 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the evolution of galaxies and especially how that works through mergers of galaxies and how galaxies have built up from smaller galaxies over time. So let's go ahead and look, first of all how can we observe changes in galaxies? Well just like when we looked at stars, the changes occur too slow to be able to observe even over a human lifetime or multiple human lifetimes. What we see as a galaxy today, we are going to see essentially exactly the same thing a hundred or even a thousand years from now. However, because light travels at a finite speed, that means we can see galaxies at various stages of their evolution. So a galaxy at a distance of 12 billion light years is seen not as it is today, but as it was when the universe was only 2 billion years old. And this helps us to study galactic evolution. So how can we study these galaxies? What do we look at? Well, we look at their spectra. And this can be difficult because the galaxies are faint. That makes them difficult. However, one thing we can observe when we look at galaxies such as the ones pictured here are the colors of the galaxies. Now remember what this tells us. A blue galaxy tells us there has been recent star formation. A red galaxy says there has not been recent star formation. So looking at the galaxies in the image, we can at least tell which of them have had star formation recently and which have not. The earliest generation of stars we now know that the heavy elements formed in the first billion years of the history of the universe. It did not take long for those early generations of stars to form heavier elements. How do we know? Well, even really distant galaxies show lines of heavy elements that were not created in the Big Bang. So how do galaxies evolve? How do they change over time? Well, what were the early galaxies like? How can we see what a galaxy was like billions of years ago? We can see them. We can actually see these galaxies. And what we find is that early galaxies do not look like today's galaxies. Meaning the nearby galaxies the ones that we see as they are closer to the current time. Galaxies in the very early, sorry galaxies very early in the history of the universe were smaller and bluer than galaxies that we see today. And they were also much more chaotic. There was a lot more distortion of galaxies due to collisions going on. So the Hubble classification was made based on the galaxies that we see today. Those are nearby within maybe a hundred million light years or a little more or something like that. And that was what the Hubble classification was based on. Earlier galaxies look quite different than this. So what do we learn about galaxy mergers? Galaxies combining together. And we now believe that mergers and interactions between galaxies are very important in the evolution of galaxies. This is the primary way that galaxies evolve over time. We talk about galaxies colliding. Why did we never talk about stars colliding when we looked at stars or even now? That's because stars do not collide. Galaxies are very large compared to the distances between them. Stars are incredibly tiny compared to the distances between them. If you imagine a decent sized room and have a dozen beach balls that could bounce around and never slow down, you can imagine that they would collide with each other. That would be comparable maybe to roughly the idea of what a galaxy might be. If you also had a dozen BBs that could somehow surround that room, again never stopping, the odds of them bumping into each other is very, very small by comparison. And by doing this we are certainly vastly understating the difference in size between the galaxies and the stars. And we also, as we show here, many instances of galactic collisions have now been observed. So here we see a few of those, but there are a lot of galaxies that are seen to collide and are distorted because of that. So what happens when galaxies collide? It is not like an ordinary collision. It takes hundreds of millions of years. As I said, the individual stars do not collide. However, gas clouds will collide which enhances star formation, giving us sometimes what we call a starburst galaxy with extra material forming around, here we see in the blue, extra material that has formed. And streams of material will, streams of material can be expelled as those gravitational fields interact. So this distorts the galaxies and gives us the title stream such as we looked at with our Milky Way. The shapes of the galaxies can be vastly distorted from what we're used to. So here we see a galaxy with a ring pattern around it possibly because of a splash almost when a galaxy passed right through another galaxy. So we see a lot of enhanced star formation in the blue and we see this ring of material around that could have been distorted by that collision. Now what else do galaxies do? We talk about galaxy collisions. We also talk about galactic cannibalism. So friction often causes these colliding galaxies to merge. This is Centaurus A which we've looked at previously which is two galaxies in the process of colliding. They will merge together to become a single even larger galaxies. So the way we look at it is large sized galaxies merge, large galaxies will cannibalize, will consume those smaller galaxies that exist. And we will see that some galaxies show multiple nuclei from these collisions where the galaxies have collided but here we see one example where the two nuclei are still separate and have not yet coalesced together. That will eventually happen where we can see multiple nuclei even if the galaxies may not we may not see them colliding anymore. We can see those from previous mergers. So let's look at the timeline of these galaxy collisions and in fact collisions are relatively rare today less than 5% of galaxies. They were much more common in the past. Distant galaxy clusters sometimes show multiple collisions going on. So here we see a number of those in Hubble Space Telescope images from a variety of different galaxies, some showing relatively small amounts of interaction, others showing significant distortion. So what has happened is that galaxies have built up from those small blue galaxies that existed long ago to the larger spiral and elliptical galaxies that we see today. Now we've looked previously at active galaxies and how does that tie into this? Well collisions can feed the black hole at the center of a galaxy making it become an AGN, an active galactic nucleus. These AGNs were much more common in the early universe. Why? Because collisions were more common. So forming active galactic nucleus core and the jets may impact the star formation in the remainder of the galaxy. So this may actually impact what happens with the galaxy in the long run as well. And here we see one example in that collision with the jets of material coming out from the central core. So let's go ahead and finish up with our summary and what we've looked at this time is that galaxy evolution is driven by collisions and by galactic mergers and cannibalism. Galaxy collisions take a long time to occur. The individual stars do not collide. And the galaxy collisions were much more common in the early history of the universe and are relatively rare today. So that concludes this lecture on galaxy evolutions and mergers. We'll be back again next time for another topic in astronomy. So until then have a great day everyone and I will see you in class.