Stars are formed from large clouds of interstellar dust and hydrogen gas left over from the creation of the Universe.
Disturbances in the clouds cause a clumping of the gas and dust which then pulls more matter in due to gravity. The particles of dust and gas will rub against each other, and heat up due to friction. They will also begin to rotate around the centre of the clump. The rotating centre of the clump of gas and dust is called a ‘protostar’.
As more of the matter falls into the centre, the protostar continues to heat up. If it reaches a critical temperature (about 10,000,000 C), nuclear fusion will begin in its core. Hydrogen atoms fuse to form helium. This provides all of the star’s energy, which is given out as heat and light. A star that is ‘burning’ hydrogen in a nuclear reaction is known as a Main Sequence star. It will spend most of its lifetime like this.
The hydrogen in the star’s central region or ‘core’ will eventually run out and the star will then expand to become a red giant. When this happens to the Sun it will expand out to a distance of the Earth’s orbit. If the star is hot enough in the centre, it will begin a new fusion reaction, turning helium into the heavier element carbon.
At this point, the star’s evolution depends on its mass. A low mass star like our Sun will stop fusion reactions as soon as the helium has run out. When this happens the core of the star will contract. This throws off the outer layers of the star to form an expanding shell of glowing dust and gas called a ‘planetary nebula.’
The star that is left in the centre is known as a ‘white dwarf.’ At this point the star is about the same size as the Earth, but it is incredibly dense. A sugar-cube sized piece of the star would weigh as much as two fully-grown polar bears. The white dwarf starts at a temperature of about 10,000 C but will eventually cool and fade until it can no longer be seen. It is then known as a ‘black dwarf.’ It will stay like this forever.
A star more massive than the Sun will be hot enough in the centre for further fusion reactions to begin, so will continue to burn carbon into oxygen, and oxygen into silicon and so on, until either the star is not hot enough to start another reaction or iron is created. Once the fuseable elements run out, the star will collapse and explode in a supernova. This leaves behind a very dense core called a neutron star. This is even more dense than a white dwarf. A cubic centimetre of neutron star would weigh as much as all of the people on the Earth put together – that’s 6 billion people packed into a volume the size of a sugar cube!
An even more massive star, one more than 25 times the mass of the Sun, would collapse even further to form a black hole.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001