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It’s time for school! The Astro101 series will cover some of the most important questions in astronomy. In today’s lesson, we’ll have: What is a galaxy? What does our own galaxy look like? What are the different kinds of galaxies, and how did they get that way? I discuss these questions and more in today’s Ask a Spaceman!

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Hosted by Paul M. Sutter, astrophysicist and the one and only Agent to the Stars (http://www.pmsutter.com).

 

EPISODE TRANSCRIPTION (AUTO-GENERATED)

there are stars and there are, well not stars. And and that just about sums up all of the astronomy that we've explored so far in our Astro 101 series. They're the story of the stars and then everything else. And yes, this is part of a series on getting schooled in basic astronomy, and it also just about sums up all of astronomy prior to say, I don't know, like 1900. There were the things that were stars. And then there was a vast collection of other and all the other thing things seem vastly less important than the stars like. And I know I've been talking a lot in this series about the early 19 hundreds because it's such a pivotal moment in astronomy. It's where a lot of things click into place where astronomy really goes from just measuring positions and recording things and making spreadsheets to actually explaining what is happening out there in nature, that I I see the early 19 hundreds as the advent of astrophysics as we understand it, and since I'm kind of basically an astrophysicist, it's very near and dear to my heart.

This origin of like a lot of modern day astronomy and astrophysics and cosmology. And in the early 19 hundreds, uh, you know, a century ago or I don't know when you're listening to this podcast, it could be two centuries ago or a millennia ago there was so much interest in the stars and how they worked in classifying them. And then there was just all the other stuff. There were the planets. There were the comets. There were the asteroids and there were the nebulae. I've talked about nebule before, and while some nebulae have been known for centuries, like, uh, I don't know the Andromeda nebule, Uh, but some hadn't been known were just just back then getting discovered. I mean, discovered in terms of the European astronomers who are doing this work of astrophysics of trying to explain what is going on up there. Uh, obviously, there are a bunch of that have been known to humans for ever since there have been humans, but they weren't a part of the European scientific enterprise until, say, like the 17 hundreds and 18 hundreds, I'm talking about things like the large Magellanic Cloud and the small Magellanic Cloud because they're in the Southern Hemisphere.

And so, if you're a European scientist, uh, you rely on data. And if not a lot of people are sending exploratory expeditions south of the equator, you don't know what's up so. But my point is that astronomers have known about some nebulae for a very long time. I mean, even the Andromeda Nebula. It's visible to the naked eye. It's the size of a outstretched fist is huge. It's the biggest nebulae you can see. And so yeah, of course, astronomers forever have known about some nebulae, and then astronomers in the 18 hundreds were starting to become aware of a lot more nebulae. For a long time, they were just nebulae. Yeah, they're just like, over. OK, whatever. There's these cloudy things, and so we'll even use the Greek word for cloudy thing to describe them. But they're just whatever. Let's let's focus on the stars, man. That's where all the action is. That's where we had all this cool stuff going on with the class and the dwarfs and the MS and the gas and the other random letters of the alphabet.

And we're starting to organize stars along the Hertz sprung Russell diagram and we were starting to make sense of nuclear fusion. We were actually getting getting somewhere with the stars, and some people were also on the side studying the nebulae. I mean, there are only so many problems to work out with stars, and there's enough people working on it that you figure OK, someone's gonna crack it. Maybe there's something else worth looking at. And and in the 18 hundreds, folks started getting serious about cataloging the nebulae and categorizing them and studying them in more detail. And one of the challenging things about the nebule is, well, the fact that they're cool, they're interesting. They're pretty, Uh, but there are no two are alike, so it's not like the stars where once you see one Red Star, you've basically seen every Red Star where you see a a white star like, and it's like every other white star, you know, just a little bit different. It's useful to have that kind of boring uniformity because it helps you build classification schemes.

And then, as we saw the classification schemes helps us understand what is going on in nature. But the nebulae, they're just like every single one is different. They're different colors. They're different shapes. They're different sizes. They have different structures inside of them. And so classification is kind of hard. And because classification is hard, it makes it hard to decide what the heck they are. And what are they doing up there? Why are their nebule? Why did they have different shapes and colors and sizes and structures? Why we didn't know and one of the most mysterious kinds of nebulae, the ones that we that astronomers were like really talking about 100 years ago were the spiral nebulae like the Andromeda Ne Nebula. The Andromeda Nebula is a spiral nebula. Why is it called that? Because it looks like a spiral. OK, it's it's It's not that apparent to the naked eye. To the naked eye, it just looks like a fuzzy patch. But as soon as you start looking at it through a telescope and then especially once you start taking pictures of it with a photographic plate, you start to see some very interesting structures.

And Andromeda wasn't alone. There were a bunch of these nebulae that had this strange spiral structure and people as well, not people astronomers. I mean astronomers are people, too. You get my point. Astronomers were really starting to debate in the 18 hundreds. Late 18 hundreds, early 19 hundreds. What are these? Spiral nebulae. What do they mean? What do they represent? How did they get there? And how far away are they? This was a pretty pretty loaded question because at the time astronomers were trying to figure out how big the universe is. I mean, astronomers are still trying to figure out how big the universe is, but back then we were We were also doing it. It's kind of a common theme. They were trying to figure out how big the universe is and what are the contents of the universe like? OK, we've got our stars. We got our asteroids, We got our planets, our comets and our nebulae. I'll shoot. I guess we have white dwarfs, too. I guess we'll allow those, uh, but then that's it.

And then and let me see the planet. And asteroids and comets are close to us. Uh, the the A bunch of stars are can vary in distance from close to us to not so close to us and then the nebulae while they're close and then with the spiral ones. Well, what's interesting? I stop doing that voice that what's interesting about the spiral nebulae is that they only had faint stars in them. They only had faint stars. And when if you look at another random, non spiral nebula like, say, I don't know the Orion Nebula, you see some obviously big, bright stars, and then you see some a bunch of dim stars, and so there's like a healthy mix. But then, when you look at the Spiral Nebula, there's only dim stars, and so you might be tempted to think that maybe the Spiral Nebula are really far away. Maybe this is maybe the Spiral nebula look different. They are different because it's a different category of nebula, and they're a category of nebula that are really, really, really, really, really, really far away from us.

The problem is, if you're gonna argue that they're really far away because they only have dim stars, you start running some estimates of how far away these things are. They are very far away, like tens of thousands of times farther away than the farthest star we've ever cataloged. At the time, and that seemed unacceptably large and that seemed rudely large, rudely far away. But some astronomers were claiming that they were able to see the spiral nebula rotate. And if they're super duper far away, then no way could we ever watch them rotate in real time over the course of years. And so what's going on related to this question was the nature of the Milky Way, and I'll talk a little bit about the the Milky Way that we see on the sky. But here I'm talking about the Milky Way, as in the name of our galaxy. Our galaxy was obviously something. It was made up of stars and nebulae and dust and junk and white dwarves.

But did the stars fill up the universe as one big galaxy like everywhere you go in the universe, are you gonna see some collection of stars or do the stars stop? And then there's some vast expanse of nothing, and then that's it for the universe. Or are these spiral nebulae like something else? Are they their own Galaxies completely separated from us? And also related to that question is, how big is the whole thing? Different results were giving different answers. If you do a survey of stars and just map out, just look in random directions and start counting stars and figuring out how far away they are and you build up a map of the universe. It looks like we're in the center. We're surrounded by a big ball of of hundreds of thousands of stars. The ball is relatively spherical, a little bit flatter, but hey, we know our surveys aren't incomplete. But if you look at other things like, say, globular clusters, globular clusters are clumps of old, red, dead, dying stars and they're all grouped together.

Hence the name cluster. I don't know why astronomers chose the name globular, but that's not my business. We see a few of these clumps of old red stars, and they all move together. They're obviously gravitationally attracted. They're bound together as a ball of red stars. And then the whole ball is moving and we can actually watch those things move. And they're all like orbiting. They're all moving. They're all, uh, orbiting around circling around some central point, and that central point is not us. So if you look at just the stars, it looks like we're at the center of the universe. If you look at the globular clusters, it looks like we're at the edge of the universe. If you just count stars, then the universe isn't very big. If these spiral nebulae are really far away, then the universe is gigantic. It was It was a mess, but maybe the spiral that aren't so far away. Maybe they are. Maybe the observations of how they were rotating turn out, turn out to be legit. Here's spoiler alert. They were not legit. Um, but like we don't.

But this is just confusing mess. Just what's going on with the universe? It this all came to a head in something called the Curtis Shapeley Debates or the Great Debate, which was a debate that was very widely published and publicized about the size of the universe and where we sit in it. Do we have a large universe with without us at the center and the universe is synonymous with the galaxy? Or do we live in a small galaxy or slash universe with a S at the edge? Are the spiral nebulae really far away? Are they really close? It was a complicated topic. It was actually the second most complicated topic at the time. The organizers of the debate originally wanted to, and this was like in 1910 or something UH, originally wanted to have a debate about special relativity and whether it was correct. But they decided that, like, nobody would be able to follow the discussion because special relativity was so wacky out there. And also they couldn't find competent people to debate it because no one really understood special relativity.

So they they tabled that and instead have held a debate about the structure and size of the universe because that was an easier thing to talk about. And this debate lasted for like, a decade. Just just how big is the universe? What is the nature of our galaxy? What's going on with these spiral nebulae? Just what's going on? The person who held the keys to all of this in her hands was an astronomer named Henri, Anna Swan, Levitt or Lovett. I don't know if it's Leavitt or Levitt, and so you might hear me float around the different pronunciations. Henri Anna discovered something very, very cool about our universe. She discovered that there's this particular kind of star. Remember in the early 19 hundreds, everyone's classifying star stars left and right, and there's one kind of star that changes with brightness over time, it it over the course of days or weeks. It can get brighter, and then it gets dimmer and then gets brighter. And then it gets dimmer, and it does it on a fairly regular basis in a very fairly predictable way.

The first one of these kinds of stars was identified in the constellation Cus, and hence so hence they get the name Seid variables the variable stars, which are just stars that vary and who cares how they work. So that's not important for what we're trying to do here in astronomy. We'll leave that to the astrophysicists. What Henrietta found. She found a absolutely remarkable thing. Everybody wanted to know. How far away are these spiral nebulae? But the stars themselves were too far away. They were too small, too dim for us to do any, like parallax measurement. We, uh, all of our distance measuring techniques were coming up short literally. But what Henrietta Swan Levitt did was unlock a new tool for finding and measuring distances in the universe because she found that the longer a seid variable takes to get brighter and dimmer to go through its cycles, the period of its cycle, then the brighter overall it is in an absolute sense in the faster variable cycles between bright and dim, bright and dim, bright and dim.

The dimmer overall it is, is in an absolute sense. So what this means is that once you know for sure the distance to a few Seid variables and people were starting to do this soon, soon after she published, she published this relationship. Hey, Hey, everyone. Hey, Hey guys, Uh, the longer a sad variable takes to vary the brighter it is Take it away. Then other astronomers realized started getting some distance measurements to seid variables so they could calculate their absolute distance. And this gave them a calibration. Once they had a calibration, they can go out and find any other seid variable anywhere in the universe slash galaxy. Watch it very brighter and dimmer, brighter and dimmer. And then use this relation that Henry and a swan Levett came up with to calculate a true brightness. So you're like Oh, OK, OK. It take. It took 14 days for that one to cycle. So that means it's gonna be this bright, as if you were standing up right up close to it. That means it's this bright. And so you compare that brightness to the brightness that you see on our sky, which is gonna be a lot dimmer because it's super far away.

But you can compare those two do a little bit of trigonometry and you get a distance. Henry Levitt delivered the keys to unlocking the universe to the astronomical community in the form of this relationship. Now, at the time, I should say, not everyone bought this relationship. A bunch of people thought it was super sketch. But that's life. It turns out it's very legit. Edwin Hubble came along in 1922 and at the time he was using the world's most powerful telescope. It was the hooker telescope at Mount Wilson. It was 100 inches across back when we measured telescope diameters in inches. It was a big telescope. And what he did, he is. He found a whole bunch of stuff. He had variables in the Andromeda Nebula. You watch them very brighter and dimmer, brighter and dimmer calculate their true brightness. Compare that to the brightness that he saw on the telescope. Use that to a distance. The Andromeda Nebula, according to his calculations, was millions of light years away, which is way farther away than anyone was thinking, even the people who thought it was far away.

For some time, the Andromeda Nebula maintained its name as the Andromeda Nebula. But nowadays we call it the Andromeda Galaxy. Because that one observation exploded our view of the cosmos and exploded our view of what a galaxy is. We thought Galaxies were just the there was just the galaxy and it filled up the universe. But now that we start seeing other Galaxies, other regions of the universe that are crammed full of stars that are separated from us by vast gulfs of absolute nothing to the tune of millions of light years compare that to the typical size for a large galaxy of like, I don't know, 100,000 light years. That's a big distance. The universe is more empty. Then it is not. And it was Hubble's observations that allowed us to make that realization.

Before I continue. I want to let you know that this episode of Ask a Spaceman is brought to you by my friends at better help. Better help provides easy, convenient, affordable access to online counseling and therapy. And, you know, the therapy has been an important part of my life. Experience is something I'm absolutely not ashamed to talk about. I wish more people used the therapists and counselors to take better care of their own mental health, just like they take care of their physical health. Uh, I know a lot of you turn tune into this show for Astro Thera as a word, but maybe if you're having a really tough time, you should talk to an actual professional, and so I encourage you to go to better help. They are convenient and professional. It's real therapy and counseling, and it is affordable and you connect online. You don't have to wait in a waiting room or any of that. You just talk to someone who who cares and and knows what they're talking about. As a listener, you'll get 10% off your first month by visiting better help at better help dot com slash spaceman, and I want you to join over 1 million people who have taken charge of their mental health again, that's better help HE LP dot com slash spaceman Now back to our Astro Thera session brought to you by not a licensed psychologist, but an astrophysicist.

But good enough, right? At least for now. There is one more thing I do want to tell you about, and that is the great courses. Plus, now we're all nerds, right? We're all geeks. I mean, you're listening to a podcast about astrophysics. OK? Like like don't be ashamed. It's OK, but we all love learning. I love just being a sponge and absorbing cool new information and the great courses. Plus is a font of cool new information. There's one course I really want you to check out the search for exoplanets what astronomers know. You know, I geek out about all that exo planet stuff. I think it's wild. I think it's beautiful. I think it's amazing. I think it's a crazy new window into our universe, and I think you will benefit a lot from listening to that course or watching it and and you can go on a website. There's an app. I mean this is 2020 was pretty rough, right? Let's try to make 2021 a little bit better. I want you to sign up for the great courses plus and learn something new.

That's a special URL the great courses plus dot com slash spaceman. You'll get an entire month of unlimited access for free. That's the great courses plus dot com slash spaceman. I'm gonna say it again. The great courses plus dot com slash spaceman We realized, with Hubble's observations that Galaxies are a thing. They are entities. They are structures. They are a new kind of pattern that this, at least this one kind of nebula, the Spiral Nebula, turned out to be island universes, clumps of enormous numbers of stars. And the only reason we can see them, despite their incredible distance, is because they contain enormous numbers of stars. So armed with this information, we can go back to look at our own galaxy, which we call the Milky Way galaxy. And from our perspective, the Milky Way is just a band of light that stretches across the sky.

It's just this diffuse glowy thing. If you live in an urban area or even suburban area. Good luck seeing the Milky Way. I encourage you to go out to a dark sky so you can actually see the Milky Way for yourself. It is stunning, of course, people have wondered. It's visible with the naked eye. So people have wondered through the ages what is going on with the Milky Way. What is it so many strange and cool and wonderful ideas through history of of what it could be. And, of course, some people accidentally got it right. Most people got it wrong about what actually is this Milky Way, this band of light. But some people got it right, and the first person to demonstrate the rightness was Galileo because he looked at with the telescope. And what did you see when he pointed his telescope in the Milky Way? He saw loads of stars. And so yeah, ancient peoples, some of them randomly guess. I wonder if it's a bunch of stars that are so far away that we only see it as a diffuse band of light as no, not as individual points, but they were just guessing.

And because we only see it as a band of light, we already get a clue as to what our own galaxy looks like. And then when we start looking out at other Galaxies, like the Andromeda Nebula, eventually the Andromeda Galaxy, we also pick up clues as to what our own Milky Way galaxy is like. The Milky Way galaxy is named for the Milky Way band of Light. It turns out they're the same thing. Our galaxy is the thin disk, and we're inside of it. When you look at that band of light in the sky, you're looking through the disk and all the loads of stars in that direction. And when you're not looking at the Band of Light, you're looking out away from the plane of our thin disc of our galaxy, and there aren't as many stars in that direction, so you don't get the band of light. So already, just from the fact that there is a band of light in our sky, we discover that our galaxy is flat like a pancake and contains loads of stars. Of course, we can do more sophisticated things to to map the Milky Way. It's kind of hard to do it because we're right in the thick of it, but we can do surveys of stars, millions, hundreds of millions of stars as far away as we can possibly see.

We can look at things that orbit in the Milky Way, like those globular clusters we were talking about earlier, Uh, also dwarf Galaxies, which I haven't talked about yet. You can also turn on your neutral hydrogen glasses and look at radio emission from neutral hydrogen. Get a sense of what's going on. You can do radio emissions. You can look at infrared and X-RAY. Uh, just doing whatever you can to try to study the galaxy from inside of it, because it's kind of hard to get a satellite up above the plane of the galaxy and just snap a picture. We don't know exactly what the Milky Way galaxy looks like, you know the same way. We know what the Andromeda Galaxy looks like because we get a nice view of it. But we do know it looks something like this. It's what's called a barred spiral. It is a spiral galaxy like if there are astronomers in the Andromeda Galaxy looking over at the Milky Way galaxy, they would call it a spiral nebula, too. Barred spiral means we have a We have a dense core, our galaxy and most galaxy. Almost all Galaxies have a very, very dense core. Uh, but our core isn't round.

It's not a ball, it's it's stretched out. It looks like a I don't know, like a bar. Actually, to me, it looks more like like a can of soda or something. But soda spiral isn't exactly gonna turn a lot of heads in the astronomical community, so I'll keep that to myself as to how big it is. Remember, this was the whole point of those Curtis Shapeley great debates it's about. I mean, it depends on how you define the edge, because it's not like, Oh, there's a bunch of stars, a bunch of stars, a bunch of stars, a bunch of stars, and then it just stops. And then it's the absolute void of nothing until you get to another galaxy. And that would be easy. But nature isn't always easy. There's just stars and then slightly fewer stars and then slightly fewer stars and then slightly fewer stars, and then just like a handful of stars. And then and then there's some random gas that just floats around orbiting the Milky Way. And then it's like kind of vaguely nothing. But there's still kind of something there. A thin spread of diffuse gas between the Galaxies, so it gets kind of hard, depending on how you define it.

If you're just looking at the stars, it's about 100,000 light years across. If you're including the gas that's associated with our galaxy, it's about 200,000 light years across our Milky Way Galaxy has a few 100 billion stars anywhere between, say, two on the very low side and five on the very high side. So hundreds of billions of stars, roughly the same number of planets and roughly here means anywhere from 100 billion planets to a trillion planets. So we're still working on that number, and now we have a pretty good indication of where we are in the galaxy. Our solar system is 27,000 light years away from Galactic Center is on the inside edge of a spur of one of the spiral arms known as the Orion arm. Yeah, we live. We live in in in, like the suburbs of the galaxy. We don't live in that dense downtown core. We're too old for that. We want a house and a yard, so we're in the suburbs pretty nice over here. We're in a spiral arm, but not like in the main spiral arm we're on like some small subdivision off to the side.

By the way, we're orbiting the center of the Milky Way at about 200 kilometers per second, which means one orbit takes about 240 million years, and that's too fast. This is one of the surprising things we learned about Galaxies in the 19 seventies is that they are rotating, literally rotating too fast. Like if you if you if you add up all the stars and gas and dust all the stuff that's glowing in a galaxy, you can use that to weigh because you're like you know how bright a star is. You know how much, Uh, certain reservoir of gas weighs and you can connect that to observations in our own galaxy you can come up with, like mass estimates of other Galaxies. It's not the easiest calculation, and it's not the most precise calculation, but it gives you a pretty decent ballpark and then you can say OK for that amount of mass. Uh, that's in certain amount of gravity so it can support a certain maximum rotational speed. But if the stars and stuff inside of a galaxy are rotating faster for all that, then all that gravity can support. It should just like tear itself apart.

And that's kind of exactly what is happening in every galaxy that we observe. The stars, including our own, are orbiting the centers of their Galaxies at too high of a speed. Given the amount of matter that we can see, hence there is matter that we can't see hence dark matter. It turns out that all Galaxies are embedded in a much larger blob of dark matter. This is matter that does not interact with light. We do not fully understand it, but it makes up about 80% of the mass of the universe, so we better start figuring it out. Oh, we don't call them blobs, by the way, we call them halos. Our Milky Way galaxy is embedded in a halo of dark matter because that sounds way fancier than blob. And as to the center of the Milky Way, you know, I mentioned we're 27,000 light years away from Galactic Center. It's in the direction of the constellation Sagittarius. So if you happen to look at that constellation you are looking in the direction of the center of the Milky Way galaxy. It doesn't seem like much invisible light. You can see the the Milky Way band on the sky maybe looks a little bit denser there.

There's some stars there, but otherwise not nothing too special. But when you look at with radio, there's this incredibly bright source of radio emission in the direction of the constellation Sagittarius. And we call this Sagittarius a star. Yes, it's a horrible name, but you can blame the radio astronomers for that one. That's a asterisk, Asterisk. Whatever it's called, Sagitta is a star that is the center of the Milky Way galaxy. That is the core, and that's deep in the core. Like the core itself is this big bulge like soda canny, bulbous thing in the center and then surrounded by a relatively thin disc of spiral arms and other stars in between the arms. We'll get to that in a second, but if you look in the radio. There's this insanely bright radio source we call a sagitta Say star. It turns out that's where a giant black hole lives. Nobody asked for that 100 years ago. It's a black hole about 4 million times more massive than our sun and all that radio emission is coming from all the stuff flowing onto that giant black hole.

But even though that black hole is massive, Sagittarius is a star is massive 4 million times the mass of the sun. It's peanuts compared to the Milky Way itself. It's like less than 1% of the whole mass of the galaxy, like our galaxy has hundreds of billions of stars, plus gas and dust. But these giant black holes do play a role in their host Galaxies, and they do it through something called feedback. So even though they're small, they they actually dictate the evolution of their host Galaxies. And by the way, we think every galaxy has a supermassive black hole at its center and that the evolution of that black hole is intimately tied to the evolution of the galaxy. Here's what happens if if say, you're a galaxy is forming and there's a lot of material flowing in and everything's looking great, and you get to make a lot of stars and get to be really big. A bunch of that gas is gonna wind its way down to that central, supermassive black hole. It's going to accrete onto that black hole. It's going to form a giant accretion disk.

Its energies are gonna go out of control. It will blast intense bursts of radiation and jets and all sorts of cool, gross things we call these things. Quasars are active galactic nuclei. When these events happen, it blasts out so much radiation that it actually heats up the rest of the galaxy and prevents new material from falling in and kind of slows down and sometimes even shuts off star formation and making the galaxy bigger. And then eventually the gas settles down, cools off, feeds down onto the center, get a big explosive event, which heats up the gas. The black hole shuts down because there's no news reservoir of gas, and then it cools and you get this constant cycling process that actually keeps galactic evolution and growth in check so that Galaxies don't just gobble up every single little bit of atom that ever comes near it, so that's pretty cool. And that pretty much sums up the contents of a galaxy. There's a giant black hole. There's a load of stars.

There's nebulae of various shapes and sizes, which we eventually learned our connection connected to the birth and death of stars. There's random bits of dust, which we call the interstellar medium. There's a gassy halo, which is just some hydrogen and helium just hanging out near the Milky Way, but not really. Inside the disk, there are fountains of material launched by, say, the random supernova explosion. There's AAA Patreon Page, where you can go to patreon dot com slash PM Sutter to learn how you can support the Milky Way galaxy and also this show. Every galaxy has its own patreon page. There's a weak magnetic field. It's and when I say weak, it's like a millionth the strength of the Earth's magnetic field. But it stretches to extend the whole entire galaxy, which is super cool. Every galaxy is way more dark matter than anyone realized. Like 80% of the mass of a galaxy is really in the form of dark matter and that generally describes just about every galaxy like there are different shapes and sizes of Galaxies, but they all have the same components.

They're all large, you know, tens of thousands of light years across minimum giant black hole loads of stars, bunch of nebulae, an interstellar medium, a gassy halo, magnetic fields, cosmic rays. You know, they're they're like they're like cities, you know, all sorts of interesting characters and people and groups and businesses and activity. And then you get out on the highway and there's a whole bunch of corn fields, and then you get to another city and it's super exciting and it's dazzling and there's lots of lights, and then you leave the city limits and it's back to the cornfields for the next five hours. The Galaxies are the cities of the universe. The Milky Way as a city is pretty average pretty average, just pretty. OK, as Galaxies go not the biggest, but not the smallest. It's it's just fine, just fine. It's like a Pittsburgh of cities. It's just you know what? You're good. You're good. Not too big, not too small.

You're just you're just Pittsburgh. But like I mentioned the Milky Way is a spiral galaxy. There are three kinds of Galaxies overall, which I call beautiful, boring and ugly. But the astronomers call spiral elliptical and irregular. I love that word. Irregular, like we didn't want to hurt their feelings or something. And, of course, like just like the stars, Remember, we learned about the stars. There are dwarf stars, and then there are giant stars. There are also dwarf Galaxies. And then there are like giant Galaxies. Uh, so we're talking. The number of stars here is anywhere from 10 to the eight, Like 100 million stars on the very, very low side to 10 to the 14 stars. We're talking trillions of stars on the high side. This question of the three different kinds of Galaxies spiral, elliptical or regular or beautiful, boring and ugly like Yeah, like spiral are very pretty. These are the ones with all the cool spiral arms obvious elliptical are just big blobs of stars not doing much. They tend to be redder while the spirals tend to be bluer.

And then they're the irregular, which is just all sorts of things. We think that the different kinds of Galaxies are determined by their history, which is very un, uh, surprising and very unexpected and also very cool. We actually think that a galaxy left to its own devices or, at worst, suffering only small mergers where, like little tiny dwarf Galaxies, get gobbled up by a bigger galaxy. If you just leave a galaxy to its own devices, it would naturally form spiral arms. That's because the spiral arms are actually density waves that have piled up on top of themselves and twisted themselves around. And a galaxy left to its own devices will just naturally generate density waves from, like a random dwarf galaxy passing by or a bunch of supernova going off or just existing. You can get these very cool spiral structures that just appear out of their own. So, like if we look at the Milky Way or the Andromeda galaxy with our beautiful spiral arms, they are pretty normal Galaxies that have not suffered any major collisions.

They just get their spiral arms. They just get their density wives, the spiral arms. I did a whole episode on them, but let me let me give you a very brief version here. The spiral arms, even though they look amazing in pictures and really stand out, they actually aren't very much more dense than the places between the spiral arms. They're only like 10% on average, more dense than the places in between the arms. But because these are slightly denser regions, these are places that trigger star formation. And when you get a whole bunch of star formation, you get the usual mix. You get the small, red dim ones, you get the medium ones and you get the big blue ones. So the spiral arms are places where stars are forming actively in a galaxy. And that's why you see the blue ones. And that's why they look very, very pretty when you look at through a telescope because you're seeing all those bright, beautiful stars. But then eventually the spiral arm passes. Stars do their thing. The bright, bright blue stars die. All the normal medium stars and the dwarf stars keep on living, but they're not as bright and as impressive, so they move out of the spiral arms.

They move into the gaps between the spiral arms and not unlike what our solar system is doing right now, and make it harder to see. But sometimes Galaxies suffer big mergers like equal size Galaxies slamming into each other. Which is exactly what is going to happen with the Milky Way and the Andromeda Galaxies in about 4 to 5 billion years. When these collisions happen, they are ugly. There's no big crash or boom, but there is a tremendous release of energy. Uh, but Galaxies will get distorted. They will get stretched out. They will get ripped apart. They will intermingle and then pass through each other and then do it again and again. And it's just nasty when they're in the middle of an a massive merger event or recently suffered a merger event. That's when we call them irregular because they look really messed up. And then, eventually, after the collision after the merger, the merger event itself triggers so much star formation that the Galaxies actually use up almost all of their available gas reservoirs for making stars like our galaxy.

The Milky Way pumps out like I don't know, like 10 stars a year or something like 1 to 10 stars a year. This in the middle of a merger event. This can get cranked up to like hundreds of stars a year, and that just uses up all the material. So Galaxies that have merged together become much larger, but they pay a price. They aren't able to manufacture new stars as efficiently. And of course, they lose their spiral structure because who would? Who would be able to keep that after slamming? It is like being able to keep your fender after slamming into another car at 60 miles an hour or your nice, shiny paint job. And so, ultimately, what happens is you get a big blob of stars that with no new star formation so you don't get the bright blue ones anymore. You just have the old dead and red ones, and you turn into an elliptical galaxy. So the spiral Galaxies or Galaxies have largely been able to scoot through these billions of years relatively unscathed.

The elliptical Galaxies are what happens when you've suffered too many collisions, and then the irregular Galaxies are what happens when you are actively suffering a collision. And those are the Galaxies. And the immediate question you wanna ask is now that you realize that Galaxies exist is, uh why did they get there but that is another class because this one is dismissed. Thank you so much to my top patreon contributors this month. That's patreon dot com slash PM Saturday, and you can support me, too. Yes, you can. Matthew K, Justin G, Justin Z, Justin G, Kevin O, Duncan M, Coy D, barque Dude, Robert M, Nate H, Andrew F, Chris Cameron, NAIA Aarones, Tom B, Scott M and Rob H is their contributions, and everyone else is that keep this show going and I can't thank you enough. Speaking of thanks, if you have a chance, please go to the iTunes, ask a spaceman director and leave a review. It actually does help the show visibility or you know what? Why don't you just leave me a question? Use hashtag. Ask the spaceman.

Go to ask us spaceman at gmail dot com or ask us spaceman dot com hit me up with some questions. Uh, we've only got a couple more episodes left in this series that I'm exploring this Astro 101. And then we will return to business as usual in our quest for complete knowledge of time and space

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