Stars are formed from large clouds of dust and hydrogen gas left over from the creation of the Universe.
If the clouds are disturbed, the dust and gas will start to clump together and rub against each other, making them heat up. The cloud will start to spin as it collapses. This hot, spinning ball is called a ‘protostar’.
As more dust and gas falls into the middle, the protostar will keep heating up. If it gets hot enough, a reaction called nuclear fusion will start in the centre. This changes hydrogen into helium, and all of the star’s energy comes from this reaction. We see and feel this energy as light and heat. This is what is happening in our Sun at the moment.
When the hydrogen in the star runs out, it will grow and become a red giant. This will happen to the Sun in about 5 billion years, and it will become so big that it will reach the Earth. If the star is hot enough in the centre, a new nuclear reaction can start. This will turn helium into carbon. If it gets even hotter it may start to turn carbon into oxygen, and then oxygen into silicon.
Our Sun will not get this hot, so the nuclear reactions will stop when all the helium has turned to carbon. When this happens, the middle of the star will shrink and the outer parts will fly off into space. We call this a ‘planetary nebula.’
The star that is left is called a ‘white dwarf’. The white dwarf is about the same size as the Earth, but is much, much heavier. A chunk the size of a sugar cube would be as heavy as two large polar bears. At first the white dwarf is very hot, but it cools down and gets dimmer until it can no longer be seen. It is then called a ‘black dwarf.’ It will stay like this forever.
In bigger stars, the middle gets hot enough for other nuclear reactions to happen. When these reactions stop, the middle of the star shrinks very quickly and the outside explodes. This is called a ‘supernova’. There is then a very small and very heavy star left behind. This is called a neutron star. A lump of neutron star the size of a sugar cube would be as heavy as all of the people on the Earth put together.
An even bigger star, one more than 25 times heavier than the Sun, would shrink even more and become a black hole.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001