The study of the resolved stellar populations in nearby galaxies and star clusters through the analysis of colour-magnitude diagrams provides the most detailed and quantitative determination of the star formation histories of these systems. The properties of different age populations provide an insight into distinct physical processes taking place during the entire history of the stellar system. The detection of the oldest main sequence turn-offs is currently restricted to stellar systems within the Local Group due to the limitations in spatial resolution and flux sensitivity of available telescopes. Individual stars need to be detected and accurately distinguished from their neighbours. To improve this situation we need to build new telescopes with larger primary mirrors that can deliver a very stable image quality at the diffraction limit. Over the next decade we can look forward to new larger telescope in space: the James Webb Space Telescope, currently scheduled to be launched in 2021; and several large telescope projects, the largest of which is the 39m ESO extremely large telescope on Cerro Armazones in Chile, currently scheduled to start operations in 2024.

Stars in low-mass dwarf galaxies show a larger range in their chemical properties than those in the Milky Way halo. The slower star formation efficiency make dwarf galaxies ideal systems for testing nucleosynthetic yields. Not only are alpha-poor stars found at lower metallicities, and a higher fraction of carbon-enhanced stars, but we are also finding stars in dwarf galaxies that appear to be iron-rich. These are compared with yields from a variety of supernova predictions.

In and around the Milky Way halo there are a number of low mass low luminosity dwarf galaxies. Several of these systems have been studied in great detail. I describe recent photometric and spectroscopic studies of the Sculptor dwarf spheroidal galaxy made as part of the DART survey of nearby dwarf spheroidal galaxies.

We present the first results of a comprehensive HST study of the star-formation history of Fornax dSph, based on WFPC2 imaging of 7 Fornax fields. Our observations reach the oldest main-sequence turnoffs, allowing us to address fundamental questions of dwarf galaxy evolution, such as the spatial variations in the stellar content, and whether the old stellar population is made up of stars formed in a very early burst or the result of a more continuous star formation.

Within the Local Universe galaxies can be studied in great detail star by star. The Color-Magnitude Diagram synthesis analysis method is well established as the most accurate way to determine the detailed star formation history of galaxies going back to the earliest times. This approach received a significant boost from the exceptional data sets that wide field CCD imagers on the ground and the Hubble Space Telescope could provide. Spectroscopic studies using large ground based telescopes such as VLT, Magellan, Keck and HET have allowed the determination of abundances and kinematics for significant samples of stars in nearby dwarf galaxies. These studies have shown directly how properties can vary spatially and temporally, which gives important constraints to theories of galaxy formation and evolution.

We observed two fields near M32 with the ACS/HRC (Program GO-10572, PI: T. Lauer) on board the Hubble Space Telescope, located at distances of about 1.8' and 5.4' (hereafter F1 and F2, respectively) from the center of M32. To obtain a very detailed and deep color-magnitude diagram (CMD) and to look for short period variability, we obtained time-series imaging of each field in 32-orbit-long exposures using the F435W (B) and F555W (V) filters, spanning a temporal range of 2 days per filter. We focus on our detection of variability on RR Lyrae variable stars, which represents the only way to obtain information about the presence of a very old population (larger than 10 Gyr) in M32 from optical data. Here we present results obtained from the detection of 31 RR Lyrae in these fields: 17 in F1 and 14 in F2. We claim we detected 7+4−3 RR Lyrae variables belonging to M32 in F1 thus indicating the presence of a metal-poor ancient population in M32.

We have obtained metallicities from near-infrared calcium triplet spectroscopy for nearly a thousand red giants in 28 fields spanning a range of radial distances from the center of the bar to near the tidal radius. We have used these data to investigate the radius-metallicity and age-metallicity relations. A powerful application of these data is in conjunction with the analysis of deep HST color–magnitude diagrams (CMDs). Most of the power in determining a robust star-formation history from a CMD comes from the main-sequence turnoff and subgiant branches. The age-metallicity degeneracy that results is largely broken by the red giant branch color, but theoretical model RGB colors remain uncertain. By incorporating the observed metallicity distribution function into the modelling process, a star-formation history with massively increased precision and accuracy can be derived. We incorporate the observed metallicity distribution of the LMC bar into a maximum-likelihood analysis of the bar CMD, and present a new star formation history and age–metallicity relation for the bar. The bar is certainly younger than the disk as a whole, and the most reliable estimates of its age are in the 5–6 Gyr range, when the mean gas abundance of the LMC had already increased to [Fe/H] ≳ −0.6. There is no obvious metallicity gradient among the old stars in the LMC disk out to a distance of 8–10 kpc, but the bar is more metal-rich than the disk by ≈0.1–0.2 dex. This is likely to be the result of the bar's younger average age. In both disk and bar, 95% of the red giants are more metal-rich than [Fe/H] = −1.2.

Dwarf galaxies offer an opportunity to understand the properties of low metallicity star formation both today and at the earliest times at the epoch of the formation of the first stars. Here we concentrate on two galaxies in the Local Group: the dwarf irregular galaxy Leo A, which has been the recent target of deep HST/ACS imaging (Cole et al. 2007) and the Sculptor dwarf spheroidal, which has been the target of significant wide field spectroscopy with VLT/FLAMES (Battaglia 2007).

This is a review of the progress of ESO/WFI Imaging and VLT/FLAMES spectroscopy of large numbers of individual stars in nearby dwarf spheroidal galaxies primarily by the Dwarf Abundances and Radial-velocities Team (DART). These observations have allowed us to show that neither the kinematics nor the abundance nor the spatial distributions are easy to explain in a straight forward manner for these smallest galaxies. The main result is that dwarf galaxies show complex and highly specific evolutionary and metal enrichment processes, especially at ancient times. This conclusively proves that these small galaxies are not the building blocks of the larger galaxies in the Local Group.

Top of the wish list of any astronomer who wants to understand galaxy formation and evolution is to resolve the stellar populations of a sample of giant elliptical galaxies: to take spectra of the stars and make Colour-Magnitude Diagrams going down to the oldest main sequence turn-offs. It is only by measuring the relative numbers of stars on Main Sequence Turnoffs at ages ranging back to the time of the earliest star formation in the Universe that we can obtain unambiguous star formation histories. Understanding star formation histories of individual galaxies underpins all our theories of galaxy formation and evolution. To date we only have detailed star formation histories for the nearest objects in the Local Group, namely galaxies within 700kpc of our own. This means predominantly small diffuse dwarf galaxies in a poor group environment. To sample the full range of galaxy types and to consider galaxies in a high density environment (where much mass in the Universe resides) we need to be able to resolve stars at the distance of the Virgo ($\sim$17 Mpc) or Fornax ($\sim$18 Mpc) clusters. This ambitious goal requires an Extremely Large Telescope (ELT), with a diameter of 50–150m, operating in the optical/near-IR at its diffraction limit.

The Fornax dSph is an interesting case as it contains five old globular clusters and its field stars, although predominantly of intermediate (3-8 Gyr old) age, cover a wide range of age and metallicity. Detailed abundance analysis is crucial to our understanding of the earliest star formation epoches, where classic CMD analysis fails to provide a unique answer. It also allows us to measure the chemical evolution of the stellar population following tracers of different enrichment mechanisms through time, e.g. SN type II (alpha elements); AGB stars (s-process elements) etc. With our large sample of abundance measurements we will obtain a detailed picture of the evolution of Fornax and of the role played by small galaxies in the building up of larger ones.

I will review the progress of VLT spectroscopy of large numbers of individual stars in nearby dwarf spheroidal galaxies. This spectroscopy has allowed us to obtain detailed insights into the chemical and dynamical properties of the resolved stellar population in these nearby systems.

Here we describe some of our latest results from measuring detailed abundances in Local Group dwarf galaxies with the VLT. Combining spectroscopic abundances with Color-Magnitude diagrams allows the effective measurement of detailed chemical evolution with time in these galaxies. Although there are not yet significant numbers of individual stars observed in local group dwarf galaxies, the uniformity of the abundance patterns of the majority of stars in galaxies with very different star formation histories must hint at general properties of all star formation in these small systems.

The relative abundances of elements in galaxies can provide valuable information on the stellar and chemical evolution of a galaxy. While nebulae can provide abundances for a variety of light elements, stars are the only way to directly determine the abundances of iron-group and s-process and r-process elements in a galaxy. The new 8m and 10m class telescopes and high-efficiency spectrographs now make high-quality spectral observations of bright supergiants possible in dwarf galaxies in the Local Group. We have been concentrating on elemental abundances in the metal-poor dwarf irregular galaxies, NGC 6822, WLM, Sextants A, and GR 8. Comparing abundance ratios to those predicted from their star formation histories, determined from color-magnitude diagrams, and comparing those ratios between these galaxies can give us new insights into the evolution of these dwarf irregular galaxies. Iron-group abundances also allow us to examine the metallicities of the stars in these galaxies directly, which affects their inferred mass loss rates and predicted stellar evolution properties.

Unless the nearby universe is subtly anomalous, it should contain a relatively normal selection of galaxies whose histories are representative of field galaxies in general throughout the Universe. We can therefore take advantage of our ability to resolve the nearby galaxies into individual stars to directly, and accurately, measure star formation histories. The star formation histories are determined from model-fitting analysis, based on stellar evolution tracks, of colour-magnitude diagrams. The most accurate information on star formation rates extending back to the earliest epochs can be obtained from the structure of the main sequence. However, the oldest main sequence turnoffs are very faint, and it is often necessary to use the more evolved stars to infer the star formation history. These can then be compared with the properties of galaxies seen over a large range of lookback times at moderate to high redshifts. There is considerable evidence that the faint blue galaxies seen in large numbers in cosmological redshift surveys are the progenitors of the small late-type irregular galaxies seen in copious numbers in the Local Group, and we concentrate on these here.

Stellar Evolution theory is based upon well understood physics and provides clear predictions as to how a Colour-Magnitude Diagram (CMD) will change due to effects of age and metallicity. The theory has been tested by looking at nearby coeval star clusters. The power of applying CMD analysis to galaxies has been demonstrated in studies of the Carina dSph (Smecker-Hane et al. 1996). In Carina the observation of separate, distinct Main Sequence (MS) Turnoffs has forced us to believe that this small, nearby companion of our Galaxy has had a very complex star formation history. No similar direct evidence for “bursting” behaviour on a global scale has been seen indisputably in larger systems. Partly this is due to the greater distances of larger systems, but also to the complications in distinguishing old star formation events (> 1 Gyr old) in systems which are currently forming stars. Where we lack MS turnoffs we have to resort to statistical modeling of the CMD. This has been applied using a a number of different approaches, but all generally based on Monte-Carlo techniques (e.g. Tosi et al. 1992; Bertelli et al. 1992; Tolstoy &; Saha 1996). However, even using these more sophisticated analysis techniques, it is difficult to find unique solutions. This is mostly due to the age-metallicity degeneracy on the Red Giant Branch (RGB). The RGB is usually the most populated, easiest to observe phase of stellar evolution. The Carina CMD reveals the dangers of blindly interpreting the RGB, because from the RGB alone it is impossible to extract the information revealed by the MS Turnoffs.

The complex effects that determine the shape of an observed Colour Magnitude Diagram (CMD) are best disentangled through numerical simulation. We make synthetic CMDs by randomly extracting stars from theoretical stellar evolution tracks using a series of adopted initial conditions, including an Initial Mass Function. Utilizing reliable error estimates on our photometry provided by the program DoPHOT we apply a Maximum Likelihood technique to quantitatively determine which of the numerous different possible models is the most probable match to the data. From these comparisons we obtain a better understanding of how star formation proceeds in the relatively simple environments provided by Dwarf galaxies.