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


I briefly review the relevance of radioactive nuclei for timingevents of interest in the evolution of the Solar System. The relatedtechniques are based on nuclear radio-chronometry of terrestrial ormeteoritic samples. A special attention is paid to short lived($\bar \tau \le$ 100 Myr) isotopes produced by stellarnucleosynthesis: I mention the indirect information we have on theirpresence in the early solar nebula, and discuss their use both forestablishing very precise isochrons in the initial evolutionarystages of the Solar System, and for inferring the history ofnucleosynthesis prior to the Sun's formation. In particular, fromthe existing record of a few important nuclei (26Al, 60Fe,205Pb, 41Ca among others) it appears that the protosolarnebula was subject to late contaminations both by spallationreactions in the fast winds of the early Sun, and by a nearby star.The recent suggestions that this last was of low or intermediatemass are also discussed.

We present the basic theory of nuclear reactions in stars and sketchthe general rules of stellar evolution. Then we shortly review thesubject of supernova explosions both by core collapse in massivestars (type II) and carbon-deflagration in binary systems when oneof the components is a White Dwarf accreting mass from the companion(type Ia). We also present elementary notions of s- and r-processnucleo-synthesis. Finally, we shortly review the topic of galacticchemical evolution and highlight some simple solutions aimed atunderstanding the main observational data on abundances andabundance ratios.

Primordial nucleosynthesis is one of the three observational evidences for the Big–Bang model. Even though they span a range of nine orders ofmagnitude, there is indeed a good overall agreement between primordial abundances of 4He, D, 3He and 7Li either deduced from observation or primordialnucleosynthesis. This comparison was used for the determination ofthe 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 probethe physics of the early Universe.

Cross section measurements for quiescent stellar H-burning arehampered mainly by extremely low counting rate and cosmicbackground. An effective method is to perform cross sectionmeasurements in an underground laboratory where cosmic ray flux isreduced by several orders of magnitude. The main features of ununderground nuclear reaction measurement are presented focusingmainly on accelerators and detectors techniques taking examplesfrom the LUNA experiment at the Gran Sasso National Laboratory inItaly. Finally some astrophysical consequences of LUNA results arediscussed.

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, orfitting experimental cross sections. Both variants are presented in a common framework. The methodis 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 instellar explosions. Nuclear energy production is a key agent in these objects;the production of new isotopes drives the chemical evolution of gas throughoutthe universe and of the objects which form from this gas over cosmic times.Radioactive isotopes, ejected into interstellar space by cosmicnucleosynthesis events, are observed with new space telescopes.Gamma-ray lines from the radioactive ejecta of such cosmic nuclear reactorsneed to be sufficiently bright so they can be observed with current telescopes;this limits allgamma-ray astronomy studies to present and nearby nucleosynthesisprocesses, 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 isotopes56Co, 22Na, 44Ti, and 26Al, detecting supernova radioactivityand 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 detailsof these cosmic gamma-ray lines.Current results include an all-sky map and line shape measurement of positron annihilationemission, 26Al emission from the inner Galaxy and from the Cygnus region, a detection of60Fe gamma-rays, and limits on 44Tiemission from Cas Aand 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 providesinformation to determine astrophysical factors for binary rearrangement processes x + A → b + Bat astrophysically relevant energies by measuring the cross section for the Trojan Horse (TH) reactiona + 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/Euis 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 environmentcompared to other stable elements between the second and third peak, including Pb.It is impossible to determine the Th/Eu production ratioby theoretical study at this point, though it is essential for the chronometer.Besides, observed universality of r-process abundances does notguarantee 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 modelsstill 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 modelproduce 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 crosssections for a wide range of nuclear capture reactions. Direct laboratory measurements ofimportant stellar reaction rates are hindered by low cross sections and, in some cases, the needfor radioactive targets. Indirect techniques have been developed to determine reaction rates forsystems that are particularly difficult to measure in a direct experiment. One of the indirecttechniques involves measurements of Asymptotic Normalization Coefficients (ANCs) whichdetermine the direct-capture contribution to a capture reaction. Also ANCs can be used tounderstand the role of subthreshold states in stellar capture. This essay gives an introduction toANCs and describes how they are used in nuclear astrophysics. Examples are given ofmeasurements which have been carried out with both stable and radioactive beams.

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