A Type Ia supernova is a type of supernova that occurs in binary systems in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf. From Wikipedia
Discovery of two concentric calcium shells in SNR 0509-67.5 validates a sub-Chandrasekhar double-detonation as a viable explosion pathway, setting the stage for new surveys of young supernova remnants.