Nuclear astrophysics

TL;DR

Nuclear astrophysics connects laboratory nuclear physics with the diverse environments of the cosmos to explain how elements form and evolve. The paper surveys experimental techniques (underground labs, recoil separators, storage rings, THM, laser-plasma and photon facilities) and theoretical tools (R-matrix, Hauser-Feshbach, nuclear mass models) used to derive astrophysical reaction rates across stellar interiors and explosive events. It highlights central reactions like ^12C-induced processes and the challenging CNO and carbon-burning regimes, and discusses how observational pillars—gamma-ray lines, neutrinos, stardust, and gravitational waves—test and constrain these nuclear pathways. The work emphasizes an integrated, multi-messenger framework and chemical-evolution context to map nucleosynthesis from local stellar processes to the chemical makeup of galaxies, while outlining key future avenues in experiments, theory, and astronomy.

Abstract

Reactions between atomic nuclei are measured in great detail in terrestrial laboratory experiments; transferring and extrapolating this knowledge to how the same reactions act within cosmic environments presents major challenges. Cross-disciplinary efforts are needed in view of the many nuclear reactions that govern the chemical evolution of the universe, and occur in a broad range of stellar plasma conditions that require astrophysical exploration. Since the early identification of 'processes' of nucleosynthesis, new insights have been obtained on the complexity of nuclear reaction mechanisms. We use 12C induced capture and fusion processes to illustrate the challenge of low-energy measurements and of using theoretical methods to extrapolate measurements towards energy regimes within cosmic sources. Particle beam experiments at accelerator facilities above and deep underground simulate stellar reactions, new experimental facilities and methods complement these, and this is further complemented by improved theoretical tools to calculate the quantum effects of nuclear reactions at the various cosmic conditions. Astronomical signatures of cosmic nuclear reactions are deduced from light curves characterizing cosmic explosions through gamma-ray lines and presolar grains to the detection of rare neutrino particles from our Sun to distant cosmic events. High resolution spectroscopy of stars has been expanded to objects measured in the X-ray and the gamma energy range of the electromagnetic spectrum. Astro-seismology and isotopic analysis of meteoritic inclusions provide new tools. Chemical-evolution models describe the complex dynamics during the evolution of galaxies. This article summarizes the experimental and theoretical work, and the broad range of observational tools that test the experimental data and the theoretical interpretation of nuclear processes in the cosmos.

Figures (18)

• Figure 1: Table of known isotopes with their proton (vertical axis) and neutron (horizontal axis) numbers, showing the stable isotopes (black squares) as well as unstable isotopes (colored squares). The total number of isotopes that are expected to exist is more than 7000, with$\simeq$ 3000 of them known, and $\simeq$ 300 of them being stable. Characteristic nuclear-reaction networks in cosmic environments can be approximated through reaction paths, or processes. These are discussed in Section 2.6 and illustrated in this figure of the table of isotopes by (white) arrows, reflecting the charged-particle induced reactions and the s process along the valley of stable isotopes, the r-process path on the neutron rich side, the rp process on the proton-rich side, and the p (or $\gamma$-) process pointing horizontally away from the higher end of the stability valley. Magic numbers, which characterize enhanced nuclear stability, are indicated for proton and neutron numbers, characteristically aligning with kinks in the r-process path.
• Figure 2: Illustration of proceeding knowledge about isotopes on the neutron-rich side of the stability valley. The table of known isotopes with their proton (vertical axis) and neutron (horizontal axis) numbers shows the stable isotopes (black squares), where knowledge about nuclear properties is best as experiments can provide the required masses and energy levels. 'AME 2016' indicates how far the knowledge of nuclear masses extended beyond stable isotopes in the regularly-updated tables Audi:1996Kondev:2021Wang:2021a of atomic mass evaluations (AME) in 2016. For nuclei further away from the valley of stability, capabilities of future radioactive-ion beam (RIB) facilities are indicated. The colored squares reflect abundances of an r-process calculation after freeze-out of neutron captures, and illustrate the region of interest. (From Cowan:2021)
• Figure 3: The fusion of $\alpha$ particles to Carbon, as well as the fusion of Carbon in Carbon burning, may be facilitates by nuclear states in the compound nucleus, which are characterized by a cluster substructure of the nucleus (from Adsley:2022).
• Figure 4: The Na-Mg-Al region of the table of isotopes features a cycle of reactions, that processes material from a Na-dominated composition up to heavier isotopes up to Si. (p,$\alpha$)-reactions return lighter nuclei and thus provide the reverse path compared to the (p,$\gamma$) reactions that characterize the path towards heavier isotopes, thus being properly describes as a cycle. Beyond $^{27}$Al, no such cycles occur, as (p,$\alpha$) reactions are prohibited by the high Coulomb barrier for the $\alpha$ particles with their relatively low energy.
• Figure 5: Comparison of nuclear binding models away from the region of stable nuclei and towards the neutron drip line. Two-neutron separation energies are shown for Gd isotopes, comparing different mass models (see text) with experimental values as represented by AME2016 (from Horowitz:2019). Avoiding spin effects, the 2-neutron separation energy, S$_{2n}$, is determined in measurements along sequences of isobars, and characteristically reveals shell closure effects.