In Chapter 6 we have shown that the ordinary matter of the universe consists of about 87% protons and 13% neutrons (plus, of course, electrons). It was stated that the neutrons virtually all ended up in the form of helium-4 after a few minutes, and very few other nuclei were formed. In this Chapter the rates of the relevant nuclear fusion reactions are used to calculate directly the final abundances of the light nuclei: deuterium, helium-3 and helium-4.
The bulk of the nuclear reactions occur in the period 100 to 200 seconds. The reactions virtually cease at ~500 seconds. It is shown that the deuterium and helium-3 abundances are about four orders of magnitude less than that of the dominant nuclei (protons and helium-4) and consistent with observed cosmic abundances. The primordial composition of the universe was therefore about 92% protons and 8% helium-4 (by particle density), or 75% protons and 25% helium-4 (by mass).
Early workers in this field (e.g. Gamow) had the idea that the Big Bang fireball might be responsible for synthesising all the chemical elements. This Chapter demonstrates that this is not the case.
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The Horsehead Nebula in Orion (Barnard 33, nebula IC434) located just below Alnitak, the star farthest left on Orion's Belt. The darkness of the Horsehead is caused mostly by thick dust. The red glow originates from hydrogen gas ionized by the nearby bright star Sigma Orionis.