How Do Stars Explode?

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 secondstar can be another white dwarf, a redgiant, 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 sameprocess that ignites thermonuclear weapons. Eventually, the star converts most of its hydrogen into helium. The starthen must fuel itself by fusing helium intocarbon. 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 theiron phase.

It’s during the iron phase that things getreally heavy. Fusing iron does not producemore energy—in fact, iron fusion requiresenergy. Without the fusion pressure thatcounteracted the star’s gravity, the core ofthe star, which is approximately the size of Earth, collapses into a space less than 10miles (16 km) in diameter at about one- quarter light speed. When the stellar massbounces back into space (crashing into theouter shell of the doomed star), theresultant shock wave is what we on Earthwitness as a supernova.

Upon going supernova, the star may tear
itself apart entirely or leave behind an
extremely dense neutron star. If the core ofthe star is heavy enough, the supernovaleaves behind one of the most mysteriousobjects in the known universe: a blackhole.

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