Stars fuse hydrogen into helium, converting a tiny fraction of mass into energy via E=mc². Two pathways compete:
pp-chain: 4 ¹H → ⁴He + 2e⁺ + 2ν + γ (rate ∝ T⁴)
CNO cycle: 4 ¹H → ⁴He + 2e⁺ + 2ν + γ (rate ∝ T¹⁶, catalyzed by C/N/O)
L ∝ core density × reaction rate
Below about 18 million K the slow pp-chain dominates (the Sun's core, ~15 MK). Above that, the CNO cycle's steep temperature dependence takes over — it powers stars more massive than about 1.3 M☉. Photons produced in the core random-walk outward for thousands of years before escaping as starlight.
- pp-chain / CNO toggle — previews each pathway's particle choreography and its very different temperature sensitivity.
- Core temperature — the strongest lever on fusion rate; CNO's rate rises far faster with temperature than pp-chain's.
- Stellar mass — scales core size and gravitational compression, shown as core radius.
- Core density — a direct multiplier on collision (fusion) rate.
- Time scale — speeds up the visualization; real fusion timescales are far slower than any animation.
This is the same physics that sets a star's main-sequence lifetime, its color, and ultimately whether it ends as a white dwarf, neutron star, or black hole.