Is The Milky Way On A Collision Course?

The Andromeda galaxy https://www.nasa.gov/image-article/nasas-chandra-shares-a-new-view-of-our-galactic-neighbor/ 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 https://www.npr.org/people/1082526815/regina-g-barber and computational astrophysicist Arpit Arora https://astro.washington.edu/people/arpit-arora explain why a recent paper out in the journal Nature Astronomy https://www.nature.com/articles/s41550-025-02563-1suggests this cosmic game of bumper cars may never come to a head at all. Interested in more space episodes? Email us your question at [email protected] mailto:[email protected]. Listen 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 http://plus.npr.org/shortwave. See pcm.adswizz.com https://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 https://www.npr.org/about-npr/179878450/privacy-policy

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:12Emily Kwong?

  You're listening to Short Wave from NPR. Hi, Short Wavers. Emily Kwong here with my co-host and our resident astrophysicist Regina Barber. Hey, Gina.

• 0:23Regina Barber?

  Hey, Em.

• 0:24Emily Kwong?

  OK, we are talking about, am I reading this right, neighbors?

• 0:27Regina Barber?

  That's right, Em. Are you ready for an astronomy quiz about neighbors?

• 0:32Emily Kwong?

  As a PBS kid raised on Mr. Rogers, yes, I love neighbors. Let's go.

• 0:35Regina Barber?

  OK, so which planets are the nearest neighbors to Earth?

• 0:39Emily Kwong?

  Uh, Mars and Venus.

• 0:42Regina Barber?

  Yes. OK, let's zoom out past our solar system.

• 0:46Emily Kwong?

  OK.

• 0:46Regina Barber?

  Which star is the closest to our sun?

• 0:49Emily Kwong?

  I don't know. What is it?

• 0:51Regina Barber?

  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?

• 1:03Emily Kwong?

  OK, I was paying attention to this in astronomy class in college. Andromeda.

• 1:07Regina Barber?

  Good, yes. So Andromeda is the nearest big galaxy to us, our nearest, like, big galaxy neighbor. And for the past 100 years, scientists have thought the Andromeda Galaxy, its destiny was to crash into us.

• 1:21Emily Kwong?

  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.

• 1:35Regina Barber?

  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.

• 1:53Emily Kwong?

  OK.

• 1:53Speaker 03

  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.

• 2:03Emily Kwong?

  It's like young people in the club. They're just like, they gotta, they gotta be close.

• 2:07Regina Barber?

  True.

• 2:08Speaker 03

  This is something that actually makes these massive, massive galaxies that we see today.

• 2:13Regina Barber?

  That's Arpit Arora. He's a computational astrophysicist at the University of Washington in Seattle, and he creates models of galaxies.

• 2:20Emily Kwong?

  Cool.

• 2:21Regina Barber?

  Yeah. He explained that the Milky Way has collided with smaller galaxies in the past.

• 2:26Emily Kwong?

  Oh.

• 2:27Regina Barber?

  And?

• 2:27Speaker 03

  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.

• 2:46Emily Kwong?

  OK, and Andromeda's next, right?

• 2:48Regina Barber?

  That's what I thought. That's what I was taught. It was like 100% it's going to happen.

• 2:52Emily Kwong?

  Yeah.

• 2:53Regina Barber?

  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.

• 3:03Emily Kwong?

  Huh.

• 3:03Regina Barber?

  This study claims that the odds now are more like a coin toss, 50/50.

• 3:11Emily Kwong?

  So today on the show, we go on a galaxy quest.

• 3:14Regina Barber?

  One of my favorite movies. Arpit 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?

• 3:26Emily Kwong?

  All that on this episode of Short Wave, the science podcast from NPR.

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• 5:09Emily Kwong?

  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?

• 5:17Regina Barber?

  Yeah, I don't want to steal his thunder, so I'm going to let you listen to our conversation yourself.

• 5:21Emily Kwong?

  Okay.

• 5:21Regina Barber?

  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. They're mostly open space.

• 5:32Emily Kwong?

  Interesting. Just like moving through the universe?

• 5:35Regina Barber?

  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?

• 5:50Speaker 03

  So imagine a boat driving through water, right? You see these ripples trailing behind the boat.

• 5:57Emily Kwong?

  Wow.

• 5:58Speaker 03

  Something similar like that happens to the stars where, well, it depends on the collision type, but most of the smaller dwarf galaxies that are 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.

• 6:19Regina Barber?

  Right, like, 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 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?

• 6:41Speaker 03

  So what actually drives all these collisions into 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 when two spheres now, well, 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 that galaxies keep on merging along with the dark matter content, creating these more massive systems.

• 7:22Regina Barber?

  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 gonna happen?

• 7:30Speaker 03

  I think the idea that the Andromeda was gonna 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 a Doppler shift. And then 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

• 8:12Speaker 03

  of the Andromeda was that it's moving towards us, but it's not really moving across the sky too much.

• 8:19Regina Barber?

  Hmm.

• 8:20Speaker 03

  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.

• 8:35Regina Barber?

  Mm hmm. 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?

• 8:50Speaker 03

  So first of all, whenever we measure some positions or some distances or some velocities or motion, we measure it with some uncertain DNA.

• 8:59Regina Barber?

  Yeah.

• 8:59Speaker 03

  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 more, you know, in a more new setting, and especially ever since the Gaia Space Telescope.

• 9:21Regina Barber?

  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.

• 9:30Speaker 03

  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.

• 9:49Regina Barber?

  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?

• 10:05Speaker 03

  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.

• 10:22Regina Barber?

  Because they're part of something called the Local Group. They're all kind of gravitationally bound.

• 10:26Speaker 03

  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.

• 10:37Regina Barber?

  Okay. We still might merge. So what could happen if Andromeda Galaxy does kind of ram into us?

• 10:44Speaker 03

  Well, first of all, the night sky will be unbelievably magical right now. Imagine this massive galaxy so close to us in some ways.

• 10:57Regina Barber?

  Yeah, and the stars might change.

• 11:00Speaker 03

  It would be amazing. We will see a lot more structure of the Andromeda itself.

• 11:05Regina Barber?

  Right.

• 11:06Speaker 03

  Maybe to the future scientists, this is an opportunity to see a galaxy, another galaxy from so close.

• 11:12Regina Barber?

  Hmm.

• 11:13Speaker 03

  And naively for the galaxy, we can expect more stars forming because of the Andromeda merging in.

• 11:20Regina Barber?

  Oh, because it would push on gas.

• 11:22Speaker 03

  It would bring us on gas in the system now that can possibly form more stars. And in the end of, I mean, this merger will take a very long time. It's not that it happens in five billion years. It will take like 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 milky meta or milchrometa or something like that.

• 11:56Regina Barber?

  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.

• 12:05Speaker 03

  Yes.

• 12:06Regina Barber?

  And that's an elliptical galaxy.

• 12:08Speaker 03

  Yeah.

• 12:09Regina Barber?

  And I like how optimistic you are. You're like, humans would be around and they would see, you know, the Andromeda galaxy very close.

• 12:18Speaker 03

  I mean, someone might be around. Who knows if it's humans or...

• 12:21Regina Barber?

  I think we're going to be like human alien hybrids, you know, in like five billion years.

• 12:27Speaker 03

  Fair enough.

• 12:27Regina Barber?

  Arpit, thank you so much for talking to us today about galaxies.

• 12:31Speaker 03

  Yeah, definitely.

• 12:33Emily Kwong?

  Gina, I love how every time you talk to a fellow astrophysicist, you steer in a direction of, but this is proof of aliens, right? Like we, even if those aliens are we someday.

• 12:44Regina Barber?

  Yeah, you know, like I like to leave the door open. What's wrong with that, you know?

• 12:49Emily Kwong?

  To our alien neighbors.

• 12:51Regina Barber?

  Yes, exactly. It's all about neighbors.

• 12:54Emily Kwong?

  Gina, thanks so much for this.

• 12:55Regina Barber?

  You're welcome. I love talking about galaxies.

• 13:03Emily Kwong?

  Short Wave is, if you want to hear more Galaxy Quests with Gina, please follow Short Wave on the NPR app or whatever app you're using to listen to the show.

• 13:11Regina Barber?

  Today's episode was produced by Rachel Carlson and it was edited by our showrunner, Rebecca Ramirez. Tyler Jones checked the facts. Jimmy Keeley was the audio engineer.

• 13:20Emily Kwong?

  Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy.

• 13:25Regina Barber?

  I'm Regina Barber.

• 13:26Emily Kwong?

  And I'm Emily Kwong. Thank you for listening to Short Wave from NPR.

• 13:43Regina Barber?

  You can actually see Andromeda with your naked eye.

• 13:46Emily Kwong?

  Really?

• 13:47Regina Barber?

  Yeah. Do you want me to show you how?

• 13:48Emily Kwong?

  Yeah.

• 13:48Regina Barber?

  Okay, so go clear night. You know, it's the summer, so it's probably easier now. 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.

• 14:02Emily Kwong?

  Oh, right.

• 14:03Regina Barber?

  And squint your eyes.

• 14:04Emily Kwong?

  It's of a lady hanging upside down.

• 14:06Regina Barber?

  It is, yeah, as punishment.

• 14:08Emily Kwong?

  Terrible story. Cool constellation, though.

• 14:10Regina Barber?

  Easy constellation to find, actually.

• 14:12Emily Kwong?

  Yeah.

• 14:13Speaker 04

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