Nova

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

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 ageing 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.[1] (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.[2] 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.[3]

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.[1] Nonetheless, this is enough energy to accelerate nova ejecta to velocities as high as several thousand kilometers per second—higher for fast novae than slow ones—with a concurrent rise in luminosity from a few times solar to 50,000–100,000 times solar.[1][4]

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 20 to 60 novae per year, with a likely rate of about 40.[1] The number of novae discovered each year is much lower, probably due to great distance and observational biases.[5] 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.[6]

Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium.[1] The contribution of novae to the interstellar medium is not great; novae supply only 1/50th the amount of material to the Galaxy as supernovae, and only 1/200th that of red giant and supergiant stars.[1]

Recurrent novae like RS Ophiuchi (those with periods on the order of decades) are rare. Astronomers theorize however that most, if not all, novae are recurrent, albeit on time scales ranging from 1,000 to 100,000 years.[7] 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.[1] Consequently, the interval is shorter for high-mass white dwarfs.[1]

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. Though this was a supernova and not a classical nova, the terms were considered interchangeable until the 1930s.[1]

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.[8]

References

1. ^ a b c d e f g h i Prialnik, Dina. "Novae", pp. 1846-56, in Paul Murdin, ed. Encyclopedia of Astronomy and Astrophysics. London: Institute of Physics Publishing Ltd and Nature Publishing Group, 2001. ISBN 1-56159-268-4

2. ^ AAVSO Variable Star Of The Month: May 2001: Novae

3. ^ Brian Warner. Cataclysmic Variable Stars. 052154209X.

4. ^ Zeilik, Michael. Conceptual Astronomy. New York: John Wiley & Sons, Inc., 1993. ISBN 0-471-50996-5

5. ^ Muirden, James. "Searching for Novae", pp. 259-79. In James Muirden, ed., Sky Watcher's Handbook. New York: W.H. Freeman and Company Ltd., 1993. ISBN 0-7167-4502-X

6. ^ W. Liller, B. Mayer, July 1987, "The rate of nova production in the Galaxy", Publications Astronomical Society of the Pacific, vol. 99, pp. 606-609.

7. ^ Seeds, Michael A. Horizons: Exploring the Universe, 5th ed. Belmont: Wadsworth Publishing Company, 1998, ISBN 0-534-52434-6, p.194.

8. ^ 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 3-540-20128-9.

Bright novae since 1890

Year	Nova	Maximum brightness
1891	T Aurigae	3.8 mag
1898	V1059 Sagittarii	4.5 mag
1899	V606 Aquilae	5.5 mag
1901	GK Persei	0.2 mag
1903	Nova Geminorum 1903	6 mag
1905	Nova Aquilae 1905	7.3 mag
1910	Nova Lacertae 1910	4.6 mag
1912	Nova Geminorum 1912	3.5 mag
1918	V603 Aquilae	−1.8 mag
1919	Nova Lyrae 1919	7.4 mag
1919	Nova Ophiuchi 1919	7.4 mag
1920	Nova Cygni 1920	2.0 mag
1925	RR Pictoris	1.2 mag
1934	DQ Herculis	1.4 mag
1936	CP Lacertae	2.1 mag
1939	BT Monocerotis	4.5 mag
1942	CP Puppis	0.3 mag
1943	Nova Aquilae 1943	6.1 mag
1950	DK Lacertae	5.0 mag
1960	V446 Herculis	2.8 mag
1963	V533 Herculis	3 mag
1970	FH Serpentis	4 mag
1975	V1500 Cygni	2.0 mag
1975	V373 Scuti	6 mag
1976	NQ Vulpeculae	6 mag
1978	V1668 Cygni	6 mag
1984	QU Vulpeculae	5.2 mag
1986	V842 Centauri	4.6 mag
1991	V838 Herculis	5.0 mag
1992	V1974 Cygni	4.2 mag
1999	V1494 Aquilae	5.03 mag
1999	V382 Velorum	2.6 mag
2006	RS Ophiuchi	4.5 mag
2007	V1280 Scorpii	3.9 mag [1],[2]

Note: Please add all novae brighter than 6 mag [3]

Recurrent novae

* RS Ophiuchi

* T Coronae Borealis

* T Pyxidis

Notes

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