 Greetings, and welcome to the Introduction to Astronomy. In this lesson, we are going to talk about the different types of spectra that are observed in astronomical sources and what they are able to tell us. Now we've looked at, spectra is one way that we can split up the light and learn more about an astronomical object. Light itself is the only way we can learn about objects out in space, whether that be visible light or infrared or ultraviolet or x-rays or gamma rays, but they are the only ways we can learn about things, and by splitting light up into its spectra allows us to learn even more about those objects. So let's start off here. We're going to look at how we create a spectrum in the first place. So how does that spectrum get created? And it is created by taking white light, which is composed of all of the colors of the rainbow, and sending them through an object like a prism or a diffraction grating that astronomers use that splits the light into its component colors. So we see here white light coming in, and each of those wavelengths of light, remember that light will have different wavelengths, blue light having a shorter wavelength, red light having a longer wavelength, each of those is bent by differing amounts. So as they cross into this prism, the blue light gets bent a little bit more, and then a little bit more coming out, and when we look at the screen here, we end up seeing the blue on this side, and the red on this side, because the blue has been bent a lot more than the red in this time. So now we're able to see not just the light and get pictures coming from the stars, but we're able to see other details of the spectrum as well. Now, why is this so useful? Well, when we talked about the black body spectrum, that was able to tell us things about temperature and brightness of the stars. So we were able to look at just images of the stars, we could see how much light they were emitting or how bright they are, we could use the colors to tell us what the temperature was, but when we actually get the spectrum, we can learn things about the composition, what our stars made of. We can learn how the stars are moving. And more, but these are just a couple of the very important ones, how do we find out what a star is made of when we can't go get a sample of it and bring it back to the laboratory? Well, we use its spectrum to be able to determine that, and we're going to look a little bit about how that works here. So what are the different types of spectra that we see? Well, first of all, we can see an example of a continuous spectrum. A continuous spectrum is formed by a solid, a liquid, or a dense gas, and you might realize that takes care of lots of objects that we're used to thinking of here on Earth, but not as much when we look out in space. A continuous spectrum is just that, is continuous with no breaks in it. So it goes from all of the colors, from the violet over on one end to the red on the other, without any breaks in between it. And even though I'm showing visible spectrum here, it does include all electromagnetic radiation. It includes gamma rays through radio waves, so all of it is concluded here. In a visible spectrum, we look at the red through the violet, and we could also look at a radio spectrum or spectrums across other types of electromagnetic radiation. These are emitted by black body sources. Recall that those are objects that emit light based only on their temperature. So things like an incandescent light bulb with a filament inside would give you a continuous spectrum. Things like stars, if we ignore their atmosphere, would give us a continuous spectrum. They are just a solid black body source and give off all energy based only on its temperature. So that is one type of spectrum that we can see. Others that we can see are an emission spectrum. So an emission spectrum here is formed by a diffuse gas. So we covered almost everything in the first one. Now we look at a diffuse gas. We looked at dense gases giving us a continuous spectrum. A diffuse gas gives us an emission spectrum. And in an emission spectrum, sometimes called a bright line spectrum, we only see specific wavelengths. So only specific wavelengths are visible. So if we look at this here, and this is a hydrogen, we see a line here in the red, one off in the blue, one in the deeper blue, and then off into the violet, and that's about it. So it's very clean by comparison to what we looked at with the continuous spectrum where you had all of the wavelengths visible. Here we only see very specific wavelengths, so not everything able to be seen. These would be emitted by things like nebulae. Excited gases out in space, and any time a gas is excited in a nebula like that, we would then be able to see emission lines. And that becomes very important what these specific lines are. But let's first look at one more type of spectrum, and that is what we call the absorption spectrum. So an absorption spectrum is actually what we do see with a star when we add its atmosphere in. So we are able to see, we look at a continuous source, you need a source of a continuous spectrum, and view it through a cooler gas. So this could be, for example, the star, and this could be the atmosphere of the star. And that means we call this sometimes a dark line spectrum, and specific wavelengths are now removed. So there are wavelengths missing from this. So when we see them here, we see that there are specific lines. We can see the underlying continuous spectrum that we'd be used to, going from violet over here, through red over here. But we are having specific lines missing. Now these lines are very specific to the elements that create them. Each element will have its own fingerprint. So that when we look at something like hydrogen, here, hydrogen has its very specific set of lines that we can see. When we see this pattern in something in space, we then know that hydrogen is present. When we see other patterns, we could be able to tell whether there was helium or neon or carbon or iron in these objects as well. Now it actually gets a lot more complicated than that, because it is not very easy. When we start looking at all of these, all of the other objects, we see that it's a lot more complicated, that there aren't just, things are not just made up of one object, of one element, but of multiple elements, and those overlap each other. So it can be a difficult process to try to decipher, you have to try to decipher each of these wavelengths and match them up with the various atomic patterns. But that is a way that we can learn what things are made up of out in space. Now we can put these together into the set of radiation laws, and those are called Kirchhoff's Radiation Laws, and we have those up here, Kirchhoff's Radiation Laws, tell us what type of spectrum will be given off by various objects. Now I've summarized these, put these in before, but we want to summarize them to get here. So we have a continuous spectrum which is given by a solid, liquid, or dense gas. We have an emission spectrum which is a diffuse gas. And then we have an absorption spectrum that is emitted when the light from a continuous source here passes through a cooler gas. And we can see that in the picturing here. We have a light source, so that could be a light bulb, it could be the surface of a star, and that would give us a continuous spectrum. So this would be the continuous spectrum is this one. An emission spectrum would be over here on the other side, where you're looking at just the gas cloud that's been excited. The specific lines that it gives off are then telling you what that gas cloud is made of. And finally you can have an absorption spectrum when you look at the continuous source through a gas cloud, and now instead of them being bright lines we will get dark lines, but they correspond to the same element. So in either of these cases we can tell what the object is made up of. With a continuous spectrum we cannot do that, a black body we cannot tell what that is made up of unless we're able to see some various lines. And in reality when we're seeing this we're not learning the composition of the light source itself, we're learning the composition of the cloud. So this cloud here is what is doing the absorption, and that is what we are learning the composition of. So let's summarize a little bit what we've gone over here, and that is that the spectra can be used to determine many properties of the stars. So we can look at stars, we can look at galaxies, we can look at nebulae, and we can learn about all of these different astronomical objects and what they are made, and what they are composed of, how they're moving as a couple of examples. We talked about three different types of spectra, they were continuous, emission and absorption, and we talked about the various conditions you need to form each of those, and those were summarized as Kirchhoff's radiation laws that tell us under what circumstances each of these will occur. But again, the key thing is we're learning these properties such as composition is very important to be able to find out what things are made up of out in space. We have no other ways to get a sample, for example, of a star. We have to use the light that comes from it to be able to find out what it's made of. So that completes our lecture on the various radiation laws and types of spectra. So 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.