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: Energy production in stars.
Stars generate energy through nuclear fusion in their cores, where high temperatures and pressures allow light nuclei to overcome electrostatic repulsion and combine into heavier nuclei. This process releases energy due to the mass defect, as per \( E = mc^2 \), primarily converting hydrogen into helium, emitting heat and light. Step 2: Examples of nuclear reactions.
- Proton-Proton Chain (Step 1): In stars like the Sun, two protons fuse: \[ ^1\text{H} + ^1\text{H} \to ^2\text{H} + e^+ + \nu_e + 0.42 \, \text{MeV} \] A deuteron (\( ^2\text{H} \)), positron (\( e^+ \)), and neutrino (\( \nu_e \)) are produced, releasing 0.42 MeV of energy.
- CNO Cycle (Step 1): 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. The cycle continues to form helium.