Stars
The Life of the Stars depend upon the amount of Hydrogen gas present in them. As the amount of Hydrogen gas converted into helium, the temperature and colour of the star changes. The life of the star ends when there is very little or no hydrogen left with them. The final stage of the star depends upon their mass. Some are converted into red giants, some to white dwarfs and some ends with neutron star and some with black Hole.
Red Giants
The stars produce heat and light energy continuously due to the fusion of hydrogen nuclei to helium nuclei. As the hydrogen of the star keeps on decreasing, the surface of the star goes on decreasing along with its surface temperature. The Outward pressure created by the fusion reaction becomes less and less. As a result, the core of the sun contracts on its own gravity. However, the outer shell of the star which consists of hydrogen gas, expands due to the energy released in fusion reaction. This continues and finally the star becomes red and very giant. In this way red giant is formed.
White Dwarf
When the amount of hydrogen gas becomes very low, a star becomes red giant. If the mass of the star is comparable to the mass of solar system, the core part of the star goes on contracting and temperature at the core goes on increasing . It causes expansion of the atmosphere of the star and the star explodes. This is known as SuperNova. After nova exploision, the atmosphere of the star is lost and the core part continues to contract. Finally, the temperature of the core becomes very high, the helium nuclei start to fuse to form carbon/nitrogen nuclei. In this reaction, the star produces white visible light and thus make itself look white . In this way white dwarf is formed.
