 A scientist might tell you that the distant stars are made of hydrogen and helium. But we've never been to a distant star, so how do we know they're not made of cheese? I'm going to show you how a CD in a cereal box can help us to understand one of the most important tools in astrophysics. First we need to understand why it is that we use a CD. You might have noticed that when you move a CD around you can often see a rainbow in the reflections. The reason for that is that the music on a CD is stored in millions and millions of tiny little grooves. And these grooves mean that when light falls on the CD different colours are reflected at different angles. That's why you can see the rainbows. And it's also going to be crucial for the device that we're about to make. To make your CD spectrometer you need to get your cereal box and cut a slit into it at about 30 degrees from the bottom of the box. And then take your CD, shiny side up and slide it into the slot you just made. Then you need to make another slit on the opposite side of the box to the CD. And the quality of this slit is going to determine the quality of the images you produce. So you can make it just by cutting straight into the cardboard. That'll work okay. Or you can do what I've done here, which is get two bits of aluminium foil and put them really close together for a nice clean edge. So the light comes in through this slit. It goes across the box and bounces off the CD. And so if you want to see what's going on you're going to need to make a window in the top of the box. And that's it. You've made your CD spectrometer. Now you've just got to point this slit at any light sources you want to examine. So we might try a tungsten light bulb. That's one of the eco unfriendly ones that get very hot when you turn them on. For a tungsten light bulb you should see a complete rainbow. Red, orange, yellow, green, blue, indigo and violet. And that's because the white light given off by the tungsten bulb is actually a mixture of all those different colours. It's just that you can only see it when it's split up by the CD. Next let's try an energy efficient light bulb. Here although the light still looks white it's actually only made up of certain colours. Red, orange, green, blue and violet in this case. So finally we might go outside and try say a sodium street lamp. That's one of the old fashioned very yellow looking ones. Now from this you won't get a complete rainbow at all but just one very bright line. That's because sodium, the chemical element in the street lamp only gives off light of this one very specific yellow. In fact all chemical elements only give off light of certain specific colours and it's this fact that we're going to use to try to work out what the distant stars are made from. The inside of a star is a ball of very very hot gas and so like this very hot tungsten light bulb it gives off a rainbow of all the different colours simultaneously. Gases in the outer atmosphere of the star then absorb some of these colours and in fact the light they absorb is always of exactly the same colour as the light they emit when we heat them up in a light bulb here on Earth. So if this star had an outer atmosphere made entirely from sodium that would mean we'd see a dark line in the yellow in exactly the same place as the bright line we saw in the sodium street lamp with this spectrometer. By looking for dark lines corresponding to other chemical elements we can work out exactly what a star is made of without having to spend millions and millions of years travelling there. And in the process I've even managed to find a use for this ABBA compilation CD.