The understanding of our own place in the Universe is one of the most fundamental quests of humankind. A related question is if the Solar System was born under common or special circumstances because the circumstances of a stellar birth, from its location within the Galaxy to the specific nature of the birth environment, significantly determine the formation and early evolution of the associated planetary system. This is due to the effect of radiation from massive stars, of dynamical encounters with other stars, and of the heat released by radioactive decay. The latter range from the decay of aluminium-26 (within roughly one million year) impacting on the evolution of the first planetesimals, to the decay of the longer-lived U and Th (billion of years) impacting on the evolution of the Earth. The main problem is that we still do not know in which kind of environment the Sun was born. While we know that it was most likely born in a stellar cluster, we do not know if this cluster was located within a small or a giant stellar nursery - also known as molecular cloud. To investigate the origin of our Solar System we have used the abundances of radioactive nuclei produced in stars by nuclear reactions as clocks to determine the time of occurrence of different astrophysical events before the birth of the Sun, and as fingerprints of the local stellar sources present in the environment of such birth. The immediate aim of our investigation is the interpretation of measurements provided by analyses of meteoritic rocks, which demonstrate that radioactive nuclei with half-lives or the order of millions of years were present at the birth of the Sun. We have found that radioactive nuclei made by rapid neutron-captures originated most likely from a last neutron-star merger event that happened 100-200 million years before the formation of the Sun. The radioactive nuclei made by the slow neutron-captures, instead, most likely originated from giant stars. Their abundances require a relatively long time interval, 9 to 26 million years, to have elapsed from the time of the formation of the molecular cloud to the time of the formation of the first solids in the Solar System. These values are robust, as they include uncertainties from stellar and galactic physics. They confirm that the Sun was born in a long-living giant molecular cloud, such as the Orion molecular cloud complex. The very short-lived nuclei (such as the heating source aluminium-26) most likely originated from massive star sources within this cloud. We found that massive stars winds provide a possible site of origin, while analysis of all the (15) radioactive nuclei made in core-collapse supernovae confirm that the yields from these sources are not consistent with the data.
