Sun is made up of gases mainly helium and hydrogen. The energy getting from sun is due to the fusion reaction between the hydrogen molecules in the sun. After a long time, when all the hydrogen is converted into helium, the fusion reaction stops, only very few hydrogen molecules fuse which are in the interior core of the sun.
The sun starts contracting due to the gravitational pull and the pressure the temperature increases. As the temperature reaches to a certain maximum value, the fusion of helium starts and then the sun again gives us energy in different forms.
