Can galaxies ever get destroyed? What happens to their stars? Do galaxies ever die? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

Sometimes, I just want to unleash my inner 8 year old and destroy something. Just pick it up and smash it. Maybe to see what it looks like on the inside. Maybe to see how it works. Maybe just because I feel like it and that's valid enough reason.

So, since I'm in a smashing mood today, I thought it would be fun to destroy a galaxy. That's right. A galaxy. I mean, if we're going to destroy something, we might as well go big. Right?

And we've blown up stars and dismantled planets before in previous episodes, so that's no fun anymore. We're going to do everything we can to fully, and I mean totally and completely and utterly destroy a galaxy. As you know, galaxies are kind of large and kind of complex places. A typical galaxy will be home to anywhere from a 100000000 stars on the small side to trillions of stars on the large side. Our own Milky Way galaxy has about 300 to 500,000,000,000 stars, while our neighbor, Andromeda, has around a trillion.

Galaxies have a range of sizes. The smallest ones are hard to distinguish from dwarf galaxies, which deserve an episode in their own right, so go ahead and ask. So we'll go ahead and put ranges anywhere from 10 ish 1000 light years across to a few 100000 light years across on the side for anything for galaxy scale. They come in all sorts of shapes in addition to all sorts of sizes. In addition to all sorts of sizes, they also come in all sorts of different shapes.

You have the beautiful spirals, the boring ellipticals, the ugly irregulars, but a galaxy beauty contest is, as you guessed it, another episode. Because today, we are focused on smashing. But if we're going to destroy a galaxy, we're going to have to contend with more than just stars. I mean, stars are great and all. I even have a favorite one.

But altogether, they make up only a few percent of the mass of a typical galaxy. You heard me right. When you see something grand and beautiful, like the Andromeda galaxy and all of its spirally glory in front of you, you're looking at less than a few bits out of a 100 of the true contents of the galaxy. About 10% of the galaxy is just loose bags of gas and dust just floating around minding their own business, not bothering anybody and not being bothered by anything. There are stellar remnants, like the white dwarfs and the black holes.

There are the brown dwarfs. There are a bunch of other things like planets that don't really add to the total, and the rest is all dark matter. You know, this invisible form of matter that suffuses every galaxy and actually overwhelms every galaxy. When you look at a galaxy, you're just seeing the center concentration of matter. It's it's enveloped in something we call a halo, this, like, gigantic ball of dark matter.

So how do we go about destroying something as grand and beautiful and seemingly permanent as a galaxy? The first galaxies appeared over 10000000000 years ago, and they tend to stick around, so they seem like rather hardy creatures in the universe. If we're going to destroy a galaxy, we need a source of energy. We need something to power our endeavors and make this happen. The good news is that the universe is full of all sorts of sources of energy and there's plenty enough energy to go around to rip a galaxy to shreds.

And as we survey the possible avenues, we can take to destroy a galaxy like a toy we don't care about anymore, we're going to rate this on a satisfaction scale with the amount of satisfaction proportional to the amount of destruction that we can achieve. So let's see what our options are. Number 1. I know. I know.

A giant black hole. Now, giant black holes are large. Supermassive black holes are a few 1000000 times the mass of the sun, sun, all the way up to 100 of billions of times more massive than the sun. Almost every single galaxy hosts a supermassive black hole in its center. The Milky Way has one.

We call it Sagittarius a star. The star is an asterisk, by the way, different different episode. It's about 4a half 1000000 times the mass of the sun, which is gigantic. These super massive black holes are almost always the largest single object in a galaxy, and yet, compared to the total mass of a galaxy, especially including all the dark matter, they're like less than 1% of the total mass of a galaxy. So by mass, they don't do much.

By size, they don't do much either. Supermassive black hole. Just to have a picture in your head, just think solar system scales, like, maybe Mercury orbit, maybe Pluto orbit, maybe even larger orbit, but just somewhere in the vicinity of the size of a solar system is the typical size of the event horizon of one of these giant black holes. So by mass, they're not influential. By size, they're not influential.

But they are one of the most important aspects of a galaxy, and that's because of their enormous gravity. When matter falls into a black hole, say a star gets torn to shreds or there's a giant clump of gas that gets too close, It gets caught in the gravitational grip of the black hole, and then it flows inwards. And as it does, that gas compresses. It has to squeeze a whole giant volume of material down into a relatively tiny space. And, yes, I know we're talking about objects the size of solar systems.

But for galaxies, that's a tiny space. This material heats up. It compresses, heats up, and emits radiation. This radiation then blasts out through the rest of the galaxy during one of these feeding episodes. It's in this phase that we call the material surrounding a black hole.

We call it a quasar. These things are the most energetic long term events in the entire universe. You can have brief events like a gamma ray burst or a hypernova that can briefly put out more energy than a quasar, but not over long periods of time. A gamma ray burst will last, like, 2 seconds. A giant flash of flare from a magnetar or something will last, like, a a microsecond.

These things are nothing. Supernova will be bright for a couple weeks. A quasar can outshine a 1000000 galaxies at once, and they can last for 1000000 of years. They are insanely energetic, the most powerful engines in the entire universe. And what this energy does is when it goes out of control, it can kill a galaxy.

It kills a galaxy by heating up all the gas. The gas in a galaxy, this random collection of gas and dust that's just floating around, if it wants to make stars, that gas has to cool off. It has to compress. It has to reach very, very high densities. It can only do that by releasing heat.

It has to cool off and compress, and then you get star formation. But if you have these quasars, like, blasting radiation out, it heats up all the gas, and it shuts off star formation. In worst case scenarios, we have some observations of this where the quasar was able to blast away material just through sheer radiation pressure. Like, there are so many photons flooding through the galaxy that they literally push on the material, on the matter, on the gas, and push it out of the galaxy. So you don't just shut down star formation for a little bit.

You remove the ability for the galaxy to make stars altogether, and that's kind of destroying a galaxy. I'm giving it a 2 out of 5 on the satisfaction scale. It's like sticking a firecracker inside of a toy, but this toy may be losing a little bit and maybe it's, like, smoky and charred on the inside, but it's largely intact. We've killed star formation, but the dark matter is still there. The structure of the galaxy is the same.

And so okay. 2 out of 5. So let's go with something else. What if we smash a galaxy against something even bigger? You know, it's very easy to destroy a toy by smashing it against a wall or smashing it against a rock.

The trouble with smashing galaxies is if you want to smash it against something bigger, the only thing bigger than galaxies are groups and clusters. And by definition, groups and clusters, they're large. Yes. They're more massive. Yes.

But they're very they're not dense. If you look at a galaxy cluster, galaxy clusters are these dense groupings of galaxies home to, like, a 1000 galaxies or more. But this is over a diameter of, like, a few million light years. And so you have these 1,000 galaxies clustered together, hence the name cluster, and then a whole bunch of empty space. And and and the space isn't entirely empty.

It's filled with something called the intracluster medium. It's this hot, thin, diffuse gas. It's there. It's present. It's hot enough to glow in X-ray as we see it, but it's so thin it would register as a vacuum in Earth Laboratories.

So when you smash a galaxy against a galaxy cluster, honestly, it's a little bit underwhelming. What happens is something called ram pressure stripping, which is this wonderful phrase. It's it's it's one of these phrases. It's just, like, 3 random words that should not go together and were just picked out of a hat, and now they describe a physical process. It's RAM pressure stripping.

What happens is you have a galaxy falling into a cluster. It encounters the intracluster medium, which is very thin, but it's still there. The galaxy is moving pretty fast, usually 1 or 200 kilometers per second, which, yeah, fast. And that speed is enough for the interaction between the galaxy and the cluster medium to to register, to make something, and so it heats up on the front. And it heats up the gas in the galaxy itself, and it pulls that galaxy stuff off of it.

It peels it off. Like, you imagine a meteorite falling through the sky. It's doing the same thing. It's compressing the air in front of it. It's heating up the meteorite itself, and then little chunks and bits of the meteorite go go falling off.

It's it's a similar process just playing out on much larger scales. And so it's like this galaxy is swimming. It's it's it's swimming through the intracluster medium. It the gas inside of it is heating up, and then pieces of the gas come flying off. We get this amazing thing that we call jellyfish galaxies because the galaxy is there, but then little tendrils and by little, I mean, 1,000 tens of 1,000 of light years long.

Ten little tendrils of gas streaming behind it. It looks like a jellyfish. It's hilarious to look at. But the law but the galaxy is largely intact. So, yeah, satisfaction scale, 1 out of 5.

Alright? It's cute. It's like taking a toy and throwing it as hard as you can, not against a rock, but against a swimming pool, a bathtub. If you're lucky, a little piece will break off, but but that's about it. So if we want to smash a galaxy against something, it has to be against something of comparable density.

It can't be against a cluster. So what if we smash a galaxy against another galaxy? Right? They have the same size, the same mass, the same density. This is our best shot.

Right? So when galaxies merge with galaxies of roughly the same mass, we call these a major merger. Major mergers are one of the most energetic events to occur in the entire universe. This releases an enormous amount of gravitational energy. It's like taking 2 balls on opposite sides of the valley and then letting them roll down and then smashing into each other at the middle of the valley.

You're transforming all of this potential energy into kinetic energy when they fall, and then they collide together, and you're transforming that energy once again into the impact of the collision, the the connection between them. Same thing if galaxies encounter each other, merge together, they fall gravitationally towards each other, and then they smash. But you start to get some interesting things even before the encounter. When the galaxies are getting closer together, you get these things called tidal tails. The galaxies elongate along their direction pointing to the other galaxy, and you get essentially, like, spiral arms detaching from the galaxy because the the gravitational forces get really weird and complex.

And so you get these giant arms arcing away from the main galaxy. We have another cute name for this. Sometimes we call these tadpole galaxies because we see 2 galaxies close to each other, and then they have these big, long arms extending in opposite directions. They look like tadpoles, like, smooshing against each other. It's amazing.

We're it's gonna happen to us. The Milky Way is on a collision course with Andromeda. In about 5000000000 years, we will merge. The merging takes forever. It takes 100 of 1000000 of years because even though galaxies are, when they encounter each other, are roughly the same mass and same density, most of the galaxy is just empty space.

The stars just, you know, they just they just swim by each other. Yeah. Don't there there will be gravitational interactions. There will be scattering events. There will be some mergers.

But by and large, the the stars just swim past each other. The dark matter by definition, dark matter doesn't talk to anybody, not even itself, so the dark matter just goes by it itself. That leaves the gas to get all tangled up. All the random clouds of gas and dust that just inhabit a galaxy, They get all tangled up with each other. There are shock waves.

There are density waves. There are collisions that ignite a fantastic round of star formation. In fact, when galaxies merge, it uses up almost all the available gas at once in one giant round of star formation. Usually, the something like the Milky Way is producing 1 solar mass worth of stars every year on average. You know, just nice and steady, just just just spitting out the stars.

During a merger, this will shoot up to 100, sometimes even 1000 of times faster, pump out a lot of stars, depletes it. When we see red and dead galaxies, these are the results of recent major mergers. That's what will happen to the Milky Way and Andromeda when we collide together. For a moment, for a few 100000000 years, we'll be brilliant, we'll be intense, we'll be of we'll have so many stars. It will look amazing, and then we'll go to make new stars, and there won't there won't be any gas left in the tank.

The black holes merge. There's a new round of quasar activity, which will strip material away. The merger itself will scatter material. Patreon will shut down. That's patreon.com/pmsutter.

It's how you keep this show going, and we are gonna going to keep this show going all the way, at least until the merger of Andromeda and Milky Way, 5000000000 years from now. And we need I need your help to keep that going. That's patreon.com/pmsutter. On the satisfaction scale, I'm going to give galaxy mergers a 3 out of 5. It's fun for a while, and there are some fireworks.

It's exciting. Black holes merging, quasars activating, tidal tails, star formation, out the wazoo. It's crazy. But then it just kinda fades away, and then you're left with a lump of mixed up galaxy that just kinda sits there. Doesn't quite get the job done.

Okay. That's major mergers. What about minor mergers? Like, taking a small toy and smashing it up against a big toy. See, the key here is we can't we can't rely on galaxy clusters because they're too thin.

We can't rely on any internal mechanisms because they're not powerful enough. Smashing one toy against another toy, Galaxies, they just don't have the right kinds of stuff to make this interesting. But what if I take a small galaxy and smash it against a big galaxy? Well, we know what happens. We never get to observe galaxy mergers play out in real time.

Like I said, these things take 100 of 1000000 of years to play out for the process to unfold. So we don't get to see it happen in real time, but we can see the remnants of these kinds of processes. These remnants we find in our own galaxy. We find what are called stellar streams or stellar associations. When we look out at the 100 of billions of stars in the Milky Way, of which we've mapped and cataloged 1 to 2000000000, a small fraction of them, Sometimes, groups of stars just stick out, or I shouldn't even say groups of stars because they're not even associated with each other.

Like, we'll look out. We'll scan a big area, and we see stars of all sorts of different colors, metallicities, speed velocities. But then let's say there'll be, like, a star over here that has a certain metallic a certain amount of heavy elements inside of it at a certain age and a certain orbit around the core of the Milky Way. And then there's another star over here that has the same kind of orbit, the same proportion of heavy elements, the same age. And then there's this other star over here that, again, shares similar properties.

The hypothesis, the going hypothesis, is that if you look at a bunch of stars and they have similar sets of properties, the the the same age, the same metallicity, the same orbital parameters, then they probably came from the same origin point. They got mixed up, but they they all came from the same birthplace. Think of your last name, and think of all your family members and your distant cousins. And, presumably, maybe they've lived in the same spot. But if you're, like, an come from an immigrant family, you can like, I can look at my own last name, Sutter.

I, you know, I know where my ancestors came from in, you know, Swiss Germany, came to the United States. I can go across the United States, and I encounter other Suttters, other people with the same last name, and we share something in common. We share the same last name. So we know we can trace our ancestry back to the same, like, little town in Switzerland or whatever, but we've been mixed up. We've been scattered around, but we share some common properties that allow us to trace back our common ancestry.

It's the same thing with stars. Stars don't have last names, but they do have other properties that we can measure. So even though they get mixed up in the galaxy, we say, hold on. Hold on. I think these came from the same origin point.

And we have identified certain collections, certain populations of stars that even though they're distributed around the galaxy, they came from a common origin point, and they came from a common origin point that was outside the Milky Way. How do we know this? Well, one, they have ages, metallicities, properties that are well outside the average for a typical Milky Way star, and their orbital parameters, how they orbit around the center, what the ellipticity, how elongated their orbit is, the inclination, what the angle is relative to everybody else. There's no way that they could have been born inside of the Milky Way and have that kind of orbit. They had to come from somewhere else.

The most famous of these collections is known as the I'm not making this up. Known as the Gaia sausage. The Gaia part comes from the Gaia European Space Agency Telescope that's mapped nearly 2,000,000,000 stars by now and found it. The sausage comes from it when you think Gaia sausage well, honestly, I can't tell you what you would normally think when you hear the phrase Gaia sausage. It's kind of a new one for all of us, but it's not a clump of stars.

You can't point to one place in the Milky Way and say, there's the Gaia sausage. Instead, when we make a plot of velocity, inward and outward, we call that radial velocity, and then velocity along the line of sight. There's this, like you get all sorts of stars with these kinds of velocities in different directions, and then you get this lump sitting out here. We call that the Gaia sausage. I'm sorry.

It's just it's just what we call it. The people who came up with it thought it was a good idea at the time. The Gaia Sausage is the remnant of a smaller galaxy that collided with the Milky Way and was totally destroyed by it 1000000000 of years ago. It's not obvious. It's not a clump anymore.

But all the stars in the Gaia sausage have similar metallicity, similar ages, similar orbits, similar velocities. Radially means similar velocity going inwards or outwards relative to the center of the galaxy, similar velocities, going across our line of sight. They stick out. This is a remnant of a galaxy that collided with ours 1000000000 of years ago, and that galaxy was destroyed. It was torn to shreds by the gravitational might of the Milky Way.

Its stars were dispersed throughout the Milky Way. We can only find the remnants through this detective work by going to galaxy ancestry.com and figuring out that we all share, like, the like, these stars share the same DNA sequence. But it was a galaxy. It was a dwarf galaxy, but still a galaxy that was destroyed and cannibalized and incorporated into the Milky Way. The dark matter component is just is just now mixed up with the Milky Way, indistinguishable.

There might be a Gaia sausage dark matter particle that was one time member of a of a different galaxy and then got mixed up with the Milky Way that's passing through you right now. There are other associations, other streams, other collections. Sometimes we can actually see the physical stream of stars. They're not fully dispersed. They're not fully mixed in with the Milky Way.

There's the archeron stream, the stick stream, the most one, specter, many more. These ghostly tenuous remnants of galaxies, some of them are still hanging on, still have some semblance of existence, and some that are totally, totally destroyed. So satisfaction scale, 5 out of 5. Maximum gore. Maximum destruction.

We are taking a small toy, smashing it into a larger one, and the pieces are so small now that it's hard to tell they were even a part of a toy to begin with. A separate toy altogether. That is that's peak satisfaction. There is one more option though, and that option is to wait. And folks, we need to take a brief break because I need to mention that this show is sponsored by BetterHelp, and I want to talk about comparison.

They say that comparison is the thief of joy, and that is so true. I have spent so many years in my life in a hypercompetitive environment. Academic research is beyond competitive. You're constantly comparing yourself. Oh, they're doing better research.

They're they're publishing more papers. They gave a better talk, or they have a better tie than me. It's like and and you just sink yourself lower and lower and lower because you constantly compare another person's best to your internal worst, and that is so damaging. It is one of the hardest things to overcome. So the way I went around that and I worked with my own therapist to develop these kinds of skills and tools was not to was to keep comparing, but not compare myself to others, but to compare myself to my past self.

What was old Paul doing 10 years ago, 5 years ago, last week? Am I a better person? Am I more kind and giving? Am I more, generous? Like like, am I more successful in my career?

Am I having more fun in my career, in my job, in my daily life than I was last week, a year ago, 10 years ago, 20 years ago? And if the answer is yes, then I'm winning. I'm winning the game of life, and the only competition is me. It's like when you play a racing game and you're fighting the ghost car of your previous best lap. That's that's the comparison that I like to make.

And therapy helped guide me to that conclusion. If you're thinking of starting therapy, give BetterHelp a try. It's all online, designed to be convenient, flexible, suited to your schedule. Fill out a brief questionnaire, and you're good to go. Stop comparing and start focusing with BetterHelp.

Visit betterhelp.com/spaceman today to get 10% off your 1st month. That's betterhelphelp dotcom/spaceman. Galaxies are survivors. Milky Way galaxy has been around for, what, 11000000000 years now. It's cannibalized and destroyed many other galaxies.

It will merge with Andromeda and it but it will still be an object after that. It'll just have a new name. Some people have proposed. I hate that. But we'll we'll save that for a later discussion.

We have a few 1000000000 years to decide that one. But it will still be a galaxy. It will still exist. But if we wait long enough I mean, the universe has only been around for 13.8000000000 years. What if we wait a 100000000000 years?

A trillion years. A 10000000000 years. Some measure of time that would put the make the current age of the universe peanuts, insignificant. Well, when you're willing to wait an extremely long time, interesting things start happening. Gravity starts to do its work.

You know, a galaxy is mostly empty space. But sometimes, stars get close to each other. They interact briefly. Maybe they collide, they destroy each other. Maybe they just pass close by and they swing past each other, and there's a a slingshot in exchange of energy.

One of those stars gets to stick around, and one of those stars gets ejected from the galaxy. We see this. We we call them hypervelocity stars. These are stars that have been ejected from their host galaxy. It happens all the time.

It's it's rare, but it happens. Give yourself a 10000000000 years. It happens to a lot. Eventually, all stars in a galaxy will either be scattered away or scattered into the supermassive black hole at the center. As for everything else like planets, well with enough time they will just dissolve through quantum mechanical tunneling.

Eventually, all bound structures will evaporate given timescales of 100 of trillions of years. All microscopic objects just dissolve, evaporate, expand away from each other, get caught up in the expansion of the universe. And that is a sure fired way to destroy a galaxy. As a satisfaction scale, I'll put that 4 out of 5. It's incredibly slow, incredibly agonizing, but you do completely and totally destroy any semblance of a galaxy.

Like, even the Gaia Sausage still maintains some of an identity. They've been they've the Gaia Sausage has immigrated into the Milky Way. They've assimilated. They've picked up all the culture and habits and norms. They celebrate the same holidays as all the the Milky Way natives, but they still have that genetic heritage that that gives them away, That signals that they came from somewhere else.

But, if you wait long enough, all distinction is gone. All the stars are there ejected or consumed. Even the individual particles that make up planets and asteroids and comets and all that dissolve, float away. Even black holes evaporate after something like 10 to a 100 years. Hawking radiation will guarantee that the black holes evaporate, and there's nothing left.

And so the only way to completely and totally eradicate a galaxy, to erase it from existence, is to wait an extremely long time. But if we're channeling our inner 8 year old, that's way too much time. Hence, the satisfaction scale of 4 out of 5. My favorite way, the winner for me is to smash a small galaxy against a larger galaxy. That that would be most satisfying to my inner 8 year old.

Thank you to Michael, c on email, and Steven d on Facebook for the questions that led to today's episode, and thank you to all my Patreon contributors. All of you are doing so much amazing things. Every contribution counts. All of it supports this show. All of it keeps this show going.

I can't thank you enough. I would like to thank my top contributors this month. They are Justin g, Chris l, Hiberto m, Duncan m, Corey d, Stargazer, Robert b, Tom g, Nyla, Sam r, John s, Joshua Scott m, Rob h, Scott m, Lewis m, John w, Alexis Gilbert m, Rob w, Dennis a, Jules r, Mike g, Jim l, Scott, j, David s, William w, Scott r, bbjj108, Heather, Mike s, Michelle r, Pete h, Steve s, Wattwatbird, Lisa r, and Koozie. Thank you so much everyone for your contributions. Thank you for the amazing questions that you're always sending me.

Keep them coming. That's askaspaceman.comoraskaspaceman@gmail.com. Send me those questions. I'll put them on the list. Someday, hopefully, before the milky way dissolves, I will get to it.

And I will see you next time for more complete knowledge of time and space.

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