Type Ia: thermonuclear detonation in binary systems
Type Ia supernovae occur in binary systems where a white dwarf (the dense remnant of a dead star) orbits a companion star. Material from the companion's outer atmosphere falls onto the white dwarf, accumulating in a layer on its surface. As the layer grows hotter and denser, it eventually reaches a temperature and pressure where carbon and oxygen fuse explosively. Unlike in normal stars where fusion is controlled, this runaway thermonuclear explosion engulfs the entire white dwarf. The entire star, typically about 1.4 solar masses, is blown apart. Type Ia supernovae are remarkably consistent in brightness because they always reach the same ignition point before detonating. This consistency makes them invaluable as standard candles for measuring cosmic distances. The 1998 discovery that distant Type Ia supernovae are dimmer than expected led to the conclusion that the universe's expansion is accelerating, a finding that won the Nobel Prize in Physics in 2011.
Type II: core collapse in massive stars
Type II supernovae result from the violent death of massive stars. When a massive star (20 or more solar masses) exhausts its fuel, it fuses elements up to iron. Iron cannot be fused further; the core collapses catastrophically. Within a few seconds, the core density reaches that of a neutron star, the infalling material rebounds off the incompressible neutron core, and a shockwave races outward through the star. For a moment, the shock falters, but neutrinos streaming from the core interact with infalling material and reignite the shock, blasting the star apart. The energy released is roughly one hundred times greater than the initial kinetic energy of the collapsed core, demonstrating the tremendous energy stored in neutrinos. The explosion leaves behind either a neutron star or, for the most massive progenitors, a black hole. Type II supernovae are fainter and more variable than Type Ia but are far more common, since massive stars occur in any young star-forming region.