 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the spectra of stars, and what we are able to learn about stars from their spectra. So when we look at these, what we see is this is how stars are classified. How do we break stars into different groups? We use their spectra. And what we find is that stars show different spectral lines, not because they have different compositions. All their compositions are essentially the same, mostly hydrogen and helium, but it because they have different temperatures. So what lines are visible depend on the temperatures. And here we see a graph showing the spectral classes from hot stars on the left to cooler stars on the right. So the very hot stars have things like helium. They are able to excite the helium. A cool star doesn't have the energy to excite helium, so it doesn't show helium lines. Hydrogen is a good example. Hydrogen is the most common element in stars. However it is the A-type stars where hydrogen is most prominent. And those are stars about 10,000 Kelvin. If the stars are too hot, they ionize the hydrogen ripping off its electron and no long making it no longer visible in the spectrum. Cool stars are not hot enough to excite the hydrogen. So the line drops off if you go to cooler stars. It drops off if you go to hotter stars. So only when you have just the right region do you see those specific lines. So just because a line is not present in the star does not mean that that element is not present in the star. Now, how do we go about classifying stars? Well, the process started back in the 1880s with Wilhelmina Fleming who classified stars based on the strength of the hydrogen line. So her strongest class was class A, then B, and so on. And then worked down through the alphabet. Now a decade or so later Annie Cannon went through and rearranged the classes. So here it was done just based on what we saw, just on the hydrogen lines. Annie Cannon then rearranged them to be based on temperature. So giving it a more physical meaning as to what they are. So now the main classifications are O, B, A, F, G, K, and M. So those are the primary classes. And that is the basis of the classification system that is used today. Now, there are also new classes that have been added. Some are in the table, some are not. But we have the class W, extremely hot stars, Wolf-Reye stars. Very hot stars would show emission lines. We have the carbon stars, class C and S, that are red giant stars that have an excess of carbon. Now, that does not mean they are made of carbon. It means they have a lot more carbon than normal typical stars. We have the class D of the white dwarf stars. And then we have the classes L, T, and Y, which are the brown dwarf stars down here. Much cooler than the M stars and dust begins to form in their atmosphere. So these are objects that are not hot enough to be able to fuse hydrogen into helium. They're kind of that in between a star and a planet. Now, here we see the mass of a brown dwarf star, about 0.075 solar masses, 79 Jupiters. These are objects that never become hot enough for nuclear reactions to begin. So Jupiter is not really a failed star. You would need 78 more Jupiters worth of mass to be able to make Jupiter into a star. So these are what we call failed stars, and they can fuse deuterium, which is a heavy form of hydrogen, but they cannot fuse hydrogen itself. Now, if we go to even lower masses, less than 0.012 solar masses or 13 Jupiter masses, then deuterium fusion is no longer possible, and the object is classified as a planet. So things 13 Jupiter masses or less would be a planet. Between 13 and 79 would be a brown dwarf, and 79 or greater Jupiter masses would then be a star. So how do we measure some of these properties of stars that we see? Well, sizes. How can we measure the size of the stars? Well, stars come in a wide variety of sizes. From tiny red dwarf stars, these red dwarf stars are much smaller than the sun, but we also have red giant, super giant, and hyper giant stars that are many times larger than the sun. And here we see some of those in images that we go from these very, very hot stars here, already many times larger than our sun, to things like Vy, Canis Majoris, and Stevenson 2-18. Which are vastly larger than the sun. We see for scale, 10 astronomical units here. That is about the distance to Saturn. So many of these stars, if you place these at the center of the solar system, would stretch out beyond Saturn. Now how do we go ahead and determine the size of a star? So we look at differences in the spectral lines. We can't see stars and see what are large stars and what are small stars just by looking at them. However, large stars are spread out and have a lower density in their atmospheres. And the spectral lines will appear narrow in that low density environment. So here we can have two stars of similar temperature, a white dwarf star, very tiny and dense, very broad spectral lines, and a blue giant star, or a much larger star, that has very, very narrow lines. So that can help us to really tell the classification of the two types of stars. Now when we look at these, we can look at, again, also the differences in ionization. So lower density in large stars means that the ions are less likely to recombine, which changes the spectrum that we see. And we see four luminosity classes here, which are the super giant stars, the bright giants, the giants, the sub giants, and the dwarfs. And those are luminosity classes, Roman numeral one through five. Now for this class, we tend to look at the dwarfs, which are the main sequence stars like our sun, the giant stars, generally often red giant stars, and the super giant stars. We will look far less at classes two and four. So we'll look at one, three, and five as the main classes we want to look at for this class. So abundances, what are stars made up of? Well, let's look at a couple of things. First of all, the presence of an absorption line shows that the element is present. We know that it is there if we see the absorption line. If we see iron lines, then iron is in that star. However, the converse is not true. An absence of absorption line does not mean the element is not present. Remember that the temperature and pressure of the atmosphere determine what elements can produce lines. So a very cool star will not show much in the way of helium lines, even though helium is still the second most abundant element in that star. All stars have essentially the same composition. 96% or more of the atoms will be hydrogen and helium. The remaining are the metals. Anything else to an astronomer, if it is not hydrogen or helium, is classified as a metal. So when we talk about a metallicity of a star, it's talking about what it has, what its composition is, in terms of heavier elements. Now, what else can we learn from the spectra? Well, we can learn something about the velocities. Let's start off by looking, though, at the proper motion of a star, which is the motion of a star across the sky. It's very slow, but is noticeable over long times. Here is the Big Dipper, an asterism in the constellation of Ursa Major, and the arrows show the directions of motions of the star on the sky. Now, will they look any different next week or next month or even next year? No, even 100 years from now, they'll barely have changed. But 50,000 years from now, those slow motions add up, and we would see that the Dipper would look a little more distorted from what we're used to looking at today. We can trace that back and see what the Dipper would have looked like 50,000 years before. So it's a very slow motion because the stars are so far away. Now, what we can measure is the radial velocities, and we can use that using the Doppler effect. Now, remember that the motion is relative. We cannot distinguish if it is Earth or the star that is moving, and most likely it's gonna be both. Remember, if it's moving away, it's a red shift. The lines are shifted toward longer wavelengths. Moving closer means a blue shift. Lines are shifted to shorter wavelengths. And we can use this and put these together to get the true velocity of a star. And that's what we'd call the space velocity. We can combine the radial velocity, the proper motion, and distance. So the radial velocity is easy to measure from the Doppler effect. The proper motion isn't too hard to measure, but it takes time. So this one takes more time to be able to measure because stars do not move very fast. That's the proper motion. And in order to convert that proper motion into a velocity, we need the distance. This gives us the true motion of the star through space. So again, radial velocity easy, transverse velocity more difficult because it requires distance. And we will look at determining distances in a future lecture. How about stellar rotation? We can also use the Doppler effect to learn about rotation. We will get a blue shift from the side of the star rotating toward us, a red shift from the side of the star rotating away from us, which will broaden the lines. The broader these lines, the faster the rotation. Now remember, we also got line broadening from the atmospheres depending on how dense they were. So low density gave us narrow lines. So we have to be able to put all of this together and make these measurements to figure out what part of it is attributed to each of these, to the rotation and to the size of the star. So that that allows us to determine not only velocities of stars, but the rotations of stars as well. So let's go ahead and finish up with our summary. And we looked about how stars are classified based on their temperatures using OBAFGKM. We've added new classes for unusual stars and the brown dwarf stars. And the spectrum of a star can tell us things about its size, velocity, composition and rotation. So that concludes this 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.