A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner. A supernova is a energetic nova that has much more energy than a nova. Supernovae are extremely luminous and cause a burst of radiation that often briefly out shines an entire galaxy, before fading from view over several weeks or months. A star can take billions of years before it will explode creating a supernova, and the light can take hundreds of years to reach the earth because the closest star is four light years away.

A supernova happens when a star dies like I said earlier. The star runs out of hydrogen to burn and when that energy has nothing to do it combusts and explodes creating the supernova. This creates deadly radiation to us in space, but it is harmless because it is so far away.

There are two types of supernovae. Type l: Results from some binary star systems in which a carbon-oxygen white dwarf is accreting matter from a companion. Type ll : These supernovae occur at the end of a massive star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy.

Supernovae are very hard to detect but astronomers have a way to do it, they use telescopes that work at various wavelengths. The telescopes also have light filters that measure the brightness or lamination of stars. They do this because supernovae are such a big explosion that it gives of more light than just a normal star. Astronomers also pass the light through a prism that emits color called a spectrum. The spectrum shows how the brightness of the light depends of the wavelength of the light.

This video by Steven Hawking, the author of A Briefer History of Time, is a great visual of what supernovae really are.

This picture of a supernova shows the energy exported by the massive explosion.
This is the life cycle of a massive star, it shows where in the life cycle the supernova fits in.