Will our Galaxy collide with its neighbour?

[Music] Hello and welcome to Radio Astronomy, the podcast from the makers of BBC Sky at Night magazine. You can subscribe to the print edition of the magazine by visiting skyatnightmagazine.com or to our digital edition by visiting iTunes or Google Play. [Music] In five billion years our Milky Way Galaxy will collide and merge with our neighbouring and dromony Galaxy. Or at least that's what we used to think. Astronomers have used two of our most powerful observatories to predict what will happen with more accuracy. So what is the likelihood of a collision? And who would win? I spoke to Alice Deeson at Durham University in the UK to find out. [Music] So I'm Professor Alice Deeson, I work at Durham University in the astronomy department. Thanks very much for coming on the podcast, Alice. It's great to have you on. Thanks, I'm happy to be here. Today we're talking about colliding galaxies. Am I come as a surprise to some people to understand that galaxies actually collide? They're not these static things in the universe. What's going on here? Why do galaxies collide? Well, the sure answer is gravity. So galaxies collide fairly often and it's all really dictated by the force of gravity. So something that has mass has gravity and the more massive something is, the more gravity it has. So things tend to fall together and merge to form bigger things. So does this mean that galaxies are actually moving through space? They're actually moving through the universe? They are. I mean actually that whole universe is expanding, but there are places where there's so much mass that those things kind of group together to form bigger things. So our own galaxy, for example, is in what we call a local group and it's got other galaxy systems around it and these are all gravitationally attracted to each other. Amazing. So what actually happens when two galaxies collide? Talk us through the process. Well, I'm going to be honest, I'm not a big fan of the word collide because it's not really what happens. It's more like what I think merger is probably a softer word because these things like the stars and galaxies when they merge don't actually hit each other. They're not like snookables. They actually gravitationally interact. So a collision is often really a gravitational collision. So when two galaxies merge they essentially redistribute the stars and that matter around them. So it's a gentle process that I think collision makes you think and that's because stars are actually very far apart. Most galaxies are actually pretty empty. There's a lot of space between stars. So it's very, very unlikely that two stars hit each other but they can gravitationally interact. And we know sort of that there are these supermassive black holes at the centers of galaxies. Has there been any instance where we've observed these colliding or do we know that these black holes could actually collide? So actually, yeah, the black holes are in the very central region of the galaxies are particularly interesting because what we think happens is that when two galaxies merge those two black holes actually merge together to form an even bigger black hole. And that event itself can even cause something called what we call an AGM which is active, galactic nuclei where the black hole actually shines. So there's so much energy involved but actually we get a lot of light coming from that. And that can have a big trigger on the new galaxy. The kind of leftover remnant galaxy which can trigger for example more stars forming and it's one of the more energetic events that can happen when galaxies collide. Yeah, that notion of sort of re-igniting points of star formation in galaxy emerges is really interesting because you would think that a galaxy merger would be mostly destructive but it's in why does a galaxy merger re-ignite star formation for example? Well, that's really to do with the gas because I said like stars kind of gravitationally interact, dark matter only interacts gravitationally so just kind of moves around. The gas and gas getting compressed is what really caused these these violent bouts of star formation. So you're gonna imagine like pushing this gas together and that's what causes these events to happen and agents actually surrounding gas if you like feeding the central back hole. And probably the most famous galaxy collision that you know, as sort of in the popular mindset is one that we're talking about today and this is this is actually our own galaxy which is going to well as far as I knew it was pretty much a dead certainty that our Milky Way galaxy would merge with our neighbouring measure galaxy, the Andromeda galaxy. What was the sort of time skill that that was supposed to happen? It wasn't going to happen anytime soon, was it? No, no time soon. So the kind of previous feeling was or understanding within the astronomy community was that we were inevitably going to merge with our neighbour Andromeda and that time scale is about five billion years. So yes, no time soon but it was considered definitely an inevitable event but what we found in our more recent work is using new measurements and actually taking into account uncertainties in these measurements that actually it's more like a 50/50 chance that this will happen and even if it does maybe not in the next 10 billion years and certainly it's not inevitable to happen in five billion years. How is it possible to predict something like that? Well the first thing is the data we use so as I said the main thing that causes galaxies to merge is gravity so that's the kind of main physics that helps understand this. So we need to know how far away things are, how massive they are and how much they're moving and these are really the only ingredients we need in order to predict what happens between these galaxies. I make that sound very easy, it's actually very hard to know these things and Andromeda is very far away and it's difficult to have precise measurements for how fast things are moving but there are exquisite now measurements for these things but they do still have uncertainties and because we don't know exactly how fast things are moving or exactly how massive they are it means that there's a small amount of wiggle room that can in some cases mean that it's a near miss they don't actually hit each other and in some cases they do. The two sort of observatories that are most prominently mentioned in the study are Gaia and Hubble. Everyone will have heard of Hubble, that'll be a telescope but Gaia is a really interesting mission isn't it? What is that mission, what was that mission doing and how did that help you to refine this? So Gaia is a more recent mission so the Gaia Space Telescope it's a telescope about in space and one nice thing about Gaia particularly in my own personal view because I study the Milky Way is that it was designed to study our galaxy which we don't have many missions that do that. Hubble for example looks at lots of different galaxies across cosmic time but Gaia is built specifically for us and it essentially maps the motions of significant fraction of stars in our galaxy to exquisite detail so it tells you how fast they're moving and it's been our way to create if you like a map of our galaxy and what the stars are doing within it and we've never had that before. So it's completely revolutionized Milky Way research in the last so it launched in about 2016 but we got our first data in about 2018 so it's been going for some time now and it's completely changed our view of the galaxy and the way Gaia contributes to this research is it helps us get those movements of far away things. Oh see yeah one of the really interesting things about Gaia is it's sort of it's giving us this view as if we were like looking at our galaxy from afar doesn't that? Yeah and even though obviously it's still the telescope is in our galaxy it's just outside the earth but it gives what we call like a 3D dimensional view of what the galaxy is doing. Before that we were very much restricted to motions in like one dimension or positions on the sky but not how far away things are or how they're moving across the sky. And how did Hubble help with the study? So Hubble is particularly interesting because it's got such a high resolution camera and because it's been going for so long actually it's been one of the great things we can do with is we take a picture of something 10 years ago and then we take a same picture of that same thing 10 years later and everything slightly moved in that time because things are moving. It's a very very small change but the longer time baseline we have the more accurate we can do that. So one of the nice things that Hubble does is we use data from a long time ago with very new data to give us a very accurate measurement of how fast things are moving on the sky. And are there any other observatories that you might use to further refine? There's definitely many coming up in the future so Hubble's been it's been going for a long time but it's coming to the end of its life so you've probably heard of JWST that's almost the successor to Hubble another space mission which has got many many science aims but it is also got very good resolution and we can even combine JWST images with Hubble ones to look at the motions of objects in space too. So I mentioned that time baseline and so it means that things we did we're doing 20 years ago are still quite valuable to us. Amazing. Let's imagine that like it did actually happen and say it happened in like five million years time. One of our sewer system even be like then and if there was anyone on earth would we notice that we were in a galaxy merger? It's a good question and I think that's where you know for us in particular if our galaxy did merge with Android we're in five billion years in terms of what would happen to our solar system we may not be impacted that much probably the most that would happen is that our sun would get nudged a little bit off its orbit so we may go to a slightly different part of the galaxy but it's unlikely for anything more drastic than that to happen but I think the sun is particularly important here because that's our life force and unfortunately in five billion years I don't think the sun is going to be helping us that much in fact because it will have evolved to be a red giant it may even have grown large enough to have engulfed the earth by then so I think we have bigger the problems to worry about rather than Andromeda coming and merging with our galaxy. And also presumably the Andromeda guy is being much bigger than the Milky Way. Preservally, if you were to say he would win and he would lose, Andromeda would win. We think so, but that's actually an interesting point, part of the uncertainty in our measurements in terms of if these things are going to emerge is that we don't know the masses of the Milky Orlander and Dromeda very well. In some studies it's been suggested that the Milky Ways, the bigger one, and some it's been Andromeda. I think there's more that suggests it's Andromeda, but it's not yet certain because it's quite difficult to measure the total mass of these things. So at this point I'd probably bet on it being Andromeda being bigger, but who knows that could change. And the other really interesting thing about like Galaxy Emergist is that whenever you see those images of them actually happening and there's sort of two galaxies almost like warping into one another. So it's like the beautiful intricate spiral structures of the spiral galaxies gets completely messed up. Billions of years from the future will that eventually settle down and they'll once again get a spiral structure and you won't even know that they were ever emerged? I think the biggest thing that happens particularly to Disguise, it's very hard to, if you have a massive merger to retain that. So you said these lovely spiral arms, they're the things that mainly get messed up by these big mergers. And what's more likely to happen is you got a bigger liptical galaxy, so something a bit more spherical on the sky probably not as pretty in a picture. But I think it will settle down the sense that everything kind of redistributes. So it won't be as messy. It will turn into something a bit more structured. And the stars will have a more structured orbits. And we look at a lot of elliptical galaxies in our universe and we would eventually just become just like those. And you mentioned at the start that we are part of a sort of gravitationally-bound collection of galaxies called the local group. Do we know or did the study look at sort of further interactions within the group? Me sort of other galaxies that might collide or? It's a good point. First I should say that part of the collision between Andromeda and Milky Way does depend on other massive things. So we have our own massive satellite called the LMC, the Large Mageonic Cloud. You can actually see with your own eye if you're in the right part of the world. Unfortunately not this part of the world. And Andromeda has its own massive satellite too. And these slightly smaller, but still fairly massive objects also affect the orbits because they have mass. So anything with mass can affect the gravitational interaction, but the more massive they are, the more impactful. We actually found that the Large Mageonic Cloud actually nudges the orbit a little bit to make it less likely that they merge. So that was one of the interesting findings that we had. But there are other satellite galaxies or smaller galaxies in our system, which we haven't modeled. So one of the things that if we were going to do more refined work on this would be to study every single mass distribution that we can in terms of this interaction. Because sometimes when we get very, very accurate measurements, we need to do much more precise predictions and able to use them. And one of the things that we could do is to do a more detailed mapping of all of the mass distribution in the local group. Do you and the team intend to do any further work on this? So that's definitely on the cards, and again that will move beyond the realm of just being able to use Newtonian gravity and write. Sometimes even write down equations, I think this will much more need large supercomputers and what we call simulations. So we should use computer simulations to help us when we need a lot more help if we want to include all those different elements. So we'll use a lot of computing time, but it definitely is on the cards. Fantastic. Yeah, it's a really interesting topic and it was so interesting when that study came through because, as I said, as far as I was concerned, as a lay person to all this, it was considered like an inevitability, so it's been interesting to see how it's possible to refine the parameters of this. But as thanks very much for coming on the podcast and for talking through the study and giving us an insight into what might happen in the next 10 billion years. Great. Thanks for having me. Thank you for listening to this episode of the Radio Astronomy Podcasts and the makers of BBC Sky at Night magazine. For more of our podcasts, visit our website at skyatnightmagazine.com or head to Acast, iTunes or Spotify.