What causes some stars to explode as supernovas?

Stars explode as supernovas through two main mechanisms. In massive stars (roughly eight or more times the Sun's mass), the core runs out of fuel and can no longer sustain nuclear fusion. Without the outward pressure from fusion, gravity causes the core to collapse catastrophically in seconds, and the resulting shockwave blasts the outer layers into space as a core-collapse supernova. The second type involves white dwarfs in binary systems. A white dwarf can accumulate matter from a companion star until it exceeds a critical mass limit (about 1.4 solar masses), triggering a runaway thermonuclear explosion that destroys the star entirely. Both processes release enormous energy, briefly outshining entire galaxies.