If you look up to the sky at night, especially a clear moonless night, you will find that the sky is dominated by stars. A star is a huge ball of glowing gas in the sky.  The sun is a star.  It is the only star close enough to the earth to look like a ball.   From a dark site, several hundred to a thousand stars are visible to the naked eye depending on the location and time of year. There are more than 200 billion billion stars.  If everyone were to count stars each person could count more than 50 billion of them without the same star being counted twice.  The stars differ in brightness and also in colour. If you look closely at the brighter stars, you will see that many are coloured. The colour of stars is an indication of their surface temperature (red=coolest, blue=hottest).  Stars twinkle because starlight comes to us through moving layers of air that surround the earth.  A star is made up of two gasses: hydrogen and helium.  Most stars began shining about 10 billion years ago.  People have studied the stars since ancient times.  Early farmers watched the stars to know when to plant their crops.

A star begins its life as a cloud of interstellar gas and dust.  The first step in the formation of a new star is the contraction of part of an interstellar cloud into a ball.  Through millions of years, the cloud of gas and dust contracts as gravity pulls it together.  As the material pulls together into a ball, the pressure of the gas increases.  When the ball reaches about 1,100,000 degrease Celsius the nuclear fusion reaction begins.  The hydrogen in the centre begins to change into helium