Supernovas can occur in one of two ways: through a process of runaway nuclear fusion or through a rapid collapse of the star’s core.

The first process occurs in binary star systems where at least one star is a white dwarf, a dense, aging star that can no longer support nuclear fusion. The second star can be another white dwarf, a red giant, or a main sequence star such as our own Sun, that fuses hydrogen atoms to form helium atoms at its core. In either case, the white dwarf siphons off (or collides with) the mass of its companion star, reigniting nuclear fusion. Once the white dwarf reignites, it gets so hot so fast that it blows apart, outshining an entire galaxy and leaving no remnant behind.

Less luminous, though no less spectacular, are core collapse supernovas. Instead of exploding in a runaway fusion reaction, this type of supernova occurs when the star’s fusion reaction grinds to a halt. For most of a star’s life, it burns by fusing hydrogen atoms. This is the same process that ignites thermonuclear weapons. Eventually, the star converts most of its hydrogen into helium. The star then must fuel itself by fusing helium into carbon. If the star is heavy enough—about eight times the mass of the Sun—it will then proceed to fuse carbon into neon and helium. The star continues to fuse heavier and heavier elements until it reaches the iron phase.

It’s during the iron phase that things get really heavy. Fusing iron does not produce more energy—in fact, iron fusion requires energy. Without the fusion pressure that counteracted the star’s gravity, the core of the star, which is approximately the size of Earth, collapses into a space less than 10 miles (16 km) in diameter at about one-quarter light speed. When the stellar mass bounces back into space (crashing into the outer shell of the doomed star), the resultant shock wave is what we on Earth witness as a supernova.

Upon going supernova, the star may tear itself apart entirely or leave behind an extremely dense neutron star. If the core of the star is heavy enough, the supernova leaves behind one of the most mysterious objects in the known universe: a black hole.