Artist's conception of a white dwarf star accreting hydrogen from a larger companion

A nova (pl. novae) is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star.

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 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 is largely unresponsive to heat, while the accreted hydrogen is not. Eventually, the pressures and temperatures within the hydrogen layer becomes great enough to trigger a nuclear fusion reaction that rapidly converts a large amount of the hydrogen into helium and other heavier elements. 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 (as in fast novae) or gradual (as in slow novae); after the peak the brightness declines steadily.^[1]

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. Nonetheless, the accreted matter is blown off the star at velocities as high as several thousand kilometers per second, with a concurrent rise in luminosity from a few times solar to 50,000-100,000 times solar.^[2]

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 five times (in 1898, 1933, 1958, 1967, 1985, and again in 2006). Eventually, however, either the companion star will run out of material, or the white dwarf will undergo a nova so powerful that it is completely destroyed in the process. This is somewhat similar to a type Ia supernova. Supernovae in general, however, involve different processes as well as much higher energies, and should not be confused with ordinary novae.

Occasionally a nova is bright enough and close enough to be conspicuous to the unaided eye. The most 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).

Historical significance

The astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, and described it in his book de stella nova (Latin for "concerning the new star"), giving rise to the name nova. In this work he argued that a nearby object should be seen to move relative to the fixed stars, and that the nova had to be very far away.

Novae as distance indicators

Novae have some promise for use as standard candles. For instance, the distribution of their absolute magnitude is bimodal, with a main peak at magnitude -7.5, and a lesser one at -8.8. Novae also have roughly the same absolute magnitude 15 days after their peak (-5.5). Comparisons of nova-based distance estimates to various nearby galaxies and galaxy clusters with those done with Cepheid variable stars have shown them to be of comparable accuracy.^[3]

References

1. ^  AAVSO Variable Star Of The Month: May 2001: Novae
2. ^  Zeilik, Michael. Conceptual Astronomy. New York: John Wiley & Sons, Inc., 1993.
3. ^  Alloin, D., and W. Gieren, eds. Stellar Candles for the Extragalactic Distance Scale. Robert Gilmozzi and Massimo Della Valle, "Novae as Distance Indicators", pp. 229-241. Berlin: Springer, 2003. ISBN 3540201289.

Bright novae since 1890

Note:- Please add all Novae brighter than 6 mag ( http://www.tsm.toyama.toyama.jp/curators/aroom/var/nova/1600.htm )

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

• Cataclysmic variable
• Hypernova
• Nova remnant
• Supernova
• Crab Nebula

External links

• General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow
• NASA Observatorium: Classical Nova