 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about stellar populations within our galaxy as well as the formation of the galaxy itself. So what do we mean by a stellar population? Well, what we do when we look at the stars we find in general two types of stars in the galaxy. Most of the stars are what we call disk stars. These orbit in the plane of the galaxy. So we look at the thin disk here. That's where most of those stars seem to orbit in a flat plane, in a piece of paper almost like our solar system. However, some stars are halo stars and orbit in halo and orbit at very unusual angles, at very high angles relative to the galactic plane. These are two different types of stars and will tell us something about the early history of our galaxy. So let's look at these two types and these were given by Walter Bada and he looked at the Milky Way at the Andromeda galaxy and found that there were two different populations of stars. That we had population one, the disk stars. And we see here that they're split up a little bit more. We have population one and extreme population one, but most of the stars in the disk are what we call population one stars. Population two are the outer stars or the halo stars. And the population one are disk stars with circular orbits, have a wide range of ages they could be very young or very old. And they have a high metal abundance so lots of metals, remember metals is something that is not hydrogen or helium. So these are stars like our own sun that have lots of heavier elements in them. Population two stars can be found in the disk or halo. They have eccentric or very tilted orbits, they're old, and they have a low metal abundance. Now they don't have zero but they have a much lower concentration of heavier elements than something like our sun. Now in the real world, yes there's probably a variation between this and we kind of see that going from the extreme population one, which would be the very highest metal stars. Very concentrated to the disk where we get a combination between population one and two and then we get further out to population two. And then finally the question, are there any population three stars? Well, it depends on what formed in the early history of the universe. These are theoretically thought to have existed. They would have been extremely massive stars, which means they wouldn't have lived very long. They would have essentially no heavier elements. These would have been the first stars to form after the Big Bang. Now if we could find some of those that would be very interesting telling us about these stars. Now the massive stars would no longer exist but if massive stars formed probably smaller stars would have formed and many stars much little more less massive than the sun would live for the entire history of the universe. So that is a search for looking for these very oldest type of stars. Now how about the formation of our galaxy and what does this tell us? Well, when we talk about a galaxy formation we think that it's similar to the formation of a star but on a much larger scale. Now we're going to look at two different models here and we're going to see which of those fits or maybe it's a combination of the two as often happens. The monolithic model as pictured here, the galactic cloud collapses to form a disk. That should sound very similar. That was the cloud that formed the sun and it collapsed down to form the central star as well as the planets around it. Now the very earliest things to form before it collapsed were old stars and globular clusters. Once they formed they did not collapse along with the rest of the disk so they remain in the halo around the rest of the collapsing galaxy. And then as material fragmented into pieces they fragmented into clusters of stars and then finally into star size pieces which would undergo the same type of process on a smaller scale and form stars and planetary systems as we've looked at previously. Well this is one model where everything formed all at once. What else could things have formed? Let's look at what we call the multiple merger model. Maybe it was not so peaceful a formation as we looked at here. We are really finding now that galaxy collisions are extremely common in the universe. In fact we have the Sagittarius dwarf galaxy and others are being torn apart by tidal forces of the Milky Way and become incorporated into our galaxy. So as a galaxy is torn apart it forms tidal streams and in the halo of the galaxy so we see some of those streams that are present around our own Milky Way that we've been able to measure. Now this is a drawing showing what it would look like from outside. Of course we can't see this from inside but we can measure those streams and see the remnants of those bits of galaxies that are now being torn apart. Now tearing the galaxy apart does not hurt the stars or the planets in it. They get pulled away but they just become incorporated within the other material of our galaxy. So we've identified multiple streams of these within our own Milky Way. So what happens with these collisions? Well the collisions will stir up the gas and dust in the disk. So the disk becomes thicker and stars form in that thicker disk. Then the gas settles back down so those collisions kind of stir everything up and thicken it up a little bit and then gravity it pulls back down to that disk. So we get some stars forming in what we call the thick disk and then the gas settles back down to that real thin disk where the newest stars will form. Those extreme population 1 stars. More collisions are coming so we see here this is an image of two things put together. This is our Milky Way on the right hand side and this is the Andromeda galaxy on the left. Now you don't see things like this in the sky and that's because the Andromeda galaxy is two and a half million light years away and it just looks like a small fuzzy patch of light. However it is visible to the naked eye and can be seen if you know where to look. We do know that it is on a collision course with our Milky Way and these will likely collide in about three to four billion years, eventually merging into a single galaxy. These will eventually so at some point this will be the view in the sky many billions of years from now as Andromeda slowly gets closer and closer. We won't notice the difference in our lifetimes or even multiple lifetimes but over billions of years it's slowly getting closer and closer to our Milky Way. What effect will that have on Earth? Probably not much. Three to four billion years from now our sun will have warmed up a bit and will have already have made Earth uninhabitable. So at that point what will happen to our sun well it will probably get it might get flung around in one of these tidal streams it just depends on the exact positioning as the two galaxies collide together. But it is something to think about that when we look at galaxies and their collisions it's something that our galaxy will show in a few billion years. But we'll have one great collision there and we have other minor collisions constantly going on as with the Canis Major dwarf galaxy is gradually merging with our Milky Way. So there can be big collisions, there can be small collisions but that can be another way that galaxies tend to grow. We tend to think it's probably a combination of these two that maybe some galaxy forms a smaller galaxy forms first and then grows through collisions. So let's go ahead and finish up with our summary and what we've looked at is that we can divide the stars in the galaxy into two populations, population one which are the disk stars and population two which are the halo stars. We think that maybe the galaxy formed by a larger version by the process we already looked at for star formation but we also talked about collision between galaxies being important and explaining some of the structures that we see in our galaxy today. So that concludes this lecture on stellar populations and formation of the galaxy. 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.