Here's why:

* Nuclear fusion is the process where light atomic nuclei combine to form heavier nuclei, releasing tremendous amounts of energy. This is what happens in the Sun's core. Hydrogen atoms fuse to form helium, releasing energy in the process.

* Nuclear fission is the process where a heavy atomic nucleus is split into two or more lighter nuclei, also releasing energy. This is the process used in nuclear power plants.

The Sun's core is incredibly hot and dense, providing the necessary conditions for nuclear fusion to occur.