Why do some stars explode as supernovae?

A star shines because nuclear fusion in its core pushes outward against the crush of its own gravity. In stars much heavier than the Sun, fusion eventually builds an iron core, and iron cannot release energy by fusing further, so the outward pressure vanishes. The core collapses in a fraction of a second and rebounds in a catastrophic explosion called a core-collapse supernova, briefly outshining an entire galaxy. A second kind occurs when a dense white dwarf star siphons matter from a companion until it crosses a critical mass and detonates in a thermonuclear blast. Supernovae forge and scatter heavy elements like gold and iron across space, seeding future stars and planets.