What Happens When Two Galaxies Collide? In four billion years, the Milky Way and Andromeda galaxies will collide, according to a study in the Astrophysical Journal. Roeland van der Marel, an astronomer at the Space Telescope Science Institute, talks about how this discovery was made, and the fate of our solar system.

What Happens When Two Galaxies Collide?

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This is SCIENCE FRIDAY. I'm Ira Flatow. If you look up in the sky at night, especially later this year, you can spot the Andromeda Galaxy, it's a small cloudy smudge in space. It's our galaxy, the Milky Way's closest galactic neighbor, about two and a half million light years away.

But if you keep watching it, yeah, go ahead, over time, keep watching it night after night, ooh, it's headed straight to us, but you're going to have to watch it for a long time because - you have to crane your neck for about four billion years before it smacks right into the Milky Way.

So how is this collision going to change our galaxy and the solar system that we live in for that matter? Will there be life on Earth four billion years from now? Here to talk about it is my guest, Roeland van der Marel. He is an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, and research appears in the latest issue of the Astrophysical Journal. Welcome to SCIENCE FRIDAY.

ROELAND VAN DER MAREL: Good afternoon, Ira.

FLATOW: Is this going to be a violent collision?

MAREL: Yeah, as these things go, it probably will be. Our Milky Way and the Andromeda Galaxy are roughly equally big. Both have lots of stars in them. Each galaxy has, you know, over, you know, 100 billion stars in them. These galaxies are going to run into each other, and this is going to totally reshape them.

So in that sense it's going to be very violent. It's not going to be violent in the sense that individual stars will run into each other because these galaxies have a lot of empty space in them, and most of the individual stars will actually pass by each other.

So our sun will survive, but our galaxy as a whole will be totally reshaped.

FLATOW: So we actually have a picture on our website, at sciencefriday.com, of what it might look like. It's a NASA drawing. Now, aren't there black holes at the centers of galaxies? I mean, don't we know one's at the Milky Way's? Is there one in Andromeda also?

MAREL: Yeah, we actually know fairly certain that both of these galaxies have a black hole in their center. That's not particularly unusual. We think many or maybe all galaxies have such a super-massive black hole in their center. The one in the center of our Milky Way is about a million times as massive as our sun. The one in the center of Andromeda is about 10 million times as massive as our sun.

When these galaxies run into each other, ultimately the black holes will slowly spiral in towards each other, and they will merge, and this will be a particularly interesting event. It'll lead to the formation of one bigger black hole, and it'll also produce what is known as gravitational radiation, gravitational waves, which astronomers have yet to detect, and physicists, but we are fairly certain exists based on Einstein's Theory of Relativity.

FLATOW: And how fast is the Andromeda headed toward us?

MAREL: Well, we've known for about 100 years that it's coming closer to us at a rate of about 250,000 miles an hour. Andromeda was actually the first galaxy in the universe for which it was possible to measure its relative motion, and interestingly, it was found to come towards us.

We live in an expanding universe, and almost all other galaxies are moving away from us. And the reason it's moving towards us is that the Milky Way and Andromeda are particularly close and particularly massive, so even though they started out expanding away from each other when the universe was young, they've actually pulled on each other hard enough that they're falling back together.

Now, what astronomers didn't know is what the sideways motion is of Andromeda with respect to us. So the velocity, you really need to know in three dimensions. The fact that the distance is getting shorter with time doesn't necessarily mean Andromeda is heading straight for us. There might be a sideways component to the motion, and that's what we've now been able to measure for the very first time.

FLATOW: That it's not going to slip by, it's going to hit us head-on.

MAREL: Indeed. Astronomers have certainly speculated it might be hitting us head-on, but this was never known. If the velocity had had a significant sideways component, it would have been a more elliptical orbit, and the galaxies would have missed each other and not directly collided. As it turns out from our measurement, we find that to within the accuracy with which we can measure this, Andromeda is actually heading straight in our direction.

So what that means is that, you know, there will be either very close passage or a completely direct hit some four billion years from now. And then six billion years from now, the galaxies will merge together to form one new galaxy, which will have a very different shape than the galaxies currently have.

FLATOW: And what shape might that be?

MAREL: Well, so today, our Milky Way is a flat, rotating system, sort of like a Frisbee. It's spinning, and the reason the Milky Way appears to us on the sky as a luminous band of light is that we sort of see a luminous system from the side. It's a flat system, we see it from the side, it appears to us as a band.

Our galaxy is also a spiral galaxy, and all of this is very similar to Andromeda. Andromeda is also a spiral galaxy, it's a flat, spinning disk. When these galaxies hit each other, the stars will really be jostled about, and their orbits will be randomized. So this creates a new galaxy, which has a more three-dimensional shape.

It'll be what we call an elliptical galaxy, and the individual stars will not simply be rotating around in circles anymore. They will be on a wide range of possible orbits, many quite elliptical. So this galaxy will look fairly different than the galaxies look today.

FLATOW: A little more chaotic?

MAREL: Well, actually on the sky, we know of other elliptical galaxies in the universe around us, and we actually think this is one mechanism for forming such galaxies. On the sky, these things look fairly smooth. It just looks like a smooth sort of clump of light, sort of American football-shaped.

So they don't look chaotic. The orbits certainly are more chaotic. Our Milky Way is sort of like a bigger version of our solar system. Things are just going around nicely in circles, everything goes in the same direction. So in that sense it's very ordered. An elliptical galaxy, you know, some stars rotate one way, other stars rotate another way. Some stars are in very elliptical orbits. So there's a much bigger variety of things.

But if you look at it from the outside, it actually looks much smoother than a spiral galaxy does, which looks fairly patchy and has these spiral patterns.

FLATOW: Let's go to Gary(ph) in Washington. Hi, Gary, welcome to SCIENCE FRIDAY.

GARY: Hi, I'm calling from the District of Columbia, and I wanted to know: Is it not true that the Milky Way galaxy, as currently constructed, is a product of a collision between two galaxies that previously occurred? And I wanted to know if the guest could talk about our local group and the Andromeda Supercluster.

MAREL: Sure, so our local universe is dominated by two big galaxies, that's our Milky Way and Andromeda. There are many smaller galaxies. So our local group has about 50 smaller galaxies. One or two of those are about 10 percent the mass of these two big galaxies. Most of them are less than one percent. So most of them, sort of, in terms of gravity, don't contribute much. But, you know, they're there.

What happens, we think, how - one of the interesting questions in astronomy today is how galaxies form. And astronomers have come to realize that the way galaxies form is by, you know, slowly growing, by merging with each other and getting bigger.

Now what usually happens is that you have a big galaxy that runs into a smaller galaxy. So we actually know that our Milky Way is currently in the process of creating one or two of these smaller galaxies, and we see this happening around us. The same is happening to Andromeda. It has some evidence that it's been, you know, eating or swallowing one or two smaller galaxies.

But when that happens, it doesn't really drastically reshape the galaxy. So our Milky Way has had such events before, but since the smaller galaxies it's swallowing up don't weigh as much, it doesn't really change the structure very much.

We actually know that our Milky Way has never in its lifetime run into something that was really the same size as itself because if that had happened, the spiral disk of our galaxy wouldn't exist anymore. If you run into another galaxy that's about as big as you are, it tends to destroy the disk of the galaxy, and it tends to make you more three-dimensional, as I described.

So from the fact that we don't see this today, we know that the galaxy has never had a major run-in. We know it's had several minor run-ins, but this is going to be the first time in the lifetime of our galaxy that it's really going to have, you know, such a major collision.

We do know that this happens to other galaxies in the universe around us. We see this happening to other galaxies sometimes. But, you know, this is going to be us. It's going to be our galaxy. It's going to be, you know, our sun, our planet. So this is really special in that sense.

FLATOW: Jerry(ph) in Mitchell, South Dakota. Hi, Jerry.

JERRY: Yeah, if that galaxy is coming at us at 250,000 miles an hour, why don't it just go right straight through and keep on going? Is there that much gravity that it can stop it going 250,000 miles an hour?

FLATOW: Good question.

MAREL: Yeah, interestingly there is. So it's going very fast, 250,000 miles an hour, that's the distance of the moon in one hour. On the other end, the Andromeda Galaxy is quite far away. It's two and a half million light years away. That's why it's going to take four billion years to get here.

As I mentioned, galaxies have a lot of empty space in them. So you're actually correct. When these galaxies hit each other, the individual stars will just keep going, and initially the galaxies will just come out on the other side and keep flying.

However, there is a process known as dynamical friction, which will slow them down sufficiently that they then fall back together, and when that happens, they completely merge.

FLATOW: What happens to the dark matter that's inside these galaxies?

MAREL: Well, so the dark matter, basically the same things happen as for the stars. So these galaxies consist of stars, that's how we see them. But we actually know that, at some level, the stars are just the tip of the iceberg. The stars only make up about 10 percent of the mass of each system, and 90 percent of the mass is in this matter called dark matter.

The dark matter is distributed in a more, you know, spherical distribution. When these galaxies hit each other, it's not only the stars that ultimately merge to form a new galaxy, the dark matter of these galaxies will also form, merge together. So you'll end up with an elliptical galaxy, which has, you know, a big halo of dark matter around it.

FLATOW: Let's go to John(ph) in Wilson, Wyoming. Hi, John.

JOHN: Yeah, I'm wondering if in 3.9 billion years, the Andromeda Galaxy is going to take over the night sky, make a spectacular sky or even be visible in the daytime.

FLATOW: Good question, yeah.

MAREL: That is a good question. When we publicized these results, we actually disseminated some animations of what the night sky might look like when this happens. And what we think will happen is that first, as Andromeda gets closer to us, it'll start, you know, look bigger on the sky.

When the galaxies actually get very close together, our Milky Way will start getting distorted, but moreover the gas in both galaxies will start getting compressed, and this actually triggers the formation of new stars.

So when this hits, or very close passage happens, about four billion years from now, we expect that, you know, the sky will light up with new regions of bright star formation. Over time, though, this will die out because the gas tends to get expelled from the galaxies as supernovae go off that blow out the gas. So over time, there won't be new star formation, but in four billion years from now, the sky will look truly spectacular if anyone is still around to see it.

And if you look around a little bit on the Internet for the results of this work, you might actually find the very nice visualizations that we've put out to support this.

FLATOW: We have that on our website, at sciencefriday.com, if you want to see a nice view of the sky. Quick question, the sun will still be shining here at that time?

MAREL: Yes, at the time Andromeda arrives, our sun will still be, you know, a regular star, shining happily. Our sun was born about five billion years ago, and it has nuclear fuel left for about another six billion years. At that point, things will change drastically, but when Andromeda arrives here, our sun will still be very similar to what it is today.

FLATOW: All right, Dr. van der Marel, thank you very much.

MAREL: Thank you, Ira.

FLATOW: Roeland van der Marel is an astronomer at the Space Telescope Science Institute in Baltimore. We're going to take a break, and we're going to still talk about dark energy, not only dark matter, and some spooky things still happening with that. So stay with us. We'll be right back after this break.


FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

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