 Greetings and welcome to the introduction to astronomy. In this video we're going to talk about the structure of our Milky Way galaxy. So look at how it is composed and what it is made up of and look at the general structure and some of the history of that. So let's go ahead and get started here and what we see is first of all how do we determine the structure of our galaxy. It's not an easy thing to do primarily because we are stuck inside our galaxy. So it's a little harder to determine that than it is if we could look at a different distant galaxy. So an example would be, it would be very hard how do you know the shape or extent of a building if you're stuck inside a single room. You might have windows to look out to give you some idea but if you could not move from that room and had not seen the building before you would have a very difficult time determining how many floors the building has. How high up does it go? How far down does it go? We have a very similar issue with our galaxy. We cannot easily see through our galaxy and in fact one astronomer William Herschel in 1785 counted stars in various directions to try to get the first rough map of the Milky Way. And here what he found was our sun relatively close to the center of the galaxy with stuff but he did notice that the galaxy was flattened that there were far more stars in this direction than there were in the up and down direction so he saw that there were far more stars here. Now one of the problems that he had is that he did not understand that there was a lot of dust. We did not realize that there was dust and in fact looking off this way would be the center of our galaxy and most of the stars there are completely blocked by the dust so there are actually a lot more stars here in our galaxy extends a lot farther off here to the left than it does to the right and while it was a first great map of the Milky Way it did not tell us the true extent of the Milky Way galaxy. So more modern measurements because as I've said Herschel's measurements were limited meaning that he did not know about dust and that limited the part of the galaxy that he actually was able to view so as we see in the image here Herschel's image is just this little black section here he was only looking at a very small part of our galaxy. However looking at the globular clusters gives us a better idea that's because they are distributed pretty much uniformly around the galaxy in a spherical halo and this tells us a much better idea of what the galaxy is like so we can see that the galaxy is now we have all of these dots in here each of these little dots represents a globular cluster they can be seen over large distances so we can see them even when they are very far away and if we can assume that they are pretty much centered on the center of our galaxy then that means that the center of our galaxy is here roughly well outside even the little section that Herschel was able to observe back in the 1700s so studying these globular clusters and Harlow-Shapley here was one who was able to do that and he was able to find that no we're not near the center of our galaxy as Herschel had found from his measurements but we are actually well away from it so let's look at what our galaxy is made up and what our galaxy is composed of so the different components of our galaxy we have several different we have several different ones here and what we have is we have the disk of the galaxy which is really most of the visible material when you look at a spiral galaxy which is what our Milky Way is most of that material is the disk material it contains things like the spiral arms and the dust in the gas and is where stars would be forming so it would be sites of star formation and that also makes it very visible because there are bright stars forming there so our disk shown down here is the very flattened area that goes right through the central portions of our galaxy and is where the galaxy is flattened now there's also the halo which is a spherical distribution that's where those globular clusters that Harlow Shapley saw were and when he measured those those Harlow's globular clusters are then centered on the Milky Way so we happen to be located here actually towards the outskirts of the galaxy but there is a halo a lot less material it has the globular clusters but has a lot less material a lot less certainly not any gas and dust that would be forming new stars now there's a couple other components as well and we have the bulge of the galaxy so the central bulge is noted here that is an area that is kind of in between the disk and the halo the disk is very flattened the bulge is extended a little bit and then the halo is a big spherical area around it so this is an extended distribution of material around the center of our galaxy the galactic center itself contains a supermassive black hole about four million solar masses so four million times the mass of our sun and then out beyond that not shown in the image is a dark matter halo now we saw the halo itself that stuff we can see but there is also much more unseen or dark matter around the halo this is very important we know that it exists because of gravitational effects and we'll take a look at that later but we know that it exists there that there must be far more mass than we can possibly see and dark matter is one of those things that astronomers are continuing to study today so what does our galaxy look like well what we'll see is that our galaxy when we look at it we actually see an edge-on view so we do not see our galaxy as a great spiral galaxy we see it edge-on because we're sitting inside it and it gives us a very limited view of what we're able to see we can use measurements and observations of other galaxies however to kind of predict what our galaxy might look like so we can make some measurements figure out where different dust clouds are figure out the extent of our galaxy where the denser portions are where the least dense portions are and use that to be able to try to determine what our galaxy might look like and an artist's conception might give us something like this that we would then be able to see our galaxy so here is the central bulge of the galaxy in towards the center the whole thing we're seeing is the disk we get the distinct spiral arms spiraling out from the center and we also in our case we see a central bar that means there is a bar of material going through the middle of the galaxy and that is important because we see a number of other galaxies that have this as well and in fact the spiral arms start from the edge of the bar not from the actual center of the galaxy itself so this is an example of what we might be able to see if we could travel hundreds a hundred thousand light years away or ten thousand light years away straight up out of our galaxy and look back down on it to get that full view of our galaxy so what do we see when we see the spiral structure when we see some of it in the image here we can take a look what do we know we now know what our galaxy looks like a little bit better we see that there is very distinct spiral structure as I showed before we have various different arms there's the Sagittarius arm into the Perseus arm out here and the Scutum Centaurus arm is the main arms but there's also some other ones going here so our galaxy doesn't have just one or two spiral arms but has a number of them that have formed we know it has those several distinct arms and the bar going through its center which again we've seen those in some galaxies and a question can be why do we get spiral arms in some galaxies but not all of them so why do certain galaxies show us a spiral arms and what we think in terms of how spiral structure forms we can actually look at that very briefly here and what we find is that you know how do spiral arms form well one thought is that maybe they form because the galaxies undergo differential rotation now that's not unusual it simply means that the inner part of the galaxy rotates faster than the outer part so you could imagine that if you had blobs of material that stretched across the galaxy with the inner parts rotating faster and the outer parts rotating much slower that they would spread out over time so you'd start to get them spread out a little bit here and more here and then finally you would form distinct spiral arms just like we saw in the estimation of what our galaxy might look like the problem is these would not last a very long time after just a few galactic rotations which might only be a billion years or so they would wind up and be completely gone so while it can form spiral arms we need some method to be able to retain the spiral structure for the lifetime of a galaxy and we're talking 10 billion years in that case since we see spiral galaxies at various ages and the way we believe this works is through what are called density waves so a spiral density wave what they are is areas of increased density much like the material material bunches up the way cars bunch up in a traffic jam so the compressed material as the density wave travels through where that compressed material is here and here then that forms that is where star formation is going to form so stars are going to begin to form in these areas where the material is compressed you push more material together you're more likely to begin to be able to form stars so while the density waves themselves would be invisible this highlighted star formation makes the density waves stand out and gives us the pattern now because the density wave would would move much slower then the actual rotation of the galaxy this would allow spiral pattern to maintain for astronomical timescales so just as a traffic jam itself can move slower than the individual cars if there are cars bunched up on the highway and you're moving through that you slow down as you're moving through it and then you pick up your speed again so overall the cars will move through it the stars would move through the density waves but this is where they would be bunched up and where new stars would form and those very bright hot new stars that form would then highlight the density waves giving rise to the spiral structure that we see in galaxies so let's go ahead and finish up here as we do with our summary and what we've looked at in this section is that our galaxy is an example of what we call a barred spiral galaxy meaning meaning that it has a bar going through its center however this is not simple to determine because we are stuck inside it when we look at other galaxies we'll be able to easily see whether they are ordinary spirals or barred spirals or other types of galaxies the composition of our galaxy contains the disk halo central bulge the galactic center and then a dark matter halo outside that and we believe that spiral arms can be maintained by density waves which bunch up the material much like cars within a traffic jam so that concludes this lecture on the structure of our 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