Nuclear astrophysics

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 elements and the energy generation in stars.

The basic principles of explaining the origin of the elements and the energy generation in stars were laid down in the theory of nucleosynthesis 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, winning the Nobel Prize for this in 1983.

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 radiative 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.

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