We used deep wide-field photometric observations to derive the fraction of binary systems in a sample of five high-latitude Galactic open clusters. By analysing the colour distribution of main-sequence stars, we derived the minimum fraction of binary systems required to reproduce the observed colour-magnitude diagram morphologies. We found that all

Aims: We analyzed the resolved stellar population of the C component of the extremely metal-poor dwarf galaxy I Zw18 in order to evaluate its distance and star formation history as accurately as possible. In particular, we aimed at answering the question of whether this stellar population is young. Methods: We developed a probabilistic approach to

The ACS Survey of Galactic globular clusters is a Hubble Space Telescope Treasury program which has obtained deep, high quality photometry of 65 clusters. The absolute ages of a sample of clusters which have low reddening and well determined chemical compositions have been determined. A Monte Carlo set of theoretical isochrones has been generated

The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon, first described in the 1960s, acknowledges that metallicity alone is not enough to describe the HB morphology of all GCs. In particular, astronomers noticed that the outer Galactic halo contains GCs with

In order to address the "cosmic downsizing problem," or the underpopulation of highly massive quasars in the current epoch, we are exploring a correlation between the pitch angle of a galaxy's spiral arms and the mass of its central supermassive black hole (SMBH). The goal is to generate a census of SMBH masses using only photographic images. This

The metallicities implied by collisionally excited lines (CELs) of heavy elements in H II regions are systematically lower than those implied by recombination lines (RLs) by factors of ~2, introducing uncertainties of the same order in the metallicities inferred for the interstellar medium of any star-forming galaxy. Most explanations of this