Суперновададағы элементтердің пайда болуы

Production of the elements in a supernova explosion
Supernova nucleosynthesis is the nucleosynthesis of chemical elements in supernova explosions. In sufficiently massive stars, the nucleosynthesis by fusion of lighter elements into heavier ones occurs during sequential hydrostatic burning processes called helium burning, carbon burning, oxygen burning, and silicon burning, in which the byproducts of one nuclear fuel become, after compressional heating, the fuel for the subsequent burning stage. In this context, the word "burning" refers to nuclear fusion and not a chemical reaction. During hydrostatic burning these fuels synthesize overwhelmingly the alpha nuclides , nuclei composed of integer numbers of helium 4 nuclei. Initially, two helium 4 nuclei fuse into a single beryllium 8 nucleus. The addition of another helium 4 nucleus to the beryllium yields carbon 12, followed by oxygen 16, neon 20 and so on, each time adding 2 protons and 2 neutrons to the growing nucleus. A rapid final explosive burning is caused by the sudden temperature spike owing to passage of the radially moving shock wave that was launched by the gravitational collapse of the core. W. D. Arnett and his Rice University colleagues suggesting that the expected shock wave nucleosynthesis is an essential component of supernova nucleosynthesis. Together, shock wave nucleosynthesis and hydrostatic burning processes create most of the isotopes of the elements carbon , oxygen , and elements with (from neon to nickel). As a result of the ejection of the newly synthesized isotopes of the chemical elements by supernova explosions, their abundances steadily increased within interstellar gas. That increase became evident to astronomers from the initial abundances in newly born stars exceeding those in earlier born stars. Elements heavier than nickel are comparatively rare owing to the decline with atomic weight of their nuclear binding energies per nucleon, but they too are created in part within supernovae. Of greatest interest historically has been their synthesis by rapid capture of neutrons during the r process, reflecting the common belief that supernova cores are likely to provide the necessary conditions. However, newer research has proposed a promising alternative (see the r process below). The r process isotopes are approximately 100,000 times less abundant than the primary chemical elements fused in supernova shells above. Furthermore, other nucleosynthesis processes in supernovae are thought to be responsible also for some nucleosynthesis of other heavy elements, notably, the proton capture process known as the rp process, the slow capture of neutrons (s process) in the helium burning shells and in the carbon burning shells of massive stars, and a photodisintegration process known as the γ process (gamma process). The latter synthesizes the lightest, most neutron poor, isotopes of the elements heavier than iron from preexisting heavier isotopes.

The papers of Hoyle (1946) and Hoyle (1954) and of B²FH (1957) were written by those scientists before the advent of the age of computers. They relied on hand calculations, deep thought, physical intuition, and familiarity with details of nuclear physics. Brilliant as these founding papers were, a cultural disconnect soon emerged with a younger generation of scientists who began to construct computer programs that would eventually yield numerical answers for the advanced evolution of stars and the nucleosynthesis within them.

A supernova is a violent explosion of a star that occurs under two principal scenarios. The first is that a white dwarf star, which is the remnant of a low mass star that has exhausted its nuclear fuel, undergoes a thermonuclear explosion after its mass is increased beyond its Chandrasekhar limit by accreting nuclear fuel mass from a more diffuse companion star (usually a red giant) with which it is in binary orbit. The resulting runaway nucleosynthesis completely destroys the star and ejects its mass into space. The second, and about threefold more common, scenario occurs when a massive star (12–35 times more massive than the sun), usually a supergiant at the critical time, reaches nickel 56 in its core nuclear fusion (or burning) processes. Without exothermic energy from fusion, the core of the pre supernova massive star loses heat needed for pressure support, and collapses owing to the strong gravitational pull. The energy transfer from the core collapse causes the supernova display. The nickel 56 isotope has one of the largest binding energies per nucleon of all isotopes, and is therefore the last isotope whose synthesis during core silicon burning releases energy by nuclear fusion, exothermically. The binding energy per nucleon declines for atomic weights heavier than , ending fusion's history of supplying thermal energy to the star. The thermal energy released when the infalling supernova mantle hits the semi solid core is very large, about 10 ergs, about a hundred times the energy released by the supernova as the kinetic energy of its ejected mass. Dozens of research papers have been published in the attempt to describe the hydrodynamics of how that small one percent of the infalling energy is transmitted to the overlying mantle in the face of continuous infall onto the core. That uncertainty remains in the full description of core collapse supernovae. Nuclear fusion reactions that produce elements heavier than iron absorb nuclear energy and are said to be endothermic reactions. When such reactions dominate, the internal temperature that supports the star's outer layers drops. Because the outer envelope is no longer sufficiently supported by the radiation pressure, the star's gravity pulls its mantle rapidly inward. As the star collapses, this mantle collides violently with the growing incompressible stellar core, which has a density almost as great as an atomic nucleus, producing a shockwave that rebounds outward through the unfused material of the outer shell. The increase of temperature by the passage of that shockwave is sufficient to induce fusion in that material, often called explosive nucleosynthesis. The energy deposited by the shockwave somehow leads to the star's explosion, dispersing fusing matter in the mantle above the core into interstellar space.

During supernova nucleosynthesis, the r process creates very neutron rich heavy isotopes, which decay after the event to the first stable isotope, thereby creating the neutron rich stable isotopes of all heavy elements. This neutron capture process occurs in high neutron density with high temperature conditions. In the r process, any heavy nuclei are bombarded with a large neutron flux to form highly unstable neutron rich nuclei which very rapidly undergo beta decay to form more stable nuclei with higher atomic number and the same atomic mass. The neutron density is extremely high, about 10 neutrons per cubic centimeter. Initial calculations of an evolving r process, showing the evolution of calculated results with time, That can happen when both massive stars in orbit with one another become core collapse supernovae, leaving neutron star remnants. The localization on the sky of the source of those gravitational waves radiated by that orbital collapse and merger of the two neutron stars, creating a black hole, but with significant ejected mass of highly neutronized matter, enabled several teams to discover and study the remaining optical counterpart of the merger, finding spectroscopic evidence of r process material thrown off by the merging neutron stars. The bulk of this material seems to consist of two types: Hot blue masses of highly radioactive r process matter of lower mass range heavy nuclei (A < 140) and cooler red masses of higher mass number r process nuclei (A > 140) rich in actinides (such as uranium, thorium, californium etc.). When released from the huge internal pressure of the neutron star, this neutron rich spherical ejecta expands and radiates detected optical light for about a week. Such duration of luminosity would not be possible without heating by internal radioactive decay, which is provided by r process nuclei near their waiting points. Two distinct mass regions (A < 140 and A > 140) for the r process yields have been known since the first time dependent calculations of the r process. Because of these spectroscopic features it has been argued that r process nucleosynthesis in the Milky Way may have been primarily ejecta from neutron star mergers rather than from supernovae.