A supernova is one of the universe's most awesome, violent events. In most cases, a supernova explosion can outshine an entire galaxy. Such catastrophic detonations leave behind spectacular legacies seen through telescopes-the Crab Nebula, as one example, is a rapidly expanding shell of beautiful, incandescent gas created by a 1054 A.D. supernova explosion. At the center of the nebula is a rapidly spinning neutron star-all that remains of a red supergiant star that once existed there.
When a massive, old star has gobbled up all of its hydrogen fuel, a new thermonuclear process begins at its core-the remaining helium and carbon then serves as the star's new fuel. With the hydrogen gone, the laws of physics seek to counterbalance an inward pull resulting from the star's heavy outer mass. However, this counterbalancing act can never be pulled off; gravity inevitably causes the core to collapse.
At the final moment of a collapsing core, a massive explosion occurs-this is called a supernova. As the explosion unfolds, plasma-ultra hot gas-is blasted far into space. For a brief moment, lasting hours or days, the supernova will outshine all the stars of its resident galaxy. As the explosion fades, the star remnant dims with only its core remaining. The remaining core is called a neutron star because it is composed entirely of densely packed neutron particles.
In some cases, supernova compression will continue past the neutron star stage and create a black hole where not even photons (light) can escape due to the extreme gravitational force of the "hole".
There are two types of supernova explosions, types I and II.
In a type-I supernova, two stars are involved. In a binary system, the larger companion star swells to a red giant, expanding dangerously close to its smaller partner. The smaller, but now more massive companion sucks gases off the red giant until what remains of the giant is a white dwarf star. In reverse roles, the companion now swells to a red giant and the dwarf sucks gases off its newly bloated companion. Finally, when all gaseous material is stripped away, two white dwarfs remain. But the odd dance of death is not yet complete. The two dwarf stars fall in toward each other and explode. According to most astronomers, type-I supernova explosions leave nothing behind.