For a long time, the people did not know the nature of the element (or elements) of which the sun was made. So, all the earlier theories put forward to explain the source of sun’s energy were only a guess-work. It was only around the year 1920 when the method of spectrum analysis was devised that the real composition of the sun became known. In order to find out the composition of the sun, the wavelengths of light emitted by the sun were compared with the wavelengths emitted by the known elements, when hot. It was then found that the wavelengths of light emitted by the sun tallied with that emitted by hydrogen element, when hot From this it was concluded that the sun is made up of hydrogen gas.
Image Source: eurasiablog.lcms.org/files/2014/02/Sun1.jpg
When it became known that the sun contains a huge amount of hydrogen gas, it was suggested that when hydrogen burns in oxygen, it produces a lot of heat energy. So, the combustion (or turning) of hydrogen gas in the sun was said to be the source of sun’s energy. It was, however, argued by others that the sun’s energy is so enormous that it just can’t be produced by burning hydrogen gas, like we burn natural gas. By now it was clear that sun’s energy comes from hydrogen, but the process by which this energy is released took some more time to reveal itself.