Nuclear fusion and stellar equilibrium
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Question
What is nuclear fusion?
Answer
Joining two or more **light** nuclei into a **heavier** one; the product is slightly lighter and the missing mass is released as energy.
Question
What is Coulomb repulsion, and why does it matter for fusion?
Answer
The electrical **push** between two positive charges. Nuclei are positive, so they repel — fusion must overcome this to bring them together.
Question
What two conditions let a star's core overcome Coulomb repulsion?
Answer
Very high **temperature** (fast-moving nuclei) and very high **density / pressure** (frequent collisions).
Question
What is the proton-proton (p-p) chain?
Answer
The series of reactions that fuses **hydrogen into helium** in stars like the Sun, releasing energy at each step.
Question
Which equation gives the energy released in a fusion reaction?
Answer
$E = mc^{2}$ — mass-energy equivalence (given in the data booklet). Here m is the mass defect.
Question
Fast way to convert a mass defect in u into energy in MeV?
Answer
Multiply Δm (in u) by **931.5**, because 1 u = 931.5 MeV c⁻².
Question
Where does the energy released in fusion actually come from?
Answer
The **mass defect** — the product is slightly lighter than the nuclei that fused, and that missing mass becomes energy.
Question
What is stellar (hydrostatic) equilibrium?
Answer
The state where the **outward** pressure from fusion's radiation and hot gas exactly **balances** gravity's **inward** pull, so the radius stays stable.
Question
What balances gravity in a main-sequence star?
Answer
The **outward pressure** from the heat of fusion — radiation pressure plus the pressure of the hot gas. (Not the reactions themselves directly.)
Question
Why is a star's equilibrium self-correcting?
Answer
If it shrinks → core heats → fusion speeds up → more pressure → it expands back. If it expands → cools → fusion slows → gravity pulls it back in.
Question
A fusion reaction has Δm = 0.0265 u. Energy released in MeV?
Answer
E = 0.0265 × 931.5 ≈ **24.7 MeV**.
Question
Three steps to find the energy released by fusion?
Answer
1) mass defect Δm = total mass of nuclei − mass of product; 2) E = mc² (or Δm × 931.5 for MeV); 3) keep the unit.
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Fusion and stars
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