It is thought that the universe started 10-13 billion years ago with a big bang. Immediately after the big bang, hydrogen atoms formed. The gravity pulls the hydrogen atoms and dust together, causing a star to form. These gases become more concentrated at the core. This causes the temperature and pressure at the centre to increase and eventually leads to nuclear fusion. The hydrogen nuclei fuse to form helium. This releases energy and the new star begins to shine. The star is now a hot ball of gas, being heated by the nuclear fusion at the centre. This period is called its main sequence.

A White Dwarf

A Red Giant

The star will continue to remain stable like this for millions of years and until all the hydrogen fuel is used up. For stars as big as the sun this could take about 10 billion years. After this it will expand and cool to become a red giant. After a few hundred million years the nuclear reactions cease and the star contracts. This leaves a small white star known as a white dwarf, which shines weakly for a long time before eventually fading from sight.