Question

Briefly explain how energy is produced in stars, giving two examples of the nuclear reactions involved.

Hint:

The proton-proton chain dominates in smaller stars like the Sun, while the CNO cycle is more efficient in hotter, more massive stars, both ultimately fusing hydrogen into helium.

Answer 1

Step 1: Stellar Energy Generation.
Stars produce energy via nuclear fusion in their cores. Extremely high temperatures and pressures facilitate the fusion of light nuclei into heavier ones, overcoming electrostatic repulsion. This process, governed by \( E = mc^2 \), liberates energy from a mass defect, primarily converting hydrogen to helium and emitting heat and light.Step 2: Nuclear Reaction Examples.
- Proton-Proton Chain (Initial Step): In stars like the Sun, two protons combine: \[^1\text{H} + ^1\text{H} \to ^2\text{H} + e^+ + u_e + 0.42 \, \text{MeV}\]This yields a deuteron (\( ^2\text{H} \)), a positron (\( e^+ \)), and a neutrino (\( u_e \)), with an energy release of 0.42 MeV.
- CNO Cycle (Initial Step): In more massive stars, a proton fuses with carbon-12: \[^{12}\text{C} + ^1\text{H} \to ^{13}\text{N} + \gamma + 1.95 \, \text{MeV}\]A gamma photon (\( \gamma \)) is emitted, releasing 1.95 MeV. Subsequent reactions in the cycle produce helium.