 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to start talking about our Milky Way galaxy and specifically the structure and how our galaxy is set up. So much is when we started talking about planets, we started with the Earth, when we started talking about stars, we started with our Sun and now we're starting to talk about galaxies and we'll look at our own galaxy first. So how do we determine the structure of our galaxy? This is difficult primarily because we're stuck inside it. How do you know what a building is like if you're stuck inside the building and can't leave it? So if you were brought into a building blindfolded, put in a single room, how might you figure out the shape or the extent of a building? Well, you might look out through a window if you have one, you could think of other, maybe other ways you could try to explore a little bit, but if you can't leave that room, you'll have a very hard time figuring out what the whole building looks like. Now the advantage of the galaxy is that it is somewhat clear and we can actually see as leads to certain ways along it. And William Herschel in 1785 measured the galaxy and made a map of the galaxy as we see here and he counted stars in various directions to give us the first rough map of the galaxy. You note that it put the Sun close to the center and that's just because, it turns out that's just because we're only seeing a portion of the galaxy and not the entire galaxy. Now as later measurements could be made, we then be able to learn more. Herschel's measurements were very limited. He did not know about dust in the galaxy and that limited how much of the galaxy he could actually see. So here we see Herschel's map and the dots are globular clusters. So globular clusters gives a better idea of the distribution of the galaxy. They are distributed around the galaxy in a spherical halo so that allows you to get a better idea of where the central portion of this galaxy might be finding the center of the globular cluster distribution and this gives us a much better idea of the distribution and the actual extent of our galaxy. When we look at the galaxy we'll look at several different components and primarily we'll have the disk of the galaxy. The disk contains a lot of the most of the material, the spiral arms, the dust and the gas. So that's most of the visible material within the galaxy is flattened down to a disk. The halo is a spherical distribution which contains those globular clusters and it's a lot easier to see out in those directions so we can see out in this direction and out in this direction a lot better than we can see along the disk and that's because this is where all the dust is. Dust is very good at blocking out visible light so if we try to look along the disk we have a much harder time seeing it and that's why when we looked at Herschel's maps Herschel was much better able to get the up and down extent of our galaxy than he was the left to right side distribution of the galaxy. So we have the disk where most of the material is we have a halo that surrounds it containing mostly globular clusters and then we also have the bulge of material so there's an extended distribution around the center of our galaxy. We have the galactic center which we will look at later which contains a supermassive black hole about four million times the mass of our sun and then out beyond all this is the dark matter halo now dark matter this is unseen matter out beyond the actual halo of the galaxy so there is a lot of material out beyond what we actually see for the galaxy how do we know something's there if we can't see it? well we notice it's gravitational effects on the rest of the stars in the system they're not moving as they should the only material there were the visible light so this is why we know that there is dark matter and we will come back and look at that again later on now what does our galaxy look like? well what we see in the sky is looking at an edge on view of the galaxy because we are limited in what we can see so if we look at other spiral galaxies as we will in the future our galaxy looks like one of those that we see edge on we're looking at the flattened disk portion we do not look down and see the great spiral arms however we can use measurements and observations of other galaxies to predict what our galaxy might look like so here's our galaxy as we see it in the sky and if we want to look at what the galaxy might look like here we can put together an image from various measurements and we can get an artist's conception of what the Milky Way would look like if we could travel out tens of thousands of light years up above the Milky Way and then look back down on it and then we would be able to see the spiral structure with the spiral arms coming out we see our sun here located near the Orion section here but we have other different spiral arms too named after the constellations where they often appear or were most prominent and we have a number of spiral arms as they swirl around the galaxy and we will look at this structure coming up later now what about the spiral structure now we have a good idea of what it looks like we know that there are several distinct arms we know that there is a bar across the center a good question is why do spiral arms form in some galaxies they don't form in all galaxies but some galaxies have spiral arms and some don't some of those spiral galaxies have a bar some do not so those are some very good questions when we want to try to better understand spiral structure in galaxies so how do we form this spiral structure well could it form through differential rotation that means that the inner part of the galaxy rotates faster than the outer part and if you had blobs of material here you could imagine as it rotates that the inner part portions swirling around faster would eventually form some kind of spiral structure the problem is this would not last a long time so that would not be able to have spiral structure holding up for billions of years and the arms would wind up very quickly just after a few galactic rotations so we need some method that allows us to retain spiral structure over the lifetimes of galaxies and what we believe does this is what we call density waves dense spiral arms are actually areas of increased density so there is more density of material in them and the material bunches up much like cars in a traffic jam so this compression leads to enhanced star formation and makes the density waves stand out so you have an area of compression everything slows down and the star clouds may collide together and start forming new stars making and those are going to be the very bright blue stars and those highlight the spiral arms so once the stars move out then they proceed back up to a more normal speed this method is able to maintain the spiral pattern on astronomical time scales so we believe that density waves something like a traffic jam where material bunches up within the spiral arms and then the density waves themselves are not visible but we can see them by the material that is stuck within those and slowed down as it moves through those cosmic traffic jams so let's go ahead and finish up with our summary and we said that our galaxy is an example of a bar spiral but it's not easy to determine that from within the galaxy the galaxy consists of a disk, halo, central bulge, the galactic center and a dark matter halo and the spiral arms may be maintained by density waves much like cars in a traffic jam so that concludes this lecture on the structure 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