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The history of our galaxy, nearby galaxies and their stellar clusters

Our own galaxy and the large variety of galaxies in its vicinity, such as the spirals and irregulars of the Local Group, offer two unique and complimentary opportunities to study the local Universe and its evolution. The arrival of the Hubble Space Telescope, HST, the Very Large Telescope, VLT, and 10 metre class telescopes has allowed great progress in the fields of photometry and spectroscopy of open and globular clusters, of single stars, and of the interstellar medium (planetary nebulae and HII regions). Thanks to this progress, it is now possible to obtain and combine important information for classes of objects representative of the various evolutionary phases, and reconstruct, on an observational basis, the history of star formation and chemical enrichment of the galaxies.

The contribution of our astronomers to this work has been very important, and many of the most interesting results at an international level have been obtained by groups with an Italian Principal Investigator (P.I.), and sometimes, entirely Italian. The most important aspects of this research can be summarised as the study of chemical abundances of the various elements in stars belonging to different environments within our galaxy, belonging to various stellar populations and in stars belonging to different galaxies.

For the study of star clusters and field stars of the Galaxy and nearby galaxies, use is made of the optical telescopes with diameters less than 8 metres, such as the TNG. Instead, both for multi-object spectroscopy or high resolution spectroscopy of resolved stellar populations, and multi-band photometric surveys, ground based 8-10 metre class telescopes, such as the VLT and Large Binocular Telescope (LBT), of which INAF is a main partner, are used.

For studies of galactic evolution, star formation, binary stars and variable stars in clusters, space telescopes such as Chandra, XMM, HST and Spitzer are preferred.

As far as the study of the formation and evolution of our galaxy is concerned, from the study of the chemical abundances of the various elements within the Galaxy, it is possible to derive its mode of formation and evolution, using an astro-archaeological approach. Another research direction in this field is the comparison of the evolution of the Milky Way with that of its satellite galaxies. The study of galaxy interactions, and in particular the study of hierarchical merging, generally thought to be one of the fundamental mechanisms of galaxy formation, has seen great progress over the last ten years, thanks to some fundamental discoveries and the availability of large, new generation surveys.

Italian researchers also have a tradition of excellence for their results and innovative methods in photometric, astrometric and spectroscopic studies of resolved stellar populations. One of the applications that has recently seen great success at an international level is the derivation of the star formation history from the evolutionary characteristics and colour-magnitude diagrams of resolved stellar populations. These diagrams are one of the most powerful theoretical "instruments" available to astrophysicists because they relate the effective temperature and luminosity of stars. The effective temperature and luminosity are physical quantities that depend strictly on the intrinsic characteristics of the star (mass, age and chemical composition), that can't be measured directly by the observer but can be derived via physical models. In recent years, these studies have led to the understanding that in the local Universe there are no examples of galaxies that are now forming their first stars, because they all contain old stars, and that star formation activity varies greatly from one region to another even within very small galaxies. Italian researchers were the first to propose and develop the numerical methods for the derivation of the star formation history from colour-magnitude diagrams of resolved stellar populations, later used by an ever growing number of foreign groups.

Another of the most recent and revolutionary results in the field of stellar astrophysics concerns the discovery of multiple stellar populations in globular and open clusters in our own and nearby galaxies. Italian researchers aren't just in the front line of this research area, but are essentially those that have opened up this whole field, that in only a few years has led to the need to reconsider the very concept of a globular cluster. Though the emerging scenario is very interesting, it is far from being understood. It is very likely that this area of research will see significant developments in the coming years, both at an observational and theoretical level, and that Italian researchers will certainly play a prominent role internationally in coordinating the efforts to understand the stellar populations in clusters.

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