Nuclear astrophysics is an interdisciplinary branch of physics involving close collaboration among researchers in various subfields of nuclear physics and astrophysics: notably stellar modeling; measurement and theoretical estimation of nuclear reaction rates; physical cosmology and 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 elements and the energy generation in stars.
The basic principles for explaining the origin of elements and energy generation in stars appear in the theory of nucleosynthesis, which came together in the late 1950s in seminal works by Burbidge, Burbidge, Fowler, and Hoyle, and by Cameron. Fowler is largely credited with initiating collaboration between astronomers, astrophysicists, and experimental nuclear physicists that we now know as nuclear astrophysics (for which he won the 1983 Nobel Prize).
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), while all other elements are formed in stars. Conversion of nuclear mass to radiative energy (per Einstein's famous mass-energy relation) is what 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 by converting gravitational potential energy into heat. Under such a model, its lifetime can be calculated relatively easily using the virial theorem — around 19 million years, which was inconsistent with the interpretation of geological records and the (then new) 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), considering the rate of thermal energy emission, its lifetime would be merely four or five thousand years, which is not even consistent with records of human civilization. Though now discredited, this hypothesis that the Sun's primary energy source is gravitational contraction was 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 that of the proton, Eddington proposed that, through an unknown process in the Sun's core, hydrogen is transmuted into helium, liberating energy. Twenty years later, Bethe and von Weizsäcker independently derived the CN cycle, the first known nuclear reaction that accomplishes this transmutation. However, the Sun's primary energy source is now understood to be proton–proton chain reactions, occurring at much lower energies and much more slowly than catalytic hydrogen fusion. The interval between Eddington's proposal and derivation of the CN cycle can mainly be attributed to an incomplete understanding of nuclear structure. A proper understanding of nucleosynthetic processes only came when Chadwick discovered the neutron in 1932 and beta decay theory developed. Nuclear physics 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 — about 4.5 billion years. The mass of stars like the Sun allow core hydrogen burning on the main sequence of the Hertzsprung-Russell diagram via pp-chains for about 9 billion years. This primarily is determined by extremely slow production of deuterium,
which is governed by the nuclear weak force.
Stellar nucleosynthesis theory estimates chemical abundances consistent with those observed in the Solar System and galaxy, whose distribution spans twelve orders of magnitude (one trillion). While impressive, these data were used to formulate the theory, but a scientific theory must be predictive to have merit. The theory has been well-tested by observation and experiment since first formulated.
The theory predicts technetium (the lightest chemical element without stable isotopes) in stars, galactic gamma-emitters (such as 26Al and 44Ti), and observation of solar neutrinos and 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 ongoing process even in our own time. 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 — a long-standing concern in the nuclear astrophysics community colloquially known as the Solar neutrino problem. The observable neutrino flux from nuclear reactors is much larger than that of the Sun, so Davis and others were primarily motivated to look for solar neutrinos for astronomical reasons.
Although the foundations of the science are bona fide, many questions still remain open. Some 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
- Abundance of the chemical elements
- Joint Institute for Nuclear Astrophysics
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