The center of an atom is held together by powerful forces. This gives them a huge amount of stored energy. This type of energy is called nuclear energy. This energy can be released and used to do work. This happens in nuclear power plants where they split apart the nucleus of an atom. This splitting apart is called nuclear fission. Another type of nuclear energy happens in the Sun. Here the atoms nuclei are not split apart. Instead, the nuclei of the atoms are fused, or joined together. This process is called nuclear fusion. Some of the suns energy travels to Earth. This energy from nuclear fusion warms the planet and provides the energy for photosynthesis. 
What type of nuclear energy process occurs in the sun?
The splitting of atoms