The nova/supernova puzzle remained a mystery until it dawned on researchers that they might not be similar phenomena. And they're not, as far as we can tell now. A supernova is indeed a massive star meeting its catastrophic end. A nova is - apparently - the result of a white dwarf star with a close companion star, most likely a giant. The giant's swollen outer layers are sucked into an "accretion disk" and then onto the dwarf's surface. When a critical mass of about 1/100,000 a solar mass is reached, the pressure and temperature of the gases triggers a nuclear explosion. The layers are blown off violently into space at 500 miles/second. The brightness goes way up, then drops back down over a period of weeks. Then the cycle starts again. Some novae have been seen multiple times.
Artist's conception of a white dwarf's accretion disk
accumulating matter from a companion star.
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