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A nova is a cataclysmic nuclear explosion casued by the accretion of hydrogen on to the surface of a white dwarf. The hydrogen ignites and sparks runaway nuclear fusion. These are separate items not related to supernovae or other phenomena.
When a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the companion’s outer atmosphere. The companion may be a main sequence star or a red giant. The captured gases are mainly hydrogen and helium. The gases are compacted on the white dwarf’s surface by its intense gravity, compressed and superheated as additional material accretes. The hydrogen is compressed and heated at the surface of the white dwarf to a temperature of some 20 million kelvin before nuclear fusion occurs. The hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other heavier elements in runaway fusion. The enormous amount of energy liberated by this process blows the remaining gases away from the white dwarf’s surface and produces an extremely bright outburst of light. The rise to peak brightness can be very rapid or gradual which is related to the speed class of the nova; after the peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used to classify a nova via its speed class. A fast nova will typically take less than 25 days to decay by 2 magnitudes and a slow nova will take over 80 days.
The amount of material ejected by novae is usually small relative to the mass of the white dwarf. Only five percent of the accreted mass is fused to power the outburst. This is enough energy to accelerate nova ejecta to velocities as high as several thousand kilometers per second with a associated rise in luminosity(from a few times solar to 100,000 times the Sun). A nova also emits gamma rays and a single white dwarf can generate several novae.
Nova are awesome events that inspire quite a bit of scientific interest. New information about novae was discovered as recently as 2010, so who knows what will be found in the future.
We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?