Q.Discuss the process of nuclear fusion and how energy is generated in stars.

Step 1. Definition. Nuclear fusion is the combination of two or more light nuclei ($A<20$ each) into a single heavier nucleus, with the mass deficit between reactants and product released as energy.

Step 2. Why extreme temperature is needed. Unlike fission, fusion never happens spontaneously at room temperature, because the mutual Coulomb repulsion between two positively-charged light nuclei keeps them apart long before the short-range strong nuclear force can act. Overcoming this repulsion requires the nuclei to carry very high kinetic energy - equivalently, temperatures of order $10^7$ K or higher - a condition called thermonuclear fusion.

Step 3. Stars as the natural setting. The one place such extreme temperatures routinely occur is deep in the core of stars; thermonuclear fusion is the sole energy source of every star, including the Sun. A star's life begins as a collapsing cloud of gas and dust, with gravitational potential energy converting to kinetic energy (heat) as it collapses, until the core becomes hot enough to ignite fusion, which then stabilises the star against further collapse.

Step 4. The Sun's proton-proton chain. According to Hans Bethe, the Sun is powered by the proton-proton chain: (i) $^{1}_{1}H+{}^{1}_{1}H\to{}^{2}_{1}H+e^{+}+\nu$; …

Explain why fusion needs extreme temperature, connect this to why stellar cores are the natural setting, and walk throu …

• Omitting the reason fusion doesn't happen at room temperature (Coulomb repulsion) - this is the essential physical constraint the whole answer hinges on. …