CoachTwo nuclei are both positive: to touch, they must first climb the hill of their electric repulsion (Chapter 14) — the Coulomb barrier. Only near do collisions carry the energy to cross it; the fuel is then a plasma of bare nuclei and electrons. Fission pays no such toll: its trigger, the neutron, is neutral, and works cold.
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Example 33.19 (The Sun runs on it)
The Sun’s core, at , fuses ordinary hydrogen step by step — the proton–proton chain, of net effect , about per helium: of the hydrogen’s mass leaves as energy. The sunlight whose spectrum we read in Chapter 2 is this mass, arriving eight minutes late; every star shines by walking the curve toward iron.