Recent advances in the determination of stellar abundances and their impact on the study of nucleosynthesis and the chemical evolution of galaxies are discussed. It is argued that detailed interpretation of the abundance patterns among chemical elements found at different metallicities still requires solid spectroscopic data. Despite current improvements in abundance analyses, these are not, however, free from systematic errors which can hide the real history of the chemical evolution. Some examples are discussed.


I briefly review the relevance of radioactive nuclei for timing events of interest in the evolution of the Solar System. The related techniques are based on nuclear radio-chronometry of terrestrial or meteoritic samples. A special attention is paid to short lived ($\bar \tau \le$ 100 Myr) isotopes produced by stellar nucleosynthesis: I mention the indirect information we have on their presence in the early solar nebula, and discuss their use both for establishing very precise isochrons in the initial evolutionary stages of the Solar System, and for inferring the history of nucleosynthesis prior to the Sun's formation. In particular, from the existing record of a few important nuclei (26Al, 60Fe, 205Pb, 41Ca among others) it appears that the protosolar nebula was subject to late contaminations both by spallation reactions in the fast winds of the early Sun, and by a nearby star. The recent suggestions that this last was of low or intermediate mass are also discussed.

We present the basic theory of nuclear reactions in stars and sketch the general rules of stellar evolution. Then we shortly review the subject of supernova explosions both by core collapse in massive stars (type II) and carbon-deflagration in binary systems when one of the components is a White Dwarf accreting mass from the companion (type Ia). We also present elementary notions of s- and r-process nucleo-synthesis. Finally, we shortly review the topic of galactic chemical evolution and highlight some simple solutions aimed at understanding the main observational data on abundances and abundance ratios.

Primordial nucleosynthesis is one of the three observational evidences for the Big–Bang model. Even though they span a range of nine orders of magnitude, there is indeed a good overall agreement between primordial abundances of 4He, D, 3He and 7Li either deduced from observation or primordial nucleosynthesis. This comparison was used for the determination of the baryonic density of the Universe. For this purpose, it is now superseded by the analysis of the CMB anisotropies. Big–Bang nucleosynthesis remains, nevertheless, a valuable tool to probe the physics of the early Universe.

Cross section measurements for quiescent stellar H-burning are hampered mainly by extremely low counting rate and cosmic background. An effective method is to perform cross section measurements in an underground laboratory where cosmic ray flux is reduced by several orders of magnitude. The main features of un underground nuclear reaction measurement are presented focusing mainly on accelerators and detectors techniques taking examples from the LUNA experiment at the Gran Sasso National Laboratory in Italy. Finally some astrophysical consequences of LUNA results are discussed.

We present a short overview of the R-matrix method in nuclear physics, with emphasis on nuclear astrophysics. The method can be applied in two ways: solving the Schrödinger equation for scattering states, or fitting experimental cross sections. Both variants are presented in a common framework. The method is illustrated with the α + α scattering, and the 12C(p,γ)13N and 3He(d,p)4He reactions.

Nuclear reactions occur in cosmic environments in the cores of stars and in stellar explosions. Nuclear energy production is a key agent in these objects; the production of new isotopes drives the chemical evolution of gas throughout the universe and of the objects which form from this gas over cosmic times. Radioactive isotopes, ejected into interstellar space by cosmic nucleosynthesis events, are observed with new space telescopes. Gamma-ray lines from the radioactive ejecta of such cosmic nuclear reactors need to be sufficiently bright so they can be observed with current telescopes; this limits all gamma-ray astronomy studies to present and nearby nucleosynthesis processes, i.e. out to few Mpc in distance and back a few million years in time. The Compton Observatory had provided a first sky survey for the isotopes 56Co, 22Na, 44Ti, and 26Al, detecting supernova radioactivity and the diffuse glow of long-lived radioactivity from massive stars in the Galaxy. High-resolution spectroscopy is now being exploited with Ge detectors, which allows to measure Doppler broadenings and line shape details of these cosmic gamma-ray lines. Current results include an all-sky map and line shape measurement of positron annihilation emission, 26Al emission from the inner Galaxy and from the Cygnus region, a detection of 60Fe gamma-rays, and limits on 44Tiemission from Cas A and other candidate young supernova remnants; 22Na from novae still has not been seen. In this paper we discuss the experimental methods for such cosmic gamma-ray spectroscopy, and the corresponding astrophysical implications.

The Trojan Horse method (THM) is a powerful indirect technique that provides information to determine astrophysical factors for binary rearrangement processes x + A → b + B at astrophysically relevant energies by measuring the cross section for the Trojan Horse (TH) reaction a + A → y + b + B in the quasifree kinematics. The theory of the THM both for direct and resonant binary subreactions is presented based on the few-body approach and Green's function formalism. The off-energy-shell effects and initial and final state interactions have been taken into account. To treat the resonance processes we developed the half-off-energy-shell R matrix approach. Examples are presented to demonstrate how THM works.

Nuclear cosmochronometry is a direct method to measure cosmic age. It has been believed that the abundance ratio of Th/Eu is a reliable chronometer and Th/Eu ratios of metal-poor stars have been studied intensively to obtain a lower limit of Galactic age. However, after the actinide-boosted metal-poor star CS 31082-001 had been found, applicability of Th/Eu chronometer to metal-poor stars became questionable. Furthermore, theoretical r-process calculations indicate that the Th/Eu production ratios strongly depend on the nucleosynthesis environment compared to other stable elements between the second and third peak, including Pb. It is impossible to determine the Th/Eu production ratio by theoretical study at this point, though it is essential for the chronometer. Besides, observed universality of r-process abundances does not guarantee a universal Th/Eu production ratio anymore. Th/Eu chronometer is now facing a serious crisis. On the other hand, the U/Th production ratio obtained by theoretical calculation does not depend on the nucleosynthesis environment significantly. Although there is only one U measurement of metal-poor stars at this point, the U/Th chronometer seems to be still robust. Many uncertainties related to data analyses and nuclear physics models still remain to be resolved.

Understanding the electron screening effect in the laboratory is critical for a correct interpretation of low-energy nuclear reactions in stars. The theory of electron screening in a stellar plasma is still based on the physics model introduced in 1954 by Salpeter, although the mathematical treatments have been improved significatly in the meantime. Extensive studies of the electron screening effect in different nuclear reactions have shown always the same discrepancy, i.e. the experimental Ue ≫ adiabatic Ue. In the recent years extensive studies of the elctron screening effect in deuterated metals (54 metals and 4 insulators) and other environments have been carried out in Bochum. Experimental results of anomalous enhancements have been interpreted in terms of the Debye plasma model, applied to quasi-free metallic electrons. Within this model, the deduced number of valence electrons per metallic atom also agrees with the corresponding number from the Hall coefficient. The expected temperature dependence of the screening potential has been verified together with the expected Zt scaling (with the metallic host) of the Debye radius.

Stellar models are a fundamental tool to investigate the physical nature of stellar interiors, otherwise hidden to a direct observation. A change in the input physics used to build up a model produce different prescriptions for observable quantities. Thus, by comparing models predictions with astronomical data, we can try to constrain fundamental physics. The case of the 12C(α, γ)16O reaction is discussed in some details. Its relevance in various astrophysical contexts is reviewed.

Our understanding of stellar evolution depends on knowing beta-decay rates and reaction cross sections for a wide range of nuclear capture reactions. Direct laboratory measurements of important stellar reaction rates are hindered by low cross sections and, in some cases, the need for radioactive targets. Indirect techniques have been developed to determine reaction rates for systems that are particularly difficult to measure in a direct experiment. One of the indirect techniques involves measurements of Asymptotic Normalization Coefficients (ANCs) which determine the direct-capture contribution to a capture reaction. Also ANCs can be used to understand the role of subthreshold states in stellar capture. This essay gives an introduction to ANCs and describes how they are used in nuclear astrophysics. Examples are given of measurements which have been carried out with both stable and radioactive beams.

Some general features of indirect approaches for determining cross sections of astrophysically relevant reactions at low energies are discussed from a theoretical point of view. The methods of Coulomb dissocation (CD), of the asymptotic normalization coefficient (ANC) and the Trojan-horse method (THM) are considered.