Definition
A supernova is the explosive death of a star. It occurs either when a massive star's core collapses at the end of its life, or when a white dwarf in a binary system is pushed past a critical mass and detonates.
In context
For a few weeks a single supernova can outshine an entire galaxy of stars. The explosion forges and scatters heavy elements, seeding the surrounding gas with the material that later forms new stars, planets, and ultimately the chemistry of life.
Related ideas
A core-collapse supernova can leave behind a dense neutron star or, for the most massive stars, a black hole. The expanding debris becomes a supernova remnant — a type of nebula.