A nova (pl. novae) is 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 Type Ia supernovae, or another form of stellar explosion first announced by Caltech in May 2007, Luminous Red Novae.

Occurrence rate, and astrophysical significance

Astronomers estimate that the Milky Way experiences roughly 30 to 60 novae per year, with a likely rate of about 40.  The number of novae discovered in the Milky Way each year is much lower, about 10.  Roughly 25 novae brighter than about magnitude 20 are discovered in the Andromeda Galaxy each year and smaller numbers are seen in other nearby galaxies.

Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium.  The contribution of novae to the interstellar medium is not great; novae supply only 1/50th as much material to the Galaxy as supernovae, and only 1/200th as much as red giant and supergiant stars.

Recurrent novae like RS Ophiuchi (those with periods on the order of decades) are rare. Astronomers theorize however that most, if not all, novae are recurrent, albeit on time scales ranging from 1,000 to 100,000 years.  The recurrence interval for a nova is less dependent on the white dwarf's accretion rate than on its mass; with their powerful gravity, massive white dwarfs require less accretion to fuel an outburst than lower-mass ones.  Consequently, the interval is shorter for high-mass white dwarfs.

Historical significance

The first record of a nova dates from 1400 BC in China, in an oracle bone inscription, which reads: "On the 7th day of the month, a new star appeared next to 'Heart Constellation II'." From 1400 BC to AD 1600, China recorded 90 novas. Among them the supernova discovered in 1054 was the first to be confirmed by modern radio astronomers. In 1731, a British astronomer discovered an oblong spot of fog over China. After observation, calculation and analysis by several astronomers, it was proved that the crab-shaped nebula found in this position was the ruins of a supernova that had shot out of a dense cluster some 900 years previously, i.e., the year of 1054. This discovery was one of the most significant astronomical findings in the 1960s.

The astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, and described it in his book de stella nova (Latin for "concerning the new star"), giving rise to the name nova. In this work he argued that a nearby object should be seen to move relative to the fixed stars, and that the nova had to be very far away. Though this was a supernova and not a classical nova, the terms were considered interchangeable until the 1930s.