Mass deficit releases energy
When final rest mass is lower, Q is positive and the difference becomes particle kinetic energy, photons, and neutrinos.
Nuclear physics · stellar evolution
Stellar Fusion: Hydrogen to Iron
Why does fusion release energy only until nuclei approach iron? Binding energy per nucleon peaks near iron. Fusing lighter nuclei moves toward that peak and releases energy; beyond it, fusion absorbs energy instead. The star works through ever hotter burning stages until its core is iron, and fusing iron cannot hold the core up.
Step through the proton-proton chain, the triple-alpha process, carbon, neon, oxygen and silicon burning, and finally nickel decay. Each step shows the temperature it needs and which way the energy goes.
Current stageHydrogen burning
Main product
Temperature gate
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. Fusing light nuclei into more tightly bound ones releases the energy of the mass deficit, but fusion beyond the peak takes energy in.
Why it mattersThis energy curve controls how bright a star is, how long it lives, how its shells are arranged and how it ends. The Sun burns only light fuels, while massive stars ignite a series of hotter, 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
- Every step up in nuclear charge raises the Coulomb barrier. When one fuel runs out, the core contracts and heats until the next set of reactions can start. At the iron peak, fusion can no longer supply this support.
Core mathematical model
Coulomb barrier scale
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. The Lab shows only the dominant burning stages and representative products, and leaves out 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 modelAt these temperatures, photodisintegration, captures and nuclear statistical equilibrium first favor nickel-56, which later decays through cobalt toward iron-56.
Run the experiment
- 01
See where a Sun-like star stops
Choose Sun-like star and advance the chain.
What to observe: The star’s mass and temperature are too low for carbon burning. Not every star makes iron. - 02
Open the massive-star chain
Choose Massive star and click through successive stages.
What to observe: The star is massive enough, but carbon, neon, oxygen and silicon burning each still need a high enough temperature to ignite. - 03
Read the binding-energy curve
Advance toward the iron peak and follow the red marker.
What to observe: Each stage that releases energy climbs the curve of binding energy per nucleon. Beyond iron, fusion no longer supplies energy.
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