The shell of gas is being blown away from, and excited to glow by far-ultraviolet radiation from, a very hot white dwarf star which was the core of the originally formed star. The inner core of a star which ends as a supernova, can be even more exotic than the white dwarf stars which form from stars of more moderate initial mass. Also, in some cases, the other member of the binary system is close enough to the compact object that some gas from the more normal star is drawn in toward the compact star by the latter s intense gravitational force. In the evolutionary cycles of stars, much of the interstellar material from which they formed is eventually returned to space, where it can be recycled to form new generations of stars. The returned material is enriched in heavy elements, to a degree dependent on the mass of the star when it was first Fig. 4.
Supernova explosions are the primary source of all elements heavier than iron.