Is The Milky Way On A Collision Course?

Summary

Short Wave Podcast Summary: "Is The Milky Way On A Collision Course?"

Introduction In the July 2, 2025 episode of NPR’s Short Wave, hosts Emily Kwong and Regina Barber delve into one of the most captivating topics in astrophysics: the impending relationship between our galaxy, the Milky Way, and its nearest large neighbor, the Andromeda Galaxy. This episode explores the longstanding belief in an unavoidable galactic collision and introduces groundbreaking research that challenges this notion.

Exploring Galactic Neighbors The episode begins with an engaging astronomy quiz segment where Emily Kwong and Regina Barber test their knowledge on celestial neighbors. Regina Barber, the show’s resident astrophysicist, initiates the discussion by highlighting the importance of understanding our galactic surroundings.

“Andromeda is the nearest big galaxy to us, our nearest, like, big galaxy neighbor.” – Regina Barber [01:14]

This sets the stage for a deeper exploration of the relationship between the Milky Way and Andromeda, emphasizing the significance of this cosmic partnership.

Historical Perspective on the Milky Way-Andromeda Collision Historically, astronomers have predicted that the Milky Way and Andromeda galaxies are on a collision course, destined to merge in approximately 4 to 5 billion years. Regina likens this predicted event to a “lover's dance of doom,” capturing the inevitable yet dramatic nature of the impending merger.

“...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 attraction would pull them into each other and they'd combine into this, like, huge galaxy.” – Regina Barber [01:43]

Recent Findings Challenging the Collision Theory A pivotal moment in the episode is the introduction of a recent study published in Nature Astronomy, which suggests that the probability of the Milky Way colliding with Andromeda is now a 50-50 proposition, significantly lower than previously thought.

“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.” – Regina Barber [03:00]

Emily Kwong underscores the impact of this study, hinting at its potential to reshape our understanding of galactic dynamics.

“It's gonna challenge a lot of what people think about Andromeda.” – Emily Kwong [05:06]

Insights from Computational Astrophysicist Arpit Arora The conversation deepens with insights from Arpit Arora, a computational astrophysicist at the University of Washington, who specializes in galaxy modeling. Arpit explains the complexities involved in predicting galactic movements and the role of dark matter in these interactions.

“What actually drives all these collisions into galaxies is basically dark matter... So in most of our current theories, that's how we actually form these massive structures that we see around us.” – Arpit Arora [06:36]

He elaborates on how dark matter influences the behavior and eventual merger of galaxies, highlighting its fundamental role in cosmic evolution.

Impact of the Gaia Space Telescope The Gaia Space Telescope, a mission by the European Agency, has been instrumental in providing more precise measurements of stellar positions and velocities. These enhanced data capabilities have introduced greater uncertainties into our predictions, thereby affecting the estimated likelihood of the Milky Way-Andromeda collision.

“Ever since the Gaia Space Telescope... we've also 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.” – Arpit Arora [09:25]

Possible Scenarios: Collision vs. No Collision The episode explores two primary scenarios based on whether the Milky Way and Andromeda collide:

1. Collision and Merger:

   • A spectacular night sky with Andromeda appearing much closer.
   • Increased star formation due to the gravitational interactions pulling gas and dust, leading to new celestial bodies.
   • The eventual formation of a massive elliptical galaxy, humorously named “Milky Medra” or “Milcromedra.”

   “We will see a lot more structure of the Andromeda itself... in the end of this merger will take, like, millions and millions and billions, maybe even billions of years to actually form this one massive clump in the end.” – Arpit Arora [10:55]

2. No Collision:

   • The Milky Way and Andromeda continue their cosmic dance, orbiting a common center of mass indefinitely.

   “We might just be in this cosmic dance or orbit for a very long time.” – Regina Barber [10:17]

Long-Term Implications for Humanity Regina Barber and Emily Kwong mused about the long-term future, contemplating the existence of humans or human-alien hybrids witnessing such grand cosmic events.

“I think we're going to be, like, human alien hybrids, you know, in like, 5 billion years.” – Regina Barber [12:17]

Observing Andromeda from Earth In a delightful conclusion, Regina Barber offers a practical guide for listeners eager to observe the Andromeda Galaxy with the naked eye. She points them towards locating the constellation Cassiopeia, providing a simple method to spot Andromeda on a clear night.

“You can actually see Andromeda with your naked eye... Find the constellation Cassiopeia. It's like this big W.” – Regina Barber [13:38]

Conclusion The episode wraps up by encouraging listeners to engage further with the topic through the NPR app and acknowledging the contributions of the production team. The discussion leaves the audience with a sense of wonder about our place in the cosmos and the dynamic future of our galactic neighborhood.

Notable Quotes

• “Andromeda is the nearest big galaxy to us, our nearest, like, big galaxy neighbor.” – Regina Barber [01:14]
• “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.” – Regina Barber [03:00]
• “What actually drives all these collisions into galaxies is basically dark matter... So in most of our current theories, that's how we actually form these massive structures that we see around us.” – Arpit Arora [06:36]
• “We might just be in this cosmic dance or orbit for a very long time.” – Regina Barber [10:17]
• “You can actually see Andromeda with your naked eye.” – Regina Barber [13:38]

Final Thoughts This episode of Short Wave masterfully balances expert insights with accessible explanations, making complex astrophysical concepts understandable to a broad audience. By challenging long-held assumptions with new research, Emily Kwong and Regina Barber not only inform but also inspire curiosity about the vast and ever-changing universe we inhabit.

Transcript

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[Emily Kwong]

You're listening to shortwave from npr. Hi, shortwavers. Emily Kwong here with my co host and our resident astrophysicist, Regina Barber. Hey, Gina.

[Regina Barber]

Hey, Em.

[Emily Kwong]

Okay, we are talking about. Am I reading this right?

[Regina Barber]

Neighbors. That's right. Em, are you ready for an astronomy quiz about Neighbors?

[Emily Kwong]

As a PBS kid raised on Mr. Rogers. Yes. I love neighbors.

[Regina Barber]

Let's go. Okay, so which planets are the nearest.

[Emily Kwong]

Neighbors to Earth, Mars and Venus?

[Regina Barber]

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

[Emily Kwong]

Okay.

[Regina Barber]

Which star is the closest to our sun?

[Emily Kwong]

I don't know. What is it?

[Regina 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?

[Emily Kwong]

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

[Regina Barber]

Good. 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. Was to crash into us.

[Emily 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. A lover's dance of doom. They're just drawn to each other, but to. To their. To their peril.

[Regina Barber]

I mean, we'll talk about if it's doom or not, but, like, 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. That gravitational, like, attraction would pull them into each other and they'd combine into this, like, huge galaxy.

[Emily Kwong]

Okay.

[Arpit Arora]

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.

[Emily Kwong]

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

[Regina Barber]

True.

[Arpit Arora]

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

[Regina Barber]

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

[Emily Kwong]

Cool.

[Regina Barber]

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

[Emily Kwong]

Oh.

[Arpit Arora]

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.

[Emily Kwong]

Okay, and Andromeda's next, right?

[Regina Barber]

That's what I thought. That's what I was taught. It was like, 100% it's gonna happen.

[Emily Kwong]

Yeah.

[Regina 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.

[Emily Kwong]

Huh.

[Regina Barber]

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

[Emily Kwong]

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

[Regina Barber]

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.

[Emily Kwong]

All that. On this episode of Shortwave, the science podcast from N.

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[Emily Kwong]

Is gonna challenge a lot of what people think about Andromeda. So what did you learn from your conversation with Arpit?

[Regina Barber]

Yeah, I don't wanna steal his thunder, so I'm gonna 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. They're mostly open space.

[Emily Kwong]

Interesting. Just like moving through the universe.

[Regina Barber]

Well, yeah, I mean, they contain a lot too dispersed within that. 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.

[Arpit Arora]

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

[Emily Kwong]

Wow.

[Arpit Arora]

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.

[Regina Barber]

Right? Like. 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 gather galaxy mergers. Like what, what's the role there?

[Arpit Arora]

So what, 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 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.

[Regina 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 going to happen?

[Arpit Arora]

I think the idea that the Antrobeta 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, one of the motions is the direct, 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 given 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 4 to 5 billion years.

[Regina Barber]

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?

[Arpit Arora]

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

[Regina Barber]

Yeah.

[Arpit Arora]

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

[Regina Barber]

Gaia is the satellite telescope from the European Agency. And the idea was that they wanted to study individual stars of our galaxy in detail.

[Arpit Arora]

Right. We, as astronomers or astrophysicists, also computational astrophysicists, 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.

[Regina 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?

[Arpit Arora]

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.

[Regina Barber]

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

[Arpit Arora]

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.

[Regina Barber]

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

[Arpit Arora]

Well, first of all, the night sky will be unbelievably magical. Right. Now, imagine this massive galaxy so close to us in some ways.

[Regina Barber]

Yeah. And the stars might change.

[Arpit Arora]

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

[Regina Barber]

Oh, because it would push on gas.

[Arpit Arora]

It was. It will bring its own gas 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, like, millions and millions and billions, 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 Milcromedra or something like that.

[Regina 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.

[Arpit Arora]

Yes.

[Regina Barber]

And that's an elliptical galaxy.

[Emily Kwong]

Yeah.

[Regina Barber]

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

[Arpit Arora]

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

[Regina Barber]

I think we're going to be, like, human alien hybrids, you know, in like, 5 billion years.

[Arpit Arora]

Fair enough.

[Regina Barber]

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

[Arpit Arora]

Yeah, definitely do.

[Emily Kwong]

You know, 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. We. Even if it. Even if those aliens are we someday.

[Regina Barber]

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

[Emily Kwong]

You know, to our alien neighbors.

[Regina Barber]

Yes, exactly. It's all about neighbors.

[Emily Kwong]

Gina, thanks so much for this.

[Regina Barber]

You're welcome. I love talking about galaxies.

[Emily Kwong]

Short Wavers. 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 this show.

[Regina 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.

[Emily Kwong]

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

[Regina Barber]

I'm Regina Barber.

[Emily Kwong]

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

[Regina Barber]

You can actually see Andromeda with your naked eye.

[Emily Kwong]

Really?

[Regina Barber]

Yeah. Do you want me to show you how?

[Emily Kwong]

Yeah.

[Regina Barber]

Okay. So go. Clear night. You know, it's the summer, so it's probably easier now. Okay. Get out of the city. Get 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.

[Emily Kwong]

It's of a lady hanging upside down.

[Regina Barber]

It is, yeah. As punishment.

[Emily Kwong]

Terrible story. Cool constellation, though.

[Regina Barber]

Easy constellation to find, actually.

[Emily Kwong]

Yeah.

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