Supernova nucleosynthesis

Supernova nucleosynthesis is a stellar evolution theory about the origin of the natural abundances of the chemical elements as created in supernovae. It explains how the nucleosynthesis of elements, from carbon to nickel, are made in the cores of massive stars. Knowledge of this process was a major stepping-stone in understanding how supernovae deposit these elements into the surrounding environment by enriching the interstellar medium and providing recycling materials for the birth of new stars. Supernova nucleosynthesis was first postulated by Fred Hoyle in 1954.[1]

Creation of the elements in supernovae

In massive stars, the nucleosynthesis of lighter elements into heavier ones by fusion occurs during sequential hydrostatic burning processes called helium burning, carbon burning, oxygen burning, and silicon burning, in which the ashes of one nuclear fuel become, after compressional heating, the fuel for the subsequent burning stage. During hydrostatic burning these nuclear fuels synthesize overwhelmingly the alpha-nucleus (A = 2Z) products. A rapid final explosive burning[2] is caused by the sudden temperature spike owing to passage of the radially moving shock wave launched by the gravitational collapse of the core. W. D. Arnett and his Rice University colleagues[3][2] demonstrated that the final shock burning would synthesize the non-alpha-nucleus isotopes more effectively than hydrostatic burning was able to do,[4][5] 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 (Z = 6), oxygen (Z = 8), and elements with Z = 10–28 (from neon to nickel).[5][6] As a result of the ejection of the newly synthesized isotopes of the chemical elements by supernovae their abundances steadily increased within interstellar gas. Explaining this temporal increase of the natural abundances of the elements is the main goal of stellar nucleosynthesis. Hoyle's paper was the founding paper of that theory; however, ideas about nuclear reactions in stars providing power for the stars is often confused with stellar nucleosynthesis. Realize that nuclear fusion in stars can occur with negligible impact on the abundances of the chemical elements.

Creation of heavier elements in supernovae

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. But see the r-process below for a recently discovered alternative.

The r-process isotopes are roughly a 100,000 times less abundant than the primary chemical elements fused in supernova shells above. Furthermore, other nucleosynthesis processes in supernovae are thought to also be responsible 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.

History

The theory that nucleosynthesis of the chemical elements occurred primarily during advanced evolution of massive stars was first proposed by Hoyle in 1954,[1] in which he predicted the existence of the excited state in the 12C nucleus that enables the triple-alpha process to burn resonantly, enabling it to heat the helium cores of stars while synthesizing massive quantities of carbon and oxygen; and he introduced the thermonuclear sequels of carbon-burning synthesizing Ne, Mg and Na and of oxygen-burning synthesizing Si, Al and S. Hoyle could not yet convincingly discern how silicon burning would happen, although he foresaw that it must be the final core fusion prior to operation of his thermal-equilibrium picture of iron formation.[7] He also predicted that the collapse of the evolved cores of massive stars was "inevitable" owing to their increasing rate of energy loss by neutrinos. This work was so advanced relative to the state of astrophysics that it was hard to digest. Hoyle's 1954 theory fell into obscurity for decades after the more-famous B2FH paper[8] was published in 1957 and, surprisingly, did not include Hoyle's original description of nucleosynthesis in massive stars. Donald D. Clayton has attributed the obscurity also to Hoyle's 1954 paper describing its key equation only in words,[9] and a lack of careful review by Hoyle of the B2FH draft by coauthors who had themselves not adequately studied Hoyle's paper.[10] During his 1955 discussions in Cambridge with his coauthors in preparation of the B2FH first draft in 1956 in Pasadena,[11] Hoyle's modesty had inhibited him from emphasizing to them the great achievements of his 1954 theory.

Thirteen years after the B2FH paper, W. D. Arnett and colleagues[3][2] demonstrated that the final burning in the passing shock wave launched by collapse of the core could synthesize non-alpha-particle isotopes more effectively than hydrostatic burning could,[4][5] suggesting that explosive nucleosynthesis is an essential component of supernova nucleosynthesis. A shock wave rebounded from matter collapsing onto the dense core, if strong enough to lead to mass ejection of the mantle of supernovae, would necessarily be strong enough to provide the sudden heating of the shells of massive stars needed for explosive thermonuclear burning within the mantle. Understanding how that shock wave can reach the mantle in the face of continuing infall onto the shock that became the theoretical difficulty. Supernova observations assured that it must occur.

Era of computer models

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[12] that would eventually yield numerical answers for the advanced evolution of stars[13] and the nucleosynthesis within them (for examples see,.[14][15] Most of this new generation never digested Hoyle (1954) carefully and in any case forgot what they had read in their focus on the immense task of computerizing massive stars. They usually did not cite Hoyle (1954), but they did cite B2FH as a needed default citation for stellar nucleosynthesis. This computer cultural revolution began in late 1960s. The upshot in regard to the puzzling confusion over Hoyle and B2FH that followed was made possible by the B2FH review's failure to describe Hoyle's picture. Understandable was the feeling by the new generation of themselves discovering the correct picture that Hoyle had presented, albeit with huge numerical details that Hoyle could not provide. The computer models of massive stars demonstrated that core burning in massive stars occurred in smaller cores than the previous burning phase had. This shrinking of successive cores yielded an onion shell model of the sequence of burning phases, a shell model that was necessary for Hoyle's 1954 picture to work as simultaneous ejection of the abundances from each burning phase. Understanding this computer cultural revolution takes one far in understanding why Hoyle (1954) was forgotten and B2FH appeared to have been the work that founded stellar nucleosynthesis, as many even claimed. The field of working astronomers became devoted to B2FH owing to that paper's citation of about 100 research papers by astronomers showing evidence of abundance changes in stars owing to nuclear reactions. Such abundance alterations, which were visible at the telescopes, became confused with Hoyle's goal of understanding the origin of the huge interstellar abundances of the elements.

Cause

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 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.[16] 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 A = 56, 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 1053 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 in falling 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.[3][17] 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.

Silicon burning

After a star completes the oxygen burning process, its core is composed primarily of silicon and sulfur.[18] If it has sufficiently high mass, it further contracts until its core reaches temperatures in the range of 2.7–3.5 billion Kelvin (230–300 keV). At these temperatures, silicon and other isotopes suffer photoejection of nucleons by energetic thermal photons (γ) ejecting especially alpha particles (4He).[18] The nuclear process of silicon burning differs from earlier fusion stages of nucleosynthesis in that it entails a balance between alpha-particle captures and their inverse photo ejection which establishes abundances of all alpha-particle elements in the following sequence in which each alpha particle capture shown is opposed by its inverse reaction, namely, photo ejection of an alpha particle by the abundant thermal photons:

28Si + 4He 32S + γ;
32S + 4He 36Ar + γ;
36Ar + 4He 40Ca + γ;
40Ca + 4He 44Ti + γ;
44Ti + 4He 48Cr + γ;
48Cr + 4He 52Fe + γ;
52Fe + 4He 56Ni + γ;
56Ni + 4He 60Zn + γ.

The alpha-particle nuclei 44Ti and those more massive in the final five reactions listed are all radioactive, but they decay after their ejection in supernova explosions into abundant isotopes of Ca, Ti, Cr, Fe and Ni. This post-supernova radioactivity became of great importance for the emergence of gamma-ray-line astronomy.[19]

In these physical circumstances of rapid opposing reactions, namely alpha-particle capture and photo ejection of alpha particles, the abundances are not determined by alpha-particle-capture cross sections; rather they are determined by the values that the abundances must assume in order to balance the speeds of the rapid opposing-reaction currents. Each abundance takes on a stationary value that achieves that balance. This picture is called nuclear quasiequilibrium.[20][21][22] Many computer calculations, for example,[23] using the numerical rates of each reaction and of their reverse reactions have demonstrated that quasiequilibrium is not exact but does characterize well the computed abundances. Thus the quasiequilibrium picture presents a comprehensible picture of what actually happens. It also fills in an uncertainty in Hoyle's 1954 theory. The quasiequilibrium buildup shuts off after 56Ni because the alpha-particle captures become slower whereas the photo ejections from heavier nuclei become faster. Non-alpha-particle nuclei also participate, using a host of reactions similar to 36Ar + neutron ⇌ 37Ar + photon and its inverse which set the stationary abundances of the non-alpha-particle isotopes, where the free densities of protons and neutrons are also established by the quasiequilibrium. However, the abundance of free neutrons is also proportional to the excess of neutrons over protons in the composition of the massive star; therefore the abundance of 37Ar, using it as an example, is greater in ejecta from recent massive stars than it was from those in early stars of only H and He; therefore 37Cl, to which 37Ar decays after the nucleosynthesis, is called a "secondary isotope". The silicon burning in the star progresses through a temporal sequence of such nuclear quasiequilibria in which the abundance of 28Si slowly declines and that of 56Ni slowly increases. This amounts to a nuclear abundance change 2 28Si ≫ 56Ni, which may be thought of as silicon burning into nickel in the nuclear sense. In interest of economy the photodisintegration rearrangement and the nuclear quasiequilibrium that it achieves is referred to as silicon burning. The entire silicon-burning sequence lasts about one day in the core of a contracting massive star and stops after 56Ni has become the dominant abundance. The final explosive burning caused when the supernova shock passes through the silicon-burning shell lasts only seconds, but its roughly 50% increase in the temperature causes furious nuclear burning, which becomes the major contributor to nucleosynthesis in the mass range 28–60.[2][20][21][24] The star can no longer release energy via nuclear fusion because a nucleus with 56 nucleons has the lowest mass per nucleon of all the elements in the sequence. The next step up in the alpha-particle chain would be 60Zn, which has slightly more mass per nucleon and thus is less thermodynamically favorable. 56Ni (which has 28 protons) has a half-life of 6.02 days and decays via β+ decay to 56Co (27 protons), which in turn has a half-life of 77.3 days as it decays to 56Fe (26 protons). However, only minutes are available for the 56Ni to decay within the core of a massive star. This establishes 56Ni as the most abundant of the radioactive nuclei created in this way. Its radioactivity energizes the late supernova light curve and creates the pathbreaking opportunity for gamma-ray-line astronomy.[19] See SN 1987A light curve for the aftermath of that opportunity. Clayton and Meyer[23] have recently generalized this process still further by what they have named the secondary supernova machine, attributing the increasing radioactivity that energizes late supernova displays to the storage of increasing Coulomb energy within the quasiequilibrium nuclei called out above as the quasiequilibria shift from primarily 28Si to primarily 56Ni. The visible displays are powered by the decay of that excess Coulomb energy.

During this phase of the core contraction, the potential energy of gravitational compression heats the interior to roughly three billion degrees K, which briefly maintains pressure support and opposes rapid core contraction. However, since no additional heat energy can be generated via new fusion reactions, the final unopposed contraction rapidly accelerates into a collapse lasting only a few seconds. The central portion of the star is now crushed into either a neutron star or, if the star is massive enough, a black hole. The outer layers of the star are blown off in an explosion triggered by the outward moving supernova shock, known as a Type II supernova whose displays last days to months. The escaping portion of the supernova core may initially contain a large density of free neutrons, which may synthesize, in about one second while inside the star, roughly half of the elements in the universe that are heavier than iron via a rapid neutron-capture mechanism known as the r-process. See below.

Nuclides synthesized

Stars with initial masses less than about eight times the sun never develop a core large enough to collapse and they eventually lose their atmospheres to become white dwarfs, stable cooling spheres of carbon supported by the pressure of degenerate electrons. Nucleosynthesis within those lighter stars is therefore limited to nuclides that were fused in material located above the final white dwarf. This limits their modest yields returned to interstellar gas to carbon-13 and nitrogen-14, and to isotopes heavier than iron by slow capture of neutrons (the s-process). A significant minority of white dwarfs will nonetheless explode, however, because they formed in a binary orbit with a giant companion star that loses mass to the stronger gravitational field of the white dwarf, which then grows past its Chandrasekhar limit and explodes as a Type Ia supernova, synthesizing about a solar mass of radioactive 56Ni isotopes. Its radioactive decay to iron keeps Type Ia optically very bright for weeks and creates more than half of all iron in the universe. Virtually all of the remainder of stellar nucleosynthesis occurs, however, in more frequent stars that are massive enough to end as Type II supernovae.[24][25] In the presupernova massive star this includes helium burning, carbon burning, oxygen burning and silicon burning. Much of that yield may never leave the star but instead disappears into its collapsed core. The yield that is ejected is substantially fused in last-second explosive burning caused by the shock wave launched by core collapse.[2] Prior to core collapse, fusion of elements between silicon and iron occurs only in the largest of stars, and then in limited amounts. Thus the nucleosynthesis of the abundant primary elements[26] defined as those that could be synthesized in stars of initially only hydrogen and helium (left by the Big Bang), is substantially limited to core-collapse supernova nucleosynthesis.

The r-process

Nucleosynthesis periodic table
A version of the periodic table indicating the main origin of elements found on Earth. All elements past plutonium (element 94) are manmade.

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 1022-24 neutrons per cubic centimeter. First calculation of an evolving r-process, showing the evolution of calculated results with time,[27] also suggested that the r-process abundances are a superposition of differing neutron fluences. Small fluence produces the first r-process abundance peak near atomic weight A = 130 but no actinides, whereas large fluence produces the actinides uranium and thorium but no longer contains the A = 130 abundance peak. These processes occur in a fraction of a second to a few seconds, depending on details. Hundreds of subsequent papers published have utilized this time-dependent approach. The only modern nearby supernova, 1987A, has not revealed r-process enrichments. Modern thinking is that the r-process yield may be ejected from some supernovae but swallowed up in others as part of the residual neutron star or black hole.

Entirely new astronomical data about the r-process was discovered in 2017 when the LIGO and Virgo gravitational-wave observatories discovered a merger of two neutron stars that had previously been orbiting one another[28] That can happen when both massive stars in orbit with one another become core-collapse supernovae, leaving neutron-star remnants. Everyone could "hear" the replay of the increasing orbital frequency as the orbit became smaller and faster owing to energy loss by gravitational waves. 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 spun off mass of highly neutronized matter, enabled several teams[29][30][31] 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 lanthanides (such as uranium, thorium, californium etc.). When released from the huge internal pressure of the neutron star, these neutralized ejecta expand and radiate 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.[27] 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.[32] These results offer a new possibility for clarifying six decades of uncertainty over the site of origin of r-process nuclei. Confirming relevance of this discovery by gravitational-wave astronomy to the r-process is the radiogenic power from radioactive decay of r-process nuclei that maintains the visibility of these spun off r-process fragments. Otherwise they would dim quickly. Unquestionably this discovery raised support for such mergers being the main sources of the r-process nuclei rather than core-collapse supernovae; but that debate continues.

See also

References

  1. ^ a b Hoyle, F. (1954). "On Nuclear Reactions Occurring in Very Hot STARS. I. The Synthesis of Elements from Carbon to Nickel". The Astrophysical Journal Supplement Series. 1: 121. Bibcode:1954ApJS....1..121H. doi:10.1086/190005.
  2. ^ a b c d e Woosley, S. E.; Arnett, W. D.; Clayton, D. D. (1973). "The Explosive burning of oxygen and silicon". The Astrophysical Journal Supplement Series. 26: 231–312. Bibcode:1973ApJS...26..231W. doi:10.1086/190282.
  3. ^ a b c Arnett, W. D.; Clayton, D. D. (1970). "Explosive Nucleosynthesis in Stars". Nature. 227 (5260): 780–784. Bibcode:1970Natur.227..780A. doi:10.1038/227780a0. PMID 16058157.
  4. ^ a b See Figures 1, 3, and 4 in Arnett & Clayton (1970) and Fig. 2, p. 241 in Woosley, Arnett & Clayton 1973
  5. ^ a b c Woosley, S. E.; Weaver, T. A. (1995). "The Evolution and Explosion of Massive Stars. II. Explosive Hydrodynamics and Nucleosynthesis". The Astrophysical Journal Supplement Series. 101: 181. Bibcode:1995ApJS..101..181W. doi:10.1086/192237.
  6. ^ Thielemann, Fr.-K.; Nomoto, K.; Hashimoto, M.-A. (1996). "Core-Collapse Supernovae and Their Ejecta". The Astrophysical Journal. 460: 408. Bibcode:1996ApJ...460..408T. doi:10.1086/176980.
  7. ^ Hoyle, F. (1946). "The Synthesis of the Elements from Hydrogen". Monthly Notices of the Royal Astronomical Society. 106 (5): 343–383. Bibcode:1946MNRAS.106..343H. doi:10.1093/mnras/106.5.343.
  8. ^ Burbidge, E. M.; Burbidge, G. R.; Fowler, W.A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars" (PDF). Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547.
  9. ^ Clayton, D. D. (2007). "Hoyle's Equation". Science. 318 (5858): 1876–1877. doi:10.1126/science.1151167. PMID 18096793.
  10. ^ See footnote 1 in Clayton 2008, p. 363
  11. ^ See B2FH Paper
  12. ^ Donald D. Clayton, Principles of Stellar Evolution and Nucleosynthesis, McGraw-Hill (1968) Chapter 6. Calculation of Stellar Structure
  13. ^ for example I. Iben, Jr. Astrophys J. 147, 624 (1967) description of helium burning
  14. ^ Woosley, S. E.; Weaver, T. A. (1995). "The Evolution and Explosion of Massive Stars. II. Explosive Hydrodynamics and Nucleosynthesis". The Astrophysical Journal Supplement Series. 101: 181. doi:10.1086/192237.
  15. ^ Thielemann, Fr.-K.; Nomoto, K.; Hashimoto, M.-A. (1996). "Core-Collapse Supernovae and Their Ejecta". The Astrophysical Journal. 460: 408. doi:10.1086/176980.
  16. ^ Heger, A.; Fryer, C. L.; Woosley, S. E.; Langer, N.; Hartmann, D. H. (2003). "How Massive Single Stars End Their Life". The Astrophysical Journal. 591 (1): 288–300. arXiv:astro-ph/0212469. Bibcode:2003ApJ...591..288H. doi:10.1086/375341.
  17. ^ Clayton, D. D.; Woosley, S. E. (1974). "Thermonuclear astrophysics". Reviews of Modern Physics. 46 (4): 755–771. Bibcode:1974RvMP...46..755C. doi:10.1103/RevModPhys.46.755.
  18. ^ a b Clayton, D. D. (1983). Principles of Stellar Evolution and Nucleosynthesis. University of Chicago Press. pp. 519–524.
  19. ^ a b Clayton, D. D.; Colgate, S. A.; Fishman, G. J. (1969). "Gamma-Ray Lines from Young Supernova Remnants". The Astrophysical Journal. 155: 75. Bibcode:1969ApJ...155...75C. doi:10.1086/149849.
  20. ^ a b Bodansky, D.; Clayton, D. D.; Fowler, W. A. (1968). "Nucleosynthesis During Silicon Burning". Physical Review Letters. 20 (4): 161–164. Bibcode:1968PhRvL..20..161B. doi:10.1103/PhysRevLett.20.161.
  21. ^ a b Bodansky, D.; Clayton, D. D.; Fowler, W. A. (1968). "Nuclear Quasi-Equilibrium during Silicon Burning". The Astrophysical Journal Supplement Series. 16: 299. Bibcode:1968ApJS...16..299B. doi:10.1086/190176.
  22. ^ Clayton, D. D. (1968). Principles of Stellar Evolution and Nucleosynthesis. University of Chicago Press. Chapter 7.
  23. ^ a b Clayton, D. D.; Meyer, B. S. (2016). "The secondary supernova machine: Gravitational compression, stored Coulomb energy, and SNII displays". New Astronomy Reviews. 71: 1–8. Bibcode:2016NewAR..71....1C. doi:10.1016/j.newar.2016.03.002.
  24. ^ a b Clayton, D. D. (2003). Handbook of Isotopes in the Cosmos. Cambridge University Press.
  25. ^ François, P.; Matteucci, F.; Cayrel, R.; Spite, M.; Spite, F.; Chiappini, C. (2004). "The evolution of the Milky Way fromits earliest phases: Constraints on stellar nucleosynthesis". Astronomy & Astrophysics. 421 (2): 613–621. arXiv:astro-ph/0401499. Bibcode:2004A&A...421..613F. doi:10.1051/0004-6361:20034140.
  26. ^ Clayton, D. D. (2008). "Fred Hoyle, primary nucleosynthesis and radioactivity". New Astronomy Reviews. 52 (7–10): 360–363. Bibcode:2008NewAR..52..360C. doi:10.1016/j.newar.2008.05.007.
  27. ^ a b Seeger, P. A.; Fowler, W. A.; Clayton, D. D. (1965). "Nucleosynthesis of Heavy Elements by Neutron Capture". The Astrophysical Journal Supplement Series. 11: 121–126. Bibcode:1965ApJS...11..121S. doi:10.1086/190111.
  28. ^ Abbott, B. P.; et al. (2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral". Physical Review Letters. 119 (16): 161101. arXiv:1710.05832. Bibcode:2017PhRvL.119p1101A. doi:10.1103/PhysRevLett.119.161101. PMID 29099225.
  29. ^ Arcavi, I.; et al. (2017). "Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger". Nature. 551 (7678): 64–66. arXiv:1710.05843. Bibcode:2017Natur.551...64A. doi:10.1038/nature24291.
  30. ^ Pian, E.; et al. (2017). "Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger". Nature. 551 (7678): 67–70. arXiv:1710.05858. Bibcode:2017Natur.551...67P. doi:10.1038/nature24298. PMID 29094694.
  31. ^ Smartt, S. J.; et al. (2017). "A kilonova as the electromagnetic counterpart to a gravitational-wave source". Nature. 551 (7678): 75–79. arXiv:1710.05841. Bibcode:2017Natur.551...75S. doi:10.1038/nature24303. PMID 29094693.
  32. ^ Kasen, D.; Metzger, B.; Barnes, J.; Quataert, E.; Ramirez-Ruiz, E. (2017). "Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event". Nature. 551 (7678): 80–84. arXiv:1710.05463. Bibcode:2017Natur.551...80K. doi:10.1038/nature24453. PMID 29094687.

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Abundance of the chemical elements

The abundance of the chemical elements is a measure of the occurrence of the chemical elements relative to all other elements in a given environment. Abundance is measured in one of three ways: by the mass-fraction (the same as weight fraction); by the mole-fraction (fraction of atoms by numerical count, or sometimes fraction of molecules in gases); or by the volume-fraction. Volume-fraction is a common abundance measure in mixed gases such as planetary atmospheres, and is similar in value to molecular mole-fraction for gas mixtures at relatively low densities and pressures, and ideal gas mixtures. Most abundance values in this article are given as mass-fractions.

For example, the abundance of oxygen in pure water can be measured in two ways: the mass fraction is about 89%, because that is the fraction of water's mass which is oxygen. However, the mole-fraction is 33.3333...% because only 1 atom of 3 in water, H2O, is oxygen. As another example, looking at the mass-fraction abundance of hydrogen and helium in both the Universe as a whole and in the atmospheres of gas-giant planets such as Jupiter, it is 74% for hydrogen and 23–25% for helium; while the (atomic) mole-fraction for hydrogen is 92%, and for helium is 8%, in these environments. Changing the given environment to Jupiter's outer atmosphere, where hydrogen is diatomic while helium is not, changes the molecular mole-fraction (fraction of total gas molecules), as well as the fraction of atmosphere by volume, of hydrogen to about 86%, and of helium to 13%.The abundance of chemical elements in the universe is dominated by the large amounts of hydrogen and helium which were produced in the Big Bang. Remaining elements, making up only about 2% of the universe, were largely produced by supernovae and certain red giant stars. Lithium, beryllium and boron are rare because although they are produced by nuclear fusion, they are then destroyed by other reactions in the stars. The elements from carbon to iron are relatively more abundant in the universe because of the ease of making them in supernova nucleosynthesis. Elements of higher atomic number than iron (element 26) become progressively rarer in the universe, because they increasingly absorb stellar energy in their production. Also, elements with even atomic numbers are generally more common than their neighbors in the periodic table, due to favorable energetics of formation.

The abundance of elements in the Sun and outer planets is similar to that in the universe. Due to solar heating, the elements of Earth and the inner rocky planets of the Solar System have undergone an additional depletion of volatile hydrogen, helium, neon, nitrogen, and carbon (which volatilizes as methane). The crust, mantle, and core of the Earth show evidence of chemical segregation plus some sequestration by density. Lighter silicates of aluminum are found in the crust, with more magnesium silicate in the mantle, while metallic iron and nickel compose the core. The abundance of elements in specialized environments, such as atmospheres, or oceans, or the human body, are primarily a product of chemical interactions with the medium in which they reside.

B2FH paper

The B2FH paper, named after the initials of the authors of the paper, Margaret Burbidge, Geoffrey Burbidge, William A. Fowler, and Fred Hoyle, is a landmark paper on the origin of the chemical elements published in Reviews of Modern Physics in 1957. The title of that paper is "Synthesis of the Elements in Stars", but as that paper grew in influence, it came to be referred to only as "B2FH". The B2FH paper spread stellar nucleosynthesis theory widely in the scientific community, especially among astronomers who saw everyday relevance to their quest, at a time when it was appreciated by only a handful of experts in nuclear physics. But it did not create the theory of stellar nucleosynthesis as much as bring it vividly to life.

The paper comprehensively outlined and analyzed several key processes that are responsible for the nucleosynthesis of the elements heavier than iron and their relative abundance by the capture within stars of free neutrons. It advanced much less the understanding of the synthesis of the very abundant elements from silicon to nickel. A puzzle about that is that despite Hoyle's coauthorship of B2FH and being its chief conceptual architect, the paper did not include the carbon-burning process, the oxygen-burning process and the silicon-burning process, each of which contributes massively to the growth of stellar metallicity from magnesium to nickel in the interstellar gas. The supernova nucleosynthesis that achieves that had been published by Hoyle in 1954. Donald D. Clayton has attributed the severe undercitations of Hoyle's 1954 paper relative to the voluminous citations of B2FH to several factors: the advanced difficulty of digesting Hoyle's 1954 paper even for his B2FH coauthors, as it proved to be for the world of astronomy generally; to Hoyle's having described its key equation only in words rather than writing it prominently in his paper; and finally to a lack of careful review by Hoyle himself of the B2FH draft written by two junior coauthors who had themselves not adequately digested Hoyle's paper.

Cassiopeia A

Cassiopeia A (Cas A) is a supernova remnant (SNR) in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred approximately 11,000 light-years (3.4 kpc) away within the Milky Way. The expanding cloud of material left over from the supernova now appears approximately 10 light-years (3 pc) across from Earth's perspective. In wavelengths of visible light, it has been seen with amateur telescopes down to 234mm (9.25 in) with filters.It is estimated that light from the stellar explosion first reached Earth approximately 300 years ago, but there are no historical records of any sightings of the supernova that created the remnant. Since Cas A is circumpolar for mid-Northern latitudes, this is probably due to interstellar dust absorbing optical wavelength radiation before it reached Earth (although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae by John Flamsteed on August 16, 1680). Possible explanations lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. These outer layers would have cloaked the star and re-absorbed much of the light released as the inner star collapsed.

Cas A was among the first discrete astronomical radio sources found. Its discovery was reported in 1948 by Martin Ryle and Francis Graham-Smith, astronomers at Cambridge, based on observations with the Long Michelson Interferometer. The optical component was first identified in 1950.Cas A is 3C461 in the Third Cambridge Catalogue of Radio Sources and G111.7-2.1 in the Green Catalog of Supernova Remnants.

Chemical element

A chemical element is a species of atom having the same number of protons in their atomic nuclei (that is, the same atomic number, or Z). For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have exactly 8 protons.

118 elements have been identified, of which the first 94 occur naturally on Earth with the remaining 24 being synthetic elements. There are 80 elements that have at least one stable isotope and 38 that have exclusively radionuclides, which decay over time into other elements. Iron is the most abundant element (by mass) making up Earth, while oxygen is the most common element in the Earth's crust.Chemical elements constitute all of the ordinary matter of the universe. However astronomical observations suggest that ordinary observable matter makes up only about 15% of the matter in the universe: the remainder is dark matter; the composition of this is unknown, but it is not composed of chemical elements.

The two lightest elements, hydrogen and helium, were mostly formed in the Big Bang and are the most common elements in the universe. The next three elements (lithium, beryllium and boron) were formed mostly by cosmic ray spallation, and are thus rarer than heavier elements. Formation of elements with from 6 to 26 protons occurred and continues to occur in main sequence stars via stellar nucleosynthesis. The high abundance of oxygen, silicon, and iron on Earth reflects their common production in such stars. Elements with greater than 26 protons are formed by supernova nucleosynthesis in supernovae, which, when they explode, blast these elements as supernova remnants far into space, where they may become incorporated into planets when they are formed.The term "element" is used for atoms with a given number of protons (regardless of whether or not they are ionized or chemically bonded, e.g. hydrogen in water) as well as for a pure chemical substance consisting of a single element (e.g. hydrogen gas). For the second meaning, the terms "elementary substance" and "simple substance" have been suggested, but they have not gained much acceptance in English chemical literature, whereas in some other languages their equivalent is widely used (e.g. French corps simple, Russian простое вещество). A single element can form multiple substances differing in their structure; they are called allotropes of the element.

When different elements are chemically combined, with the atoms held together by chemical bonds, they form chemical compounds. Only a minority of elements are found uncombined as relatively pure minerals. Among the more common of such native elements are copper, silver, gold, carbon (as coal, graphite, or diamonds), and sulfur. All but a few of the most inert elements, such as noble gases and noble metals, are usually found on Earth in chemically combined form, as chemical compounds. While about 32 of the chemical elements occur on Earth in native uncombined forms, most of these occur as mixtures. For example, atmospheric air is primarily a mixture of nitrogen, oxygen, and argon, and native solid elements occur in alloys, such as that of iron and nickel.

The history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur, copper and gold. Later civilizations extracted elemental copper, tin, lead and iron from their ores by smelting, using charcoal. Alchemists and chemists subsequently identified many more; all of the naturally occurring elements were known by 1950.

The properties of the chemical elements are summarized in the periodic table, which organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties. Save for unstable radioactive elements with short half-lives, all of the elements are available industrially, most of them in low degrees of impurities.

Cosmic ray spallation

Cosmic ray spallation is a naturally occurring nuclear reaction causing nucleosynthesis. It refers to the formation of chemical elements from the impact of cosmic rays on an object. Cosmic rays are highly energetic charged particles from beyond Earth, ranging from protons, alpha particles, and nuclei of many heavier elements. About 1% of cosmic rays also consist of free electrons.

Cosmic rays cause spallation when a ray particle (e.g. a proton) impacts with matter, including other cosmic rays. The result of the collision is the expulsion of large numbers of nucleons (protons and neutrons) from the object hit. This process goes on not only in deep space, but in Earth's upper atmosphere and crustal surface (typically the upper ten meters) due to the ongoing impact of cosmic rays.

Fred Hoyle

Sir Fred Hoyle FRS (24 June 1915 – 20 August 2001) was an English astronomer who formulated the theory of stellar nucleosynthesis. He also held controversial stances on other scientific matters—in particular his rejection of the "Big Bang" theory, a term coined by him on BBC radio, and his promotion of panspermia as the origin of life on Earth. He also wrote science fiction novels, short stories and radio plays, and co-authored twelve books with his son, Geoffrey Hoyle.

He spent most of his working life at the Institute of Astronomy at Cambridge and served as its director for six years.

Hertzsprung–Russell diagram

The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective temperatures. More simply, it plots each star on a graph plotting the star's brightness against its temperature (color).

The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell and represents a major step towards an understanding of stellar evolution.

The related color–magnitude diagram (CMD) plots the apparent magnitudes of stars against their color, usually for a cluster so that the stars are all at the same distance.

Iron peak

The iron peak is a local maximum in the vicinity of Fe (Cr, Mn, Fe, Co and Ni) on the graph of the abundances of the chemical elements, as seen below.

For elements lighter than iron on the periodic table, nuclear fusion releases energy while fission consumes it. For iron, and for all of the heavier elements, nuclear fusion consumes energy, but nuclear fission releases it. Chemical elements up to the iron peak are produced in ordinary stellar nucleosynthesis. Heavier elements are produced only during supernova nucleosynthesis. This is why we have more iron peak elements than in its neighbourhood.

Neutron capture nucleosynthesis

Neutron capture nucleosynthesis describes two nucleosynthesis pathways: the r-process and the s-process, for rapid and slow neutron captures, respectively. R-process describes neutron capture in a region of high neutron flux, such as during supernova nucleosynthesis after core-collapse, and yields neutron-rich nuclides. S-process describes neutron capture that is slow relative to the rate of beta decay, as for stellar nucleosynthesis in some stars, and yields nuclei with stable nuclear shells. Each process is responsible for roughly half of the observed abundances of elements heavier than iron. The importance of neutron capture to the observed abundance of the chemical elements was first described in 1957 in the B2FH paper.

Nickel

Nickel is a chemical element with the symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile. Pure nickel, powdered to maximize the reactive surface area, shows a significant chemical activity, but larger pieces are slow to react with air under standard conditions because an oxide layer forms on the surface and prevents further corrosion (passivation). Even so, pure native nickel is found in Earth's crust only in tiny amounts, usually in ultramafic rocks, and in the interiors of larger nickel–iron meteorites that were not exposed to oxygen when outside Earth's atmosphere.

Meteoric nickel is found in combination with iron, a reflection of the origin of those elements as major end products of supernova nucleosynthesis. An iron–nickel mixture is thought to compose Earth's outer and inner cores.Use of nickel (as a natural meteoric nickel–iron alloy) has been traced as far back as 3500 BCE. Nickel was first isolated and classified as a chemical element in 1751 by Axel Fredrik Cronstedt, who initially mistook the ore for a copper mineral, in the cobalt mines of Los, Hälsingland, Sweden. The element's name comes from a mischievous sprite of German miner mythology, Nickel (similar to Old Nick), who personified the fact that copper-nickel ores resisted refinement into copper. An economically important source of nickel is the iron ore limonite, which often contains 1–2% nickel. Nickel's other important ore minerals include pentlandite and a mixture of Ni-rich natural silicates known as garnierite. Major production sites include the Sudbury region in Canada (which is thought to be of meteoric origin), New Caledonia in the Pacific, and Norilsk in Russia.

Nickel is slowly oxidized by air at room temperature and is considered corrosion-resistant. Historically, it has been used for plating iron and brass, coating chemistry equipment, and manufacturing certain alloys that retain a high silvery polish, such as German silver. About 9% of world nickel production is still used for corrosion-resistant nickel plating. Nickel-plated objects sometimes provoke nickel allergy. Nickel has been widely used in coins, though its rising price has led to some replacement with cheaper metals in recent years.

Nickel is one of four elements (the others are iron, cobalt, and gadolinium) that are ferromagnetic at approximately room temperature. Alnico permanent magnets based partly on nickel are of intermediate strength between iron-based permanent magnets and rare-earth magnets. The metal is valuable in modern times chiefly in alloys; about 68% of world production is used in stainless steel. A further 10% is used for nickel-based and copper-based alloys, 7% for alloy steels, 3% in foundries, 9% in plating and 4% in other applications, including the fast-growing battery sector. As a compound, nickel has a number of niche chemical manufacturing uses, such as a catalyst for hydrogenation, cathodes for batteries, pigments and metal surface treatments. Nickel is an essential nutrient for some microorganisms and plants that have enzymes with nickel as an active site.

Nuclear technology

Nuclear technology is technology that involves the nuclear reactions of atomic nuclei. Among the notable nuclear technologies are nuclear reactors, nuclear medicine and nuclear weapons. It is also used, among other things, in smoke detectors and gun sights.

Nucleosynthesis

Nucleosynthesis is the process that creates 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. Seventeen minutes later the universe had cooled to a point at which these processes ended, so only the fastest and simplest reactions occurred, leaving our universe containing about 75% hydrogen, 24% helium by mass. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. The universe still has approximately the same composition today.

Heavier nuclei were created from these, by several processes. Stars formed, and began to fuse light elements to heavier ones in their cores, giving off energy in the process, known as stellar nucleosynthesis. Fusion processes create many of the lighter elements up to and including iron and nickel, and these elements are ejected into space (the interstellar medium) when smaller stars shed their outer envelopes and become smaller stars known as white dwarfs. The remains of their ejected mass form the planetary nebulae observable throughout our galaxy.

Supernova nucleosynthesis within exploding stars by fusing carbon and oxygen is responsible for the abundances of elements between magnesium (atomic number 12) and nickel (atomic number 28). Supernova nucleosynthesis is also thought to be responsible for the creation of rarer elements heavier than iron and nickel, in the last few seconds of a type II supernova event. The synthesis of these heavier elements absorbs energy (endothermic process) as they are created, from the energy produced during the supernova explosion. Some of those elements are created from the absorption of multiple neutrons (the r-process) in the period of a few seconds during the explosion. The elements formed in supernovas include the heaviest elements known, such as the long-lived elements uranium and thorium.

Neutron star mergers and collisions are also very responsible for many heavy elements, via the r-process ("r" stands for "rapid"). Neutron stars are extremely dense remnants of supernovae, and as their name suggests, they consist of a complex state of matter, predominantly made of tightly packed neutrons. When two such dense stars collide, a large amount of neutron-rich matter may be ejected at extremely high temperatures and under exotic conditions, and heavy elements may form as the ejecta begins to cool. In 2017, the merger of GW170817 led to the detection of substantial signatures of gold, platinum and other heavy elements over an extended period.

Cosmic ray spallation, caused when cosmic rays impact the interstellar medium and fragment larger atomic species, is a significant source of the lighter nuclei, particularly 3He, 9Be and 10,11B, that are not created by stellar nucleosynthesis.

In addition to the fusion processes responsible for the growing abundances of elements in the universe, a few minor natural processes continue to produce very small numbers of new nuclides on Earth. These nuclides contribute little to their abundances, but may account for the presence of specific new nuclei. These nuclides are produced via radiogenesis (decay) of long-lived, heavy, primordial radionuclides such as uranium and thorium. Cosmic ray bombardment of elements on Earth also contribute to the presence of rare, short-lived atomic species called cosmogenic nuclides.

Pair-instability supernova

A pair-instability supernova occurs when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal pressure supporting a supermassive star's core against gravitational collapse. This pressure drop leads to a partial collapse, which in turn causes greatly accelerated burning in a runaway thermonuclear explosion, resulting in the star being blown completely apart without leaving a black hole remnant behind. Pair-instability supernovae can only happen in stars with a mass range from around 130 to 250 solar masses and low to moderate metallicity (low abundance of elements other than hydrogen and helium – a situation common in Population III stars). The recently observed objects SN 2006gy, SN 2007bi, SN 2213-1745, and SN 1000+0216 are hypothesized to have been pair-instability supernovae.

R-process

The rapid neutron-capture process, or so-called r-process, is a set of nuclear reactions that in nuclear astrophysics is responsible for the creation of approximately half of the atomic nuclei heavier than iron; the "heavy elements". The other half are produced by the p-process and s-process. The r-process usually synthesizes all of the two most neutron-rich stable isotopes of each heavy element.

The heavy elements typically have six to ten stable isotopes. Chemical elements are defined by the number of protons in their atomic nucleus, e.g. all xenon atoms have 54 protons. But all elements also have neutrons in their atomic nucleus. Each isotope is characterized by the number of neutrons that it contains, e.g. xenon can have 70, 72, 74, 75, 76, 77, 78, 80, and 82 neutrons, and thus has 9 stable isotopes. The r-process contributes to the abundances of the heaviest four isotopes: 131Xe, 132Xe, 134Xe and 136Xe, and is solely responsible for the heaviest two of those. The s-process contributes to xenon's middle five isotopes: 128Xe, 129Xe, 130Xe, 131Xe, and 132Xe. The lightest two isotopes, 124Xe, and 126Xe, are produced by other processes.

The r-process can typically synthesize the heaviest four isotopes of every heavy element, and the two heaviest isotopes, which are referred to as r-only nuclei, can only be created via the r-process. The r-process abundances peak near atomic weights A = 82 (elements Se, Br and Kr), A = 130 (elements Te, I, and Xe) and A = 196 (elements Os, Ir and Pt).

The r-process entails a succession of rapid neutron captures (hence the name) by one or more heavy seed nuclei, typically beginning with nuclei in the abundance peak centered on 56Fe. The captures must be rapid in the sense that the nuclei must not have time to undergo radioactive decay (typically via β- decay) before another neutron arrives to be captured. This sequence can continue up to the limit of stability of the increasingly neutron-rich nuclei (the neutron drip line) to physically retain neutrons as governed by the short range nuclear force. The r-process therefore must occur in locations where there exist a high density of free neutrons. Early studies theorized that 1024 free neutrons per cm3 would be required, for temperatures about 1GK, in order to match the waiting points, at which no more neutrons can be captured, with the atomic numbers of the abundance peaks for r-process nuclei. This amounts to almost a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations. Traditionally this suggested the material ejected from the reexpanded core of a core-collapse supernova, as part of supernova nucleosynthesis, or decompression of neutron-star matter thrown off by a binary neutron star merger. The relative contributions of these sources to the astrophysical abundance of r-process elements is a matter of ongoing research.A limited r-process-like series of neutron captures occurs to a minor extent in thermonuclear weapon explosions. These led to the discovery of the elements einsteinium (element 99) and fermium (element 100) in nuclear weapon fallout.

The r-process contrasts with the s-process, the other predominant mechanism for the production of heavy elements, which is nucleosynthesis by means of slow captures of neutrons. The s-process primarily occurs within ordinary stars, particularly AGB stars, where the neutron flux is sufficient to cause neutron captures to recur every 10–100 years, much too slow for the r-process, which requires 100 captures per second. The s-process is secondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei by a slow sequence of captures of free neutrons. The r-process scenarios create their own seed nuclei, so they might proceed in massive stars that contain no heavy seed nuclei. Taken together, the r- and s-processes account for almost the entire abundance of chemical elements heavier than iron. The historical challenge has been to locate physical settings appropriate for their time scales.

Silicon-burning process

In astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8-11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main sequence on the Hertzsprung-Russell diagram. It follows the previous stages of hydrogen, helium, carbon, neon and oxygen burning processes.

Silicon burning begins when gravitational contraction raises the star's core temperature to 2.7–3.5 billion Kelvin (GK). The exact temperature depends on mass. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and may explode in what is known as a Type II supernova.

Star

A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion (3×1023) stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for some stars by supernova nucleosynthesis when it explodes. Near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and many other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. The total mass of a star is the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram (H–R diagram). Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined.

A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core. As the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.

Binary and multi-star systems consist of two or more stars that are gravitationally bound and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.

Stellar nucleosynthesis

Stellar nucleosynthesis is the theory explaining the creation (nucleosynthesis) of chemical elements by nuclear fusion reactions between atoms within stars. Stellar nucleosynthesis has occurred continuously since the original creation of hydrogen, helium and lithium during the Big Bang. It is a highly predictive theory that today yields excellent agreement between calculations based upon it and the observed abundances of the elements. It explains why the observed abundances of elements in the universe grow over time and why some elements and their isotopes are much more abundant than others. The theory was initially proposed by Fred Hoyle in 1946, who later refined it in 1954. Further advances were made, especially to nucleosynthesis by neutron capture of the elements heavier than iron, by Margaret Burbidge, Geoffrey Burbidge, William Alfred Fowler and Hoyle in their famous 1957 B2FH paper, which became one of the most heavily cited papers in astrophysics history.

Stars evolve because of changes in their composition (the abundance of their constituent elements) over their lifespans, first by burning hydrogen (main sequence star), then helium (red giant star), and progressively burning higher elements. However, this does not by itself significantly alter the abundances of elements in the universe as the elements are contained within the star. Later in its life, a low-mass star will slowly eject its atmosphere via stellar wind, forming a planetary nebula, while a higher–mass star will eject mass via a sudden catastrophic event called a supernova. The term supernova nucleosynthesis is used to describe the creation of elements during the evolution and explosion of a pre-supernova massive star (12–35 times the mass of the sun). Those massive stars are the most prolific source of new isotopes from carbon (Z = 6) to nickel (Z = 28).

The advanced sequence of burning fuels is driven by gravitational collapse and its associated heating, resulting in the subsequent burning of carbon, oxygen and silicon. However, most of the nucleosynthesis in the mass range A = 28–56 (from silicon to nickel) is actually caused by the upper layers of the star collapsing onto the core, creating a compressional shock wave rebounding outward. The shock front briefly raises temperatures by roughly 50%, thereby causing furious burning for about a second. This final burning in massive stars, called explosive nucleosynthesis or supernova nucleosynthesis, is the final epoch of stellar nucleosynthesis.

A stimulus to the development of the theory of nucleosynthesis was the discovery of variations in the abundances of elements found in the universe. The need for a physical description was already inspired by the relative abundances of isotopes of the chemical elements in the solar system. Those abundances, when plotted on a graph as a function of atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions (see history of nucleosynthesis theory). This suggested a natural process that is not random. A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it was realized that the energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source of heat and light.

Supernova

A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stellar evolutionary stages of the life of a massive star, whose dramatic and catastrophic destruction is marked by one final, titanic explosion. This causes the sudden appearance of a "new" bright star, before slowly fading from sight over several weeks or months or years.

Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931.

Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been observed in other galaxies. The most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but the remnants of recent supernovae have also been found. Observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, and that any galactic supernova would almost certainly be observable with modern astronomical telescopes.

Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion, completely disrupting the star. In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics have been established and accepted by most astronomers for some time.

Supernovae can expel several solar masses of material at speeds up to several percent of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, sweeping up an expanding shell of gas and dust observed as a supernova remnant. Stars that eventually explode as supernovae are a major source of elements heavier than nitrogen in the interstellar medium, and the expanding shock waves can directly trigger the formation of new stars. Supernova remnants might be a major source of cosmic rays. Supernovae might produce strong gravitational waves, though, thus far, the gravitational waves detected have been from the merger of black holes and neutron stars, such as those that can be left behind by supernovae.

Supernova Early Warning System

The SuperNova Early Warning System (SNEWS) is a network of neutrino detectors designed to give early warning to astronomers in the event of a supernova in the Milky Way, our home galaxy, or in a nearby galaxy such as the Large Magellanic Cloud or the Canis Major Dwarf Galaxy.

As of October 2018, SNEWS has not issued any supernova alerts. This is unsurprising because supernovae appear to be rare: the most recent known supernova remnant in the Milky Way was around the turn of the 20th century, and the most recent supernova confirmed to have been observed was Kepler's Supernova in 1604.

Powerful bursts of electron neutrinos (νe) with typical energies of the order of 10 MeV and duration of the order of 10 seconds are produced in the core of a red giant star as it collapses on itself via the "neutronization" reaction, i.e. fusion of protons and electrons into neutrons: pe−→nνe. It is expected that the neutrinos are emitted well before the light from the supernova peaks, so in principle neutrino detectors could give advance warning to astronomers that a supernova has occurred and may soon be visible. The neutrino pulse from supernova 1987A arrived 3 hours before the associated photons – but SNEWS was not yet active and it was not recognised as a supernova event until after the photons arrived. However, SNEWS is not able to give advance warning of a type Ia supernova, as they are not expected to produce significant numbers of neutrinos. Type Ia supernovae, caused by a runaway nuclear fusion reaction in a white dwarf star, are thought to account for roughly one-third of all supernovae.There are currently seven neutrino detector members of SNEWS: Borexino, Daya Bay, KamLAND, HALO, IceCube, LVD, and Super-Kamiokande. SNEWS began operation prior to 2004, with three members (Super-Kamiokande, LVD, and SNO). The Sudbury Neutrino Observatory is no longer active as it is being upgraded to its successor program SNO+.

The detectors send reports of a possible supernova to a computer at Brookhaven National Laboratory to identify a supernova. If the SNEWS computer identifies signals from two detectors within 10 seconds, the computer will send a supernova alert to observatories around the world to study the supernova. The SNEWS mailing list is open-subscription, and the general public is allowed to sign up; however, the SNEWS collaboration encourages amateur astronomers to instead use Sky and Telescope magazine's AstroAlert service, which is linked to SNEWS.

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