Big Bang Nucleosynthesis (BBN) is on the verge of undergoing a transformation now that extragalactic deuterium is being measured. Previously, the emphasis was on demonstrating the concordance of the Big Bang Nucleosynthesis model with the abundances of the light isotopes extrapolated back to their primordial values using stellar and Galactic evolution theories. Once the primordial deuterium abundance is converged upon, the nature of the field will shift to using the much more precise primordial D/H to constrain the more flexible stellar and Galactic evolution models (although the question of potential systematic error in 4He abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bulk of the baryons are dark and also that the bulk of the matter in the universe is non-baryonic. Comparison of baryonic density arguments from Lyman-α clouds, x-ray gas in clusters, the Sunyaev-Zeldovich effect, and the microwave anisotropy are made and shown to be consistent with the BBN value.

The remarkable finding that B follows Fe in almost direct proportion from the earliest times to the present, with little change of slope (if any) between halo and disk metallicities (cf Figure 1, open circles, Duncan et al. 1997), has stimulated the need for a revision of the classical cosmic-ray (CR) spallation scenario. A straightforward interpretation of Duncan et al. results is that the net rate of production of B (and Be too, since it shows a similar trend) does not depend on the CNO abundances in the ISM. It has been suggested that the CR spallation most important for light element production is not primarily protons and alpha particles colliding with CNO nuclei in the ISM (Reeves et al. 1970; Meneguzzi et al. 1971). Rather, it is C and O nuclei colliding with ambient protons and alpha particles, probably in regions of massive star formation (Cassé et al. 1995). This decouples light element production from the metallicity of the ISM and results in the approximately linear relationships observed, but some aspects are difficult to justify (see Prantzos, this volume).

The possible nature of the first generation of stars is considered, using a star of 25M⊙ as an example. General nucleosynthesis and the production of CNO catalysts is examined in detail. The increase in neutron excess and its significance for yields from explosive burning is discussed. An estimate of the ratio of ionizing photons to heavy elements produced is derived, for use in early universe simulations.

The cosmic/galactic chemical evolutions have been modeled with the early metal enrichment by Type II supernovae (SNe II) and the delayed enrichment of Fe by Type Ia supernovae (SNe Ia). However, the exact nature of SN Ia progenitors have been obscure. Here we present the currently most plausible scenario of the progenitor binary systems of SNe Ia. This scenario involves strong winds from accreting white dwarfs, which introduces important metallicity effects, namely, low-metallicity inhibition of SNe Ia. Resultant predictions for the Galactic/cosmic chemical evolution and the cosmic SNe Ia rate are presented. Another importance of identifying the SN Ia progenitors lies in the use of SNe Ia as a “standard candle” to determine cosmological parameters. To examine whether the “evolution” of SNe Ia with redshift and metallicity is significant, we discuss how the metallicity affects the properties of the C+O white dwarfs such as the C/O ratio, and find the metallicity dependence is rather weak.

Progress in the theory of galactic chemical evolution has been very slow and it is only in the solar neighborhood that observations constrain seriously the parameters of the various models. The history revealed on the basis of these data allows only for a small depletion of deuterium (D), less than a factor of 3 from its pregalactic value (Sec. 2.1). The observational data for the rest of the Milky Way disk are much less constraining for the models. They suggest, however, that a much larger astration (and, hence, D depletion) has taken place in the inner Galaxy; the resulting D gradient, measurable by the future FUSE-LYMAN mission, should provide invaluable information as to the past history of the disk (Sec. 2.2). Also, assuming that our Galaxy is a typical spiral, one can calculate the properties of disk galaxies as a function of redshift (in the framework of a given cosmological model) and compare to the observed properties of the extragalactic universe: global star formation rate, gas content and metal abundances in gas clouds. It turns out that D can be considerably depleted in galaxy disks, but only at low redshifts (Sec. 2.3).

The production of elements from helium-3 to fluorine in low- and intermediate-mass stars is reviewed and compared to chemical abundances observed at the surface of both red giant branch and asymptotic giant branch stars. It is highlighted that, while the trends predicted by standard models are generally well confirmed, many chemical abundances observed at the surface of red giants require the operation of non-standard mixing in the stellar interior. In addition, chemical abundance predictions from presently available asymptotic giant branch models further suffer from the uncertainties affecting the third dredge-up phenomenon, the source of neutrons and the hot bottom burning process.

The role of Wolf-Rayet and asymptotic giant branch stars in the production of the Solar System fluorine abundance is analyzed. It is shown that both these stars can be important sources of galactic fluorine. However, the uncertainties affecting the predictions, especially those from asymptotic giant branch stars, do not allow to give quantitative figures yet.

Excellent HST/GHRS spectra of interstellar hydrogen and deuterium Lyman-α absorption toward nearby stars allow us to identify systematic errors that have plagued earlier work and to measure accurate values of the D/H ratio in local interstellar gas. Analysis of 12 sightlines through the Local Interstellar Cloud leads to a mean value of D/H = (1.50 ± 0.10) x 10−5 with all data points lying within ±1σ of the mean. Deciding whether or not the D/H ratio has different values elsewhere in the Galaxy and beyond will be one of the major objectives of the FUSE mission.

The quest to define early galactic nucleosynthesis has drawn on the efforts of many observers and theoreticians over the past few decades. Significant insights on galactic halo chemical evolution have been revealed through the use of high resolution, high S/N spectroscopy to determine abundances in stars with metallicities ranging over three orders of magnitude. In the process, many galactic nucleosynthesis ideas that were firmly established only a decade ago now are either disproved, or are currently under challenge, or are weighed down with multiple known exceptions. The galactic halo has a rich chemical history, and we are just beginning to understand just how much work still remains before we can decipher its evolution.

The empirical study of the build-up of chemical abundances in the galactic disk is an important route to our understanding of the history of the Galaxy. It supplies a wealth of constraints to the models and simulations being done. There are, however, still many unanswered questions concerning the details of the abundance patterns, and many uncertainties and possibly inconsistencies in the data at hand. Ideally one would like to map the abundances of individual elements as a function of time and 3-dimensional position in the disk. The study of the ISM today and of the surfaces of individual stars as probes of the ISM from the time and position of their formation help us in this endeavour.

The open cluster NGC 6791 is more metal-rich than any other open cluster, yet several hot blue horizontal branch (BHB) stars are probable members. We have performed an abundance analysis of the coolest such star, 2-17, which has a radial velocity and a proper motion which support cluster membership. We find parameters typical of a metal-rich BHB star, with rotational velocity = 16 ± 1 km/s, an effective temperature Teff = 7250 ± 150 K, log gravity = 3.5 ±0.5 dex, and an iron abundance more than twice solar, [Fe/H] = +0.4 ± 0.1 dex. The light even-Z elements Mg, Ca, and Si are further enhanced, ruling out A-star peculiarities. Its analysis thus establishes the cluster metallicity accurately, free from serious blending which is problematical in metal-rich giants. Abundances of C, N, and O suggest little mixing. Visual comparison of the spectra of red HB stars in the cluster with that of the field star μ Leo indicates that the spectra are indistinguishable except for stronger CN in the former.

The high iron abundance, the light-element overabundances, the spectral similarity to strong-lined field giants, and the extensive presence of hot stars all highlight the fact that this old open cluster provides an excellent template for the study of metal-rich extragalactic systems. Properties of cluster members should establish not only accurate abundances for comparison with scenarios of nucelosynthesis and star formation, but also provide a testbed for ideas of the production of hot stars in a metal-rich environment, and so for the generation of the ultraviolet upturn in elliptical galaxies.

Supersoft X-ray sources (hereafter SSS) are a class of luminous (bolometric luminosity ~ 1037 − 1038 erg s−1) objects with a characteristic radiation temperature of 30 to 60 eV (Hasinger 1994; Kahabka & Trümper 1996). The most popular model for SSS is that they are massive white dwarfs steadily burning nuclear fuel accreted from a more massive binary companion at a rate near or above the Eddington limit (van den Heuvel et al. 1992).

Interstellar abundances are compared for the Milky Way disk, the Milky Way halo, the Large and Small Magellanic Clouds and the damped Lyman alpha systems among the QSO absorption line systems. While a new set of observational aspects of element formation in the Universe is emerging, including a dearth of formation activity from z=5 to z=3, the predicted signal of [Si/Fe] decreasing from high z to low z, as Type I supernovae start contributing to Fe production, has not yet been seen.

We study the roles of type Ia (SNIa) and type II supernovae (SNII) in enrichment of the intra-cluster medium (ICM). Considering stellar [Fe/H] of ellipticals, iron mass in ICM to luminosity ratios, and [α/Fe] of ICM, we show that ~ 53% of iron in ICM comes from SNIa. While clusters keep all of ejecta from ellipticals, we obtain the observational evidence for that groups of galaxies lose parts of the gas by supernovae-driven winds. This ‘group wind’ scenario is confirmed by an 1D hydrodynamical model.

Several recent observations indicate that even the low H I column density Lyα absorption lines may not be primordial, as they were previously thought to be. High-resolution observations using the Keck telescope (Cowie et al. 1995; Tytler et al 1995.; Songaila & Cowie 1996) have revealed that a large fraction of Lyα absorption lines with H I column density as low as NHI ~ 1014 cm−2 at redshift z ~ 3 are associated with metal lines (C IV). This fraction, hereafter denoted as fm, is estimated as 0.5-0.75. On the other hand, there is an indication that at low z a large fraction of Lyα absorption lines are directly associated with halos of visible galaxies. This fraction is hereafter denoted as fg. Lanzetta et al. (1995, hereafter LBTW) claimed fg ~ 0.6 at z ≲ 1 (see also Bowen, Pettini & Boyle 1997).

Looking for and studying very distant galaxy clusters, clusters at z > 1, are one of the prime subjects of the modern observational cosmology. If the metallicity of the hot intra-cluster medium in very distant galaxy clusters is measured for example, it provides fruitful informations for us to understand the formation and evolution of galaxies. However, difficulty of the study is that there is few confirmed very distant galaxy clusters yet. We first have to search for very distant clusters but it requires very deep observations. A random selection of sky is not practical. We have to select the sky. In this article, it is demonstrated that missing lens problem has close connection with very distant cluster of galaxies and dark lens searches could open a new window for studying very distant cluster of galaxies.

In elliptical galaxies, where most of the stars - and therefore most of the heavy elements - were formed at an early epoch, the total mass, spatial distribution, and relative abundances of metals are intimately connected to the galaxy formation process.

ISM abundances in nearby spiral galaxies are well known from HII region studies (Zaritsky et al. 1994). While early type spirals, Sa, Sb, have rather uniform abundances and a narrow range of present star formation rates (SFR) the galaxy-to-galaxy variations both in HII region abundances and in present SFR increase towards late spiral types Sc, Sd (see e.g. Kennicutt & Kent 1983). ISM abundances of spiral galaxies or their progenitors up to the highest redshifts can be studied via the absorption properties imprinted in the spectra of background QSOs. While MgII- and CIV- absorption lines are produced in the low column density gas of the extended haloes around galaxies, the Damped Lyα Absorption (DLA) is believed to originate in (proto-)galactic disks. High resolution spectroscopy of a large number of metal lines associated with DLA systems reveal the redshift evolution of ISM abundances from z ≳ 4 to z ~ 0.6.

The age of a high-redshift QSO can constrain the cosmology by placing a limit on the cosmological time. We use the observed FeII (UV+optical)/MgIIλ2798 flux ratio from the broad-line gas of a QSO at z=3.62 to estimate the abundance ratio [Mg/Fe] and predict that the QSO is at least 1.3 Gyr old from the chemical analysis using the abundance ratio [Mg/Fe] as a cosmic clock. Our result favors a nonzero cosmological constant and thus conflicts with a flat, matter-dominated universe.