The early solar system.

This chapter reviews the early history of the solar system from radioactive nuclei of very different half-lives, which were recognized to have been present alive in pristine solids. Such radioactivities open a unique window on the evolution of the solar nebula and provide tools for understanding the crucial events that determined and accompanied the formation of the Sun. The understanding of the astrophysical origin of these unstable isotopes is still not complete and leaves puzzles and questions to our nucleosynthesis and stellar evolution models. We need to consider following aspects, among others: i) The determination of an age for solar system bodies, as it emerged especially from the application of radioactive dating from long-lived isotopes. ii) A synthetic account of the measurements that proved the presence of shorter-lived radioactive nuclei (especially those of half-life lower than about 100 Myr) in the Early Solar System (hereafter ESS). iii) An explanation of their existence in terms of nuclear processes. We often only have incomplete and/or qualitative views of such complex processes. Additionally, these may have occurred at a galactic scale (providing a galactic inheritance), or at the level of the molecular cloud in which the Sun was formed, or also locally, either through a single, late stellar contamination affecting the pre-collapse solar nebula or through high-energy phenomena induced by the same early sun, in its flares or in the bombardment of pristine solids with the energetic particles of its intense winds. We cannot give a complete analysis of the many branches of research on the early phases of our Solar System. Rather, we focus on those more directly connected with the general theme of this book, as the study of the early solar system promoted and opened new views on the importance of radioactive isotopes as tools for the study of stellar and galactic nuclear astrophysics. For reviews of aspects such as geochemistry and cosmochemistry, see e.g. Arnould and Prantzos (1999); Busso et al (1999); Kratz et al (2004); McKeegan and Davis (2005); Goswami et al (2005); Wasserburg et al (2006); Wadhwa et al (2007); Huss et al (2009); Davies et al (2014); Davis and McKeegan (2014), for stellar condensation in clusters, and nuclear enrichment from ejecta in sequential star formation see Adams et al (2014); Pfalzner et al (2015).