Nucleosynthesis

up to sulfur may be made in stars of all masses by charged-particle fusion reactions. Iron group elements originate mostly from the nuclear-statistical equilibrium process in supernova explosions. Elements beyond iron are made in stars with slow neutron capture , and by rapid neutron capture in the , with origins being debated among rare supernova variants and collisions. Note that this graphic is a first-order simplification of an active research field with many open questions. 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) 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.. 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. 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 . 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 Margaret Burbidge|, Geoffrey Burbidge|, 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 Volatile (astronomy)| 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. s, and (2) a general decrease in abundance, as elements become heavier. Within this trend is a peak at abundances of iron and nickel, which is especially visible on a logarithmic graph spanning fewer powers often, say between and . ==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 Big Bang nucleosynthesis occurred within the first three minutes of the beginning of the universe and is responsible for much of the abundance of (protium), (D, deuterium), (helium-3), and (helium-4). Although continues to be produced by stellar fusion and alpha decays and trace amounts of continue to be produced by spallation and certain types of radioactive decay, most of the mass of the isotopes in the universe are thought to have been produced in the BigBang. The nuclei of these elements, along with some and are considered to have been formed between 100and 300seconds after the BigBang when the primordial froze out to form protons and neutrons. Because of the very short period in which nucleosynthesis occurred before it was stopped by expansion and cooling (about 20minutes), no elements heavier than beryllium (or possibly boron) could be formed. Elements formed during this time were in the plasma state, and did not cool to the state of neutral atoms until much later. {{Image frame|align=center|width=400|caption=Chief nuclear reactions responsible for the relative abundances of light atomic nuclei observed throughout the universe. \begin{array}{ll} \ce{n^0 -> p+{} + e^-{} + \overline{\nu}_e} & \ce{p+{} + n^0 -> ^2_1D{} + \gamma}\\ \ce{^2_1D{} + p+ -> ^3_2He{} + \gamma} & \ce{^2_1D{} + ^2_1D -> ^3_2He{} + n^0}\\ \ce{^2_1D{} + ^2_1D -> ^3_1T{} + p+} & \ce{^3_1T{} + ^2_1D -> ^4_2He{} + n^0}\\ \ce{^3_1T{} + ^4_2He -> ^7_3Li{} + \gamma} & \ce{^3_2He{} + n^0 -> ^3_1T{} + p+}\\ \ce{^3_2He{} + ^2_1D -> ^4_2He{} + p+} & \ce{^3_2He{} + ^4_2He -> ^7_4Be{} + \gamma}\\ \ce{^7_3Li{} + p+ -> ^4_2He{} + ^4_2He} & \ce{^7_4Be{} + n^0 -> ^7_3Li{} + p+} \end{array}}} Stellar nucleosynthesis Stellar nucleosynthesis is the nuclear process by which new nuclei are produced. It occurs in stars during stellar evolution. It is responsible for the galactic abundances of elements from carbon to iron. Stars are thermonuclear furnaces in which hydrogen and helium are fused into heavier nuclei by increasingly high temperatures as the composition of thecore evolves. Of particular importance is carbon because its formation from He is a bottleneck in the entire process. Carbon is produced by the triple-alpha process in all stars. Carbon is also the main element that causes the release of free neutrons within stars, giving rise to the , in which the slow absorption of neutrons converts iron into elements heavier than iron and nickel. The products of stellar nucleosynthesis are generally dispersed into the through massloss episodes and the of stars. The massloss events can be witnessed today in the planetary nebula phase of star evolution, and the explosive ending of stars, called supernovae, of those with more than eight times the mass of the Sun. The first direct proof that nucleosynthesis occurs in stars was the astronomical observation that interstellargas has become enriched with heavy elements as time passed. As a result, stars that were born from it late in the galaxy, formed with much higher initial heavy element abundances than those that had formed earlier. The detection of technetium in the atmosphere of a star in1952, by spectroscopy, provided the first evidence of nuclear activity within stars. Because technetium is radioactive, with a much less than the age of the star, its abundance must reflect its recent creation within that star. Equally convincing evidence of the stellar origin of heavy elements is the large overabundances of specific stable elements found in stellar atmospheres of asymptotic giant branch stars. Observation of barium abundances some times greater than found in unevolved stars is evidence of the operation of the within such stars. Many modern proofs of stellar nucleosynthesis are provided by the isotopic compositions of stardust, solid grains that have condensed from the gases of individual stars and which have been extracted from meteorites. Stardust is one component of and is frequently called presolar grains. The measured isotopic compositions in stardust grains demonstrate many aspects of nucleosynthesis within the stars from which the grains condensed during the star's episodes. Explosive nucleosynthesis Supernova nucleosynthesis occurs in the energetic environment in supernovae, in which the elements between silicon and nickel are synthesized in established during fast fusion that attaches by reciprocating balanced nuclear reactions to 28Si. can be thought of as almost equilibrium except for a high abundance of the 28Si nuclei in the feverishly burning mix. This concept Further nucleosynthesis processes can occur, in particular the (rapid process) described by the and first calculated by Seeger, Fowler and Clayton, in which the most isotopes of elements heavier than nickel are produced by rapid absorption of free neutrons. The creation of free neutrons by electron capture during the rapid compression of the supernova core along with the assembly of some seed nuclei makes the a primary process, and one that can occur even in a star of pure H and He. This is in contrast to the B2FH designation of the process as a secondary process. This promising scenario, though generally supported by supernova experts, has yet to achieve a satisfactory calculation of abundances. The primary has been confirmed by astronomers who had observed old stars born when galactic metallicity was still small, that nonetheless contain their complement of nuclei; thereby demonstrating that the metallicity is a product of an internal process. The is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most isotopes of each heavy element. The rp-process (rapid proton) involves the rapid absorption of free protons as well as neutrons, but its role and its existence are less certain. Explosive nucleosynthesis occurs too rapidly for radioactive decay to decrease the number of neutrons, so that many abundant isotopes with equal and even numbers of protons and neutrons are synthesized by the silicon quasi-equilibrium process. The most convincing proof of explosive nucleosynthesis in supernovae occurred in1987 when those lines were detected emerging from . lines identifying 56Co and 57Co nuclei, whose half-life| limit their age to about a year, proved that their radioactive cobalt parents created them. This nuclear astronomy observation was predicted in1969 Neutron star mergers As of the mid-2020s, the merger of binary neutronstars(BNSs) is believed to be the main source of elements. Being by definition, mergers of this type had been suspected of being a source of such elements, but definitive evidence was difficult to obtain. In2017 strong evidence emerged, when , Virgo, the Fermi SpaceTelescope and , along with a collaboration of many observatories around the world, detected both and signatures of a likely neutronstar merger, GW170817, and subsequently detected signals of numerous heavy elements such as gold as the ejected degenerate matter decayed and cooled. The first detection of the merger of a neutronstar and came in July2021 and more after but analysis seem to favor over as the main contributors to heavy metal production. Black hole accretion disk nucleosynthesis Nucleosynthesis may happen in accretion disks of . Cosmic ray spallation Cosmic ray spallation process reduces the atomic weight of interstellar matter by the impact with , to produce some of the lightest elements present in the universe (though not a significant amount of deuterium). Most notably spallation is believed to be responsible for the generation of almost all of 3He and the elements lithium, beryllium, and boron, although some and are thought to have been produced in the BigBang. The spallation process results from the impact of cosmicrays (mostly fastprotons) against the interstellar medium. These impacts fragment carbon, nitrogen, and oxygen nuclei present. The process results in the light elements beryllium, boron, and lithium in the cosmos at much greater abundances than they are found within stellar atmospheres. The quantities of the light elements 1H and 4He produced by spallation are negligible relative to their primordial abundance. Beryllium and boron are not significantly produced by stellar fusion processes, since 8Be has an extremely short of seconds. ==Empirical evidence==

Theories of nucleosynthesis are tested by calculating isotope abundances and comparing those results with observed abundances. Isotope abundances are typically calculated from the transition rates between isotopes in a network. Often these calculations can be simplified as a few key reactions control the rate of other reactions. ==Minor mechanisms and processes==

Tiny amounts of certain nuclides are produced on Earth by artificial means. Those are our primary source, for example, of technetium. However, some nuclides are also produced by a number of natural means that have continued after primordial elements were in place. These often act to create new elements in ways that can be used to date rocks or to trace the source of geological processes. Although these processes do not produce the nuclides in abundance, they are assumed to be the entire source of the existing natural supply of those nuclides. These mechanisms include: • Radioactive decay may lead to radiogenic daughter nuclides. The nuclear decay of many primordial isotopes, especially , , and produces many intermediate daughter nuclides before they too finally decay to isotopes of lead. The Earth's natural supply of elements like radon and polonium is via this mechanism. The atmosphere's supply of is due mostly to the radioactive decay of in the time since the formation of the Earth. Little of the atmospheric argon is primordial. is produced by alpha decay, and the helium trapped in Earth's crust is also mostly . In other types of radioactive decay, such as cluster decay, larger species of nuclei are ejected (forexample, ), and these eventually become newly-formed stable atoms. • Radioactive decay may lead to spontaneous fission. This is not cluster decay, as the fission products may be split among nearly any type of atom. , , and are primordial isotopes that undergo spontaneous fission. Natural technetium and promethium are produced in this manner. • Nuclear reactions: Naturally-occurring nuclear reactions powered by radioactive decay give rise to nucleogenic nuclides. This process happens when an energetic particle from radioactive decay, often an alpha particle, reacts with a nucleus of another atom to change the nucleus into another nuclide. This process may also cause the production of further subatomic particles, such as neutrons. Neutrons can also be produced in spontaneous fission and by neutron emission. These neutrons can then go on to produce other nuclides via neutron-induced fission, or by neutron capture. For example, some stable isotopes such as and are produced by several routes of nucleogenic synthesis, and thus only part of their abundance is primordial. • Nuclear reactions due to : By convention, these reaction products are not termed "nucleogenic" nuclides, but rather cosmogenic nuclides. Cosmicrays continue to produce new elements on Earth by the same cosmogenic processes discussed above that produce primordial beryllium and boron. One important example is , produced from in the atmosphere by cosmicrays. is another example. ==See also==