What gives the sun necessary energy for nuclear fission?

In the sun, nuclear fusion dominates over nuclear fission. The nuclear fusion process is likely because the sun’s core is full of hydrogen gases at temperatures where fusion is possible. The energy released from fusion in turn drives other fusion and fission reactions. Hence it is a continuing cycle where reactions drive other reactions keeping the sun burning for millions of years. Normally, initiating these reactions require energy.

However, at the high temperatures present in the sun, the energy is already provided and is not a limiting factor anymore. Hence, the reactions continue to initiate other chain reactions keeping the cycle turning.