Nucleosynthesis is the process that creates new atomic nuclei from nucleons (protonsand neutrons) and nuclei. According to current theories, the first nuclei were formed a fewminutes after the through nuclear reactions in a process called BigBang nucleosynthesis. After about 20minutes, the universe had expanded and cooled to a point at which these collisions among nucleons ended, so only the fastest and simplest reactions occurred, leaving our universe containing hydrogen and helium, traces of other elements, such as lithium, and the hydrogen isotope deuterium. Nucleosynthesis in stars and stellar events such as novas and supernovas later produced the variety of elements and isotopes that we have today, in a process called cosmic chemical evolution. The amounts of totalmass in elements heavier than hydrogen and helium (called "metals" by astrophysicists) remain small (afew percent), so that the universe still has approximately the same composition.
Stars fuse light elements to heavier ones in their cores, giving off energy in the process known as stellar nucleosynthesis. Nuclear fusion reactions create many of the lighter elements, up to and including iron and nickel in the most massive stars. Products of stellar nucleosynthesis remain trapped in stellar cores and remnants except if ejected through and explosions. The neutron capture reactions of the and create heavier elements, from iron upwards.
Supernova nucleosynthesis within exploding stars is largely responsible for the elements between oxygen and rubidium: from the ejection of elements produced during stellar nucleosynthesis; through explosive nucleosynthesis during the supernova explosion; and from the (absorption of multiple neutrons) during the explosion.
Neutron star mergers are a recently-identified major source of elements produced in the . When two neutronstars collide, a significant amount of matter may be ejected which then quickly forms heavy elements.
Cosmic ray spallation is a process wherein impact nuclei and fragment them. It is a significant source of the lighter nuclei, particularly 3He, 9Be and 10,11B, that are not created by stellar nucleosynthesis. Cosmicray spallation can occur in the interstellar medium, on asteroids and meteoroids, or on Earth in the atmosphere or in the ground. This contributes to the presence on Earth of cosmogenic nuclides.
On Earth new nuclei are also produced by radiogenesis, the decay of , primordial radionuclides such as uranium, thorium, and .
History
Timeline The first protons and neutrons were formed from the plasma around ago during the , as the universe cooled below about ten billion Kelvin, or one MeV. As the universe continued to cool over the next few minutes, these protons and neutrons could combine to form hydrogen (deuterium) and helium nuclei. Trace amounts of lithium were also formed, but given the high amount of radiation in proportion to matter at this time, the high binding energies of heavier elements meant they were not produced. Free electrons were combined with these existing nuclei around 380,000 years after the Big Bang, when matter and radiation were decoupled, producing the Cosmic Microwave Background. This first process, BigBang nucleosynthesis (BBN), was the first type of nucleogenesis to occur in the universe. The vast majority of matter in the universe even today (about 74%) is hydrogen produced by BBN, while another 24% is helium. The first stars (called Population III stars) to form in the universe were therefore formed almost entirely of hydrogen and helium, with trace amounts of lithium, beryllium, and boron. (Some Li, Be, and B currently found in the universe was formed through BBN, but most was made either through cosmic ray spallation or through stellar fusion.)
The subsequent nucleosynthesis of heavier elements (called "metals" by astronomers) requires the extreme temperatures and pressures found within stars and supernovae. These processes began as hydrogen and helium created by BBN collapsed into the first stars, called Population III stars, a few hundred million years after the Big Bang. Starformation has been occurring continuously in galaxies since that time, often called "Cosmic Dawn". Heavier elements in today's universe are synthesized through stellar nucleosynthesis (e.g. carbon and nitrogen), supernova nucleosynthesis (e.g. magnesium, aluminum), or in exotic events such as neutronstar collisions (e.g. europium). Some elements are synthesized through multiple of these processes. Other nuclides, such as Ar, formed through radioactive decay.
Because the processes by which different elements form happen at different points in a given galaxy's life, astronomers can sometimes use the abundances of these elements to date the formation of a star or structure within a galaxy. For example, most magnesium found in the interstellar medium (ISM) was ejected into it by core-collapse supernovae, whereas much of the universe's iron was formed in type Ia supernovae. Type Ia supernovae occur when one star in a binary dies, leaving a white dwarf remnant. Material from the white dwarf's normal binary companion accretes onto it, and its temperature and density increase to the point of ignition, causing it to violently explode. Typically, we see the effects of type Ia supernovae on the composition of the ISM around 1 billion years after the start of star formation in that galaxy. Meanwhile, core-collapse supernovae are the deaths of massive stars, which typically live only a few million years. This means that as a general rule, the ratio of magnesium to iron abundance in stars that form earlier than 1 billion years after the start of star formation in a region will be higher than the same ratio in stars that form later, since magnesium began to be incorporated into the ISM earlier than iron.
History of nucleosynthesis theory The first ideas on nucleosynthesis were simply that the chemical elements were created at the beginning of the universe, but no rational physical scenario for this could be identified. Gradually it became clear that hydrogen and helium are much more abundant than any of the other elements. All the rest constitute of the mass of the , and of other star systems as well. At the same time it was clear that oxygen and carbon were the nexttwo mostcommon elements, and also that there was a general trend toward high abundance of the light elements, especially those with isotopes composed of whole numbers of nuclei .
Arthur Stanley Eddington first suggested in1920 that stars obtain their energy by fusing hydrogen into helium and raised the possibility that the heavier elements may also form in stars. This idea was not generally accepted, as the nuclear mechanism was not understood. In the years immediately before , first elucidated those nuclear mechanisms by which hydrogen is fused into helium.
Fred Hoyle's original work on nucleosynthesis of heavier elements in stars, occurred just after .Actually, before the war ended, he learned about the problem of spherical implosion of plutonium in the . He saw an analogy between the plutonium fission reaction and the newly discovered supernovae, and he was able to show that exploding supernovae produced all of the elements in the same proportion as existed on Earth. He felt that he had accidentally fallen into a subject that would make his career. Autobiography William A. Fowler His work explained the production of all heavier elements, 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 the1960s by contributions from WilliamA. Fowler, Cameron, and DonaldD. Clayton, followed by many others. The seminal1957 "B2FH"review paper by , , Fowlerand Hoyle is a summary of the state of the field in1957. That paper defined new processes for the transformation of one heavynucleus into others within stars, processes that could be documented by astronomers.
The Big Bang itself had been proposed in1931, long before this period, by , a Belgian physicist, who suggested that the evident expansion of the Universe in time required that the Universe, if contracted backwards in time, would continue to do so until it could contract no further. This would bring all the mass of the Universe to a single point, a "primeval atom", to a state before which time and space did not exist. Hoyle is credited with coining the term "BigBang" during a 1949BBC radio broadcast, saying that 's theory was "based on the hypothesis that all the matter in the universe was created in one bigbang at a particular time in the remotepast". It is popularly reported that Hoyle intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the twomodels. '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 space. As it happened, both Lemaître and Hoyle's models of nucleosynthesis would be needed to explain the elemental abundances in the universe.
The goal of the theory of nucleosynthesis is to explain the vastly differing abundances of the chemical elements and their several isotopes from the perspective of natural processes. The primary stimulus to the development of this theory was the shape of a plot of the abundances versus the atomic number of the elements. Those abundances, when plotted on a graph as a function of atomic number, have a jagged sawtooth structure that varies by factors up to tenmillion. Avery influential stimulus to nucleosynthesis research was an abundance table created by and that was based on the unfractionated abundances of the elements found within unevolved meteorites. Such a graph of the abundances is displayed on a logarithmic scale , where the dramatically jagged structure is visually suppressed by the many powers often spanned in the vertical scale of this graph.
Processes
There are several astrophysical processes which are believed to be responsible for nucleosynthesis. The majority of these occur within stars, and the chain of those nuclear fusion processes are known as hydrogen burning (viathe proton–proton chain or the ), helium burning, carbon burning, neon burning, oxygen burning and silicon burning. These processes are capable of creating elements up to and including iron and nickel. This is the region of nucleosynthesis within which the isotopes with the highest binding energy pernucleon are created.
Heavier elements can be assembled within stars mainly by the slow neutron capture process known as the or in explosive environments, such as supernovae and neutronstar mergers, by the , which involves rapid neutron captures (faster than the half-lifes of the intermediate isotopes). There is also a minor contribution from processes involving proton capture, such as the , and the . These processes allow the synthesis of some proton-rich isotopes that cannot be created by neutron capture and subsequent beta-decays.
Major types
Big Bang nucleosynthesis