Stellar Core: Nuclear Fusion
Cosmology & the Sky arc, page 145 of 193. Back to The Pixelated Physics Universe
Who they are
The Sun doesn’t burn. It fuses, and loses weight doing it. In the core, at about 15.7 million kelvin, four protons become one helium-4 nucleus, releasing 26.73 MeV, counting the annihilation of the two positrons it makes. The Sun turns about 4.3 million tonnes of mass into energy every second. Hans Bethe worked out the reactions in 1939. Stellar Core on a pixel-art unisex tee; equation, symbols and sources in the panel below.
Stellar Core is a character of the Cosmology & the Sky arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
The physics
In the Sun's core, four hydrogen nuclei are turned step by step into one helium nucleus. The helium weighs a little less than what went in, and that missing mass comes out as energy, which is what makes the Sun shine.
$$ \begin{gathered} 4\,{}^{1}\mathrm{H} \to {}^{4}\mathrm{He} + 2e^{+} + 2\nu_e \\ Q = \left(4m_p + 2m_e - m_\alpha\right)c^{2} \approx 26.73\ \text{MeV} \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
| $m_p$ | proton mass | kg |
| $m_e$ | electron mass | kg |
| $m_\alpha$ | helium-4 nucleus (alpha) mass | kg |
| $Q$ | total energy released per helium-4, including annihilation of the two positrons with electrons (2 × 1.022 MeV); about 2% (roughly 0.5 MeV) is carried off by neutrinos; quoted in MeV | J |
Sources
- Bethe (1939) Energy Production in Stars, Phys. Rev. 55, 434
- NobelPrize.org — Hans Bethe, Physics 1967, facts
- Encyclopaedia Britannica — Nuclear fusion
- NIST CODATA 2022 — proton mass energy equivalent in MeV
- NIST CODATA 2022 — electron mass energy equivalent in MeV
- NIST CODATA 2022 — alpha particle mass energy equivalent in MeV
- NASA NSSDCA — Sun Fact Sheet (mass, central temperature; Earth comparison)
Crossovers
Stellar Core meets Ignia (Lawson Criterion). Ignia confines a plasma with magnets. Stellar Core confines it with gravity. Both need high density, temperature and confinement time for fusion to pay its way.
Stellar Core meets Gamowyn (Narrow Window). Stellar Core fuses protons in the Sun at about 15.7 million kelvin, which is too cool for most protons to climb the Coulomb barrier. Gamowyn opens the narrow window where the thermal tail and quantum tunnelling overlap, and fusion squeezes through.
Wear the lore
- Stellar Core – Nuclear Fusion Physics T-Shirt
- Related infographic poster: The pp-I Chain – Plate 069 Physics Poster
Concept keywords: Nuclear Fusion, Stellar Core, Cosmology & the Sky, physics lore, Pixelated Physics