Next Contents


The study of resolved stellar populations provides a powerful tool to follow galaxy evolution directly in terms of physical parameters such as age (star formation history, SFH), chemical composition and enrichment history, initial mass function, environment, and dynamical history of the system. Photometry of individual stars in at least two filters and the interpretation of Colour-Magnitude Diagram (CMD) morphology gives the least ambiguous and most accurate information about variations in star formation within a galaxy back to the oldest stars. Some of the physical parameters that affect a CMD are strongly correlated, such as metallicity and age, since successive generations of star formation may be progressively enriched in the heavier elements. Careful, detailed CMD analysis is a proven, uniquely powerful approach (e.g., Tosi et al. 1991; Tolstoy & Saha 1996; Aparicio et al. 1996; Mighell 1997; Dohm-Palmer et al. 1997, 1998; Hurley-Keller et al. 1998; Gallagher et al. 1998; Tolstoy et al. 1998) that benefits enormously from the high spatial resolution of HST to the point that ground based CMD analysis is only worthwhile in ideal conditions beyond about the distance of the Magellanic Clouds.

Because of the tremendous gains in data quality and thus understanding which have come from recent high quality CMDs of nearby galaxies it is now clearly worthwhile and fundamentally important to complete a survey of the resolved stellar populations of all the galaxies in our Local Group (LG). This will provide a uniform picture of the global star formation properties of galaxies with a wide variety of mass, metallicity, gas content etc. (e.g. Mateo 1998), and will make a sample that ought to reflect the SFH of the Universe and give results which can be compared to high redshift survey results (e.g., Madau et al. 1998). Initial comparisons suggest these different approaches do not yield the same results (Fukugita et al. 1998), but the errors are large.