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Nuclear physics · stellar evolution

Stellar Fusion: Hydrogen to Iron

Step through the proton-proton chain, triple-alpha, carbon, neon, oxygen, and silicon burning, followed by nickel decay, with each temperature gate and energy direction visible.

Interactive modelStellar Fusion: Hydrogen to Iron
Current stageHydrogen burning
Main product4He{}^4\mathrm{He}
Temperature gate≥1.5×107 K\ge 1.5\times10^7\,\mathrm K
Energy resultReleased
Core conditionReaction active

Physics tutorial

Stellar nucleosynthesis: why evolution moves toward the iron peak

BackgroundBinding energy per nucleon rises rapidly with mass number and peaks near iron. Combining light nuclei into more tightly bound products releases mass deficit, but fusion beyond the peak requires energy.

Why it mattersThis energy landscape controls stellar luminosity, lifetime, shell structure, and fate. The Sun burns only light fuels, while massive stars ignite progressively hotter and shorter stages.

Start with the essentials

Focus question
Why can a star not fuse hydrogen directly into iron, instead passing through increasingly hot multi-stage burning?
One-sentence intuition
Each increase in nuclear charge raises the Coulomb barrier. Exhausting one fuel lets the core contract and heat until the next reaction network opens; the iron peak ends this source of support.

Core mathematical model

Mass deficit releases energy

Q=(minitial−mfinal)c2Q=(m_{\rm initial}-m_{\rm final})c^2

When final rest mass is lower, Q is positive and the difference becomes particle kinetic energy, photons, and neutrinos.

Coulomb barrier scale

VC(r)≈Z1Z2e24πε0rV_C(r)\approx\frac{Z_1Z_2e^2}{4\pi\varepsilon_0r}

Larger nuclear charges repel more strongly, so carbon, oxygen, and silicon burning require successively hotter cores.

Common difficulties

The sequence is not one proton at a time

Typical misconceptionA star synthesizes every element in periodic-table order.

Better mental modelThe real process is a branching reaction network. This lab keeps dominant burning stages and representative products while omitting many isotopes and side reactions.

Silicon does not fuse directly into iron once

Typical misconceptionSilicon burning is simply two silicon nuclei colliding into one iron nucleus.

Better mental modelPhotodisintegration, captures, and nuclear statistical equilibrium first favor ⁵⁶Ni, which later decays through cobalt toward ⁵⁶Fe.

Run the experiment

  1. 01

    Test the endpoint of a Sun-like star

    Choose Sun-like star and advance the chain.

    What to observe: Low mass and temperature block carbon burning; not every star manufactures iron.
  2. 02

    Open the massive-star chain

    Choose Massive star and click through successive stages.

    What to observe: The mass gate is met, but sufficient temperature is still required to ignite carbon, neon, oxygen, and silicon burning.
  3. 03

    Read the binding-energy curve

    Advance toward the iron peak and follow the red marker.

    What to observe: Every exothermic stage climbs in binding energy per nucleon; beyond iron, fusion no longer supplies energy.