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By this time the universe had cooled to a few billion kelvins (10 K) and the rate of nucleosynthesis had slowed down significantly.
By the time the universe was three minutes old the process had basically stopped and the relative abundances of the elements was fixed at ratios that didn't change for a very long time: 75% hydrogen, 25% helium, with trace amounts of deuterium (hydrogen-2), helium-3, and lithium-7.
However, there are other processes thought to be responsible for some of the element nucleosynthesis, notably a proton capture process known as the rp process and a photodisintegration process known as the gamma (or p) process.
The latter synthesizes the lightest, most neutron-poor, isotopes of the heavy elements.
The second, and more common, cause is when a massive star, usually a red giant, reaches Nickel-56 in its nuclear fusion (or burning) processes.
This isotope undergoes radioactive decay into Iron-56, which has one of the highest binding energies of all of the isotopes, and is the last element that can be produced by nuclear fusion, exothermically.
The neutron flux is astonishingly high, about 10 also suggested that the r process abundances are a superposition of differing neutron fluences.
Small fluence produces the first r process abundance peak near atomic weight A=130 but no actinides, whereas large fluence produces the actinides uranium and thorium but no longer contains the A=130 abundance peak.
Fusion of elements between silicon and iron occurs only in the largest of stars, which end as supernova explosions (see Silicon burning process).
A neutron capture process known as the s process which also occurs during stellar nucleosynthesis can create elements up to bismuth with an atomic mass of approximately 209.