Know that energy is released by nuclear fusion in the Sun

1.7.3 Energy Resources – Nuclear Fusion in the Sun

What is Nuclear Fusion?

Nuclear fusion is the process in which two light atomic nuclei combine to form a heavier nucleus. During this process a small amount of the total mass is converted into a large amount of energy.

Why the Sun is a Fusion Reactor

The Sun’s core is extremely hot (≈ \$1.5\times10^{7}\,\text{K}\$) and under immense pressure. These conditions allow hydrogen nuclei (protons) to overcome their electrostatic repulsion and fuse together.

Proton–Proton Chain Reaction

The dominant fusion process in the Sun is the proton–proton (p‑p) chain, which can be summarised in three main steps.

1. Two protons fuse to form a deuterium nucleus, a positron and a neutrino:
   

   \$\mathrm{^{1}H} + \mathrm{^{1}H} \rightarrow \mathrm{^{2}H} + e^{+} + \nu_{e}\$

2. The deuterium nucleus captures another proton, producing a helium‑3 nucleus and a gamma ray:
   

   \$\mathrm{^{2}H} + \mathrm{^{1}H} \rightarrow \mathrm{^{3}He} + \gamma\$

3. Two helium‑3 nuclei combine to give a helium‑4 nucleus and two protons:
   

   \$\mathrm{^{3}He} + \mathrm{^{3}He} \rightarrow \mathrm{^{4}He} + 2\,\mathrm{^{1}H}\$

Energy Released

The total mass of the four original protons is slightly greater than the mass of the resulting helium‑4 nucleus. The missing mass \$\Delta m\$ is converted into energy according to Einstein’s equation:

\$E = \Delta m\,c^{2}\$

For the complete p‑p chain the energy released is about \$26.7\ \text{MeV}\$ per helium‑4 nucleus formed, which corresponds to \$4.3\times10^{-12}\ \text{J}\$.

Comparison with Nuclear Fission

• Fusion combines light nuclei; fission splits heavy nuclei.
• Fusion releases energy because the binding energy per nucleon increases for light elements up to iron.
• Fission releases energy because the binding energy per nucleon decreases for very heavy elements.

Relevance to Earth

The Sun’s fusion provides a continuous, renewable source of energy that reaches Earth as sunlight. Understanding fusion helps explain why solar power is a viable, sustainable energy resource.

Summary Table – Proton–Proton Chain

Key Points to Remember

• Fusion in the Sun occurs via the proton–proton chain.
• Mass loss during fusion is converted to energy (\$E = mc^{2}\$).
• The Sun releases about \$3.8\times10^{26}\ \text{W}\$ of power, all from fusion.
• Solar energy is a renewable resource that can be harnessed on Earth.