Birth and death of a star

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.