RADIOACTIVE AL-26 IN THE GALAXY - OBSERVATIONS VERSUS THEORY

Al-26 is the first cosmic radioactivity ever detected, more than ten y ears ago, through its characteristic 1.8 MeV gamma-ray line. Its simil ar to 10(6) yr lifetime, much shorter than the similar to 10(10) yr of galactic evolution, convincingly demonstrates that nucleosynthesis is currently active in our Galaxy. Current models of nucleosynthesis are still too uncertain to allow identification of the sites of that nucl eosynthetic activity, despite their continuous improvement in the past ten years. The recent results of the Compton Gamma-Ray Observatory sh ed, for the first time, some light on the origin of galactic Al-26, fa voring massive stars as the main sources. The various measurements of 1.8 MeV emission and the theoretical models of Al-26 sources are prese nted in this review, along with the implications of the latest results for nuclear, stellar and galactic astrophysics.