E.5.2—Fusion in stars

Use fusion as a stellar source

In stellar fusion, light nuclei combine to form more tightly bound nuclei. The mass difference is released as energy, which powers the star and supports its pressure balance.

E_{\text{released}}=B_{\text{products}}-B_{\text{reactants}}=\Delta mc^2

Worked example — deuterium–tritium fusion

For $^2\mathrm{H}+{}^3\mathrm{H}\rightarrow{}^4\mathrm{He}+n$ , the binding energies are $2(1.11)=2.22\,\mathrm{MeV}$ , $3(2.83)=8.49\,\mathrm{MeV}$ and $4(7.07)=28.28\,\mathrm{MeV}$ . Therefore $E=28.28-2.22-8.49=17.57\,\mathrm{MeV}$ . The product is more tightly bound, so energy is released.

Follow nucleosynthesis

Fusion in stars can build elements up to iron through successive reactions. Elements heavier than iron are mainly formed in explosive environments and neutron-capture processes rather than ordinary core fusion.

Compare fusion and fission

Fusion can offer high energy per mass and potentially fewer long-lived waste products, but it requires extreme temperature and confinement conditions.

Common trap

Do not say all heavy elements are made by fusion in ordinary stars. The pathway changes around iron.

How it is assessed

Questions compare fusion with fission or outline how elements heavier than hydrogen and helium formed.

Command terms

Outline / State

What earns marks

Mention stellar nucleosynthesis/fusion for elements up to iron, then supernova or neutron capture for heavier elements.

Watch for

Claiming fusion alone forms every element or ignoring the comparison condition in an advantage question.

Retrieve stellar balance

Fusion releases energy, outward thermal/radiation pressure balances inward gravity, and high temperature and density allow fusion in the core.

Retrieve stellar inference

Mass controls evolution; HR regions classify stars; parallax gives distance; and $L=4\pi R^2\sigma T^4$ gives stellar radius from luminosity and temperature.

E.5.2—Fusion in stars

Trace Fusion in Stars

E.5.2 Exam Analysis

Assessment in practice

Retrieve the Stellar Model