(I) When their nuclear fuel is exhausted, stars die, and the residual iron core collapses on itself. (II) The outcome of a star's death depends on mass, however. (III) Stars with between 10 and 20 times the mass of the Sun collapse in a spectacular explosion known as a supernova, leaving behind a neutron star, whereas those larger than 20 solar masses implode to form black holes in a hypernova. (IV) In both cases, copious bursts of neutrinos are released along with optical, x-ray and gamma radiation. (V) Recent studies indicate that some massive stars may be rotating only slowly or not at all.