In 1938, H. Bethe and F.V. Weizacker independently put forward the first detailed mechanism for energy production in the stars. They concluded that the temperature at the core of the star is enough for thermonuclear fusion (that is at ~ ).
In fusion reactions, light elements are transformed into heavier ones. The final reaction products have a smaller total mass than initial nuclei. This mass difference is released as energy according to Einstein relation . This difference is called binding energy. The binding energy per nucleon is,
In above equation,
A is Atomic weight, is proton mass, is neutron mass and is mass of the necleus
The energy production mechanism proposed by Bethe and Weizacker are as follows: