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Nuclear Astrophysics (A few Problems and Perspectives)
I review the present status of our knowledge on some of the processes occurring in stars: those currently known under the name of Nuclear Astrophysics. They descend from the conclusion, achieved in the first half of the XXth century, that only nuclear fusion reactions can provide enough energy for powering the Sun; I also briefly mention some of the roots from which these views derived. I emphasize those processes for which we have a reasonable chance of (finally!) reaching a satisfactory quantitative description within the next ten years or so. This forcedly limits the discussion to the "simplest" evolutionary phases, those through which low mass stars (like our Sun) are passing now or will undergo in the future. They involve hydrogen and helium burning stages, in which the nuclear energy production and the gradual change of the plasma composition are coupled to complex hydrodynamic and magneto-hydrodynamic phenomena, providing various types of matter transport. The stars involved are long-lived (from a significant fraction of a Gyr to several Gyrs) and offer us many options to observe directly the outcomes of the nucleosynthesis processes. Thanks to modern astero-seismology, we now have also probes for retrieving information on the interior stellar structure, thus allowing us to look for a detailed understanding of how weak and strong interactions occur in stars and how they are affected by plasma processes, covering ranges of physical parameters which are very unusual for terrestrial laboratories. Some consequences on the nuclear evolution of the Universe are outlined. Many unanswered questions remain and I try to argue that they require now a wide interdisciplinary approach, involving various fields of mathematical, physical and chemical sciences operating together.