Question

does nuclear fusion and fission both occur in the sun?
why will the sun eventually change its state from yellow dwarf to other form
why is the temperature of the sun gradually decreasing

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Answer 1

only nuclear fusion occure

Answer 2

Both fission and fusion are nuclear reactions that produce energy, but the applications are not the same. Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine together releasing vast amounts of energy and
Although the energy produced by fission is comparable to what is produced by fusion, the core of the sun is dominated by hydrogen and at temperatures where hydrogen fusion is possible, so that the dominant source of energy per cubic meter is in fusion rather then the fission of very low abundance radioisotopes.