Stars and Galaxies
We're not just talking about UCAS though, oh no! We're talking stars and galaxies as well. Well we were getting withdrawal symptoms...
So, what happens when a star dies? It depends on the size of the star, but our own Sun is likely to blow its outer layers off and form a planetary nebula. You say you don't know what one is? You've probably seen a picture of one before, a famous one is the crab nebula shown below.
Mike doesn't study such pretty nebulas though, the ones he looks at are much further away and appear as green blobs. This is because when a star reaches the end of its life it only really emits green light in its spectrum. Our own Sun actually peaks in the green, but it appears more yellow than anything due to the way our eyes work.
The spectrum of light can be easily seen by yourself with just a CD. You've probably done it by accident before! Just reflect the light onto a handy near by wall and you'll see something like this: