In general,the energy producted and radiated by the sun,more specifically the term refers to the sun's energy that reaches the earth. Solar energy ,receieved in the form of radiation, can be converted directly or indirectly into other forms of energy ,such as heat and electricity which can be utilized by man. Since the sun is expected to radiate at an essentially constant rate for a few billion years,it may be regarded as an in exhaustible source of useful enegy. The sun is a large sphere of very hot gases,the heat being generated by various kinds of fusion reactions.