nova n : a star that ejects some of its material in the form of a cloud and become more luminous in the process [also: novae (pl)]

English

Etymology

Feminine nominative singular of the Latin adjective novus (new). The feminine is used since the Latin word for star, stella is feminine; thus nova is a shortening of nova stella (new star).

Pronunciation

• Rhymes: -oʊva

Noun

1. any sudden brightening of a previously inconspicuous star

Translations

• Finnish: nova
• Italian: nova

Derived terms

• supernova

Esperanto

Adjective

1. new

   Oni ne povas instrui al maljuna hundo novajn ruzojn.

   You cannot teach an old dogs new tricks.

Ido

Adjective

nova

1. new

Italian

Noun

1. nova

Derived terms

• supernova

Latin

Adjective

nova

novā

Sicilian

Adjective

nova

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.

Development

If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the star's outer atmosphere. The companion may be a main sequence star, or one that is aging and expanding into a red giant. The captured gases consist primarily of hydrogen and helium, the two principal constituents of ordinary matter in the universe. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so does not inflate at increased heat, while the accreted hydrogen is compressed upon the surface. The dependence of the hydrogen fusion rate on temperature and pressure means that it is only when it is compressed and heated at the surface of the white dwarf to a temperature of some 20 million K that a nuclear fusion reaction occurs; at these temperatures, hydrogen burns via the CNO cycle. For most binary system parameters, the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other heavier elements in a runaway reaction. (Hydrogen fusion can occur in a stable manner on the surface, but only for a narrow range of accretion rates.) The enormous amount of energy liberated by this process blows the remaining gases away from the white dwarf's surface and produces an extremely bright outburst of light. The rise to peak brightness can be very rapid or gradual which is related to the speed class of the nova; after the peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used to classify a nova via its speed class. A fast nova will typically take less than 25 days to decay by 2 magnitudes and a slow nova will take over 80 days.

In spite of their violence, the amount of material ejected in novae is usually only about 1/10,000th of a solar mass, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused to power the outburst.

A white dwarf can potentially generate multiple novae over time as additional hydrogen continues to accrete onto its surface from its companion star. An example is RS Ophiuchi, which is known to have flared six times (in 1898, 1933, 1958, 1967, 1985, and again in 2006). Eventually, the white dwarf could explode as a type Ia supernova if it exceeds the Chandrasekhar limit.

Occasionally a nova is bright enough and close enough to be conspicuous to the unaided eye. The brightest recent example was Nova Cygni 1975. This nova appeared on August 29, 1975 in the constellation Cygnus about five degrees north of Deneb and reached magnitude 2.0 (nearly as bright as Deneb). The most recent was V1280 Scorpii which reached magnitude 3.7 on February 17, 2007.

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. By comparison, the number of novae discovered each year in the nearby Andromeda Galaxy is much lower; roughly ½ to ⅓ that of the Milky Way.

Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. 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.

References

Bright novae since 1890

Note: Please add all novae brighter than 6 mag ]

Recurrent novae

• RS Ophiuchi
• T Coronae Borealis
• T Pyxidis

Notes

• Under the Morgan-Keenan spectral classification scheme, novae are classified as Type-Q.

See also

• Dwarf nova
• Supernova
• Hypernova
• Luminous Red Nova
• Nova remnant
• Cataclysmic variable
• Crab Nebula
• Guest star (astronomy)
• Extragalactic Distance Scale

External links

• [http://www.sai.msu.su/groups/cluster/gcvs/gcvs/ General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow
• NASA Observatorium: Classical Nova

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