Last week we looked at the Big Bang, this week we look at what followed - the beginning of our universe and the birth of stars. After the Big Bang, the universe took hundreds of thousands of years to cool, and even then it was still as hot as our sun. And all the time it was expanding. Scientists say that about 300,000 years after the Big Bang, hydrogen and helium atoms began to form. These atoms clumped together in huge clouds of dust and gas called nebulae. The clouds became so hot and dense that there were nuclear reactions within them and the hydrogen (the lightest atom) fused with the helium (the heaviest atom) - and the clouds exploded into huge balls of fire. These were the first stars. Nebulae are known as star nurseries, because this is where stars are born - they are still being born, 15 billion years after the Big Bang. Stars don't survive forever - eventually they burn out, but it takes a very long time (think billions of years). The bigger a star is, the faster it burns up all its fuel and dies. Our sun is a star, and this process is happening to our sun too, but there's no need to worry, scientists say we've got at least another four billion years before our sun burns itself out. So what happens when a star dies? It depends on the size of the star. When our sun eventually burns itself out, it will go through a series of explosions, gradually shedding its atmosphere. If a star is a few times bigger than our sun, then the results are more dramatic - the temperatures soars to a billion degrees Celsius and then the whole star explodes into a supernova. There is a tremendous flash of light, and it leaves behind a spinning neutron star. But the most incredible death goes to the even bigger stars. Rather then just burning out and collapsing in on themselves, monster stars do something incredible - they bend the fabric of time and space to create one of the most fascinating galactic phenomenon: Black Holes. Black holes are spectacular and deserve a whole column of their own, so we will look at those next week.