavity, and we can see all the lacy tendrils of gas, and it's a gorgeous nebula. Single supernova are can affect their local environment. They can blow out bubbles, like, a 100 light years across. And during their lifetimes, these bright stars can also evacuate cavities because they have so much radiation.

They're pouring out so much light. The light itself pushes on those hydrogen atoms, those helium atoms, those dust grains, literally pushes them out. It takes a lot of light to do it, but that's exactly what large stars have going for them. So we know that individual supernova can carve out pretty decent sized cavities in the interstellar medium. And we know that supernova can go off.

That clusters of supernova can go off at relatively similar times. You can have tens of supernova, 100 of supernova, maybe even in some rare cases, thousands of supernova going off at relatively the same time. And we think the local bubble is a product of it. This this cavity that we have found ourselves in, this crater in the interstellar medium. We think, 1000000 of years ago, multiple supernova went off.

100 of supernova, maybe a 1000 supernova went off at relatively the same time, and they all worked together to blow this giant bubble in the interstellar medium. But before I continue, I need to mention that this show is sponsored by BetterHelp. One of my favorite, favorite things about doing this podcast is how much I get to learn. I mean, yes. I I I went to school.

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That's betterhelp.com/spaceman. Okay. How does that fit with the rest of the evidence? That explains why we have an evacuated region in the interstellar medium, why the local interstellar medium is super low compared to the average. It also explains why there's a higher density shell at the edge of the bubble because all the material that was blown away by all these supernova blasts and imagining these supernova blasts, I just I think of one supernova that, like, this titanic release of energy, and then another and another and another and another, and they're all close together.

Like, the amount of energy is is insane. They have pushed all that material out to the edge, so it's now on the edge. We are surrounded by this thicker shell. But what about the other clues like the radioactive elements on the Earth or that runaway star? Well, the sun, well, the sun wasn't inside the bubble when it was created.

We were somewhere else in the Milky Way galaxy. We were doing our own thing. Then all these supernova went off and created the local bubble, and then we passed through it. But to enter the local bubble, we are now deep inside. We are in the depths of this local bubble right now.

But in order to enter the local bubble, we had to cross through that shell of material on the outside. And that shell of material is a high density region where even more stars are forming. In fact, all the nearest star forming regions to the Earth are located at the edge of the Local Bubble. And it seems likely that all newborn stars in the neighborhood of the solar system were born in the shell surrounding the Local Bubble. The shell is itself is created by material that is swept up and blown out by all these simultaneous supernova, but that itself, the shell itself serves as a site of star formation because if you're, say, a molecular cloud and you're hanging out, you're minding your own business, you're chilling, you're being a molecular cloud, you're enjoying life, and then 500 light years away, 500 supernova go off.

They start blowing material out. They blow this high density region out this shell. It expands outward and outward and outward, and then it hits you. Well, that's exactly the recipe to start triggering you to collapse and start forming stars. So there's a lot of star formation in the shell.

We think all of the newborn stars surrounding the solar system got their start in the shell surrounding the local bubble. All the nearest star forming regions to the solar system are at the edge of the local bubble. There's a lost star formation. There's a lost star death. There's a lost supernovas happening in the shell of the local bubble.

So it's this interesting feedback, a cycle that can happen in galaxies where you get a bunch of supernova, all going off, they create a bubble. But then that expanding shell itself, you can imagine it like a little shockwave that's like triggering new star formation, triggering new rounds of stellar births. Along with that comes the birth of new giant stars. Those those stars themselves go supernova, so it's like this chain reaction that works itself outwards in every galaxy. Starting at the center of one of these supernova associations.

So the shell, because it's a region of star formation, is also a region of recent supernova. This shell is laced with fresh radioactive elements. The heaviest elements, the radioactive elements, come from supernova. So all these newb formed stars at the shell of the bubble, they die. They leave behind their ash including radioactive elements, and then the solar system, our sun, plows through this shell.

Some of that dust, some of that material, some of those radioactive elements are simply going to filter their way into the solar system, filter their way towards the inner solar system, filter their way to the Earth, filter their way through the atmosphere, and get in our water, in our ice, in our land, in our trees. If you're ever walking around outside and you feel like a little speck of dirt in your eyeball, maybe it's some radioactive element from a long dead supernova. You never know. That explains these spikes in iron 60. It's because we swam through the shell of the local bubble which is a slightly radioactive place, you know, compared to the surroundings.

And so we get these spikes from the ash left over from the supernova at the shell. Okay. What about the runaway star? What about Zeta Ophioki? What about him?

Well, new star formation, active star formation on the bubble, lots of things happening, complicated place. One of 2 possibilities for this star for it to become a runaway. Either it had a companion, and the companion died earlier and went supernova and literally blasted it away like a cannonball, or it was born along with a bunch of other siblings in a litter of stars and got mixed up, gotten a tangle, gotten a gravitational scrap, and got kicked out. But we know runaway stars come from recent star formation activity, because you get either a lots of supernova or lots of gravitational interactions, and that provides the energy to kick stars out altogether. This is our story of the local bubble.

We're not exactly sure. We're not 100% sure. This is our best guess based on all the clues we have that the solar system is located in a region of lower density than average. Far lower density than average. Like, 1 tenth the average density of the Milky Way.

Surrounding that bubble, which is about a 1000 light years wide, is a thick shell. All new star formation is happening in that shell. It's not happening in the local bubble. The only thing that can have the energy to drive a bubble like that is something like a cluster of supernova. 100, maybe even a 1000 supernova going off at once.

Detonating relatively simultaneously within, like, a few 100000 years of each other, pushing away this material. This bubble will not last forever. In fact, we think it might be not an enclosed bubble, but more like a chimney. It might be open at the top and bottom where it intersects the disk of our galaxy with material flowing up, either up out of the galaxy or down out of the galaxy. Whether it's currently closed or not, the bubble will eventually disperse and melt away.

And after enough time, after 1000000 of years, the local bubble, this scene of an incredible violence and activity and dynamism in our universe will be lost in the great cosmic reshuffling. It will be lost in the great cosmic reshuffling of gas and material that defines the history of our galaxy. Like a great city that navigated wars and famines and earthquakes and fires and and countless episodes of human drama just becomes buried underneath the dirt. And 1000000 of years later and 1000 of years later, you would never know the drama of what happened here. Thank you to to Brian w and Tim for the questions that led to today's episode.

And please keep those questions coming. Askaspaceman.comoraskaspaceman@gmail.com if you want to reach out directly to me. And thank you so much to all my Patreon contributors who really do keep this show going. I can't thank you enough. But I would like to thank, at least starting with, the top contributors this month.

We've got Justin g, Chris l, Lothian 53, Barbara k, Alberto m, Duncan m, Corey d, Stargazer, Robert b, Tom g, Nyla, Mike Santa, Sam r, John s, Joshua Scott m, Rob h, Lewis m, John w, Alexis Gilbert m, Rob w, Valerie h, Demetrius j, Jules r, Mike g, Jim l, Scott j, David s, Angelo l, William w, Scott r, Dean c, Miguel, bbjj108, barely wires, Heather, Mike s, Michelle r, Pete h, Steve s, Nathan, and Wat Wat Bird. Patreon.com/pmsutter. I can't thank you enough, and thank you for dropping reviews on iTunes or Spotify, your favorite podcast platform. It helps to show visibility, and I will see you next time for more complete knowledge of time and space.

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