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A type of cataclysmic variable that exhibits a sudden, unpredictable outburst, with a typical amplitude of 11–12 mag.; abbr. N. Novae are interacting binaries, and usually consist of a main-sequence or slightly evolved secondary and a white-dwarf primary. Mass transfer occurs from the secondary to an accretion disk and thence to the white dwarf. The accumulation of hydrogen-rich gas on the white dwarf's surface eventually leads to a thermonuclear runaway (a sudden initiation of nuclear reactions), producing the outburst and ejecting much of the outer envelope from the system.

Novae are divided into three subtypes, based on their speed of decline from maximum (see also fast nova): fast (NA), declining by 3 magnitudes in less than 100 days; slow (NB), 3 magnitudes in 150 days or more; very slow (NC), persisting at maximum for years. Very slow novae probably have a giant or supergiant secondary, and are sometimes referred to as symbiotic novae or RR Telescopii stars.

No pre-nova has been studied in detail, but there may be a pre-outburst rise of some magnitudes, perhaps with fluctuations and activity like that of a dwarf nova. The initial rise is rapid (less than 1 day in most cases, 2–3 days in slow novae) to a pre-maximum halt, about 2 mag. below maximum. After a few hours to days this is followed by the final rise in 1–2 days or (in slow novae) a few weeks. About 3–4 mag. below maximum, a transition region lasting 2–3 months may occur; some novae (e.g. Nova Herculis 1934 = DQ Herculis) undergo a deep fade of 7–10 mag. and a subsequent recovery, while others exhibit quasi-periodic oscillations. The final decline into the post-nova phase is generally steady.

The spectrum of a nova at maximum shows a continuum, overlain by a sequence of absorption lines arising in the expanding envelope which indicates velocities of hundreds to thousands of kilometres per second. In the final stages the continuum fades, leaving a nebular spectrum with forbidden lines. There is an extended period of mass loss in which at least 10−4 solar masses is ejected. The mass transfer rate through the accretion disk is estimated at about 10−8 solar masses a year, so the system should undergo a further outburst after about 10 000 years. The ejecta may become visible after a period of time.

Estimates suggest that 25 to 50 novae occur each year in our Galaxy, most of which go undetected because of interstellar extinction and other factors. They occur throughout the Galaxy (including the outermost halo) and among systems of all ages from old Population II to extremely young Population I stars.

Subjects: Astronomy and Astrophysics.

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