As part of the ACS Survey of Galactic Globular Clusters, we present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy. Our deep (F606W ~ 26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing

Most of what we know about the stellar population of nearby, resolved galaxies comes from the interpretation of their color-magnitude diagrams, by comparison with stellar evolutionary models. We review how well current stellar evolution models reproduce the properties of simple stellar populations. Emphasis is given to the regions of the color

We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42

AG Carinae is a massive, evolved supergiant which is thought to be in transition from an O star to a Wolf-Rayet (WR) star and is currently identified as a luminous blue variable (LBV) with logL/solar=6.0. We present an abundance study of the ejecta nebula surrounding AG Car with the aim of elucidating the evolutionary history of the central star

We present a study of the lithium abundances of a sample of evolved components of Chromospherically Active Binary Systems. We show that a significant part of them have lithium excesses, independently of their mass and evolutionary stage. Therefore, it can be concluded that Li abundance does not depend on age for giant components of CABS. These

We produce a comprehensive field star age-metallicity relationship (AMR) from the earliest epoch until ˜1 Gyr ago for three fields in the Fornax dwarf spheroidal galaxy by using VI photometric data obtained with FORS1 at the VLT. We find that the innermost one does not contain dominant very old stars (age > 12 Gyr), whereas the relatively outer