Nuclear astrophysics
Encyclopedia
Nuclear astrophysics is an interdisciplinary branch of physics involving close collaboration among researchers in various subfields of nuclear physics
and astrophysics
, with significant emphasis in areas such as stellar modeling
, measurement and theoretical estimation of nuclear reaction
rates, cosmology
, cosmochemistry
, gamma ray, optical and X-ray
astronomy
, and extending our knowledge about nuclear lifetimes
and masses. In general terms, nuclear astrophysics aims to understand the origin of the chemical element
s and the energy generation in stars.
which came together in the late 1950s from the seminal works of Burbidge, Burbidge, Fowler
, and Hoyle
in a famous paper and independently by Cameron. Fowler is largely credited with initiating the collaboration between astronomers, astrophysicists, and experimental nuclear physicists which is what we now know as nuclear astrophysics.
The basic tenets of nuclear astrophysics are that only isotopes of hydrogen
and helium
(and traces of lithium, beryllium
, and boron
) can be formed in a homogeneous big bang
model (see big bang nucleosynthesis
), and all other elements are formed in stars. The conversion of nuclear mass to kinetic energy (by merit of Einstein's famous mass-energy relation in relativity
) is the source of energy which allows stars to shine for up to billions of years. Many notable physicists of the 19th century, such as Mayer
, Waterson, von Helmholtz, and Lord Kelvin, postulated that the Sun
radiates thermal energy based on converting gravitational potential energy into heat
. The lifetime of the Sun under such a model can be calculated relatively easily using the virial theorem, yielding around 19 million years, an age that was not consistent with the interpretation of geological records or the then recently proposed theory of biological evolution
. A back-of-the-envelope calculation indicates that if the Sun consisted entirely of a fossil fuel
like coal
, a source of energy familiar to many people, considering the rate of thermal energy emission, then the Sun would have a lifetime of merely four or five thousand years, which is not even consistent with records of human civilization
. The now discredited hypothesis that gravitational contraction is the Sun's primary source of energy was, however, reasonable before the advent of modern physics
; radioactivity itself was not discovered by Becquerel
until 1895 Besides the prerequisite knowledge of the atomic nucleus
, a proper understanding of stellar energy is not possible without the theories of relativity and quantum mechanics
.
After Aston
demonstrated that the mass of helium is less than four times the mass of the proton, Eddington
proposed that in the core of the Sun, through an unknown process, hydrogen was transmuted into helium, liberating energy. 20 years later, Bethe
and von Weizsäcker
independently derived the CN cycle
, the first known nuclear reaction cycle which can accomplish this transmutation; however, it is now understood that the Sun's primary energy source is the pp-chains, which can occur at much lower energies and are much slower than catalytic hydrogen fusion. The time-lapse between Eddington's proposal and the derivation of the CN cycle can mainly be attributed to an incomplete understanding of nuclear structure
, and a proper understanding of nucleosynthetic processes was not possible until Chadwick
discovered the neutron
in 1932 and a contemporary theory of beta decay
developed. Nuclear physics gives a self-consistent picture of the energy source for the Sun and its subsequent lifetime, as the age of the solar system derived from meteoritic
abundances of lead
and uranium
isotopes is about 4.5 billion years. A star the mass of the Sun has enough nuclear fuel to allow for core hydrogen burning on the main sequence
of the HR-diagram via the pp-chains for about 9 billion years, a lifetime primarily set by the extremely slow production of deuterium,
which is governed by the nuclear weak force.
reproduces the chemical abundances observed in the solar system and galaxy, which from hydrogen to uranium, show an extremely varied distribution spanning twelve orders of magnitude (one trillion). While impressive, these data were used to formulate the theory, and a scientific theory
must be predictive in order to have any merit. The theory of stellar nucleosynthesis has been well-tested by observation and experiment since the theory was first formulated.
The theory predicted the observation of technetium
(the lightest chemical element with no stable isotopes) in stars, observation of galactic gamma-emitters such as 26Al and 44Ti, observation of solar neutrinos, and observation of neutrinos from supernova 1987a
. These observations have far-reaching implications. 26Al has a lifetime a bit less than one million years, which is very short on a galactic timescale
, proving that nucleosynthesis is an on-going process even in our own time. Work which lead to the discovery of neutrino oscillation
, implying a non-zero mass for the neutrino and thus not predicted by the Standard Model
of particle physics
, was motivated by a solar neutrino flux about three times lower than expected, which was a long-standing concern in the nuclear astrophysics community such that it was colloquially known simply as the Solar neutrino problem
. The observable neutrino flux from nuclear reactor
s is much larger than that of the Sun, and thus Davis and others were primarily motivated to look for solar neutrinos for astronomical reasons.
, there are still many remaining open questions. A few of the long-standing issues are helium fusion (specifically the 12C(α,γ)16O reaction), the astrophysical site of the r-process
, anomalous lithium
abundances in Population III stars, and the explosion mechanism in core-collapse supernovae
.
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
and astrophysics
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
, with significant emphasis in areas such as stellar modeling
Stellar structure
Stars of different mass and age have varying internal structures. Stellar structure models describe the internal structure of a star in detail and make detailed predictions about the luminosity, the color and the future evolution of the star....
, measurement and theoretical estimation of nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
rates, cosmology
Cosmology
Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...
, cosmochemistry
Cosmochemistry
Cosmochemistry or chemical cosmology is the study of the chemical composition of matter in the universe and the processes that led to those compositions. This is done primarily through the study of the chemical composition of meteorites and other physical samples...
, gamma ray, optical and X-ray
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, and extending our knowledge about nuclear lifetimes
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
and masses. In general terms, nuclear astrophysics aims to understand the origin of the chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s and the energy generation in stars.
History
The basic principles of explaining the origin of the elements and the energy generation in stars were laid down in the theory of nucleosynthesisNucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
which came together in the late 1950s from the seminal works of Burbidge, Burbidge, Fowler
William Alfred Fowler
William Alfred "Willy" Fowler was an American astrophysicist and winner of the Nobel Prize for Physics in 1983. He should not be confused with the British astronomer Alfred Fowler....
, and Hoyle
Fred Hoyle
Sir Fred Hoyle FRS was an English astronomer and mathematician noted primarily for his contribution to the theory of stellar nucleosynthesis and his often controversial stance on other cosmological and scientific matters—in particular his rejection of the "Big Bang" theory, a term originally...
in a famous paper and independently by Cameron. Fowler is largely credited with initiating the collaboration between astronomers, astrophysicists, and experimental nuclear physicists which is what we now know as nuclear astrophysics.
The basic tenets of nuclear astrophysics are that only isotopes of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
(and traces of lithium, beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, and boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
) can be formed in a homogeneous big bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
model (see big bang nucleosynthesis
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...
), and all other elements are formed in stars. The conversion of nuclear mass to kinetic energy (by merit of Einstein's famous mass-energy relation in relativity
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....
) is the source of energy which allows stars to shine for up to billions of years. Many notable physicists of the 19th century, such as Mayer
Hans Ferdinand Mayer
Hans Ferdinand Mayer was a German mathematician and physicist and perhaps most notable for the Oslo Report which revealed German technological secrets to the British Government shortly after the start of World War II.-Biography:Hans Ferdinand Mayer studied mathematics, physics and astronomy at...
, Waterson, von Helmholtz, and Lord Kelvin, postulated that the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
radiates thermal energy based on converting gravitational potential energy into heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
. The lifetime of the Sun under such a model can be calculated relatively easily using the virial theorem, yielding around 19 million years, an age that was not consistent with the interpretation of geological records or the then recently proposed theory of biological evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
. A back-of-the-envelope calculation indicates that if the Sun consisted entirely of a fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
like coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
, a source of energy familiar to many people, considering the rate of thermal energy emission, then the Sun would have a lifetime of merely four or five thousand years, which is not even consistent with records of human civilization
Civilization
Civilization is a sometimes controversial term that has been used in several related ways. Primarily, the term has been used to refer to the material and instrumental side of human cultures that are complex in terms of technology, science, and division of labor. Such civilizations are generally...
. The now discredited hypothesis that gravitational contraction is the Sun's primary source of energy was, however, reasonable before the advent of modern physics
Modern physics
The term modern physics refers to the post-Newtonian conception of physics. The term implies that classical descriptions of phenomena are lacking, and that an accurate, "modern", description of reality requires theories to incorporate elements of quantum mechanics or Einsteinian relativity, or both...
; radioactivity itself was not discovered by Becquerel
Henri Becquerel
Antoine Henri Becquerel was a French physicist, Nobel laureate, and the discoverer of radioactivity along with Marie Curie and Pierre Curie, for which all three won the 1903 Nobel Prize in Physics.-Early life:...
until 1895 Besides the prerequisite knowledge of the atomic nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
, a proper understanding of stellar energy is not possible without the theories of relativity and quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
.
After Aston
Francis William Aston
Francis William Aston was a British chemist and physicist who won the 1922 Nobel Prize in Chemistry for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule...
demonstrated that the mass of helium is less than four times the mass of the proton, Eddington
Arthur Stanley Eddington
Sir Arthur Stanley Eddington, OM, FRS was a British astrophysicist of the early 20th century. He was also a philosopher of science and a popularizer of science...
proposed that in the core of the Sun, through an unknown process, hydrogen was transmuted into helium, liberating energy. 20 years later, Bethe
Hans Bethe
Hans Albrecht Bethe was a German-American nuclear physicist, and Nobel laureate in physics for his work on the theory of stellar nucleosynthesis. A versatile theoretical physicist, Bethe also made important contributions to quantum electrodynamics, nuclear physics, solid-state physics and...
and von Weizsäcker
Carl Friedrich von Weizsäcker
Carl Friedrich Freiherr von Weizsäcker was a German physicist and philosopher. He was the longest-living member of the research team which performed nuclear research in Germany during the Second World War, under Werner Heisenberg's leadership...
independently derived the CN cycle
CNO cycle
The CNO cycle is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain. Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle. Theoretical models show that the CNO cycle is the dominant source of energy in stars...
, the first known nuclear reaction cycle which can accomplish this transmutation; however, it is now understood that the Sun's primary energy source is the pp-chains, which can occur at much lower energies and are much slower than catalytic hydrogen fusion. The time-lapse between Eddington's proposal and the derivation of the CN cycle can mainly be attributed to an incomplete understanding of nuclear structure
Nuclear structure
Understanding the structure of the atomic nucleus is one of the central challenges in nuclear physics. This article is written from a nuclear physics perspective; as such, it is suggested that a casual reader first read the main nuclear physics article....
, and a proper understanding of nucleosynthetic processes was not possible until Chadwick
James Chadwick
Sir James Chadwick CH FRS was an English Nobel laureate in physics awarded for his discovery of the neutron....
discovered the neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
in 1932 and a contemporary theory of beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
developed. Nuclear physics gives a self-consistent picture of the energy source for the Sun and its subsequent lifetime, as the age of the solar system derived from meteoritic
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
abundances of lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
and uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
isotopes is about 4.5 billion years. A star the mass of the Sun has enough nuclear fuel to allow for core hydrogen burning on the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
of the HR-diagram via the pp-chains for about 9 billion years, a lifetime primarily set by the extremely slow production of deuterium,
| → |
Predictions
The theory of stellar nucleosynthesisStellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...
reproduces the chemical abundances observed in the solar system and galaxy, which from hydrogen to uranium, show an extremely varied distribution spanning twelve orders of magnitude (one trillion). While impressive, these data were used to formulate the theory, and a scientific theory
Scientific theory
A scientific theory comprises a collection of concepts, including abstractions of observable phenomena expressed as quantifiable properties, together with rules that express relationships between observations of such concepts...
must be predictive in order to have any merit. The theory of stellar nucleosynthesis has been well-tested by observation and experiment since the theory was first formulated.
The theory predicted the observation of technetium
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
(the lightest chemical element with no stable isotopes) in stars, observation of galactic gamma-emitters such as 26Al and 44Ti, observation of solar neutrinos, and observation of neutrinos from supernova 1987a
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...
. These observations have far-reaching implications. 26Al has a lifetime a bit less than one million years, which is very short on a galactic timescale
Galactic year
The galactic year, also known as a cosmic year, is the duration of time required for the Solar System to orbit once around the center of the Milky Way Galaxy. Estimates of the length of one orbit range from 225 to 250 million "terrestrial" years....
, proving that nucleosynthesis is an on-going process even in our own time. Work which lead to the discovery of neutrino oscillation
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...
, implying a non-zero mass for the neutrino and thus not predicted by the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, was motivated by a solar neutrino flux about three times lower than expected, which was a long-standing concern in the nuclear astrophysics community such that it was colloquially known simply as the Solar neutrino problem
Solar neutrino problem
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002...
. The observable neutrino flux from nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s is much larger than that of the Sun, and thus Davis and others were primarily motivated to look for solar neutrinos for astronomical reasons.
Future work
Although the foundations of the science are bona fideBona Fide
Bona Fide is a studio album from rock band Wishbone Ash. It is the first studio album in six years and is the only studio album to feature guitarist Ben Granfelt...
, there are still many remaining open questions. A few of the long-standing issues are helium fusion (specifically the 12C(α,γ)16O reaction), the astrophysical site of the r-process
R-process
The r-process is a nucleosynthesis process, likely occurring in core-collapse supernovae responsible for the creation of approximately half of the neutron-rich atomic nuclei that are heavier than iron. The process entails a succession of rapid neutron captures on seed nuclei, typically Ni-56,...
, anomalous lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
abundances in Population III stars, and the explosion mechanism in core-collapse supernovae
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
.
See also
- Nuclear physicsNuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
- AstrophysicsAstrophysicsAstrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
- NucleosynthesisNucleosynthesisNucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
- Joint Institute for Nuclear AstrophysicsJoint Institute for Nuclear AstrophysicsThe Joint Institute for Nuclear Astrophysics in USA is a collaboration between Michigan State University, the University of Notre Dame, and the University of Chicago to address a broad range of experimental, theoretical, and observational questions in nuclear astrophysics...