A nova is a star that suddenly brightens to many times the magnitude that it formerly was in a short period of time. The crew of the USS Enterprise-D determined that the star in orbit around Kataan went nova approximately 1,000 years previously while Captain Jean-Luc Picard was under the influence of a Kataanian probe. When the star went nova, it destroyed all life on Kataan. (TNG: "The Inner Light"). When Doctor Timicin was making experiments on Praxillus to revitalize the sun he miscalculated some variables on Praxillus went nova. (TNG: "Half a Life").

A nova is actually a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. Novae are not to be confused with supernovae, or another form of stellar explosion. A nova is a cataclysmic binary star that undergoes a sudden, spectacular brightening, by a factor of up to a million or so, before dimming to its pre-nova state. Far from being "new" stars, as the historical name suggests, novae consist of a white dwarf primary in close orbit around an orange/red dwarf or (in some cases) giant secondary, the fuel for the outbursts being gas plundered from the larger star by the white dwarf. In complete contrast with supernovae, which are one-time events accompanied by the total destruction of a star, novae leave the host stars essentially intact and capable of repeating the show. How often a nova recurs, together with the details of its behavior, determines how it is categorized. There are three main varieties: classical novae, recurrent novae, and dwarf novae.

A classical nova is characterized by an abrupt brightening of a thousand-fold to a million-fold (roughly 8 to 15 magnitudes) and the ejection of a shell of matter from the primary star. As a white dwarf, the primary would normally be a spent force in terms of making new energy by nuclear fusion, its surface rich in carbon, nitrogen, and oxygen, which the little star can't burn. However, thanks to its nearby, hydrogen-rich neighbor, it can temporarily restock its fusable reserves. Hydrogen flows from the secondary into an accretion disk around the primary, ad then down onto the primary's surface. As hydrogen (plus some helium) builds up on the white dwarf's surface, it compresses and thereby heats up the underlying material. At some point, a critical temperature, of about 10 million K, is reached in this base layer, at which the overlying hydrogen-rich layer is caused to ignite. The result is a thermonuclear runaway that produces a huge surge in luminosity and rips away the surface material to form a fast-expanding shell.