 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about different types of galaxies that we see in the universe and how they have been classified. But let's start off looking at how galaxies were discovered. Well when were galaxies discovered? Well as early as the 1700s it was suggested that they might be similar to our Milky Way and might be other galaxies. But it really was not known. They were called the spiral nebulae and there was a great debate as to whether they were part of our Milky Way galaxy and nebulae like other types of nebulae that we see. And as late as 1924 astronomers were very divided as to what these spiral nebulae were. Were they part of our galaxy or were they external galaxies like our Milky Way? In 1925 Hubble announced the discovery of Cepheid variables in Andromeda. That meant he could calculate the distance and found out that they were definitely not part of the Milky Way. They were much too distant. Now Hubble also worked on a classification scheme for galaxies and he classified them based on their appearance. So what did they look like? Now that's not all that difference than how stars were originally classified based on the appearance of their spectra. Now what we saw here was that there were elliptical galaxies on one arm of the classification and those are ellipticals and then that split into spiral and barred spiral galaxies. Spiral galaxies forming one arm here and barred spirals splitting apart. There were some similarities between the different types of spiral galaxies. And we had the S0 or lenticular galaxies which were kind of the border in between those and the irregular galaxies which really didn't fit any other classification. So let's take a look at each of these types of galaxies and what do we see? We see spiral galaxies and they have a very distinct spiral or pinwheel type structure. They consist like our galaxy of the disk, a central bulge, a halo and the spiral arms. These have ongoing star formation so are currently forming stars and they have emission nebulae and open clusters of stars that show that star formation is occurring. So we can see the pink color of hydrogen regions there and the bluish color of the overall of the spiral arms which show that stars have to be forming currently in these galaxies. They also have a population of old stars in the bulge and halo of the galaxy. So star formation has occurred over an extended period of time. So if we were formed stars long ago these galaxies are still forming stars today. Now similarly we have the barred spiral galaxies. This is what our Milky Way is. They have a similar properties to what we call normal spiral galaxies with the difference being the bar running through the center of the galaxy. So instead of the spiral arms going straight down to the center there seems to be a bar of stars through the middle and the spiral arms seem to pick up off the end of that bar. About two thirds of spiral galaxies have a bar. So while we call them normal and barred spirals it's the barred spirals that are actually the more common galaxy. The reason for the bar is still something we do not know. That's a big question mark as to why some of these galaxies form a bar and others do not. So how can we classify spiral galaxies? Well they are given the letter S to represent spiral or SB to represent a barred spiral. So that divides them as spiral and spiral with a bar and then we use the lower case letters A to C after the S or the SB. So you'd have SA, SB, and SC proceeding this way across the diagram and SBA, SBB, and SBC proceeding down on the lower arm. Those with the A mean that they have the largest bulge and the most tightly wound spiral arms. Those with the C classification would have the smallest bulge and the least tightly wound spiral arms. So you see that how tightly wound the spiral arms are for an SA and how wide open they are for an SC. And of course an SB would then be in between those. Our Milky Way would be classified as an SB for barred spiral and a lower case B. It's intermediate. It's not very tightly wound but it's not wide open as well. The next type of galaxy we want to look at are the elliptical galaxies. Elliptical galaxies are called that because they have an ellipsoidal or spherical shape. Although they are squashed spheres. And there is no trace, none of a spiral arms or a disc in these galaxies. They also have no dust, no young population 1 stars, and no emission nebulae. So none of the things that we talked about with spiral galaxies that showed that they had current star formation. These galaxies had star formation long ago and have not had star formation for billions of years. They come in a wide range of sizes from giant ellipticals which are the largest galaxies in the universe to dwarf ellipticals which are among the smallest galaxies in the universe. Now how do we classify an elliptical galaxy? Well they are given the letter E for an elliptical galaxy and these are given the number 0 through 7 to specify how squashed the galaxy appears. So an E0 is a spherical galaxy and an E7 is an ellipsoidal shape about the thickness of say an American football. So they are much flatter but not near as flat as you would expect in a disc galaxy. A disc galaxy would be far flatter than these. Now how about the other types of galaxies that we look at? We have irregular galaxies as you may guess from the name do not have any regular shape. But they do have cold gas and dust meaning that they have star formation. Like a spiral galaxy they have population 1 and population 2 stars and they are classified as IRR for irregular galaxies. They can be further subdivided but we are not going to go into that detail here. And then finally we have the lenticular galaxies. Lenticular galaxies are an interesting combination of both spiral and elliptical galaxies. They are classified as either S0 or SB0 meaning yes they can have a bar, the S meaning not spiral in this case but referring to the fact that they are a disc galaxy. So they are sort of a combination as to how they are named as well. They are disc galaxies with no cold gas and no dust meaning no star formation in these. So they would look in terms of their stellar populations like an elliptical galaxy. But their shape is closer to that of a spiral galaxy. So they are a very different type of galaxy. Now how do galaxies evolve? How do they change over time? Well perhaps originally this was thought to be an evolutionary sequence that galaxies might go from one to another. You might be able to imagine an elliptical galaxy compacting down into a disc galaxy and then somehow developing perhaps spiral arms. Now that was what was originally thought although we now know that this could not happen. So where is an elliptical galaxy going to find cold gas and dust to become a spiral? There is a big difference in terms of the properties and star formation. Can a spiral un-collapse to become an elliptical? Could you instead go this direction to the left on the diagram? That's hard to think of although it's quite possible that two spiral galaxies in a massive collision could end up forming an elliptical galaxy. A spiral galaxy on itself will not evolve into an elliptical nor will an elliptical evolve into a spiral. So we do believe that galaxies evolve but they evolve through collisions not just on their own. They will pretty much remain whatever shape they currently happen to be. So let's go ahead and finish up with our summary. And what we've looked at this time is that galaxies are classified by their appearance and we gave five types. The spiral, the spiral, the barred spiral, the elliptical, the irregular and the lenticular. Each of these types is divided into subclasses and we discussed galaxy evolution as being caused by collisions between galaxies that cause them to merge together. So that concludes this lecture on galaxy types. 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.