So this is a very, very long story. What happens is that through our galaxy, which is huge, our galaxy is a collection of stars and gas and dust, that is so big that light would take more than 100,000 years to travel from one side to the other. That's huge, so light would travel to the moon in one second, it travels 300,000 kilometres every second. So some of these clouds of gas and dust are floating around in our galaxy, this collection of stars 100,000 light years across, are themselves really vast. There may be 100 light years from side to side, so I mean that light would take 100 years to travel from one side to the other. And these clouds contain an unimaginable amount of material, gas and dust. Sometimes those clouds start to collapse under their own gravity. They start, the dust starts to be pulled together by the gravitational pull of all of the other dust, and that makes them start to collapse. And as they collapse down, they break up into clumps and fragments in the middle, and where those fragments are, they keep collapsing and the material in the middle keeps getting hotter and denser. That's just the same thing as if you ever get a bike pump and you're comparing to it, the end of the bike pump will get hot. And that's because you're compressing air so you're heating it up. This is the same process, but you're compressing things down from very, very big to relatively big, still a million kilometres, ten million kilometres across. But that's a few light seconds, rather than 100 light years. So you shrunk things down, the huge amount, which means they're really, really hot. When you get gas like hydrogen, very hot and you get the pressure very high, you start something magical called nuclear fusion. And this is an incredible power source. This is something that scientists are still trying to work out how to do on earth, because it will be the best way to power as the cleanest energy there is. To put it in perspective, how powerful this is. If you've got one bucket of sea water, just a little bucket, and we're able to do nuclear fusion on the hydrogen in the water in that bucket of sea water, that will power Australia for 300 years. That's how cool fusion is. In the middle of every star, there's a fusion furnace burning, turning hydrogen to helium and causing the stars to shine. So what you've had is a huge cloud of gas and dust that collapse down to very small objects, still much bigger than the earth, but a million kilometres across say. And in the middle of those balls of gas, the temperature has got so high that hydrogen atoms are being fused to make helium, giving a huge amount of energy causing the stars to shine. And at this point, all that energy they throw out into space, so like they're ready, they'll wind their blow away. That clears the gas and dust from around the stars, sweeping the gas and dust away, sweeping the stars behind, and obstruct it so that we can see them in the next sky.