Short Wave - Is The Milky Way On A Collision Course?

Episode Date: July 2, 2025

The Andromeda galaxy lies just beyond (...OK, about 2.5 million light-years beyond) our galaxy, the Milky Way. For the past hundred years or so, scientists thought these galaxies existed in a long-term dance of doom — destined to crash into one another and combine into one big galactic soup. But today on the show, Regina and computational astrophysicist Arpit Arora explain why a recent paper out in the journal Nature Astronomy suggests this cosmic game of bumper cars may never come to a head at all. Interested in more space episodes? Email us your question at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hi, Shortwavers. 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. Am, 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. 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. So Andromeda is the nearest big galaxy to us, our nearest like, big galaxy neighbor. And for the past hundred years, a scientist have thought the Indromeda galaxy, its destiny was to crash into us. I think that's why I remember it because the, 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 just drawn to each other, but to their, 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 like over billions of years, they would get close enough to each other that that gravitational like 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 phenomena in the early universe. When the universe is young and small and hot, these mergers are bound to happen. It's like young people in the club. They're just like they got it. They got to be close. True. This is something that actually

makes these massive, massive galaxies that we see today. That's Arpit Aurora. 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. Oh.

And? Our current understanding is that it collided with three major, well, somewhat big galaxies, about 8 to 9 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 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 of 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. Arpit, Aurora, and I get into 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.

Okay, Gina, this is going to challenge a lot of what people think about Andromeda. So what did you learn from your conversation with Arpit? Yeah, I don't want to say. 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? Well, yeah. I mean, they contain a lot, too. Dispers 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 trading 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, like, 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 influences the gas and the streams of stars you'd mentioned.

And that dark matter is also important to galaxy mergers. Like, what's the role there? So what actually drives all these collisions into galaxies is basically dark matter. Dark Martyr 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 fluid these 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-martyrgy. the 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 Antrobeda 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. So I think the initial estimates were about four to five billion years.

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 Indromeda 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 and time can lead to widely drastic results. But more, you know, in a more new setting, and especially ever since, It's 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 astrophysicist, also computational astrophysicist, we've 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 over 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 orbits for a very long time. Okay. We still might merge.

So what could happen if Indromeda Oxy 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. But 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 will bring us on chaos in the system now that can possibly form more stars. And in the end of this merger will take a very long time. It's not that it happens in 5 billion years. It will take millions and millions and maybe even billions of years to actually form this one massive lump in the end, which will basically be this massive elliptical galaxy. Some people like to call it milk,

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 syndrome of galaxy very close.

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. Gene, I love how every time you talk to a fellow astrophysicist, you steered 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 Empire 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 Keely was the audio engineer. Beth Donovan is our senior director, and Colin Campbell's our senior vice president, of podcasting strategy. I'm Regina Barber. And I'm Emily Kwong. Thank you for listening to Shorewave

from NPR. You can actually see Indromeda 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 a 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.