Answer:
Fusion of Hydrogen to form Helium
Step-by-step explanation:
The sun is a big ball of gas and it provides solar energy that drives external processes on the earth surface.
The pressure and temperature within the sun provides energy to fuse abundant hydrogen atoms together to form helium atoms. This nuclear fusion process releases a huge amount of energy.
The fusion of hydrogen atoms to form helium is the source of radioactivity in the sun.