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. 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. 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, 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