nova

A nova (sometimes abbreviated/symbolized by N) is a white dwarf explosion less energetic than a supernova. It is a thermonuclear explosion (CNO cycle) of surrounding compressed hydrogen accreted from a binary companion. Novae appear as brighter stars that gradually lose their brightness, losing 2 magnitudes in a few weeks, generally in the range of 25 to 80 days. The white dwarf survives and can produce a subsequent nova. Novae are regularly tracked in Andromeda and the Magellanic Clouds as well as the Milky Way.

The term classical nova (CN) is sometimes used to refer to the above case: the word nova basically means "new", and was coined for any apparently new stars in the sky, the above being the common underlying mechanism. (Nova is now often used for the star producing the transient and terms like nova eruption are used for the transients.) Aside from distinguishing it from supernovae and other less common cases, the term classical nova is sometimes used specifically to mean a one-time nova rather than a recurrent nova, i.e., a star that undergoes this brightening repeatedly. However, it is thought that classical novae are generally recurrent, the non-recurrent novae merely being highly infrequent: such a nova's recurrence rate can be once a year or can be once a gigayear.

Gamma rays have been detected in some recent classical novae, producing research interest, suggesting shock in surrounding material. Previous lack of detection is probably because gamma rays are blocked by Earth atmosphere, and the emissions are faint.

The term X-ray nova is used for something different: a type of X-ray transient ascribed to material falling into a black hole or neutron star, material from a companion star or perhaps the entire star.

Novae are sometimes identified with a nova-specific designator, "Nova" followed by the constellation name in Latin genitive form (as is used in a Bayer designation) followed by a year, optionally followed by a letter. An abbreviated form uses the letter "N" and the constellation abbreviation. Such designators for novas in other galaxies generally cite the galaxy rather than the constellation, e.g., Nova LMC 1968. Novae are also commonly designated by a variable star designation or other designation of the star.

The term nova is still sometimes used intending its original, more general sense, of a brightening that could bring a source up to detectability for some equipment, appearing to be a new star in the sky; the white-dwarf mechanism described above is the most common mechanism for such brightening. The term X-ray nova described above is one particular usage of nova in which the above white-dwarf mechanism is not implied.