nova

(nō'və)
n., pl., -vae (-vē), or -vas.
A star that suddenly becomes much brighter and then gradually returns to its original brightness over a period of weeks to years.

[New Latin (stēlla) nova, new (star), nova, feminine of Latin novus, new.]

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A type of cataclysmic binary 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 their 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: dwarf novae, classical novae, and recurrent novae. For more on dwarf novae, see the separate entry on U Geminorum stars, which is the name by which these objects are more specifically known.


nova An artist's conception of a classical nova binary system just before an explosion on the surface of the white dwarf component. M. Weiss/CXC


A classical nova is marked 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, and 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, which causes the overlying hydrogen-rich layer 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. (A similar phenomenon, involving neutron stars instead of white dwarfs, explains X-ray bursters.)

Differences in the light curves of classical novae have prompted astronomers to distinguish between three main varieties:

Fast novae rise very steeply to maximum brightness, stay at maximum for a few days at most, then decline, rapidly to begin with (typically fading by a factor of 10 in three months) then trailing off. The fading may involve a prolonged series of marked fluctuations. A classic example is Nova Persei 1901.

Slow novae rise gradually to maximum brightness, then remain there for several weeks or months before declining. They tend to fade slowly at first with fluctuations, after which the rate of fading quickens. As these novae continue to decrease in brightness, it is common to see them brighten slowly and irregularly to a second maximum, followed by a return to the minimum state. A factorof-10 dimming from maximum typically takes 150 days or more. Some slow novae show a deep, wide minimum in brightness about 2 to 5 months after maximum, which may be due to dust condensation that blocks the visible light. As the ejected material dissipates, the nova recovers to a brightness approximately equal to that expected in an undisturbed decline. Nova Herculis 1934 is a well-known example.

Very slow novae are a small group with maxima that extend over years and with declines that also take place extremely slowly. In 1915, RT Serpentis, the first of this type to be observed, rose slowly to magnitude 10.5, remained at this level for almost 10 years, and then began to fade very slowly, reaching magnitude 14 in 1942. Very slow novae are also referred to as symbiotic novae or RR Telescopii stars.

Closely related to classical novae are recurrent novae, which show similar or slightly lower levels of brightening but have been seen to put on more than one display. In the final analysis, all novae are probably recurrent if observed over a long enough period. The seven known examples of recurrent novae cover a broad spectrum of behavior, showing brightness increases of 4 to 9 magnitudes and intervals between outbursts of 10 to 100 years. A well-studied example, RS Ophiuchi, varies in magnitude from 12.5 at minimum to a 4.8 at maximum, when it can be seen with the unaided eye. Over a century of observations, it has erupted five times: in 1898, 1933, 1958, 1967, and 1985. Typically, it rises to peak magnitude within 24 hours, then returns to minimum over 100 days or so, brightening slightly around 700 days after the onset of the outburst. The intervals between outbursts seem to have no common period and, at minimum, the light curve shows irregular brightness variations of between 1 and 3 magnitudes. Other examples that tend to have outbursts every couple of decades or so include the Blaze Star (T Coronae Borealis) and U Scorpii. On the other hand, T Pyxis has a much slower recurrence time of about 80 years. These differences have led to the idea that there are two kinds of recurrent novae. Type A, exemplified by T Pyx, result from thermonuclear runaway on the white dwarf primary, and are just classical novae that have been observed in more than one outburst. Type B, by contrast, may be driven primarily by instability and eruptions in the accretion disk, and so have more in common with U Geminorum stars.

For more information on nova, visit Britannica.com.

The sudden brightening of a previously inconspicuous star. The name, short for nova stella (new star), formerly included objects now classified as supernovae and as other kinds of cataclysmic variables. Classical novae now include only those events where the energy source is hydrogen fusion (burning) on the surface of a white dwarf in a close binary system and the white dwarf is not destroyed in the process.

A handful of novae are discovered each year in the Milky Way Galaxy, and the total rate is probably 20–50 per year. A comparable number are found in other, nearby galaxies. The system consists of a normal, hydrogen-burning star in a close orbit (periods of a few days or less) around a white dwarf or degenerate star. A stream of gas flows from the normal star into a disk around the white dwarf and then accretes onto its surface. Hydrogen gradually builds up there until it is hot and dense enough for nuclear burning, normally with carbon, oxygen, neon, or magnesium from the white dwarf itself acting as a catalyst. Any nuclear fuel ignited under degenerate conditions explodes, because energy released does not cause the gas to expand, so temperature rises rapidly. See also Binary star; White dwarf star.

Novae brighten in a few days and fade in months to years. The peak brightness is more than 100 times the solar luminosity, and the total energy release more than 10⁴⁵ ergs (10³⁸ joules). Novae recur every 10⁴–10⁵ years. See also Cataclysmic variable; Light curves; Variable star.

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A minicomputer series from Data General. When introduced in 1969, it was the first 16-bit mini to use four CPU accumulators, quite advanced for its time. Novas and its RDOS operating system were used extensively in the OEM marketplace.

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(noh-vuh)

In astronomy, the appearance of a new star in the sky (nova is Latin for “new”). Novae are usually associated with the last stages in the life of a star. (See supernova.)

For a list of words related to nova, see:

• Celestial Bodies - nova: star that increases in brilliance by several magnitudes in a few hours due to explosive ejection of surface material

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See words rhyming with "nova."

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - nova

Nederlands (Dutch)
nova (soort ster)

Français (French)
n. - nova

Deutsch (German)
n. - Nova, neuer Stern

Ελληνική (Greek)
n. - (αστρον.) νεολαμπής (αστήρ), νόβα

Italiano (Italian)
nova

Português (Portuguese)
n. - estrela (f) nova, tipo de salmão defumado

Русский (Russian)
новая звезда

Español (Spanish)
n. - nova, tipo de estrella

Svenska (Swedish)
n. - nova

中文（简体）(Chinese (Simplified))
新星

中文（繁體）(Chinese (Traditional))
n. - 新星

한국어 (Korean)
n. - 새로 생긴 별

idioms:

• nova Scotia    노바스코샤 (캐나다 남동부의 반도; 이를 포함하는 주; 주도 Halifax; (약) N.S.)

日本語 (Japanese)
n. - 新星

العربيه (Arabic)
‏(الاسم) المستسعر, نجم‏

עברית (Hebrew)
n. - ‮כוכב הבוהק לפתע ושוקע, נובה‬

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