Is The Milky Way On A Collision Course?
12:44 Share

This message comes from ShipBob. For brands scaling from hundreds of millions to billions of dollars in sales, there's ShipBob Plus, a fulfillment solution tailored to your ever-scaling needs. Discounted shipping and custom reporting. Go to ShipBob.com slash plus for a free quote. You're listening to Shortwave from NPR.

Hi, Short Wavers. Emily Kwong here with my co-host and our resident astrophysicist, Regina Barber. Hey, Gina. Hey, Em. Okay, we are talking about, am I reading this right, neighbors? That's right, Em. Are you ready for an astronomy quiz about neighbors? As a PBS kid raised on Mr. Rogers, yes, I love neighbors. Let's go. Okay, so which planets are the nearest neighbors to Earth? Mars and Venus. Yes. Okay, let's zoom out past our solar system. Okay.

Okay. Which star is the closest to our sun? I don't know. What is it? Proxima Centauri. Let's keep zooming out, right? We got our solar system. We got our stars. Let's go beyond the galaxy. We even live in the Milky Way. What galaxy is our closest neighbor to the Milky Way? Okay. I was paying attention to this in astronomy class in college. Andromeda. Good. Yes. Yes.

So Andromeda is the nearest big galaxy to us, our nearest like big galaxy neighbor. And for the past hundred years, scientists have thought the Andromeda galaxy, its destiny was to crash into us.

I think that's why I remember it, because the professor who taught this class described the Milky Way and Andromeda as orbiting each other almost in a lover's dance of doom. They're just drawn to each other, but to their peril. I mean, we'll talk about if it's doom or not, but yeah, they're more than neighbors, right? They're gravitationally bound, and scientists have long thought that over billions of years, they would get close enough to each other that that gravitational attraction would

pull them into each other and they'd combine into this like huge galaxy. Okay. As it turns out, these are pretty common phenomenas in the early universe. When the universe is young and small and hot, it's

These mergers are bound to happen. It's like young people in the club. They're just like, they gotta be close. True. This is something that actually makes these massive, massive galaxies that we see today. That's Arpit Arora. He's a computational astrophysicist at the University of Washington in Seattle. And he creates models of galaxies. Cool. Yeah. He explained that the Milky Way has collided with smaller galaxies in the past. And... Our current understanding is that

It collided with three major big, well, somewhat big galaxies about eight to nine billion years ago.

And then more recently, it collided with two smaller dwarf galaxies that we call the LMC and then also the Sagittarius galaxy. Okay. And Andromeda's next, right? That's what I thought. That's what I was taught. It was like 100%. It's going to happen. Yeah. But there was this recent paper in Nature Astronomy, and it shows evidence that this Andromeda collision with our galaxy, the Milky Way, may not happen. Huh. This study claims that the odds now are more like a coin toss, 50-50.

So today on the show, we go on a galaxy quest. One of my favorite movies. Our pit Aurora and I get into like why galaxies merge, what would happen to our solar system if the Milky Way and Andromeda collide, and why the chances of that happening have changed. All that on this episode of Shortwave, the science podcast from NPR. ♪

This message comes from Meta AI. Meta AI is the personal AI to help you with whatever you need. Not only is Meta AI now an app, but it's also on the apps you already love. Facebook, Instagram, WhatsApp, and Messenger, plus the Ray-Ban Meta Glasses. It's easy to access wherever you are throughout the day. Experience Meta's newest AI that's tailored to you by downloading the Meta AI app.

Try the Meta AI app today on the Apple App Store and Google Play. This message comes from Dell Technologies. The new Dell AI PC with Intel Core Ultra helps fast forward through busy work, leaving more time for you time. Get a new Dell AI PC starting at $699.99 at dell.com slash AI dash PC.

This message comes from Mint Mobile. Mint Mobile took what's wrong with wireless and made it right. They offer premium wireless plans for less, and all plans include high-speed data, unlimited talk and text, and nationwide coverage. See for yourself at mintmobile.com slash switch. Okay, Gina, this is going to challenge a lot of what people think about Andromeda. So what did you learn from your conversation with Arpitz?

Yeah, I don't want to steal his thunder, so I'm going to let you listen to our conversation yourself. Okay. But first, let's start with a very important point about galaxies. They're not, like, solid. They're not billiard balls, like, hitting each other when they collide. Oh. They're mostly open space. Interesting. Just, like, moving through the universe? Yeah.

Well, yeah. I mean, they contain a lot, too, dispersed within that, like, open space. Like, the Milky Way has hundreds of billions of stars. So when we do have these collisions, the first thing I asked Arpit was, like, what's going to happen to all those stars? So imagine a boat driving through water, right?

You see these ripples trailing behind the boat. Wow. Something similar like that happens to the stars where, well, it depends on the collision type, but most of the smaller dwarf galaxies or satellite galaxies, when they fall in or when they collide with the Milky Way, they start forming these streams of stars which have very specific shapes and orbits.

Right. And galaxies, broadly speaking, have three important components in this context. Stars, the gas and dust that make stars, and dark matter. Researchers don't know what dark matter is, but they do know that it influences the gas and the streams of stars you'd mentioned. And that dark matter is also important to galaxy mergers. What's the role there? So what actually drives all these collisions in two galaxies is basically dark matter.

Dark matter in general is pretty spherical in shape. It's not purely spherical, but spherical-like in some ways. So now with two spheres now, while kind of like fluidy spheres collide, what they can do is form like a bigger sphere. So in a way, form a bigger galaxy. So in most of our current theories, that's how we actually form these massive structures that we see around us. So the galaxies keep on merging along with the dark matter content,

creating these more massive systems. Why were you and I taught that the Andromeda Galaxy would one day collide with the Milky Way? In almost certainty, like why did we think that was going to happen? I think the idea that the Andromeda was going to collide with the Milky Way is probably hundreds of years old. So what we measure as astronomers are two different motions or velocities. One of the motions is the motion towards us or away from us.

We measure this by something called the Doppler shift. And the second thing that we measure is how the galaxy or how anything moves across the sky. So there are two sorts of motion, right? One across the sky and second, if the thing is moving towards us or away from us. So I think

Our majority of our understanding of the Andromeda was that it's moving towards us, but it's not really moving across the sky too much. Which led to these simplistic theories that even that it's not really moving in the sky, but only towards us, it's bound to merge with us. And so I think the initial estimates were about four to five billion years ago.

So we have this paper, it's published in Nature Astronomy. It claims now that, you know, there's more of a 50-50 chance that the Andromeda Galaxy is going to run into us, the Milky Way Galaxy. Why is there this change now?

So first of all, whenever we measure some positions or some distances or some velocities or motion, we measure it with some uncertainty in it. So I think that is the first important point which leads to this chance because like I said that even tiniest of the deviations in these measurements over such a long distance in time can lead to widely drastic results.

But in a more new setting, and especially ever since the Gaia Space Telescope... Gaia is the satellite telescope from the European Space Agency, and the idea was that they wanted to study individual stars of our galaxy in detail. Right. We as astronomers or astrophysicists, also computational astrophysicists, we have actually started developing more complicated models of our own galaxy, of the Milky Way, for example.

And then we've also started applying that same knowledge to other galaxies. So Andromeda in this case. Okay. So what if we don't merge? What if Andromeda never actually really gets into our, you know, our sphere here, our bubble? What does that look like if we don't, if there is no merger? Like, does anything change? Well, not physically. We will just be in this cosmic dance forever and ever where...

Milky Way and Andromeda will keep on orbiting some common center of mass, essentially, for a very long time. Because they're part of something called the local group. They're all kind of gravitationally bound. Bound, yes. So we're not going to go away from each other. We might just be in this cosmic dance or orbit for a very long time.

Okay, we still might merge. So what could happen if Andromeda Galaxy does kind of ram into us? Well, first of all, the night sky will be unbelievably magical right now. Imagine this massive galaxy so close to us in some ways.

Yeah, and the stars might change. It would be amazing. We will see a lot more structure of the Andromeda itself. Right. Maybe to the future scientists, this is an opportunity to see a galaxy, another galaxy from so close. And naively for the galaxy, we can expect more stars forming because of the Andromeda merging in. Oh, because it would push on gas. It would bring its own gas in the system now that can...

Possibly for more stars. And in the end, this merger will take a very long time. It's not that it happens in five billion years. It will take millions and millions and maybe even billions of years to actually form this one massive clump in the end, which will basically be this massive elliptical galaxy.

Some people like to call it milky medra or milchromedra or something like that. And this is what you were talking about, those like fluidy spheres, right? When you have these two disk galaxies and they merge, it's going to turn into something like sphere-like. Yes. And that's an elliptical galaxy. Yeah. And I like how optimistic you are. You're like, humans would be around and they would see, you know, the syndromic galaxy very close. Yeah.

I mean, someone might be around. Who knows if it's humans or... I think we're going to be like human-alien hybrids, you know, in like five billion years. Fair enough. Arpit, thank you so much for talking to us today about galaxies. Yeah, definitely.

Gina, I love how every time you talk to a fellow astrophysicist, you steer it in a direction of, but this is proof of aliens, right? Like we, even if those aliens are we someday. Yeah, you know, like I like to leave the door open. What's wrong with that, you know? To our alien neighbors. Yes, exactly. It's all about neighbors. Gina, thanks so much for this. You're welcome. I love talking about galaxies.

Shortwavers, if you want to hear more Galaxy Quests with Gina, please follow Shortwave on the NPR app or whatever app you're using to listen to this show. Today's episode was produced by Rachel Carlson and it was edited by our showrunner, Rebecca Ramirez. Tyler Jones, check the facts. Jimmy Keeley was the audio engineer. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. And I'm Emily Kwong. Thank you for listening to Shortwave from NPR.

You can actually see Andromeda with your naked eye. Really? Yeah. Do you want me to show you how? Yeah. Okay. So you'll clear night. You know, it's the summer, so it's probably easier now. Okay. Get out of the city. Get away from the city lights. If you're in the northern hemisphere, that also very much helps. Find the constellation Cassiopeia. It's like this big W. Oh, right. Squint your eyes. It's of a lady hanging upside down. It is. Yeah. As punishment. Terrible story. Cool constellation, though. Easy constellation to find, actually. Yeah. Yeah.

This message comes from Meta.ai. The new Meta.ai app is built to get to know you, basing its responses on your preferences and interests. Get help anytime with Meta.ai, your personal AI. Download the Meta.ai app today. Now available on the Apple App Store and Google Play. This message comes from ShipBob. For e-commerce brands scaling from tens of millions to billions of dollars in sales, there's now ShipBob Plus, a

a growth-accelerating outsourced fulfillment solution that's tailored to your ambitious goals. Unlock discounted shipping rates, world-class SLAs, bespoke reporting, and direct access to specialized support teams at every level of operations, all while you leverage ShipBob's scale to save money and time. Go to ShipBob.com slash plus for a free quote.

This message comes from Bombas. Nearly 30% of marathoners end their race blistered. Bombas running socks are strategically cushioned to help say bye to blisters. Run to bombas.com slash NPR and use code NPR for 20% off your first purchase.