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High energy processes[show]
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Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons (protons and neutrons). The first nuclei were formed about three minutes after the Big Bang, through the process called Big Bang nucleosynthesis, which formed the hydrogen and helium content of the first stars, and is responsible for the general hydrogen/helium ratio of the universe today. In space, cosmic ray spallation is also a significant source of specific nuclei (9Be and 10,11B) that are not created by stellar nucleosynthesis.
With the formation of stars, heavier nuclei were created from hydrogen and helium by stellar nucleosynthesis, a process that continues today. Some of these elements, particularly those lighter than iron, are thought to be delivered to the interstellar medium in the last stages of evolution of dying low mass stars, in the non-explosive ejection of the outer envelope gases of plantetary nebulae before these stars continue to form white dwarfs.
Supernova nucleosynthesis, nuclear reactions within exploding stars, is responsible for the abundant elements between magnesium (A=24) and nickel (A=60). Supernova nucleosynthesis is also thought to be responsible for the creation of elements heavier than iron and nickel (Z > 26-28), in the last few seconds of the explosion of a supernova, such as a type II supernova event. These elements must absorb energy as they are created, and do so from energy available in the supernova explosion. Some of the elements are created from the absorption of multiple neutrons in a space of a very short time (a few seconds) during an explosion. The elements formed in supernovas include formation of the heaviest elements known in the solar system, such as the long-lived primordial element radionuclides uranium and thorium. The nuclear fission of such elements releases the energy they absorbed during their creation.
In addition to these major processes responsible for the (growing) natural abundances of elements in the galaxy, a few minor natural processes continue to produce very small numbers of new nuclides on Earth. These nuclides are unimportant for the natural abundances, but may account for the presence of specific new nuclei on Earth. These nuclides are naturally produced via the decay of long-lived primordial radionuclides such as uranium and thorium (radiogenesis), from natural nuclear reactions caused by cosmic ray bombardment of elements on Earth (cosmogenic nuclides), and from other natural nuclear reactions powered by particles from radioactive decay, (producing nucleogenic nuclides).
2 History of nucleosynthesis theory
4 The major types of nucleosynthesis
4.1 Big Bang nucleosynthesis
4.2 Stellar nucleosynthesis
4.3 Explosive nucleosynthesis
4.4 Cosmic ray spallation
5 Empirical evidence
6 Minor mechanisms and processes
7 See also
9 Further reading
It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees. A few minutes afterward, starting with only protons and neutrons, nuclei up to lithium and beryllium (both with mass number 7) were formed, but only in relatively small amounts. Some boron may have been formed at this time, but the process stopped before significant carbon could be formed, because this element requires a far higher product of helium density and time than were present in the short nucleosynthesis period of the Big Bang. The Big Bang fusion process essentially shut down at about 20 minutes after the Big Bang, due to drops in temperature and density as the universe continued to expand. This first process, Big Bang nucleosynthesis, was the first type of nucleogenesis to occur in the universe.
The subsequent nucleosynthesis of the heavier elements required stars and supernova explosions. These happened as hydrogen and helium from the Big Bang collapsed into the first stars 500 million years after the Big Bang. Star formation has occurred continuously in the galaxy since that time. The initial elements found on Earth, the so-called primordial elements, were created prior to Earth's formation by stellar nucleosynthesis and by Supernova nucleosynthesis. They range in atomic numbers from Z=6 (carbon) to Z=94 (plutonium). Synthesis of these elements occurred either by nuclear fusion (including both rapid and slow multiple neutron capture) or to a lesser degree by nuclear fission followed by beta decay.
By contrast, nuclear reactions within stars destroy deuterium and isotopes of other light elements, beryllium, lithium, and boron, which were contained in the initial compositions of the stars. Interstellar gas therefore contains declining initial abundances of these light elements, which are present only by virtue of their nucleosynthesis in the Big Bang. Larger quantities of these lighter elements in the present universe are therefore thought to have been formed mainly through billions of years of cosmic ray (mostly high-energy proton) mediated breakup of heavier elements residing in interstellar gas and dust. Fragments of these cosmic-ray collisions include isotopes of Li, Be and B.
History of nucleosynthesis theory
The first ideas on nucleosynthesis were simply that the chemical elements were created at the beginnings of the universe, but no successful physical scenario for this could be identified. Hydrogen and helium were clearly far more abundant than any of the other elements (all the rest of which constituted less than 2% of the mass of the solar system, and presumably other star systems as well). At the same time it was clear that carbon was the next most common element, and also that there was a general trend toward abundance of light elements, especially those composed of whole numbers of helium-4 nuclei.
Arthur Stanley Eddington first suggested in 1920 that stars obtain their energy by fusing hydrogen to helium, but this idea was not generally accepted because it lacked nuclear mechanisms. In the years immediately before World War II Hans Bethe first provided those nuclear mechanisms by which hydrogen is fused into helium. However, neither of these early works on stellar power addressed the origin of the elements heavier than helium.
Fred Hoyle's original work on nucleosynthesis of heavier elements in stars occurred just after World War II. This work attributed production of all heavier elements formed in stars during the nuclear evolution of their compositions, starting from hydrogen. Hoyle proposed that hydrogen is continuously created in the universe from vacuum and energy, without need for universal beginning.
Hoyle's work explained how the abundances of the elements increased with time as the galaxy aged. Subsequently, Hoyle's picture was expanded during the 1960s by creative contributions by William A. Fowler, Alastair G. W. Cameron, and Donald D. Clayton, and then by many others. The creative 1957 review paper by E. M. Burbidge, G. R. Burbidge, Fowler and Hoyle (see Ref. list) is a well-known summary of the state of the field in 1957. That paper defined new processes for changing one heavy nucleus into others within individual stars, processes that could be documented by astronomers.
The Big Bang itself had been proposed in 1931, long before this period, by Georges Lemaître, a Belgian physicist and Roman Catholic priest, who suggested that the evident expansion of the Universe in forward time required that the Universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the Universe into a single point, a "primeval atom", at a point in time before which time and space did not exist. Hoyle later gave Lemaître's model the derisive term of Big Bang, not realizing that Lemaître's model was needed to explain the existence of deuterium and nuclides between helium and carbon, as well as the fundamentally high amount of helium present, not only in stars but also in interstellar gas. As it happened, both Lemaître and Hoyle's models of nucleosynthesis would be needed to explain elemental abundances in the universe.
The goal of nucleosynthesis is to understand the vastly differing abundances of the chemical elements and their several isotopes as being a result of natural history. The primary stimulus to the development of this theory was the shape of the natural abundances. Those abundances, when plotted on a graph as a function of atomic number of the element, have a jagged sawtooth structure varying by factors up to ten million. A very influential stimulus to nucleosynthesis was an abundance table by Hans Suess and Harold Urey based on the unfractionated abundances of the non-volatile elements within unevolved meteorites. Such data, itself hard won by scientists, demanded a natural explanation to scientists, rather than as a given by God. Such a graph of the abundances is displayed on a logarithmic scale below, where the dramatically jagged structure is visually suppressed by the many powers of ten spanned in this graph. See Handbook of Isotopes in the Cosmos for more data and discussion of abundances of the isotopes.
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2 things that are very hot smashing into each other very quickly.
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