Nuclear astrophysics

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Nuclear astrophysics is an interdisciplinary part of both nuclear physics and astrophysics, involving close collaboration among researchers in various subfields of each of these fields. This includes, notably, nuclear reactions and their rates as they occur in cosmic environments, and modeling of astrophysical objects where these nuclear reactions may occur, but also considerations of cosmic evolution of isotopic and elemental composition (often called chemical evolution). Constraints from observations involve multiple messengers, all across the electromagnetic spectrum (nuclear gamma-rays, X-rays, optical, and radio/sub-mm astronomy), as well as isotopic measurements of solar-system materials such as meteorites and their stardust inclusions, cosmic rays, material deposits on Earth and Moon). Nuclear physics experiments address stability (i.e., lifetimes and masses) for atomic nuclei well beyond the regime of stable nuclides into the realm of radioactive/unstable nuclei, almost to the limits of bound nuclei (the drip lines), and under high density (up to neutron star matter) and high temperature (plasma temperatures up to 109  K ). Theories and simulations are essential parts herein, as cosmic nuclear reaction environments cannot be realized, but at best partially approximated by experiments. In general terms, nuclear astrophysics aims to understand the origin of the chemical elements and isotopes, and the role of nuclear energy generation, in cosmic sources such as stars, supernovae, novae, and violent binary-star interactions.

Contents

History

In the 1940s, geologist Hans Suess speculated that the regularity that was observed in the abundances of elements may be related to structural properties of the atomic nucleus. [1] These considerations were seeded by the discovery of radioactivity by Becquerel in 1896 [2] as an aside of advances in chemistry which aimed at production of gold. This remarkable possibility for transformation of matter created much excitement among physicists for the next decades, culminating in discovery of the atomic nucleus, with milestones in Ernest Rutherford's scattering experiments in 1911, and the discovery of the neutron by James Chadwick (1932). After Aston demonstrated that the mass of helium is less than four times that of the proton, Eddington proposed that, through an unknown process in the Sun's core, hydrogen is transmuted into helium, liberating energy. [3] Twenty years later, Bethe and von Weizsäcker independently derived the CN cycle, [4] [5] the first known nuclear reaction that accomplishes this transmutation. The interval between Eddington's proposal and derivation of the CN cycle can mainly be attributed to an incomplete understanding of nuclear structure. The basic principles for explaining the origin of elements and energy generation in stars appear in the concepts describing nucleosynthesis, which arose in the 1940s, led by George Gamow and presented in a 2-page paper in 1948 as the Alpher–Bethe–Gamow paper. A complete concept of processes that make up cosmic nucleosynthesis was presented in the late 1950s by Burbidge, Burbidge, Fowler, and Hoyle, [6] and by Cameron. [7] Fowler is largely credited with initiating collaboration between astronomers, astrophysicists, and theoretical and experimental nuclear physicists, in a field that we now know as nuclear astrophysics [8] (for which he won the 1983 Nobel Prize). During these same decades, Arthur Eddington and others were able to link the liberation of nuclear binding energy through such nuclear reactions to the structural equations of stars. [9]

These developments were not without curious deviations. Many notable physicists of the 19th century such as Mayer, Waterson, von Helmholtz, and Lord Kelvin, postulated that the Sun radiates thermal energy by converting gravitational potential energy into heat. Its lifetime as calculated from this assumption using the virial theorem, around 19 million years, was found inconsistent with the interpretation of geological records and the (then new) theory of biological evolution. Alternatively, if the Sun consisted entirely of a fossil fuel like coal, considering the rate of its thermal energy emission, its lifetime would be merely four or five thousand years, clearly inconsistent with records of human civilization.

Basic concepts

During cosmic times, nuclear reactions re-arrange the nucleons that were left behind from the big bang (in the form of isotopes of hydrogen and helium, and traces of lithium, beryllium, and boron) to other isotopes and elements as we find them today (see graph). The driver is a conversion of nuclear binding energy to exothermic energy, favoring nuclei with more binding of their nucleons - these are then lighter as their original components by the binding energy. The most tightly-bound nucleus from symmetric matter of neutrons and protons is 56Ni. The release of nuclear binding energy is what allows stars to shine for up to billions of years, and may disrupt stars in stellar explosions in case of violent reactions (such as 12C+12C fusion for thermonuclear supernova explosions). As matter is processed as such within stars and stellar explosions, some of the products are ejected from the nuclear-reaction site and end up in interstellar gas. Then, it may form new stars, and be processed further through nuclear reactions, in a cycle of matter. This results in compositional evolution of cosmic gas in and between stars and galaxies, enriching such gas with heavier elements. Nuclear astrophysics is the science to describe and understand the nuclear and astrophysical processes within such cosmic and galactic chemical evolution, linking it to knowledge from nuclear physics and astrophysics. Measurements are used to test our understanding: Astronomical constraints are obtained from stellar and interstellar abundance data of elements and isotopes, and other multi-messenger astronomical measurements of the cosmic object phenomena help to understand and model these. Nuclear properties can be obtained from terrestrial nuclear laboratories such as accelerators with their experiments. Theory and simulations are needed to understand and complement such data, providing models for nuclear reaction rates under the variety of cosmic conditions, and for the structure and dynamics of cosmic objects.

Findings, current status, and issues

Nuclear astrophysics remains as a complex puzzle to science. [10] The current consensus on the origins of elements and isotopes are that only hydrogen and helium (and traces of lithium, beryllium, boron) can be formed in a homogeneous Big Bang (see Big Bang nucleosynthesis), while all other elements and their isotopes are formed in cosmic objects that formed later, such as in stars and their explosions.[ citation needed ]

The Sun's primary energy source is hydrogen fusion to helium at about 15 million degrees. The proton–proton chain reactions dominate, they occur at much lower energies although much more slowly than catalytic hydrogen fusion through CNO cycle reactions. Nuclear astrophysics gives a picture of the Sun's energy source producing a lifetime consistent with the age of the Solar System derived from meteoritic abundances of lead and uranium isotopes —an age of about 4.5 billion years. The core hydrogen burning of stars, as it now occurs in the Sun, defines the main sequence of stars, illustrated in the Hertzsprung-Russell diagram that classifies stages of stellar evolution. The Sun's lifetime of H burning via pp-chains is about 9 billion years. This primarily is determined by extremely slow production of deuterium,

1
1H
  		+  1
1H
  		→  2
1D
  		+ 
e+
  		+ 
ν
e  		+ 0.42  MeV

which is governed by the weak interaction.

Work that led to discovery of neutrino oscillation (implying a non-zero mass for the neutrino absent in the Standard Model of particle physics) was motivated by a solar neutrino flux about three times lower than expected from theories — a long-standing concern in the nuclear astrophysics community colloquially known as the Solar neutrino problem.

The concepts of nuclear astrophysics are supported by observation of the element technetium (the lightest chemical element without stable isotopes) in stars, [11] by galactic gamma-ray line emitters (such as 26Al, [12] 60Fe, and 44Ti [13] ), by radioactive-decay gamma-ray lines from the 56Ni decay chain observed from two supernovae (SN1987A and SN2014J) coincident with optical supernova light, and by observation of neutrinos from the Sun [14] and from supernova 1987a. These observations have far-reaching implications. 26Al has a lifetime of a million years, which is very short on a galactic timescale, proving that nucleosynthesis is an ongoing process within our Milky Way Galaxy in the current epoch.

Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common. The next three elements (Li, Be, B) are rare, intermediate-mass elements such as C, O, ..Si, Ca more abundant. Beyond Fe, there is a remarkable drop beyond Fe, heavier elements being 3-5 orders of magnitude less abundant. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance of elements according to whether they have even or odd atomic numbers, 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 of ten, say between logA=2 (A=100) and logA=6 (A=1,000,000). SolarSystemAbundances.png
Abundances of the chemical elements in the Solar System. Hydrogen and helium are most common. The next three elements (Li, Be, B) are rare, intermediate-mass elements such as C, O, ..Si, Ca more abundant. Beyond Fe, there is a remarkable drop beyond Fe, heavier elements being 3-5 orders of magnitude less abundant. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance of elements according to whether they have even or odd atomic numbers, 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 of ten, say between logA=2 (A=100) and logA=6 (A=1,000,000).

Current descriptions of the cosmic evolution of elemental abundances are broadly consistent with those observed in the Solar System and galaxy, whose distribution spans twelve orders of magnitude (one trillion).[ citation needed ]

The roles of specific cosmic objects in producing these elemental abundances are clear for some elements, and heavily debated for others. For example, iron is believed to originate mostly from thermonuclear supernova explosions (also called supernovae of type Ia), and carbon and oxygen is believed to originate mostly from massive stars and their explosions. Li, Be, and B are believed to originate from spallation reactions of cosmic-ray nuclei such as carbon and heavier nuclei, breaking these apart. Unclear is, in which sources nuclei much heavier than iron are produced; for the slow and rapid neutron capture reactions, different sites are discussed, such as envelopes of stars of either lower or higher masses, or supernova explosions versus collisions of compact stars.[ citation needed ] The transport of nuclear reaction products from their sources through the interstellar and intergalactic medium also is unclear, and there is, e.g., a missing metals problem of more production of heavy elements predicted than is observed in stars. Also, many nuclei that are involved in cosmic nuclear reactions are unstable and only predicted to exist temporarily in cosmic sites; we cannot easily measure the properties of such nuclei, and uncertainties on their binding energies are substantial. Similarly, stellar structure and its dynamics is not satisfactorily described in models and hard to observe except through asteroseismology; also, supernova explosion models lack a consistent description based on physical processes, and include heuristic elements.[ citation needed ]

Future work

Although the foundations of nuclear astrophysics appear clear and plausible, many puzzles remain. One example from nuclear reaction physics is helium fusion (specifically the 12C(α,γ)16O reaction(s)), [15] others are the astrophysical site of the r-process, anomalous lithium abundances in population III stars, and the explosion mechanism in core-collapse supernovae and the progenitors of thermonuclear supernovae.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">CNO cycle</span> Catalysed fusion reactions by which stars convert hydrogen to helium

The CNO cycle is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain reaction, which is more efficient at the Sun's core temperature. The CNO cycle is hypothesized to be dominant in stars that are more than 1.3 times as massive as the Sun.

<span class="mw-page-title-main">Proton–proton chain</span> One of the fusion reactions by which stars convert hydrogen to helium

The proton–proton chain, also commonly referred to as the p–p chain, is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun, whereas the CNO cycle, the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 times that of the Sun.

<span class="mw-page-title-main">Stellar evolution</span> Changes to stars over their lifespans

Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the current age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

In physical cosmology, Big Bang nucleosynthesis is the production of nuclei other than those of the lightest isotope of hydrogen during the early phases of the universe. This type of nucleosynthesis is thought by most cosmologists to have occurred from 10 seconds to 20 minutes after the Big Bang. It is thought to be responsible for the formation of most of the universe's helium, along with small fractions of the hydrogen isotope deuterium, the helium isotope helium-3 (3He), and a very small fraction of the lithium isotope lithium-7 (7Li). In addition to these stable nuclei, two unstable or radioactive isotopes were produced: the heavy hydrogen isotope tritium and the beryllium isotope beryllium-7 (7Be). These unstable isotopes later decayed into 3He and 7Li, respectively, as above.

Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in a process called Big Bang nucleosynthesis. After about 20 minutes, the universe had expanded and cooled to a point at which these high-energy collisions among nucleons ended, so only the fastest and simplest reactions occurred, leaving our universe containing hydrogen and helium. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. Nucleosynthesis in stars and their explosions later produced the variety of elements and isotopes that we have today, in a process called cosmic chemical evolution. The amounts of total mass in elements heavier than hydrogen and helium remains small, so that the universe still has approximately the same composition.

<span class="mw-page-title-main">Stellar nucleosynthesis</span> Creation of chemical elements within stars

Stellar nucleosynthesis is the creation (nucleosynthesis) of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a predictive theory, it yields accurate estimates of the observed abundances of the elements. It explains why the observed abundances of elements change 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 and Geoffrey Burbidge, William Alfred Fowler and Fred Hoyle in their famous 1957 B2FH paper, which became one of the most heavily cited papers in astrophysics history.

<span class="mw-page-title-main">Alpha process</span> Nuclear fusion reaction

The alpha process, also known as the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements. The other class is a cycle of reactions called the triple-alpha process, which consumes only helium, and produces carbon. The alpha process most commonly occurs in massive stars and during supernovae.

The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in the cores of massive stars (at least 8 at birth) that combines carbon into other elements. It requires high temperatures (> 5×108 K or 50 keV) and densities (> 3×109 kg/m3).

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.

<i>r</i>-process Nucleosynthesis pathway

In nuclear astrophysics, the rapid neutron-capture process, also known as the r-process, is a set of nuclear reactions that is responsible for the creation of approximately half of the atomic nuclei heavier than iron, the "heavy elements", with the other half produced by the p-process and s-process. The r-process usually synthesizes the most neutron-rich stable isotopes of each heavy element. 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 be created via the r-process only. Abundance peaks for the r-process occur near mass numbers A = 82, A = 130 and A = 196.

The slow neutron-capture process, or s-process, is a series of reactions in nuclear astrophysics that occur in stars, particularly asymptotic giant branch stars. The s-process is responsible for the creation (nucleosynthesis) of approximately half the atomic nuclei heavier than iron.

The term p-process (p for proton) is used in two ways in the scientific literature concerning the astrophysical origin of the elements (nucleosynthesis). Originally it referred to a proton capture process which is the source of certain, naturally occurring, neutron-deficient isotopes of the elements from selenium to mercury. These nuclides are called p-nuclei and their origin is still not completely understood. Although it was shown that the originally suggested process cannot produce the p-nuclei, later on the term p-process was sometimes used to generally refer to any nucleosynthesis process supposed to be responsible for the p-nuclei.

Supernova nucleosynthesis is the nucleosynthesis of chemical elements in supernova explosions.

In physical cosmology, the Alpher–Bethe–Gamow paper, or αβγ paper, was created by Ralph Alpher, then a physics PhD student, his advisor George Gamow, and Hans Bethe. The work, which would become the subject of Alpher's PhD dissertation, argued that the Big Bang would create hydrogen, helium and heavier elements in the correct proportions to explain their abundance in the early universe. While the original theory neglected a number of processes important to the formation of heavy elements, subsequent developments showed that Big Bang nucleosynthesis is consistent with the observed constraints on all primordial elements.

<span class="mw-page-title-main">Type II supernova</span> Explosion of a star 8 to 45 times the mass of the Sun

A Type II supernova or SNII results from the rapid collapse and violent explosion of a massive star. A star must have at least eight times, but no more than 40 to 50 times, the mass of the Sun (M☉) to undergo this type of explosion. Type II supernovae are distinguished from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies; those are generally composed of older, low-mass stars, with few of the young, very massive stars necessary to cause a supernova.

The B2FH paper was a landmark scientific paper on the origin of the chemical elements. The paper's title is Synthesis of the Elements in Stars, but it became known as B2FH from the initials of its authors: Margaret Burbidge, Geoffrey Burbidge, William A. Fowler, and Fred Hoyle. It was written from 1955 to 1956 at the University of Cambridge and Caltech, then published in Reviews of Modern Physics in 1957.

The Hans A. Bethe Prize, is presented annually by the American Physical Society. The prize honors outstanding work in theory, experiment or observation in the areas of astrophysics, nuclear physics, nuclear astrophysics, or closely related fields. The prize consists of $10,000 and a certificate citing the contributions made by the recipient.

<span class="mw-page-title-main">Nuclear transmutation</span> Conversion of an atom from one element to another

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.

p-nuclei (p stands for proton-rich) are certain proton-rich, naturally occurring isotopes of some elements between selenium and mercury inclusive which cannot be produced in either the s- or the r-process.

<span class="mw-page-title-main">Donald D. Clayton</span> American astrophysicist

Donald Delbert Clayton is an American astrophysicist whose most visible achievement was the prediction from nucleosynthesis theory that supernovae are intensely radioactive. That earned Clayton the NASA Exceptional Scientific Achievement Medal (1992) for “theoretical astrophysics related to the formation of (chemical) elements in the explosions of stars and to the observable products of these explosions”. Supernovae thereafter became the most important stellar events in astronomy owing to their profoundly radioactive nature. Not only did Clayton discover radioactive nucleosynthesis during explosive silicon burning in stars but he also predicted a new type of astronomy based on it, namely the associated gamma-ray line radiation emitted by matter ejected from supernovae. That paper was selected as one of the fifty most influential papers in astronomy during the twentieth century for the Centennial Volume of the American Astronomical Society. He gathered support from influential astronomers and physicists for a new NASA budget item for a gamma-ray-observatory satellite, achieving successful funding for Compton Gamma Ray Observatory. With his focus on radioactive supernova gas Clayton discovered a new chemical pathway causing carbon dust to condense there by a process that is activated by the radioactivity.

References

  1. ↑ Suess, Hans E.; Urey, Harold C. (1956). "Abundances of the Elements". Reviews of Modern Physics . 28 (1): 53. Bibcode:1956RvMP...28...53S. doi:10.1103/RevModPhys.28.53.
  2. ↑ Henri Becquerel (1896). "Sur les radiations émises par phosphorescence". Comptes Rendus . 122: 420–421. See also a translation by Carmen Giunta
  3. ↑ Eddington, A. S. (1919). "The sources of stellar energy". The Observatory . 42: 371–376. Bibcode:1919Obs....42..371E.
  4. ↑ von Weizsäcker, C. F. (1938). "Über Elementumwandlungen in Innern der Sterne II" [Element Transformation Inside Stars, II]. Physikalische Zeitschrift . 39: 633–646.
  5. ↑ Bethe, H. A. (1939). "Energy Production in Stars". Physical Review . 55 (5): 434–56. Bibcode:1939PhRv...55..434B. doi: 10.1103/PhysRev.55.434 .
  6. ↑ E. M. Burbidge; G. R. Burbidge; W. A. Fowler & F. Hoyle. (1957). "Synthesis of the Elements in Stars" (PDF). Reviews of Modern Physics . 29 (4): 547. Bibcode:1957RvMP...29..547B. doi: 10.1103/RevModPhys.29.547 .
  7. ↑ Cameron, A.G.W. (1957). Stellar Evolution, Nuclear Astrophysics, and Nucleogenesis (PDF) (Report). Atomic Energy of Canada.
  8. ↑ Barnes, C. A.; Clayton, D. D.; Schramm, D. N., eds. (1982), Essays in Nuclear Astrophysics, Cambridge University Press, ISBN   978-0-52128-876-7
  9. ↑ A.S. Eddington (1940). "The physics of White Dwarf stars". Monthly Notices of the Royal Astronomical Society . 100: 582. Bibcode:1940MNRAS.100..582E. doi: 10.1093/mnras/100.8.582 .
  10. ↑ J. José & C. Iliadis (2011). "Nuclear astrophysics: the unfinished quest for the origin of the elements". Reports on Progress in Physics . 74 (9): 6901. arXiv: 1107.2234 . Bibcode:2011RPPh...74i6901J. doi:10.1088/0034-4885/74/9/096901. S2CID   118505733.
  11. ↑ P.W. Merrill (1956). "Technetium in the N-Type Star 19 PISCIUM". Publications of the Astronomical Society of the Pacific . 68 (400): 400. Bibcode:1956PASP...68...70M. doi:10.1086/126883.
  12. ↑ Diehl, R.; et al. (1995). "COMPTEL observations of Galactic 26Al emission". Astronomy and Astrophysics . 298: 445. Bibcode:1995A&A...298..445D.
  13. ↑ Iyudin, A. F.; et al. (1994). "COMPTEL observations of Ti-44 gamma-ray line emission from CAS A". Astronomy and Astrophysics . 294: L1. Bibcode:1994A&A...284L...1I.
  14. ↑ Davis, Raymond; Harmer, Don S.; Hoffman, Kenneth C. (1968). "Search for Neutrinos from the Sun". Physical Review Letters . 20 (21): 1205. Bibcode:1968PhRvL..20.1205D. doi:10.1103/PhysRevLett.20.1205.
  15. ↑ Tang, X. D.; et al. (2007). "New Determination of the Astrophysical S Factor SE1 of the C12(α,γ)O16 Reaction". Physical Review Letters . 99 (5): 052502. Bibcode:2007PhRvL..99e2502T. doi:10.1103/PhysRevLett.99.052502. PMID   17930748.
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