Nucleosynthesis
Nuclosynthesis refers to the primordial generation of the light elements when the universe was formed. The most abundant element in our present universe is hydrogen, followed by helium. The big bang theory owes much of its success to its ability to predict the relative amounts of these elements.
The big bang also holds that the first elements created in the universe were the light elements, namely helium, deuterium and lithium, which were produced in the first few instants. At that point, the temperature was so high, all matter was fully ionized and dissociated. About three minutes later, the temperature of the universe rapidly cooled from 1,032K to approximately 109K. At this temperature, nucleosynthesis began to occur: protons and neutrons collided to produce deuterium, which consists of one proton and one neutron. This newly generated deuterium then collided with other protons and neutrons to produce helium as well as a small amount of tritium, i.e. the 3-hydrogen isotope. Some trace amounts of 7-lithium from the merging of one tritium and two deuterium nuclei were also produced. The universe was then one second old and its temperature was 1,010K.
The theory initially held that all elements were generated by the big bang, but this was subsequently revised: even at the extremely high temperatures available when helium and lithium nuclei were created, they were still not high enough to smash two helium nuclei together into a heavier atom. It is now known, mostly from the work of George Gamow, that stars had to be created to provide the required temperature ranges before the remaining elements of the periodic table could be generated.
Elements heavier than helium originated in the interiors of stars, which formed much later as the universe unfolded. All stars derive their energy from the thermonuclear fusion of light elements into heavier elements. For nuclear species to be transformed into other nuclear species by reactions that add or remove protons or neutrons or both, requires very high temperatures to overcome the mutual electrostatic repulsion of the protons in each fusing atomic nucleus. For example, the temperature required for the fusion of hydrogen is 5 million degrees and elements with more protons in their nuclei necessitate even higher temperatures, such as carbon, which requires a temperature of 1 billion degrees for fusion to proceed. The big bang theory not only successfully accounts for nucleosynthesis but also for the synthesis and cosmic abundance of the elements up to iron, by successive nuclear fusion reactions, and of the elements heavier than iron, by neutron capture. Deuterium and lithium, while consumed in the nuclear reactions occurring in stars, are very rarely produced by them. Whatever amounts of these two elements are present in the universe were created some 15 billion years ago, as was most of the helium.
The relative amounts of helium, lithium, deuterium and hydrogen predicted by the big bang theory are confirmed by astronomical observations. For example, the observed hydrogen to helium ratio is of 4:1, which is in close agreement with the predicted ratio. Thus the study and observation of stellar processes not only contributes descriptive knowledge about our physical world but also provides a measure of its history and future evolution.
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