HOW STARS BURN

A star is a ball of gas squeezed by its own weight until its centre is hot enough for nuclei to fuse. Nuclei repel each other, and in the Sun's centre the electric hill between two protons is hundreds of times higher than their typical energy; they fuse only by tunnelling through it, in a narrow window of energies. Heavier nuclei face higher hills, so each stage of a star's life needs it hotter than the last: hydrogen, then helium, carbon, neon, oxygen and silicon, ending at nickel-56, which decays to iron.

• Real: every reaction and its rate, from the REACLIB library (the snapshot that the pynucastro project ships; Cyburt et al. 2010, ApJS 189, 240), 149 reactions among 38 nuclei integrated as a network; which reactions happen at each moment and how often, relative to each other; the composition, the energy released, and how long each stage takes at its conditions.
• Simplified: each stage is held at one temperature and density (round values for the Sun's core, then for the core of a star of about twenty Suns) and run until its fuel is gone, so the times are for those conditions held fixed, not a real star's stage lengths. Left out: screening by the electrons (5–20% in the Sun), the neutron side channels, partition functions in the reverse rates, and the energy neutrinos carry off (counted here).
• Cartoon: how nuclei move, and how many are drawn (64, where a real core holds about 10⁵⁰; rare ones such as the CNO cycle's carbon are drawn even when they'd be fewer than one). The clock skips, and the stats line says by how much; with "show reactions" turned up, rare reactions are shown more often than they happen, and each says how rare it really is.

Code: net.js (the network), life.js (the whole life and the knobs), fusion.js (this page), reaclib.js (the rates, made by tools/extract.py). Next: the same for four dimensions (the strong-force dial has its first part).