  In addition to long-lived radioactive nuclei like U and Th isotopes, which
have been used to measure the age of the Galaxy, also radioactive nuclei with
half-lives between 0.1 and 100 million years (short-lived radionuclides, SLRs)
were present in the early Solar System (ESS), as indicated by high-precision
meteoritic analysis. We review the most recent meteoritic data and describe the
nuclear reaction processes responsible for the creation of SLRs in different
types of stars and supernovae. We show how the evolution of radionuclide
abundances in the Milky Way Galaxy can be calculated based on their stellar
production. By comparing predictions for the evolution of galactic abundances
to the meteoritic data we can build up a time line for the nucleosynthetic
events that predated the birth of the Sun, and investigate the lifetime of the
stellar nursery where the Sun was born. We then review the scenarios for the
circumstances and the environment of the birth of the Sun within such a stellar
nursery that have been invoked to explain the abundances in the ESS of the SLRs
with the shortest lives - of the order of million years or less. Finally, we
describe how the heat generated by radioactive decay and in particular by the
abundant 26Al in the ESS had important consequences for the thermo-mechanical
and chemical evolution of planetesimals, and discuss possible implications on
the habitability of terrestrial-like planets. We conclude with a set of open
questions and future directions related to our understanding of the
nucleosynthetic processes responsible for the production of SLRs in stars,
their evolution in the Galaxy, the birth of the Sun, and the connection with
the habitability of extra-solar planets.
