 Greetings and welcome to the Introduction to Astronomy. In this video we are going to talk about the distribution of galaxies, how are they organized out in space and how they tend to group together. So we can go ahead and get started here and what we find is that what we want to look at essentially is how the galaxies are distributed, are they isolated or are they in groups and if they're in groups are they in large groups or small groups. So how are they organized together? And that leads us to what we call the cosmological principle. The cosmological principle states that the universe is isotropic and homogeneous and what that means isotropic means that the universe looks the same in all directions. Homogeneous means that every large chunk of space looks essentially the same. If you take a very big area of space it should look exactly the same as another similarly sized area of space elsewhere in the universe. Early observations of this by Hubble showed that approximately the same number of galaxies were found in any direction. So as you looked out into space it didn't matter what direction you looked you found roughly the same number of galaxies. So kind of showing that the universe is isotropic. So let's take a look at what this means and how the galaxies are distributed then. And we can start off close to us with what we call our local group. This is our local cluster of galaxies. It contains a little over 50 galaxies including three large spiral galaxies which are noted here, the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy. Those are the three large spiral galaxies and there are many dwarf elliptical galaxies and irregular galaxies that are also scattered around as well. Now even though these are the closest galaxies to us we are still discovering new members of these and in fact new faint galaxies are being found by automated surveys that look for concentrations of stars and are able to find some very faint galaxies that had been predicted but were only now being able to detect. This is a small cluster with only 50 galaxies. We actually can have much larger clusters and we have things like the Virgo cluster. The Virgo cluster is the nearest large galaxy cluster about 50 million light years away. So this one unlike ours which contained about 50 galaxies this has thousands of galaxies and when we look at images like this almost every object you're seeing here including some of the larger galaxies but almost everything that you're seeing in these images is a galaxy. So even some of the small dots there are actually galaxies, small galaxies that are part of this cluster. M87, the giant elliptical galaxy, is the dominant galaxy towards the center of this cluster. There are also even larger clusters we have the coma cluster which is even larger and that again is also dominated by elliptical galaxies. So it's an interesting difference between our cluster and things like coma and Virgo clusters. These are dominated by elliptical or E-type galaxies whereas if you recall our local group had spiral galaxies as the three most prominent members. The coma cluster contains tens of thousands of galaxies within it and is 250 million light years away and much like with the Virgo cluster everything you're seeing in this image is pretty much a galaxy that is part of that coma cluster. Now what do we see when we look in the clusters? How are the galaxies distributed? And that's something that can be very important for understanding the evolution of galaxies. And what we see is that within the galaxies we have the social galaxies and the shy galaxies. So social galaxies are the elliptical galaxies. These are found in the crowded centers of clusters. So when we look at the center of a big cluster of galaxies we have a tendency to find lots of elliptical galaxies there. Spiral galaxies are the shy ones. They are found isolated on the outskirts of clusters or in the very much smaller clusters things like our own local group. So spiral galaxies do not like to be in these big groups here and we believe that is because collisions will destroy spiral galaxies as galaxies cannibalize each other the spiral galaxies will be destroyed and will be incorporated into large elliptical galaxies which is what is predicted to happen with our Milky Way and the Andromeda galaxy that they will eventually merge into an elliptical galaxy billions of years from now. But in the outskirts of these clusters collisions are less likely so the spirals are able to hang on for a longer time. Now one of the ways we can look at clusters is through what we call gravitational lensing. Gravitational lensing is a prediction of general relativity. What essentially it says is that mass bends space and time and that means that the light paths that a star would follow will deviate when they pass near a massive object. So we see here this is the foreground galaxy this is what is doing the lensing. We have a distant galaxy here so it's light heading off in one direction will then be bent by the galaxy and come to Earth making us see an image off in the other direction and a second image perhaps out here. So we can then see these images of the galaxy and get multiple images of distant objects whether they be galaxies or quasars. The galaxy images will be distorted as things pass through the cluster and we will see as we look at some of these if we take a look at what these galaxies might look like. Here's an example of gravitational lensing and we can see that many of the galaxies their images have been distorted that this does not look like an ordinary galaxy this is a whole arc up here circled and the galaxies have been distorted and in this case they're being lensed not by an individual galaxy but by the combined gravity of all of the galaxies that we see here and the dark matter which cannot be seen. This is another way to be able to measure dark matter and count how much dark matter is there is by looking at how the lensing goes because we can use then general relativity to figure out how much mass must be there in the cluster to get what we see to get the bending that we actually see. But going back for a second looking at how this works again what we see is the light from the distant galaxy comes close to an foreground galaxy and gets bent around. That means that what we see over here is the image of this galaxy and this would be the foreground galaxy here in the blue and then we see images up on top of the galaxy and down below. The number of images will depend on the exact alignment the better the alignment the better you would better more images you will get and in fact you can actually get a ring of material around it if you had perfect alignment. So what this shows us then as we continue on and some of the things we can look at is looking at how these are grouped together is that what we begin to find is that galaxies are grouped into clusters of clusters that we call superclusters. So these clusters of galaxies group into even larger structures and we call those superclusters. So we see here a map of the entire universe and it shows all these various clusters and so all these various superclusters that we are look at used to seeing. So we saw things like the coma cluster while the coma cluster becomes part of a supercluster as well. So they become even larger groupings and those cluster superclusters are then just clusters of clusters. We also find that there are gaps between these superclusters that we call voids and there are some large voids where they do not seem to be any galaxies or very, very few galaxies. So we see some of them that we're kind of used to seeing. We mentioned coma and we mentioned the Virgo cluster as well. They also group into superclusters but we also have these great voids between the galaxies and one of the things we want to be able to understand is how this type of things these type of things formed. How come we have all these galaxies forming together like this and all of these big empty regions with no galaxies in them? Now we can use different surveys to map the universe and see what it looks like mapping the distribution of various galaxies and what we find is that the thing is we need the distances in order to get a good map we need to know the distances so we need to find the red shifts. Remember the red shifts we can then use Hubble's law to be able to determine the distances so Hubble's law will then give us the distances and what we find is the galaxies are actually concentrated into what we call filaments so we get these string-like structures that the galaxies seem to form along. We also find lots of these empty voids big regions where there are hardly any galaxies when we make this map of the universe and what we're finding is that 90% of the galaxies only fill 10% of the universe so we talk about how empty the solar system is how empty the galaxy is how empty the universe is the vast majority of the galaxies 90% of them are concentrated into just 10% of the volume of the universe making the universe even emptier than we have considered before. So let's go ahead and finish up here as we do with our summary so what we find is that galaxies group into clusters those clusters group into superclusters and the superclusters will group into the filaments between those filaments there are great empty voids where there is hardly any material very few galaxies remember that those voids only count for 10% of the galaxies in the universe and while there are a large number of them compared to what we see in the other 10% of the universe it is very very empty and then we also looked at the cosmological principle which said that the universe was both homogeneous and isotropic on the largest scales not on the small scales and if you look at the image before that we talked about what we saw was that the universe does not look homogeneous or isotropic because there seem to be these great voids however we are talking about the very largest scales when we do this we are not talking about the small scales we are actually looking at the very largest scales and if we take a whole big chunk of the universe over here and a big chunk of the universe over here we would find roughly the same number of galaxies in each of these very large chunks so that completes our lecture on the distribution of galaxies we will 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