The analyses of element abundances in a variety of stars, formed at different times in our Galaxy, are presented in an effort to determine the distribution of elemental abundances at various stages in the evolution of the Galaxy.

Stars can now mostly be separated into disk and halo members when both Galactic rotation velocity and metallicity are considered. New O/Fe data give results similar to α/Fe data and enable the O or α-element distribution function for the halo to be fitted to Hartwick's modification of the Simple Model up to nearly solar abundances, implying strong overlap with the disk and no sharp break in [O/Fe] when [Fe/H] ≃ −1. The G-dwarf problem and large-scale radial abundance gradients are discussed; in both cases, there are deficiencies in the data and too many alternative hypotheses capable of explaining them.

The oxygen-to-iron ratio in the different stellar populations is a tracer of the chemical enrichment by supernovae of types II and I: SNII/SNI along the Galaxy lifetime, given that the bulk of oxygen is produced and ejected by massive stars (M > 10 M⊙), whereas the bulk of iron is produced by SNI of intermediate masses. Otherwise, the O overabundances are generally accompanied by α-elements (24Mg, 28Si, 40Ca, 48Ti) overabundances (Wheeler et al, 1989).

In many ways, studies of the stellar populations in the Magellanic Clouds are more straightforward than in our own Galaxy because of our external vantage point of the Clouds. This allows us to study the global properties of their stellar populations, as well as the individual stars that comprise them. The resulting picture describing the stellar content of the Clouds is obviously complex, defying any attempt to present a complete description in a short review such as this. Thus, my goal is to provide a very broad overview of the global properties of the Magellanic Clouds, along with some slightly more detailed discussions on a few selected topics addressed by recent studies. Recent reviews of the global properties and stellar content of the Magellanic Clouds based on observations ranging from radio to x-ray wavelengths can be found in the Proceedings of IAU Symposium 148 (Haynes and Milne 1991) and in Westerlund (1990). More specialized reviews have been written recently by van den Bergh (1991), Freeman (1989), and Feast (1989), and by numerous authors in the proceedings of a recent European Colloquium on the Magellanic Clouds (de Boer, et al. 1989).

A stellar population survey covering a total of 130 square degrees in the outer parts (beyond 2 kpc from the optical centre) of the Small Magellanic Cloud is presented, based on colour-magnitude diagrams constructed from COSMOS measurements of a series of UKST photographic plates.

M31 and M33 are the two nearest extragalactic spirals. They are therefore particularly suitable for studies of stellar populations. From integrated photometry spiral galaxies are known to consist of four components: (1) a nucleus, (2) a nuclear bulge, (3) an (exponential) disk and (4) a halo. These stellar components are embedded within a massive invisible halo.

An active debate continues over whether elliptical galaxies are primarily old stellar systems or whether they have had major star formation events in the recent past. Not only is this question of interest with regard to understanding the stellar populations and star formation history of nearby systems, but the resolution of this issue influences the interpretation of the spectra of high-redshift galaxies and has profound consequences for our understanding of galaxy, and therefore ultimately, of cosmological evolution.

Our lack of understanding of the stellar make-up in elliptical galaxies has persisted for some time because there are no giant elliptical galaxies near enough to allow the study of their stellar populations directly. Most information on the stellar populations of elliptical galaxies rely on the interpretation of integrated light. However, direct information on the bright stellar content of low-luminosity elliptical galaxies can be obtained from a study of the Local Group dwarf ellipticals. The nearby Andromeda galaxy, M31 has four low-luminosity elliptical companions: M32, NGC 205, NGC 185 and NGC 147, the subjects of this review.

This review will begin with a broad summary of population characteristics of dwarf elliptical galaxies (dE's), it will briefly summarize what is known about the stellar populations of the four Andromeda companions, and then discuss the specific case of M32 in detail. M32, the highest surface brightness Andromeda companion, has characteristics very simliar to the giant ellipticals, and has therefore been the focus of much of the controversy surrounding the issue of the ages of elliptical galaxies. Studies of its integrated light, in combination with new studies of its brightest resolved giants, particularly in the near-infrared, may help to resolve many of the outstanding questions regarding the stellar populations in elliptical galaxies.

Our understanding of asymptotic giant branch evolution has reached the point where the timescales are well enough known that realistic luminosity functions can be computed. The Mira and OH-IR star phases of stellar evolution truncate the AGB well before Reimers' Law mass loss exhausts the envelope. These luminosity functions allow us to compute the luminosity variance, which may be sensitive to the presence of intermediate age populations.

The properties of dwarf spheroidals are reviewed with emphasis on the newly discovered Sextans system, as well as on the star formation histories, the dark matter content and the abundance - luminosity relation of these galaxies. The relation of dwarf spheroidals to other dwarf galaxies is also discussed.

We have completed a blue-green Grism survey for carbon stars in the seven “classical” dwarf spheroidal galaxies. The results for six of them have been published already, while the results for the Fornax galaxy are presented here: this galaxy contains 77 carbon stars, including 30 new objects. The bolometric luminosity function of Fornax is intermediate between that of the Small and of the Large Magellanic Cloud. We will also compare these carbon stars with the fainter objects in the galactic bulge and in the galactic halo and discuss their properties in relation with the metallicity and history of star formation in the parent galaxies.

Given the uncertainties in the derivation of the current and past SFR in galaxies of any kind, the SF regime in irregulars is subject of wide debate. Since the most direct information on the stellar populations and relative histories in any system can be derived from their CMD, we have undertaken a project for accurately studying the CMDs and luminosity functions (LF) of nearby, well resolved, irregulars. The method proceeds on two tracks: a) we have developed a numerical code for Montecarlo simulations of CMDs and b) we have taken deep and accurate CCD photometry of several galaxies and derived the corresponding CMDs and LFs. The comparison of the observational data with the corresponding theoretical expectations provides several constraints on the SF history and on the IMF of the analysed objects. We do not pretend to reach unique conclusions on the evolution of irregular galaxies, but we can sensibly reduce the range of possible interpretations.

All the irregulars in our sample were selected from the DDO Catalogue and were supposed to belong to the Local Group. Detailed descriptions of the data acquisition and reduction and of the simulation code can be found in Tosi et al. (1991). In the following we briefly summarize the results relative to Sextans B, NGC 3109 and DDO 210.

The first results from an analysis of CCD photometry of blue low surface–brightness galaxies are presented. The galaxies have a wide range of morphologies, luminosities and ages – all appear to be older than 2 Gyr. We also discuss the results from spectroscopy of ESO 146–G14, a large disk galaxy with low chemical abundances.

This paper reviews spectral libraries of stars and star clusters, together with their applications to population synthesis. The problem of abundance calibrations for metal rich populations is also addressed, in particular index definitions and non-solar CNO/Fe ratios. A stellar population data bank would be important to accelerate progress in the field and would optimize the use of future telescope time.

I have used my newly calculated iron group line list together with my earlier atomic and molecular line data, 58,000,000 lines total, to compute new opacities for the temperature range 2000K to 200000K. Calculations have been completed at the San Diego Supercomputer Center for 56 temperatures, for 21 pressures, for microturbulent velocities 0, 1, 2, 4, and 8 km/s, for 3,500,000 wavelength points divided into 1221 intervals from 10 to 10000 nm, for scaled solar abundances [+1.0], [+0.5], [+0.3], [+0.2], [+0.1], [+0.0], [-0.1], [-0.2], [-0.3], [-0.5], [-1.0], [-1.5], [-2.0], [-2.5], [-3.0], [-3.5], [-4.0], [-4.5], and [-5.0]. I have rewritten my model atmosphere program to use the new line opacities, additional continuous opacities, and an approximate treatment of convective overshooting. The opacity calculation was checked by computing a new theoretical solar model that matches the observed irradiance. Thus far I have completed a grid of 7000 model atmospheres at 2 km/s for all the abundances, for the temperature range 3500K to 50000K, and for log g from 0.0 to 5.0. This grid will allow a consistent theoretical treatment of photometry from K stars to B stars. Fluxes are tabulated from .09 to 160 micrometers. Preliminary results are reported for many photometric systems. Work is underway on grids for other microturbulent velocities. Microturbulent velocity strongly affects the interpretation of Cepheid and RR Lyrae photometry. The models, fluxes, and colors are available on magnetic tape and will also be distributed on CD-ROMs.

We review some of the contributions made by vacuum ultraviolet observations to our understanding of stellar populations, with emphasis on recent results from the HST and the Astro-1 mission.

For a true picture of the stellar content of the bulges of spiral galaxies it is necessary to combine spectroscopic and photometric observations from the ultraviolet to the infrared. In early-type spirals one generally finds a metal-rich, old population with no excess of low mass stars. In later type systems there is an increasing contribution from young stars. The population in the Galactic bulge is representative and does not contain a significant number of stars with large IR excesses.

Recent spectral data strongly suggest that elliptical galaxies represent at least a two- and probably a three-parameter sequence: light-element abundance, iron-peak abundance, and probably age. These data are discussed in the context of new ideas about elliptical formation, including hierarchical clustering, merger-caused starbursts, and a variable IMF. Non-solar abundance ratios set important constraints on elliptical formation.

In the first part of this paper it is shown that those giant ellipticals which presumably formed in a violent merging process (as inferred from their peculiar core kinematics) reflect the details of the merging in their Mg2-index profiles. In the second part it is demonstrated that, independent from other structural parameters, central Mg2 index and central velocity dispersion σo (presumably representing the mean depth of the potential well) follow a unique relationship for all types of hot stellar systems (including diffuse dwarf ellipticals, compact ellipticals, bulges and giant ellipticals).

Recent work have raised the intriguing possibility that the activity seen in most active galactic nuclei (AGN) is powered solely by young stars and supernova remnants in a burst of star formation at the time when the metal rich core of the spheroid of a normal, albeit young galaxy, was formed. The predicted emitted multifrequency spectrum, line width, variability and luminosity function of the young cores of ellipticals, are indistiguishable from those observed in Quasars. Only a small fraction (~5%) of the total mass of elliptical galaxies, the core mass, is needed to explain the observed luminosities and luminosity function of Quasars at z≥2.0.

The determination of the metallicity of an unresolved population is not easy. An obvious problem is that stars of the same mass, but different metallicities, evolve at different rates and have different luminosities. For example, metal poor red giants are considerably brighter in the optical region than corresponding metal rich stars. The “mean” abundance for a stellar population therfore depends on whether the mean is taken with respect to mass, star number or luminosity. It is inevitable that observational work weights by luminosity, while chemical evolution models usually weight by mass. The two weightings can give rather different means, which may also depend on the “metallicity structure” - i.e. the relative numbers of stars of different metallicities, and rough calculations show that metallicity indicators like colours could give 0.3 dex lower abundance than the mass-weighted mean, if uniform (i.e. single metallicity population) calibrators have been used.