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Nuclear fusion and energy in the Sun and stars sheet
5-mark answer frame

Explain how the Sun and other stars produce their energy.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Name the source. Thermonuclear fusion of hydrogen into helium.
  2. 2 · Rule out other sources. Chemical burning is far too weak, and fission needs heavy nuclei, which the Sun hardly has.
  3. 3 · Explain the conditions. Gravity squeezes the core to about 1.5 × 10⁷ K (15 million kelvin), so protons move fast enough to overcome their repulsion.
  4. 4 · Give the steps.
    • ¹₁H + ¹₁H → ²₁H + ⁰₊₁e
    • ²₁H + ¹₁H → ³₂He
    • ³₂He + ³₂He → ⁴₂He + 2 ¹₁H
  5. 5 · Write the overall reaction. 4 ¹₁H → ⁴₂He + 2 ⁰₊₁e + about 26.7 MeV.
  6. 6 · Real life. This energy reaches us as sunlight that grows our crops.
  7. 7 · Close in one line. So, the Sun shines by fusing hydrogen into helium, and it has fuel for billions of years.

Scoring tip: Write the overall equation with charges balanced (4 = 2 + 2). Say why fission is ruled out; examiners like that line.