EPS SCI 3 Lecture Notes - Lecture 13: Ecliptic, Photosphere, Geology Of The Moon
7 Dec 2016
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Document Summary
In the first millionth of a second, pair-production and annihilation of protons and neutrons proceeded at equal rates. Era of nuclei: density and temperature were too low for nuclear fusion to proceed: by the time fusion stopped, about 20-25% of the primordial nucleons had been converted to helium-4. But the temperature was still too high for electrons to bind to nuclei: photons continuously exchanged energy with charged particles in the plasma, keeping universe opaque. Fusion to fe, fission to fe (most stable atom) Helium was produced by nuclear fusion during nucleosynthesis era: nuclei composed of 5, 6, 7, and 8 nucleons are less stable than he-4. They were formed but destroyed by collisions almost instantaneously. First nucleus stable is c-12, but requires fusion of 3 x h-4. By the time enough he-4 had formed, the universe had expanded enough that [he-4] was too low to allow fusion to c-12. Big bang left us with h, deuterium, he-3, he-4 and li-7.
