Watch the three real reaction steps that fuse hydrogen into helium in the Sun's core. Click a step below to see it happen and read the real energy it releases.
Real reaction sequence (PP-I branch): Step 1 fuses two protons into a deuteron, releasing a positron and neutrino (1.44 MeV, and it happens twice per chain); Step 2 fuses that deuteron with another proton into helium-3, releasing a gamma ray (5.49 MeV, also twice per chain); Step 3 fuses two helium-3 nuclei into helium-4, releasing two protons back out (12.86 MeV).
Drag to orbit the scene and scroll or pinch to zoom. Click "Auto-play" to step through the whole chain automatically, or click any step button to jump straight to it.
Proton-Proton Chain Fusion 3D Explorer
This browser explorer walks through the proton-proton (pp) chain - the real sequence of nuclear reactions that fuses hydrogen into helium in the core of the Sun and most other main-sequence stars. Click each step to watch protons and neutrons combine and release energy, or press "Auto-play" to run the whole sequence on its own.
The pp-chain has a bottleneck built in: Step 1 needs a proton to convert into a neutron at the exact moment it fuses with another proton, a change mediated by the weak nuclear force. That conversion is so improbable per collision that a given proton in the Sun's core waits about 9 billion years on average before it happens - which is exactly why the Sun burns steadily for billions of years instead of fusing all at once.
- Click Step 1, 2, or 3 to watch that reaction happen and read its real energy release
- Step 1 and Step 2 each happen twice per full chain, building two helium-3 nuclei before Step 3 combines them
- Watch the positron, neutrino, and gamma-ray byproducts fly outward from each reaction
- Press "Auto-play" to step through the full chain automatically, or "Reset" to start over
- Runs fully in the browser with the vendored three.js engine - no account, no upload
| Step | Reaction | Releases |
|---|---|---|
| 1 (x2 per chain) | p + p → d + e+ + neutrino | 1.44 MeV |
| 2 (x2 per chain) | d + p → helium-3 + gamma | 5.49 MeV |
| 3 | helium-3 + helium-3 → helium-4 + 2 protons | 12.86 MeV |
| Net full chain | 4 protons → 1 helium-4 + 2 positrons + 2 neutrinos + 2 gamma rays | 26.73 MeV |
These figures come from standard nuclear-astrophysics Q-value tables (Adelberger et al., Reviews of Modern Physics 83, 195, 2011) and solar-model references (Bahcall & Pinsonneault, Reviews of Modern Physics 64, 885, 1992), cross-checked against NASA solar-interior fact sheets. The pp-chain supplies about 98-99% of the Sun's total energy output - the CNO cycle supplies the rest. Under this steady fusion rate the Sun converts about 600 million tonnes of hydrogen into helium every second, turning about 4 million tonnes of that mass directly into energy (E = mc squared) every second.
This page shows the reaction chemistry itself - which nuclei combine, what they release, and in what order. For the layered geography of the Sun's interior where this fusion happens (core, radiative zone, convective zone, photosphere), see Sun Structure 3D Explorer. For how a star's fusion rate determines its lifespan, see Main Sequence Lifetime 3D Explorer.
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached.
This is an educational approximation, not a physical simulation: nucleon colors (red proton, gray neutron, orange positron, cyan neutrino streak, yellow gamma streak) are a coding convention for legibility, not real particle colors, and particle identity is not tracked step to step because protons and neutrons of the same kind are indistinguishable in reality. The reaction sequence, particle counts, and energy released at each step are accurate to the cited references.
Frequently Asked Questions
What is the proton-proton chain?
It is the sequence of nuclear reactions that fuses hydrogen into helium in the core of the Sun and most other main-sequence stars. The dominant pathway, called PP-I, has three real steps: two protons fuse into a deuteron, the deuteron fuses with another proton into helium-3, and two helium-3 nuclei fuse into helium-4.
Why does Step 1 take so long?
Step 1 requires a proton to convert into a neutron via the weak nuclear force at the same instant it fuses with another proton - an extremely improbable combination. A given proton in the Sun's core waits about 9 billion years on average before this reaction happens to it, which is why the Sun fuses hydrogen slowly and steadily instead of all at once.
How much energy does the full chain release?
One complete pp-chain converts 4 protons into 1 helium-4 nucleus plus 2 positrons, 2 neutrinos, and 2 gamma rays, releasing 26.73 MeV total. About 2 percent of that escapes the Sun directly as neutrinos; the rest eventually reaches the surface as light.
Why do Steps 1 and 2 happen twice per chain?
Step 3 needs two helium-3 nuclei to fuse into helium-4. Building two helium-3 nuclei requires running Steps 1 and 2 twice first - once for each helium-3 - before Step 3 can combine them.
How much of the Sun's energy comes from the pp-chain?
About 98-99% of the Sun's total energy output comes from the pp-chain; the CNO cycle (a different fusion pathway involving carbon, nitrogen, and oxygen as catalysts) supplies the remaining 1-2%.
How is this different from the Sun Structure explorer?
Sun Structure shows the layered geography of the Sun's interior - the core, radiative zone, convective zone, and photosphere. This page shows the reaction chemistry itself: the specific particles that combine at each step and the real energy each step releases.