[BLANK_AUDIO] In this last week I'm here in an optics lab at the UK Astronomy Technology Centre to talk about spectroscopy. So what is spectroscopy? Well, light, any natural light source, is made up of light of a mixture of different wavelengths. So, red light has a longer wavelength than blue light; Infrared has an even longer wavelength; Radio waves have an even longer wavelength. Likewise ultraviolet has an even shorter wavelength than blue light, and x-rays are just rays with a very, very short wavelength. So, the art of spectroscopy is the technique of splitting light up into its component parts, so we can tell how much there is of different types. Now if I was to draw a graph, of the intensity of light plotted against the the wavelength of the different components, that shape would be the spectrum of the light, and we'll see some examples in a moment. So, later on we'll see how we actually do that; how we split the light up and, and measure it. But first of all, I wanted to show you how there are three powerful things we can do with the spectrum. [BLANK_AUDIO]. So, trick number one, is that we can use the spectrum to measure the temperature of an astronomical object. Now this works because hotter objects emit bluer light. Now we can get a rough idea of the color by using filters, as we discussed in week two. But we get a much more accurate picture if we can actually get the spectrum. So, this illustration I'm showing you here, shows how the the spectral shape of the light from the star changes, as we change the temperature of the star. We can get a pretty accurate measure of the temperature of the star. So then when we have the temperature of a range of stars and we have the luminosities as well, as we saw earlier in the course, if we plot luminosity against temperature that falls into a pattern. Thats the famous Hertzsprung-Russell diagram, that Catherine showed you, and it was the secret to understanding how stars work [BLANK_AUDIO]. So the second trick, is that we can use spectra, to tell what objects are made of. So how does that work? When we look at the spectrum of a star like the sun, as well as the smooth range of light, you can see here in this image, you also see dark features, at very particular wavelengths. Now, those dark features are caused by atoms, in the atmosphere of the sun, absorbing light at very specific wavelengths. Now, this happens because of the way atoms behave. So quantum mechanics tells us that atoms can exist in a discrete set of energy levels, and, if light is approaching at exactly the right wavelength, it can kick an atom up from one energy level to another. It has to be just the right wavelength to do that trick with a specific atom. But then once that light has done that and kicked that atom up to an excited level then that light has been absorbed by the atom, and so it's missing in the light that we see, and so we see a dark feature. Now, those sets of energy levels for an atom are unique to different kinds of atoms, different elements. So, likewise the missing lines that we see that correspond to those atomic energy levels, are a fingerprint for that atom. So, we can tell what atoms are present by seeing which wavelengths are missing in the spectrum. So, that's how we can tell What a star is made of - by those missing lines. Now that's the way it works in a star, that the atoms in the atmosphere are absorbing the bright light from the star. But if we have a glowing gas cloud as we see for example in the Orion Nebula, it can glow at discrete wavelengths which are exactly the same. And again, it's atoms, which are already excited and are dropping back down, and that makes emission lines rather than absorption lines at the same wavelengths. Now, molecules, as well as single atoms, can do a very similar trick. They likewise have a unique set of energy levels, and so produce a spectrum with discrete lines. But, in the case of molecules, those lines come out in the far-infrared and in the radio region. So, we typically measure them with radio telescopes. So, this image I'm showing you, here, this shows the far infrared spectrum, of the Orion Nebula, as measured by the Herschel space observatory. And the spectrum was superimposed on an image of the nebula. And you can see it's just chock full of lines that correspond to quite complex molecules. And if it wasn't for this technique, we would have had no idea that those complex molecules existed in space. [BLANK_AUDIO]. The third trick is that we can use spectra to measure the velocities of objects. Now, this is the trick that we are going to concentrate on for Catherine's science sections this week. So, let's see how that works. Now sometimes when we get the spectrum of an object, and we look at those dark lines or the bright emission lines, sometimes they appear at the wrong wavelength. When that happens, it's because of the motion, of the object, as you heard from Catherine in weeks three and four. The Doppler Shift is an effect that means, when an object is moving towards you or away from you, the light gets squashed up or stretched out. Now if we measure the amount of shift, then we can tell the velocity of the object and we can use that in all sorts of fascinating ways. So here's an interesting example. This is a picture of the galaxy M87. It's a Hubble Space telescope picture. And the two little circles you can see either side of the middle are places where the Hubble Space Telescope collected the light to take a spectrum. Now the line plot that you're looking at here, shows a small portion of the spectrum from each of those two spots. It shows an emission line from hydrogen atoms. Now, you see that, on one side of the nucleus of the galaxy, the line is shifted to the blue, and on the other side of the galaxy, it's shifted to the red. And that's because one side of the galaxy is moving towards us and the other side away from us. That tells us that, in fact, what we're looking at in this galaxy, is a disk of gas that's rotating. So from the size of that shift, that you see in that plot. We get the delta lambda and we can work out the velocity. So, we know the rotation speed is about 550 kilometers a second, of that gas disc in that galaxy. Then we can play the same trick that Catherine showed you earlier with Cygnus X-1. By applying Newton's laws, We know how big a mass must be causing that rotation in the centre, to make that particular speed. So we can work out the mass of the object in the centre. And it comes out, in this case, to be about 3 billion solar masses. Now, that's where it gets weird, because. when you look at the starlight in the centre, the number of stars in the middle is a thousand times less than that. So whatever this mass is, thats causing the gas to rotate, its dark; and, so we indeed think it is a supermassive black hole, and that's how we know it's there. So that's our 3 tricks, and now Catherine is going to tell you a little bit more about how we use this velocity trick, to study the acceleration of universe and dark energy.
