The first kind of nova is, well, a nova. Novae are caused by white dwarves that are accreting hydrogen and helium from a neighboring star. Accretion is merely a term that means the gravitational collecting of gas. A white dwarfs intense gravity is capable of stealing gas from nearby stars that have surpassed their Roche lobe, the area in which matter is still gravitationally bound to a star. As the white dwarf accretes more and more hydrogen, intense pressure and heat can cause an unstable fusion reaction which can blow off the outer shell of the white dwarf. These explosions are relatively small, with only 5% of the accreted matter being fused for the explosion. A white dwarf can, and probably will, go nova multiple times within its existence. One example of this is RS Ophiuchi which we have seen go nova 6 times. Novae are bright, ranging from 50,000 to 100,000 solar brightness. Novae are even classified based on their brightness. NA novae are fast, with rapid light increase and rapid light decrease. After 100 days, the novas brightness should have decreased by 3 magnitudes, to about 1/16 the original brightness. NB novae are slow, their brightness declining by 3 magnitudes in about 150 days. NC are very slow novae, their brightness can be maintained for over a decade. The final classification is NR/RN novae which are recurring novae.
White dwarf accreting gas from neighbor
Supernovae are much more serious than novae. A supernova is a much bigger explosion, and uses up most, if not all, of the exploding star. There are several mechanisms by which a supernova can happen. A supernova can emanate from a white dwarf that has been accreting matter. The core of the white dwarf can get hot enough that it ignites with carbon fusion and explodes violently. For more massive stars, their cores can reach a certain mass, after which they collapse, sending out a shock wave and blasting off their outer shells. These kinds of supernovae can create black holes or neutron stars. Supernovae are necessary for the creation of new stars. Supernovae blast out the matter necessary for the formation of new stars, so old stars must die so that new stars can live. Supernovae are classified in several ways. Type Ia supernovae are white dwarf supernovae. Types Ib and Ic supernovae are massive stars that collapsed in on themselves. Types II-P and II-L supernovae are super massive stars that collapse in on themselves, ranging from 9 times the mass of the sun to 50 times the mass of the sun.
Type Ia supernova
The final type of nova is a hypernova. Hypernovae are created when a hyper giant star reaches the end of its life and collapses in on itself. Hyper giant stars are between 100 and 300 times the mass of the sun. Hypernovae create black holes and can shoot out concentrated streams of gamma radiation at their rotational poles. It is believed that most of the light from hypernovae is provided by the unstable isotope nickel-56. Hypernovae are understandably rare, it is theorized that they occur ever 200 million years in our galaxy.
Eta Carinae, candidate for future hypernova