life history of a star

A star goes through the following stages during its life (the explanation is simplified):

	Flow-chart summarising some of the possible life-histories of a star. Its mass determines which path it follows.

1. Stars form from massive clouds of dust and gas in space.
   
   
2. Gravity pulls the dust and gas together. As the gas falls together, it gets hot.
   
   
3. A star forms when it is hot enough for a nuclear reaction to start. Hydrogen nuclei fuse (join together) to become helium nuclei. This releases a lot energy and the star 'burns' brightly. Any nuclear reaction in which nuclei fuse is called nuclear fusion.
   
   
4. The star remains stable in this phase for millions or billions of years.The nuclear reaction continues and keeps the star burning hot. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. Gravity pulls smaller amounts of outer dust and gas together, which form planets in orbit around the star. Our Sun and Solar System are in this stable phase. Our sun is a type of star called a yellow dwarf. It has been shining for nearly five billion years, and has enough hydrogen fuel to last another five billion years.
   
   
5. When the star runs out of the hydrogen nuclear fuel in its core, it may expand to become a red giant. Very big red giants are called red supergiants. During this phase, helium and then other nuclei may be burned.
   •  Stars like our Sun - small to medium mass will eventually form normal red giants.
   •  Stars like Betelgeuse - large mass will keep having nuclear fusion reactions, using helium or heavier nuclei as fuel. They'll eventually become red supergiants
   
   
6. What happens once all possible nuclear fuel is used up depends on how massive the red giant is:
   
   (a) As a red giant cools and collapses under the force of gravity, it forms a type of star called a ¨white¨dwarf¨ - millions of times denser than the matter on Earth. This in turn gradually fades and changes colour as it cools.
   
   (b) A red super giant, on the other hand, will go an getting hotter and expanding until it explodes. This phenomenon is called a supernova. The outer layers of dust and gas are flung off into space may eventually help form new stars. What's left - the core of the star - collapses to form a type of star called a neutron star, which is even denser than a white dwarf.
   
   (c) On the other hand again, an exceptionally massive neutron star will continue shrinking and eventually become what's known as a black hole - which is even denser than a neutron star.
   

Black holes

The remnants of a supernova may form a neutron star. However, if enough matter is left behind, it may contract under its own gravity to become extremely dense, with such a strong gravitational field that nothing can escape from it. This includes electromagnetic radiation such as light: that's why it is called a black hole.

Black holes cannot be seen, but their effects on the gases from neighbouring stars can be observed. For example, X-rays are emitted when these gases spiral into a black hole.

[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science ]