Welcome back, everyone. So we've already talked about, sort of the path that stars have taken in our own minds from being gods to being objects in the sky. We've talked a little bit about how stars power themselves, but now really what we want to get into is how we are able to learn so much about stars. And that comes about understanding light, understanding how we're able to take the light from stars and break it up and be able to decipher the information encoded in it. So the first thing I want to talk about is the idea of light as the electromagnetic spectrum. So it was recognized in the 1800s that light was a electromagnetic phenomena, meaning having to do with electric and magnetic fields. And then in fact if you were to take a charge like a, an electric, an electric charge like an electron, and wave it back and forth, you will generate an electromagnetic wave. And it is basically a, oscillating electric and magnetic field with the electric and magnetic fields aligned like this, the electric field lo, oscillating this way, the magnetic oscillating that way. Um,and there's a range of wavelengths of possible wavelengths associated with electromagnetic radiation, everything from very very long to very very short, and that is what we call the electromagnetic spectrum. So, in this image on this behind me, you can see a, graphical representation of the electromagnetic spectrum, and at the very long wavelength, for the electromagnetic waves. We have what are called radio waves. Radio waves have wavelengths, and their wavelength is basically the distance from the peak of a wave to the next peak, so if we imagine a nice wave here, the wavelength is the distance peak to peak. And radio waves have wavelengths that are meters of the order of meters or even more. And as we shorten the wavelength, what we get down is to microwaves, the things that are heating up your coffee in the microwave oven, infrared which is the actual light that all of our bodies are actually producing. Visible light and then ultraviolet x-rays and gamma rays. That's the progression down to shorter and shorter wavelengths. And what you can see on this image is the, you know, typical size scale associated with these different waves and as you can see radio waves as we talked about on the scale of meteors. Microwaves are on the scale of a centimeter or so which is about the size of a bee the infrared is on the order of ten to the minus sixth of a meter which is about the size of a bacteria. Visible light, all visible light from blue light which or from red light which is longest wavelength visible light to blue light which is the shortest wavelength invisible light is on the order of ten to the minus seven meters. Ultraviolet radiations stuff that can give you a burn, is on the order of 10 to the minus 8th to 10 to the minus 9th. X-ray is 10 to the minus 10th. That's a one 10 billionth of a meter. And gamma rays go all the way down to 10 to the minus 12th of a meter. So this is incredible range of different kinds of light. It's all electromagnetic radiation, from the stuff we can see. Visible light all the way out to radio waves that when we're driving around we listen to on the radio. We used to listen to, on the radio. But they're all the same basic phenomena. So what we do with stars is we have various detectors, telescopes that can gather this light. And then what we want to do is be able to analyze it. Now there's different ways to analyze the light, and there are different ways to analyze the light tells us different kinds of things. So let's first touch on one particularly important way in which light can behave or interact with matter. So, the one of the first ways scientists came to understand how matter can produce light, was what was called the Blackbody problem. And what was discovered was that any time you had a object that was dense, meaning a solid or even a dense collection of gas, that heat motions, which are just random motions of atoms bouncing around, that was enough to produce radiation as we've seen charged particles produce radiation. But with dense gas, dense gas that has some temperature, the collective motions of all those atoms will combine to create a very characteristic spectrum. And by spectrum what we mean, or what astronomers mean, is a plot of energy as a function of wavelength. So we imagine that we're you know, basically able to tune our receiver, our electromagnetic receiver to different wave lengths, which is like tuning your radio to different stations. And we see how much energy comes at those different wave lengths. So a, a plot of wave length versus or plot, excuse me, a plot of energy versus wave length is what we call a spectrum. And what we have for blackbody radiation is you always end up with the same spectrum. It's a very nice, smooth, continuous distribution of energy with wave length and there's always a peak. There's a place where the maximum amount of radiation is coming, a wavelength of which the maximum amount of radiation is being emitted. And that peak depends on temperature. Every object, every block body, every solid object will produce radiation depending on its temperature. So, for example, the human body, we're at room temperature, and the peak in our of our radiation is actually in the infrared. So right now all of us are glowing in the infrared but since we don't have infrared eyes we can't see it. So blackbody radiation was one of the first places that we came to understand. Came to think about matter and radiation, how matter produces radiation. And the great thing about it is, about blackbody radiation, is it allows us to take the temperature of the starts for example, just by looking at them. All we have to do is look to see whether a star is red or blue and we can tell what temperature it is. Because for black body radiation what we find is as you cool excuse me, as you heat an object up, the peak in its radiation will move to shorter and shorter wavelength. So a star that appears red which is a dense collection of gas which is whose surface must be relatively cool because it's producing a, a peak in the red part of the wavelength. A blue star, a star that appears blue, must have hotter, must have a hotter, hotter surface. Because its temperature, its blackbody, is peaking somewhere in the blue, or actually even over into the ultraviolet. So it appears blue to us. So that's the amazing thing. Blackbody radiation allows us to immediately take the temperature of a star just based on its color. Now we also want to note that, we've talked about light being a wave, but there's always been a debate whether light was a wave or particle. And what amazing happened in the early part of the 20th century was the recognition that light could behave as both, either a wave or particle. So we sometimes talk about light being a wave, and sometimes we'll talk about light being photons, being individual particles. Like bullets. And that may seem very strange to you. But this was, this, we came to this through you know, hard experimentation. There was something called the photoelectric effect, which actually Einstein was responsible for understanding. Where basically if you shine light on a a metal plate you could get electrons getting kicked off by the light falling onto the plate. And what was recognized that the only way to understand that effect was by thinking of light in terms of photons. Now you may say to yourself, wait, a wave and a particle are two entirely different things. How am I su, how is like, how can light be both? Well that really is the introduction to the weirdness of quantum physics. Quantum physics, which was born at the turn of the 20th century, is a branch of physics that allows us to understand the realms of the very small, of atoms, and electrons, and light, and it's interaction with atoms and electrons. And there are so many strange things about quantum mechanics, so many strange things it asks us to understand that I couldn't really go into that, all of them here. So I certainly would recommend you know, going to the library or a bookstore and looking it up. But just this idea that something could be both a wave and a particle at the same time or depending on how you look at it, it's just one place where we're going to encounter some of the strangeness of quantum mechanics in this class. We're going to encounter a little bit more in the next class so you should hold on as we get there. But now I think the most important thing for us to remember is what we've already seen, is that light is a can have a wave nature it can have a particle nature, and the interaction between large collections of matter with some temperature and light can produce this blackbody spectrum, which is of fundamental importance and allows us to gain the first handle on how the nature or what, what, the properties of the stars are in terms of temperature. Okay, very good, and we will see you in the next lecture.
