The energy released from the collapse of the gas into a protostar causes the center of the protostar to become extremely hot. When the core is hot enough, nuclear fusion commences. Fusion is the process where two hydrogen atoms combine to form a helium atom, releasing energy.

The fusion reaction is a very efficient process, releasing a huge amount of energy. This is because a single helium atom contains less mass than two hydrogen atoms. The excess mass is released as energy. Thanks to the pioneering work of Albert Einstein, the formula E = mc² tells us exactly how much energy the fusion reaction releases.

Fusion will power the star for 90% of its lifetime.