Nuclear fusion of hydrogen into helium occurs in the

There are types of nuclear reaction: nuclear fusion and nuclear fission. The difference is that fusion is a combination of two elements while fission is the breaking up of the subatomic particles of an element creating a new element. The limiting element to this is Iron. Iron-26 is the most stable element. As a result, elements lighter than Fe-26 are generally fusible. This includes hydrogen and helium.

This reaction is common in the stars, most especially the Sun. The energy of the Sun comes from its abundant hydrogen composition which becomes fusible into Helium. This occurs at a temperature of 14 million Kelvin. The nuclear reaction is a not a one-way step process as shown in the picture.