Nuclear astrophysics covers the study of all the nuclear processes that take place in stars and in other cosmic objects. To some extent it overlaps with cosmic-ray physics and neutrino astrophysics. Nuclear processes, that is, nuclear reactions and weak interactions, lead to the release (absorption) of energy and also to the formation (decay) of various chemical elements. The tasks of nuclear astrophysics are chiefly to determine the probabilities of the various nuclear processes and their energy yield. These data are used in the theory of stellar evolution and in the theory of nucleosynthesis. For the theory of stellar evolution the most important reactions are those between charged particles, including protons, alpha particles and so on. They take place inside stars under conditions of thermodynamic equilibrium, with a Maxwellian distribution of particle velocities. The rate of such thermonuclear reactions is therefore proportional to the probability of overcoming the Coulomb barrier, averaged over the equilibrium distribution of the relative velocities of the particles. As a result the intensity of thermonuclear reactions and their energy release rise sharply with temperature. It is very important to allow for electron screening in the plasma, which lowers the barrier and makes nuclear reactions easier. Alongside experimental data, various theoretical models of the nucleus are used to calculate reaction rates. Weak-interaction processes often form part of a chain of nuclear processes, in particular the first reaction of the hydrogen cycle 1H + 1H → D + e+ + ν, where e+ is the positron and ν is the neutrino. At the late stages of stellar evolution, when the electrons become degenerate, weak interactions are marked by the suppression of radioactive beta decay of nuclei. The same conditions are marked by electron captures (with a continuous energy spectrum of the electrons, unlike the K-capture usual under terrestrial conditions).
Nuclear astrophysics leads to the conclusion that there are certain distinct stages of thermonuclear burning in the course of a star's evolution. The long existence of stars on the main sequence is due to the hydrogen burning stage (the hydrogen cycle or the carbon cycle of nuclear reactions). Hydrogen burning is followed by helium burning, with the reaction that synthesises carbon from three helium nuclei. Helium burning is characteristic of stars of the giant and supergiant type. After helium burning come, in succession, the carbon, neon, oxygen and finally silicon burning stages. Each stage consists of a system of principal and secondary nuclear processes, of which only the first are significant for the energy yield. The secondary reactions are nevertheless important in nucleosynthesis. The principal reactions of the post-helium stages, of the (αγ) type, are accompanied by secondary ones: (αp), (pγ), (αn), (nγ) and so on. At the end of silicon burning the temperature at the centre of the star rises to ~3·10°K (the rise of temperature and density according to the law T~p1/3 is the essence of stellar evolution). Under these conditions the effective energy of the thermal motion of the reacting particles reaches ~1 MeV, the Coulomb barrier practically disappears and nuclear statistical equilibrium sets in. A certain difference between the concentrations of neutrons and protons as compared with the initial composition of the star is the result of non-equilibrium weak interactions. Equilibrium burning is characteristic of the onset and the course of gravitational collapse — the last stage of a star's evolution before it passes into the state of a neutron star. In the envelope of a collapsing star, however, the nuclear reactions of the earlier stages take place, but in an explosive regime. They are accompanied by explosive nucleosynthesis. These explosive processes influence the ejection of the envelope, that is, the supernova explosion, to no small degree. During gravitational collapse and a supernova explosion a noticeable quantity of free neutrons is formed, whose role at the earlier stages was small. In the presence of iron-group elements the free neutrons are rapidly captured by these elements (the so-called r-process), which leads to the formation of all the heavier chemical elements and to an increase in their share in the isotopic composition of the matter of the Universe. Reactions with free protons also contribute to the synthesis of the heavy elements.
Nuclear astrophysics studies nuclear processes in stars on the basis of the material of experimental nuclear physics, which is continuously being improved. New areas of research are appearing in nuclear astrophysics, in particular neutrino nucleosynthesis. The powerful flux of neutrinos generated by the collapse of a star causes nuclear transformations in the matter surrounding it. This process contributes to the formation of the very lightest nuclei (besides the spallation reaction) and of the bypassed nuclei (besides reactions with fast protons). One may also point to the nucleosynthesis of very heavy nuclei through fission and beta decays in clumps of matter hypothetically ejected from the interiors of neutron stars. Formerly the formation of the superheavy elements was explained by the r-process only with difficulty.