The word nova is derived from that latin word for “new”, since novae suddenly appear as new, bright stars in the night sky, then gradually fade away again over anything from a few days to a period of several years.
Novae are caused by a similar process to a type Ia supernova. A white dwarf star that is in a close binary orbit with a large companion star can pull material from the larger star into an accretion disk around itself. When the accreted material falls onto the surface of the white dwarf, it ignites a nuclear fusion reaction, via the carbon-nitrogen-oxygen (CNO) cycle, releasing enormous amounts of energy as visible light and other wavelengths of electromagnetic radiation. The gas in the outer layers of the white dwarf star are often blown outwards producing a nebula around the white dwarf star.
Unlike in type 1a supernova, however, the white dwarf star of a nova does not undergo core collapse to become a neutron star or a black hole. Consequently, in comparison to a supernova, nova in fact produce far less energy.
Many novae are observed to reoccur over periods of several decades, although it is speculated that all novae could be recurrent, with novae from higher mass white dwarfs reoccurring over shorter periods than those of lower mass, which could even take many thousands of years to reoccur.
Nova, as well as dwarf novae (see below), show good potential for use as “standard candles” – since they have roughly the same peak absolute magnitudes, their distances can be determined by comparison with their apparent magnitudes.
Dwarf novae are believed to be produced in a similar way to a nova, except that they result from gravitational potential energy released when material from the accretion disk collapses onto the white dwarf.
Luminous Red Novae
Luminous red novae result from an entirely different mechanism to standard and dwarf novae. They are believed to be caused by the merger of two main sequence stars, although only a few such novae have been observed, and it is possible that they were, in fact, type II-p supernovas.
A luminous red nova produces more energy than a standard nova, but less than a supernova and is red in colour, becoming dimmer and redder over time.