 Greetings and welcome to the Introduction to Astronomy. In this video we are going to continue talking about stars and are going to specifically talk about the spectra of stars. Now a spectrum is what happens when we take light and split it into its component colors. And this would have been discussed earlier in the course if you need to go back and review previous videos, but we can use the spectra of stars to be able to determine many properties of a star that we would otherwise be unable to determine. That would include things like their sizes, it would include their compositions, and it would include their velocities that we will talk about in this lecture. But first of all we really want to look at how stars are classified and organized based on their spectra. So getting started with that, what we find is that we do classify stars by the appearance of their spectra. What kind of lines do we see in the spectrum? And the reason that stars will show different spectral lines has nothing to do with their composition. Their compositions are all essentially the same. They are primarily hydrogen and helium, and very minor differences will say that will tell us anything else about them. So what we will see is that all stars are made primarily of hydrogen and helium, but there are differences, and those differences have to do with the stars temperatures. So the lines that are visible do depend on the temperature, and let's look at the hydrogen line for example. This graph here shows how strong the hydrogen line will be versus various spectral classes. In spectral class A it would be the strongest, but it would be very weak in a class M or a class O. Now we'll talk further about those classes later on, but let's just look at this for right now. The reason that we do not see the hydrogen gas if we look at a class M star or a very cool star, we don't see the hydrogen lines because the cool stars are not hot enough. They are not hot enough to excite the hydrogen gas and cause it to glow. A very hot star over here, hydrogen line strength has already is also dropped off. The hot stars will ionize the hydrogen. Hydrogen atom has one proton and one electron. If we rip off that electron it has nothing to form spectral lines with, so for very hot stars we will not see hydrogen lines either. However, that does not mean the composition has changed. All of these stars of whatever spectral class have the exact same composition. They are mostly hydrogen and helium with the trace of other elements. The tracing of other elements is a very minor difference between the stars. It can be important, but it is very small. It does not change the fact that every star that we look at will be mostly hydrogen and helium, and in fact 99% of the atoms in any star will pretty much be hydrogen or helium. It is those stars that are right around 10,000 degrees that are the best at exciting hydrogen and therefore give us the strongest hydrogen lines. When we look at other things like helium, helium is harder to excite because it requires higher temperatures, so we don't begin to see neutral or ionized helium lines until much hotter stars. Other things like metals, and I should say that a metal to an astronomer means anything other than hydrogen or helium, they do not appear in these very hot stars. Again, they've been very heavily ionized at those high temperatures, so they don't produce lines. But when you get to cooler stars they are at the proper temperature to excite them, so ionized and neutral metals are then visible in cooler stars, and in the very coolest stars we begin to see molecules. Molecules like titanium oxide here could not be present in the high temperatures of much hotter stars. So the spectra that we see, the spectral lines tell us about the temperatures of the star. Now let's look at how these were classified, and they were originally classified by Wilhelmina Fleming back in the 1880s. She did the original classification and the first thing that she looked at was the hydrogen line. That was the most prominent line in many stars, so she classified them this way and said that the classes with the strongest hydrogen lines were class A, then class B, and so on down through the alphabet to very weak hydrogen lines would have been classes M or so. However, this was only based on the appearance of the spectra. It had no physical meaning, and a decade or so later Annie Cannon actually came up with a better way to do this. In the 1890s she rearranged the classes that Wilhelmina had come up with and reclassified them to be based on temperature. So we use the original classes that were developed back in the 1880s, and you can see some of them here, A's, B's, F's, G's, K's, M's, and O's. However, they were changed and reordered to be based on temperature, so that the hottest stars were the O stars, and that the coolest stars were the M stars. So the classification, as we see here, became a classification of temperature with very high temperatures up here at the top, very low temperatures down here at the bottom, and this table will tell you what you would see in each of those classifications. So you'd see helium lines very strong in the hot stars, hydrogen in the intermediate stars, metal things like calcium and sodium in the intermediate and slightly cooler stars, and then molecules beginning to appear in the very coolest stars. So everything was reclassified to become temperatures, and that is the basis of the classification used today. Now we have added some new classes as new discoveries have been made, so we can look at some of those, and the new classes become the Wolf-Rea stars, which are class W. These are extremely hot stars with emission lines, so actually giving off, having brighter lines rather than dark lines, which are associated with most of the stars. We see class C and S, which are the carbon stars, and these are red giant stars that have excess carbon in their atmosphere, and perhaps later when we look at videos on stellar evolution, we can see how that might happen as carbon that is being formed in the interior can be dredged up to the surface, enriching their surface in carbon more so than we would normally see. Now what I really want to look at are the classes L, T, and Y stars. These are what are known as brown dwarfs. They are cooler than the M stars, and so they would be down at the edge of that sequence that we looked at, down at the very end of that at the very coolest stars, and at those point dust begins to form in the atmosphere, so you're getting cooler and cooler, not only forming molecules and more complex molecules, but actually beginning to form small dust particles. So what is a brown dwarf star? Well let's take a look at this. If we look at stars with a mass less than 0.075 solar masses, they never become hot enough to be able to fuse hydrogen to helium in their core. These are what we call failed stars or brown dwarf stars. They can fuse deuterium. Deuterium is an isotope of hydrogen, which has one proton and one neutron in its nucleus. So this is the distinguishing factor between a brown dwarf and a planet. So a star is able to fuse hydrogen into helium, a brown dwarf star can fuse deuterium, and a planet is unable to fuse deuterium, so that's how we can classify these things. Anything greater than 0.075 solar masses would be classified as a star. Anything between 0.075 solar masses and about 13 Jupiter masses would be classified as a brown dwarf. Anything less than about 13 times the mass of Jupiter is then classified as a planet. So sometimes we consider Jupiter being so large that it's almost a star. It's not even close. It would need to be 13. We'd need 13 Jupiters put together just to get Jupiter into the stage where it would be classified as a brown dwarf star and many many more Jupiters to get it to the point where it would actually be a star. If we took all the mass of all the other planets in the solar system and include all of the other objects within our solar system other than the Sun, we would not even have one more Jupiter mass. So all of the material left behind is not close to being able to make Jupiter even a brown dwarf, let alone a star. So let's look at what we can determine by measuring some of these properties of a star. And what we find is that stars come in a wide variety of sizes. There are extremely tiny stars that we'll look at later. But just looking at ordinary stars, typical stars can be much smaller than the Sun. So if we look at some of these here, here is Jupiter in this figure in number three here. Jupiter is there. We can have a very small star like Wolf 359 which is a small red dwarf star significantly smaller than the Sun. In fact much closer in size to Jupiter than it is to the Sun. So there are very small stars but as these overlap you can look as we've gone through go through the scales here. This is Jupiter and in the next scale we scale down to make Sirius a very small star and look at even larger stars and we get into red giant and red hypergiant stars. Things like Betelgeuse here is a very large star and Vy Canis Majoris is the largest known star. These are many many times larger than our Sun. So you can see how big our Sun would be there. This is Betelgeuse. Betelgeuse compared to something like Aldebaran. Aldebaran would be this small but Aldebaran compared to Sirius would be this small and Sirius is significantly bigger than our Sun. So many of these stars are many many times larger than our Sun. So let's look at what we can determine. How can we determine the size of a star? I've shown you some of them here but how could we actually determine that? So we'll take a look at those here. First of all we can use the spectral lines of a star to help expand our classification. So large stars are more spread out and have a lower density and the spectral lines will therefore appear to be more narrow. So we can see differences there just in looking at the spectral lines. A very large star will have very narrow spectral lines. A very small star will have a higher density and therefore much broader spectral lines. We will also see differences in ionization and how many electrons have been removed from various atoms. The lower density and the large stars, the ions are less likely to recombine and will give us a slightly different spectrum. And based on this astronomers have made five luminosity classes to go with the spectral classes that are used. There are the primary three, there are super giant stars class one, giant stars class three, and dwarf stars class five. This is most of the stars that we will see are dwarf stars. Our own sun would be classified as a dwarf. Now just because something is classified as a dwarf does not necessarily mean it is small. It's simply referring to the luminosity class based on the spectrum. But what we'll look at and what we'll look at in other lectures are super giant stars, giant stars, and dwarf stars. We really will not go into the other two in this class. So let's look at how we can use this to determine sizes by looking at the spectrum. Now how can we use it to determine abundances? What are the stars made up of? Well, what we see is that the presence of absorption lines of a specific element shows that an element is present. So if we think about that, if we see iron lines in a star then guess what? Iron must be present in the atmosphere of the star. However, the converse is not necessarily true. The absence of absorption lines does not necessarily mean the element is not present. Because the temperature and pressure in the star's atmosphere will determine what elements can produce lines. A very cool star is not going to be able to produce lines of helium. But that does not mean that helium is not there. It simply means that the temperature is not correct to be able to excite them. What we see is that all stars have essentially the same composition. Ninety-six percent or more of the atoms in any star are going to be hydrogen and helium. Anything else astronomers will call metals. So anything heavier than helium is a metal called a metal by astronomers. So when you see that terminology about metals or metallicity that simply means the percentage of a star that is not hydrogen or helium. Now the other thing that we want to be able to measure are the velocities of the stars. How are the stars moving? Well, let's take a quick look at that. We can look at that. We can use the Doppler effect to measure the radial velocity of stars. But note that this is relative motion. Who is moving? Is it the earth moving towards the star or the star moving towards the earth? Is it the earth moving away from the star or the star moving away from the earth? We cannot tell that. All we can see is the relative motion of the two. If we are moving away, then that gives us a red shift. The lines are shifted towards longer wavelength. If they're moving closer together we get a blue shift that the lines are shifted to shorter wavelengths. So that's the radial velocity that we can see and we can measure that using the Doppler effect. However, there is also a transverse velocity which we measure as proper motion. This is very slow but is noticeable over long times. And we see in the image here, here is the big dipper as we see it today. However, 50,000 years ago the big dipper looks something like this. Nothing like it does today. You can see how it's been stretched out. And what we have in the big dipper is that five of the stars, these five in the middle, are moving in roughly the same direction through space. So their relative positions stay the same. However, the other two stars, the one on the end of the handle and the one on the end of the bowl, are moving in the opposite direction. So that 50,000 years from now this handle will have curved a little more and the ball part will begin to stretch out. It's not something that one will notice in a lifetime but over tens of thousands of years the appearance of the stars do slowly change. And this would be another velocity. This was what we would call the transverse velocity. So in order to find the true velocity of a star, we need to measure both of these. Both are important and both have to be measured by different methods. So if we combine the radial velocity, the proper motion, and the distance, we can then figure out the true motion or the space velocity of the star. The radial velocity here, whether it's towards or away from us, that portion can be measured by the Doppler effect. The transverse velocity can be measured by the proper motion and combining with that with the distance will give us a transverse velocity. And then if we can add those two velocities together, the velocity here, the radial velocity here, the transverse velocity, and when we add them together we can get the true space velocity of the star. Now the radial velocity is a lot easier to get because we just have to make a direct measurement. The transverse velocity can take time. We need to look at images, sometimes decades, or even a century apart to be able to see how that star has been moving. But together those will give us the true space velocity of a star through space. Now finally we want to look at the rotation of the stars. How can we determine the rotation of stars? We can also use the Doppler effect to do this. If a star is not rotating at all, then the light will be coming the same from whether it's from the edge or from the center of the star. However, if the star is rotating, the part that's coming towards us will be blue shifted, the part that's going away will be red shifted, and we will get a blue shift from one side of the star, a red shift from another, and that will broaden the spectral lines. So in a non-rotating star we'd get a very narrow, a very narrow spectral line. In a rotating star we would get a broaden spectral line. The faster the star is rotating, the broader the line will be. So the faster that we can determine something of the rotation by looking at how wide the spectral lines are. So let's finish up here with our summary, and what we find is that stars are classified based on their temperatures. So we use the OBAFGKM classification as how we determine different types of stars. New classes have been added since that was developed in the late 1800s for more unusual stars and for the more recently discovered brown dwarf stars. And the most important thing, the spectrum of a star that we need tells us about the size, the velocity, the composition, and the rotation of stars. So just by looking at the spectrum of a star we can determine a lot of things that would otherwise be very hard to find. So that concludes our lecture on the spectra of stars. 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.