  Energy in stars is provided by nuclear reactions, which, in many cases,
produce radioactive nuclei. When stable nuclei are irradiated by a flux of
protons or neutrons, capture reactions push stable matter out of stability into
the regime of unstable species. The ongoing production of radioactive nuclei in
the deep interior of the Sun via proton-capture reactions is recorded by
neutrinos emitted during radioactive decay and detected on Earth. Radioactive
nuclei that have relatively long half lives may also be detected in stars via
spectroscopic observations and in stardust recovered from primitive meteorites
via laboratory analysis. The vast majority of these stardust grains originated
from Asymptotic Giant Branch (AGB) stars. This is the final phase in the
evolution of stars initially less massive than ~10 solar masses, during which
nuclear energy is produced by alternate hydrogen and helium burning in shells
above the core. The long-lived radioactive nucleus 26Al is produced in massive
AGB stars (>4:5 solar masses), where the base of the convective envelope
reaches high temperatures. Several other long-lived radioactive nuclei,
including 60Fe, 87Rb, and 99Tc, are produced in AGB stars when matter is
exposed to a significant neutron flux leading to the synthesis of elements
heavier than iron. Here, neutron captures occur on a timescale that is
typically slower than beta-decay timescales, resulting slow neutron captures
(the s-process). However, when radioactive nuclei with half lives greater than
a few days are produced they may either decay or capture a neutron, thus
branching up the path of neutron captures and defining the final s-process
abundance distribution. This nucleosynthesis in AGB stars could produce some
long-living radioactive nuclei in relative abundances that resemble those
observed in the early solar system.
