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. This article is about the Greek expression For the event in the Dragonlance series of books see Cataclysm (Dragonlance. In Astrophysics, the term accretion is used for at least two distinct processes A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth 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. A Type Ia supernova is a sub-category of cataclysmic Variable The California Institute of Technology (commonly referred to as Caltech) is a private, Coeducational research university located in Pasadena MAY ( also known as: Mei メイ 메이 is a Korean singer well known in South Korea for singing the song "Miracle" Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Luminous red novae (abbrev LRNe) (sing luminous red nova) are stellar explosions thought to be caused by the merger of two Stars They are characterised

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 Roche lobe is the region of Space around a Star in a binary system within which orbiting material is gravitationally bound to that star The companion may be a main sequence star, or one that is aging and expanding into a red giant. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness A red giant is a luminous Giant star of low or intermediate mass (roughly 0 The captured gases consist primarily of hydrogen and helium, the two principal constituents of ordinary matter in the universe. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Baryons are the family of Subatomic particles with a Baryon number of 1 The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy 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. Degenerate matter is matter which has sufficiently high Density that the dominant contribution to its Pressure rises from the Pauli Exclusion 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. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus The CNO cycle (for Carbon - Nitrogen - Oxygen) or sometimes Bethe-Weizsäcker-cycle, is one of two sets of fusion reactions 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. A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature leading to a destructive result ^[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. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. 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. The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand Luminosity has different meanings in several different fields of science ^[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). RS Ophiuchi ( RS Oph) is a Nova approximately 5000 Light-years away in the Constellation Ophiuchus. Year 1898 ( MDCCCXCVIII) was a Common year starting on Saturday (link will display the full calendar of the Gregorian calendar (or a Common Year 1933 ( MCMXXXIII) was a Common year starting on Sunday (link will display full calendar of the Gregorian calendar. Year 1958 ( MCMLVIII) was a Common year starting on Wednesday (link will display full calendar of the Gregorian calendar. Year 1967 ( MCMLXVII) was a Common year starting on Sunday (link will display full calendar of the 1967 Gregorian calendar. Year 1985 ( MCMLXXXV) was a Common year starting on Tuesday (link displays 1985 Gregorian calendar) Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Eventually, the white dwarf could explode as a type Ia supernova if it exceeds the Chandrasekhar limit. A supernova (plural supernovae or supernovas) is a stellar Explosion.

Occasionally a nova is bright enough and close enough to be conspicuous to the unaided eye. The brightest recent example was Nova Cygni 1975. V1500 Cygni or Nova Cygni 1975 was a bright Nova occurring in 1975 in the Constellation Cygnus. This nova appeared on August 29, 1975 in the constellation Cygnus about five degrees north of Deneb and reached magnitude 2. Events 708 - Copper coins are minted in Japan for the first time (Traditional Japanese date: August 10, 708) Year 1975 ( MCMLXXV) was a Common year starting on Wednesday (link will display full calendar of the Gregorian calendar. Cygnus (ˈsɪgnəs Swan, Κύκνος) is a northern Constellation. Deneb (α Cyg / α Cygni / Alpha Cygni is the brightest Star in the Constellation Cygnus and one of the vertices of the Summer Triangle The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value 0 (nearly as bright as Deneb). The most recent was V1280 Scorpii which reached magnitude 3. V1280 Scorpii is a Nova observed in February 2007 in the constellation Scorpius, just south of M62. 7 on February 17, 2007. Events 1500 - Battle of Hemmingstedt. 1600 - Philosopher Giordano Bruno is burned alive at Campo de' Fiori Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century.

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. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[1] The number of novae discovered each year is much lower, probably due to great distance and observational biases. Selection bias is a distortion of evidence or data that arises from the way that the data are collected ^[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. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda ^[6]

Spectroscopic observation of nova ejecta nebulae has shown that they are enriched in elements such as helium, carbon, nitrogen, oxygen, neon, and magnesium. Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Neon (ˈniːɒn is the Chemical element that has the symbol Ne and Atomic number 10 Magnesium (mægˈniːziəm is a Chemical element with the symbol Mg, Atomic number 12 Atomic weight 24 ^[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. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 Supergiants are among the most massive Stars In the Hertzsprung-Russell diagram they occupy the top region of the diagram ^[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. Tycho Brahe, born Tyge Ottesen Brahe ( December 14 1546 &ndash October 24 1601) was a Danish nobleman A supernova (plural supernovae or supernovas) is a stellar Explosion. SN 1572 ( Tycho's Supernova, Tycho's Nova) "B Cassiopeiae" (B Cas or 3C 10 was a Supernova of Type Ia in the Cassiopeia ( colloquially /ˌkæsiˈoʊpiə/ is a northern Constellation which Greek mythology considered to represent the vain queen Cassiopeia who boasted Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. 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. The 1930s were described as an abrupt shift to more radical and conservative lifestyles as countries were struggling to find a solution to the Great Depression. ^[1]

Novae as distance indicators

Novae have some promise for use as standard candles. A standard candle is an astronomical object that has a known Luminosity. For instance, the distribution of their absolute magnitude is bimodal, with a main peak at magnitude -7. In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard 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. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Galaxy groups and clusters are the largest Gravitationally bound objects to have arisen thus far in the process of cosmic structure formation A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation For the astronomical object see Variable star. Variable Star is a 2006 novel written by Spider Robinson ^[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. Since its founding in 1911 the American Association of Variable Star Observers (AAVSO has coordinated collected evaluated analyzed published and archived Variable star 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

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. RS Ophiuchi ( RS Oph) is a Nova approximately 5000 Light-years away in the Constellation Ophiuchus. T Coronae Borealis, informally nicknamed the Blaze Star, is a recurring Nova in the Constellation Corona Borealis. T Pyxidis is a recurrent Nova and Nova remnant in the Constellation Pyxis. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics

See also

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

External links

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