As stars age, many use up their fuel and fade away to oblivion. Others, however, go out with a bang as supernovae, releasing energies of up to 10⁴⁴joules—an amount of energy equivalent to 30 times the power of a typical nuclear bomb. The explosions of low-mass stars can be triggered by the accretion of mass from a companion star in a binary system to create classical, or Type Ia, supernovae. These supernovae show no hydrogen in their spectra. Massive stars, on the other hand, proceed through normal nuclear fusion but then, when their energy supply runs out, there is no outward pressure to hold them up and they rapidly collapse. The core is crushed into a neutron star or black hole, and the outer layers bounce and are then hurled outward into the surroundings at many million kilometers per hour. These are Type Ib and II supernovae. The...