The sun is the main source of light for the whole world but how it is possible no one knows till 1905 when Albert Einstein gave us the theory of E=mc2; relation of mass and energy. This theory has answered the question very nicely.
The sun actually shines because within inside the sun fusion reaction takes place where a group of four hydrogen nuclei combines to form a single nuclei. It means the process of the conversion of lighter elements into heavier elements. In course of such reaction, mass losses and this lost mass gets converted into heat, light and other electro-magnetic radiations. This process is called nuclear fusion. In simple words, the sun shines because of burning hydrogen into helium. This process is continuous. The sun converts 5 million tons of lost mass into energy every second. This energy heats the sun and makes it so shiny. Later, when the earth or anything absorbs it, it gets heated.
