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A star starts off as a huge hydrogen cloud.[[image:00027723.jpg align="left"]]
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The gas collapses inwards under the force of gravity. The gas is heated due to friction. The cloud becomes a protostar which is very hot. Nuclear fusion hasn't started in the protostar.
==Very small protostars never reach high enough tempretures for a nuclear fusion to begin. these are called brown dwarfs and they cool gradually over hundreds of millions years. however in bigger protostars there will eventually be ahigh enough tempreture for hydrogen atoms to fuse with eachother to form helium. The star is now born.==

[[image:Sun_021203.jpg width="131" height="139" align="left"]]The star then is in a stable state.
Once a star uses most of the hydrogen gas in its core then the nuclear processes then cease. Without the outward pressure generated by these reactions