A Brief History of Astronomy
Final End States of Stars
If mass accretion continues, interspersed with novae events, the total mass of the white dwarf may approach the Chandrasekhar limit. At that point, electron pressure can no longer balance gravity and the star begins a catastrophic collapse. Its carbon-oxygen material is compressed, and runaway thermonuclear reactions are triggered: C12 + C12 → Mg24 + energy; O16 + O16 → S32 + energy; C12 + O16 → Si28 + energy, and so forth, leading to the elements iron and nickel. These reactions dump so much energy into the star that it literally is blown apart in an explosion or Type I supernova. The stellar material, now enriched in large quantities of heavy elements, is returned completely to the interstellar material. In other galaxies, Type I supernovae are observed in regions dominated by old stars, which is consistent with the fact that Type I supernovae come from old, evolved white dwarfs.
