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image of the supernova remnant of Johannes Kepler Supernova, SN 1604. (Chandra X-ray Observatory)A supernova (plural: supernovae or supernovas) is a Astronomy#Stellar astronomy explosion that creates an extremely luminosity object. A supernova causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. During this short interval, a supernova can Radiation as much energy as the Sun would emit over 10 billion years. The explosion expels much or all of a star's material{{cite web | date = July 27, 2006| url = http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html | title = Introduction to Supernova Remnants | publisher = [NASA [Goddard Space Flight Center | accessdate = 2006-09-07 --> at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

Several types of supernovae exist that may be triggered in one of two ways, involving either turning off or suddenly turning on the production of energy through nuclear fusion. After the core of an stellar evolution #Massive stars star ceases to generate energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing potential energy #Gravitational potential energy that heats and expels the star's outer layers. Alternatively, a white dwarf star may accumulate sufficient material from a Binary star (usually through Accretion (astrophysics), rarely via a merger) to raise its core temperature enough to Carbon detonation Carbon burning process, at which point it undergoes Thermal runaway nuclear fusion, completely disrupting it. Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the Chandrasekhar limit, while accreting white dwarfs ignite as they approach this limit (roughly 1.38 times the [solar mass). White dwarfs are also subject to a different, much smaller type of thermonuclear explosion [CNO cycle on their surfaces called a [nova. Solitary stars with a mass below approximately nine [solar masses, such as the Sun itself, evolve into white dwarfs without ever becoming supernovae.

On average, supernovae occur about once every 50 years in a galaxy the size of the Milky Way{{cite news | date=January 4, [ | url=http://www.esa.int/SPECIALS/Integral/SEMACK0VRHE_0.html | accessdate=2007-02-02 --> and play a significant role in enriching the interstellar medium with heavy chemical element. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.{{cite web | last = Allen | first = Jesse| date = February 02, [ | url = http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980202b.html | title = Supernova Effects | publisher = [NASA | accessdate = 2007-02-02 -->

Nova (plural novae) means "new" in Latin language, referring to what appears to be a very bright new star shining in the celestial sphere; the Prefix (linguistics) "super" distinguishes supernovae from ordinary novae, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism. According to Merriam-Webster's Collegiate Dictionary, the word supernova was first used in print in 1926.

Observation history is a pulsar wind nebula associated with the SN 1054.The earliest recorded supernova, SN 185, was viewed by China astronomers in 185. The widely observed supernova of SN 1054 produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the last to be observed in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotle idea that the world beyond the Moon and planets was immutable.{{cite conference | author=D. H. Clark, F. R. Stephenson| title = The Historical Supernovae | booktitle = Supernovae: A survey of current research; Proceedings of the Advanced Study Institute | pages = 355–370 | publisher = Dordrecht, D. Reidel Publishing Co. | date = June 29, [ | location = Cambridge, England | url = http://adsabs.harvard.edu/abs/1982sscr.conf..355C | accessdate = 2006-09-24 -->

Since the development of the telescope, the field of supernova discovery has enlarged to other galaxies, starting with the 1885 observation of supernova S Andromedae in the Andromeda galaxy. Supernovae provide important information on cosmological distances.{{cite web | last =van Zyl | first = Jan Eben| year = 2003 | url = http://www.aqua.co.za/assa_jhb/new/canopus/can2003/c039litu.htm | title = VARIABLE STARS VI | publisher = [Astronomical Society of Southern Africa | accessdate = 2006-09-27 --> During the twentieth century, successful models for each type of supernova were developed, and scientists' comprehension of the role of supernovae in the star formation process is growing.

Some of the most distant supernovae recently observed appeared dimmer than expected.This has provided evidence that the expansion of the Accelerating universe. {{cite news | title=Confirmation of the accelerated expansion of the Universe| publisher=[Centre National de la Recherche Scientifique | date=September 19, [ | url=http://www2.cnrs.fr/en/45.htm?&debut=160xt/ | accessdate=2006-11-03 -->

Discovery Because supernovae are relatively rare events, occurring about once every 50 years in a galaxy like the Milky Way, many galaxies must be monitored regularly in order to obtain a good sample of supernovae to study.

Supernovae in other galaxies cannot be predicted with any meaningful accuracy. When they are discovered, they are already in progress.{{cite web | last = Bishop | first = David| url = http://www.rochesterastronomy.org/snthemes/main/images/ | title = Latest Supernovae | publisher = Rochester's Astronomy Club | accessdate = 2006-11-28 --> Most scientific interest in supernovae—as standard candles for measuring distance, for example—require an observation of their peak luminosity. It is therefore important to discover them well before they reach their maximum. Amateur astronomy, who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an optical telescope and comparing them to earlier photographs.

Towards the end of the 20th century, astronomers increasingly turned to computer-controlled telescopes and charge-coupled device for hunting supernovae. While such systems are popular with amateurs, there are also larger installations like the Katzman Automatic Imaging Telescope.{{cite web | last=Evans | first=Robert O.| year=1993 | url = http://www.aavso.org/observing/programs/sn/ | title = Supernova Search Manual, 1993 | publisher = [American Association of Variable Star Observers (AAVSO) | accessdate = 2006-10-05 --> Recently, the Supernova Early Warning System (SNEWS) project has also begun using a network of neutrino detectors to give early warning of a supernova in the Milky Way galaxy. {{cite web | url = http://snews.bnl.gov/| title = SNWES: Supernova Early Warning System | publisher = [National Science Foundation | accessdate = 2006-11-28 --> A neutrino is a Subatomic particle that is produced in great quantities by a supernova explosion, and it is not obscured by the interstellar gas and dust of the galactic disk.

Supernova searches fall into two classes: those focused on relatively nearby events and those looking for explosions farther away. Because of the Metric expansion of space, the distance to a remote object with a known emission spectrum can be estimated by measuring its Doppler shift (or redshift); on average, more distant objects recede with greater velocity than those nearby, and so have a higher redshift. Thus the search is split between high redshift and low redshift, with the boundary falling around a redshift range of z = 0.1–0.3{{cite web| last = Frieman | first = Josh | year=2006 | url =http://sdssdp47.fnal.gov/sdsssn/sdsssn.html | title =SDSS Supernova Survey | publisher =SDSS | accessdate = 2006-08-10 -->—where z is a dimensionless measure of the spectrum's frequency shift.

High redshift searches for supernovae usually involve the observation of supernova light curves. These are useful for standard or calibrated candles to generate Hubble diagrams and make cosmological predictions. At low redshift, supernova spectroscopy is more practical than at high redshift, and this is used to study the physics and environments of supernovae.{{cite web | last = Perlmutter | first = Saul| url = http://www-supernova.lbl.gov/public/ | title = High Redshift Supernova Search | publisher = [Lawrence Berkeley National Laboratory | accessdate = 2006-10-09 --> Low redshift observations also anchor the low distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies. {{cite web | last = Aldering | first = Greg| date = December 1, [ | url = http://snfactory.lbl.gov/ | title = The Nearby Supernova Factory | publisher = [Lawrence Berkeley National Laboratory | accessdate = 2006-12-01 -->

Naming convention in the NGC 4526 galaxy (bright spot on the lower left). Image by NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search TeamSupernova discoveries are reported to the International Astronomical Union's Central Bureau for Astronomical Telegrams, which sends out a circular with the name it assigns to it. The name is formed by the year of discovery, immediately followed by a one or two-letter designation. The first 26 supernovae of the year get designated with an upper case letter from A to Z. Afterward, pairs of lower-case letters are used, starting with aa, ab, and so on.{{cite web | url = http://www.cfa.harvard.edu/iau/lists/RecentSupernovae.html| title = List of Recent Supernovae | publisher = [Harvard-Smithsonian Center for Astrophysics | accessdate = 2007-10-16 --> Professional and amateur astronomers find several hundred supernovae per year (in recent years: 367 in 2005 and 551 in 2006). For example, the last supernova of 2005 was SN 2005nc, indicating that it was the 367th supernova found in 2005.{{cite web | url = http://www.cfa.harvard.edu/iau/lists/Supernovae.html| title = List of Supernovae | publisher = [International Astronomical Union (IAU) Central Bureau for Astronomical Telegrams | accessdate = 2007-10-16 --> {{cite web | url = http://web.oapd.inaf.it/supern/snean.txt| title = The Padova-Asiago supernova catalogue | publisher = Astronomical Observatory of Padua | accessdate = 2006-11-28 -->

Historical supernovae are known simply by the year they occurred: SN 185, SN 1006, SN 1054, SN 1572 (Tycho's Nova), and SN 1604 (Kepler's Star). Beginning in 1885, the letter notation is used, even if there was only one supernova discovered that year (e.g. SN 1885A, 1907A, etc.)—this last happened with SN 1947A. The standard abbreviation "SN" is an optional prefix.

Classification As part of the attempt to understand supernovae, astronomers have classified them according to the absorption lines of different chemical elements that appear in their Astronomical spectroscopy. The first element for a division is the presence or absence of a line caused by hydrogen. If a supernova's spectrum contains a line of hydrogen (known as the Balmer series in the visual portion of the spectrum) it is classified Type II; otherwise it is Type I. Among those types, there are subdivisions according to the presence of lines from other elements and the shape of the light curve (a graph of the supernova's apparent magnitude versus time).{{cite conference | author=E. Cappellaro, M. Turatto| title=Supernova Types and Rates | booktitle=Influence of Binaries on Stellar Population Studies | publisher=Dordrecht: Kluwer Academic Publishers | date=August 08, [ | location=Brussels, Belgium | url=http://adsabs.harvard.edu/abs/2000astro.ph.12455C | accessdate=2006-09-15 --> {| class="wikitable"|+Supernova taxonomy{{cite web | last = Montes | first = M.| date = February 12, [ | url = http://rsd-www.nrl.navy.mil/7212/montes/snetax.html | title = Supernova Taxonomy | publisher = [Naval Research Laboratory | accessdate = 2006-11-09 --> !Type!Characteristics|-|colspan="2" style="background: #EEEEEE; text-align: center"|Type I|-|Type Ia supernova|Lacks hydrogen and presents a singly-ionization silicon (Si II) line at 615.0 1 E-9 m, near peak light.|-|Type Ib and Ic supernovae|Non-ionized helium (He I) line at 587.6 nm and no strong silicon absorption feature near 615 nm.]|Weak or no helium lines and no strong silicon absorption feature near 615 nm.|-|colspan="2" style="background: #EEEEEE; text-align: center"|Type II|-|Type II supernova|Reaches a "plateau" in its light curve|-|Type II supernova|Displays a "linear" decrease in its light curve (linear in magnitude versus time). |}

The supernovae of Type II can also be sub-divided based on their spectra. While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of kilometres per second, some have relatively narrow features. These are called Type IIn, where the "n" stands for "narrow".

A few supernovae, such as SN 1987K and SN 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type IIb" is used to describe the combination of features normally associated with Types II and Ib.

Current models Type Ia There are several means by which a supernova of this type can form, but they share a common underlying mechanism. If a carbon-oxygen white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1.38 solar masses (for a non-rotating star), it would no longer be able to support the bulk of its plasma through electron degeneracy pressure and would begin to collapse. However, the current view is that this limit is not normally attained; increasing temperature and density inside the core Carbon detonation Carbon burning process as the star approaches the limit (to within about 1%{{cite book | last = http://www.as.utexas.edu/~wheel/ J. Craig Wheeler | first = | title = Cosmic Catastrophes: Supernovae, Gamma-Ray Bursts, and Adventures in Hyperspace | publisher = [Cambridge University Press | date = 2000-01-15 | location = Cambridge, UK | pages = p. 96 | url = http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521857147 | isbn = 0521651956-->), before collapse is initiated. Within a few seconds, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (1–2 × 1044 [joules) to unbind the star in a supernova explosion. An outwardly expanding shock wave is generated, with matter reaching velocities on the order of 5,000–20,000 km/s, or roughly 3% of the speed of light. There is also a significant increase in luminosity, reaching an absolute magnitude of -19.3 (or 5 billion times brighter than the Sun), with little variation.

One model for the formation of this category of supernova is a close binary star system. The larger of the two stars is the first to evolve off the main sequence, and it expands to form a red giant.{{cite web| last = Richmond | first = Michael | url = http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html | title = Late stages of evolution for low-mass stars | publisher = Rochester Institute of Technology | accessdate = 2006-08-04 --> The two stars now share a common envelope, causing their mutual orbit to shrink. The giant star then sheds most of its envelope, losing mass until it can no longer continue [nuclear fusion. At this point it becomes a white dwarf star, composed primarily of carbon and oxygen. {{cite conference | first = B. | last = Paczynski| title = Common Envelope Binaries | booktitle = Structure and Evolution of Close Binary Systems | pages = 75–80 | publisher = Dordrecht, D. Reidel Publishing Co. | date = July 28 – [August 1, [ | location = Cambridge, England | url = http://adsabs.harvard.edu/abs/1976IAUS...73...75P | accessdate = 2007-01-08 --> {{cite web | author=K. A. Postnov, L. R. Yungelson| year = 2006 | url = http://relativity.livingreviews.org/open?pubNo=lrr-2006-6&page=articlesu8.html | title = The Evolution of Compact Binary Star Systems | publisher = Living Reviews in Relativity | accessdate = 2007-01-08 --> Eventually the secondary star also evolves off the main sequence to form a red giant. Matter from the giant is accreted by the white dwarf, causing the latter to increase in mass.

Another model for the formation of a Type Ia explosion involves the merger of two white dwarf stars, with the combined mass momentarily exceeding the Chandrasekhar limit.{{cite web | author=Staff| url =http://cosmos.swin.edu.au/entries/typeiasupernovaprogenitors/typeiasupernovaprogenitors.html?e=1| title =Type Ia Supernova Progenitors| publisher =Swinburne University | accessdate = 2007-05-20 --> A white dwarf could also accrete matter from other types of companions, including a main sequence star (if the orbit is sufficiently close).

Type Ia supernovae follow a characteristic light curve—the graph of luminosity as a function of time—after the explosion. This luminosity is generated by the radioactive decay of nickel-56 through cobalt-56 to iron-56. The peak luminosity of the light curve is consistent across Type Ia supernovae (the vast majority of which are initiated with a uniform mass via the accretion mechanism), allowing them to be used as a secondary standard candle to measure the distance to their hostgalaxy.

Type Ib and Ic These events, like supernovae of Type II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types Ib and Ic have lost most of their outer (hydrogen) envelopes due to strong stellar winds or else from interaction with a companion.{{cite conference | last = Pols | first = Onno| title = Close Binary Progenitors of Type Ib/Ic and IIb/II-L Supernovae | booktitle = Proceedings of the The Third Pacific Rim Conference on Recent Development on Binary Star Research | pages = 153–158 | date = October 26 – [November 1, [ | location = Chiang Mai, Thailand | url = http://adsabs.harvard.edu/abs/1997rdbs.conf..153P | accessdate = 2006-11-29 --> Type Ib supernovae are thought to be the result of the collapse of a massive Wolf-Rayet star. There is some evidence that a few percent of the Type Ic supernovae may be the progenitors of gamma ray bursts (GRB), though it is also believed that any hydrogen-stripped, Type Ib or Ic supernova could be a GRB, dependent upon the geometry of the explosion.

Type II

Stars with at least nine solar masses of material evolve in a complex fashion. In the core of the star, hydrogen is fused into helium and the energy released creates an outward pressure, which maintains the core in [hydrostatic equilibrium and prevents collapse.

When the core's supply of hydrogen is exhausted, this outward pressure is no longer created. The core begins to gravitational collapse inwardly, causing a rise in temperature and pressure which becomes great enough to ignite the helium and start a helium-to-carbon fusion cycle, creating sufficient outward pressure to halt the collapse. The core expands and cools slightly, with a hydrogen-fusion outer layer, and a hotter, higher pressure, helium-fusion center. (Other elements such as magnesium, sulfur and calcium are also created and in some cases burned in these further reactions.)

This process repeats several times, and each time the core collapses and the collapse is halted by the ignition of a further process involving more massive nuclei and higher temperatures and pressures. Each layer is prevented from collapse by the heat and outward pressure of the fusion process in the next layer inward; each layer also burns hotter and quicker than the previous one – the final burn of silicon to nickel consumes its fuel in around one day, or a few days. The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells.{{cite web | last = Richmond | first = Michael| url = http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html | title = Late stages of evolution for low-mass stars | publisher = [Rochester Institute of Technology | accessdate = 2006-08-04 --> {{cite web | last = Hinshaw | first = Gary| date = August 23, [ | url = http://map.gsfc.nasa.gov/m_uni/uni_101stars.html | title = The Life and Death of Stars | publisher = [NASA [Wilkinson Microwave Anisotropy Probe (WMAP) Mission | accessdate = 2006-09-01 -->

In the later stages, increasingly heavier elements undergo nuclear fusion, and the binding energy of the relevant nuclei increases. Fusion produces progressively lower levels of energy, and also at higher core energies photodisintegration and electron capture occur which cause energy loss in the core and a general acceleration of the fusion processes to maintain equilibrium. This escalation culminates with the silicon burning process, which is unable to produce energy through fusion (but does produce iron-56 through radioactive decay). As a result, a nickel-iron core{{cite web| last=Fleurot | first=Fabrice | year=1988 | url=http://nu.phys.laurentian.ca/~fleurot/evolution/ | title=Evolution of Massive Stars | publisher=Laurentian University | accessdate=2007-08-13 --> builds up that cannot produce any further outward pressure on a scale needed to support the rest of the structure. It can only support the overlaying mass of the star through the [degeneracy pressure of [electrons in the core. If the star is sufficiently large, then the iron-nickel core will eventually exceed the [Chandrasekhar limit (1.38 [solar masses), at which point this mechanism catastrophically fails. The forces holding atomic nuclei apart in the innermost layer of the core suddenly give way, the core [Implosion (mechanical process) due to its own mass, and no further fusion process can ignite or prevent collapse this time.

Core collapse The core collapses in on itself with velocities reaching 70,000 km/s (0.23Speed of light),{{cite web | author=C. L. Fryer, K. C. B. New| date = January 24, [ | url = http://relativity.livingreviews.org/Articles/lrr-2003-2/ | title = Gravitational Waves from Gravitational Collapse | publisher = [Max Planck Institute for Gravitational Physics | accessdate = 2006-12-14 --> resulting in a rapid increase in temperature and density. The energy loss processes operating in the core cease to be in equilibrium. Through photodisintegration, gamma rays decompose iron into helium nuclei and free neutrons, absorbing energy, whilst electrons and protons merge via electron capture, producing neutrons and electron neutrinos which escape. About 1046 joules of gravitational energy—about 10% of the star's rest mass—is converted into a ten-second burst of neutrinos; the main output of the event.{{cite web , | url = http://www.aps.org/policy/reports/multidivisional/neutrino/upload/Neutrino_Astrophysics_and_Cosmology_Working_Group.pdf | title = APS Neutrino Study: Report of the Neutrino Astrophysics and Cosmology Working Group | publisher = [American Physical Society | format=PDF | accessdate = 2006-12-12 --> These carry away energy from the core and accelerate the collapse, while some neutrinos are absorbed by the star's outer layers and begin the supernova explosion.

The inner core eventually reaches typically 30 km diameter, and a density comparable to that of an atomic nucleus, and further collapse is abruptly stopped by strong force interactions and by degeneracy pressure of neutrons. The infalling matter, suddenly halted, rebounds, producing a shock wave that propagates outward. Computer simulations indicate that this expanding shock does not directly cause the supernova explosion; rather, it stalls within millisecondshttp://adsabs.harvard.edu/abs/1990ApJ...364..222M in the outer core as energy is lost through the dissociation of heavy elements, and a process that is not clearly understood is necessary to allow the outer layers of the core to reabsorb around 1044 joules (1 Foe (unit of energy)) of energy, producing the visible explosion.{{cite web | author = C. L. Fryer, K. B. C. New| date = January 24, [ | url = http://relativity.livingreviews.org/open?pubNo=lrr-2003-2&page=articlesu6.html | title = Gravitational Waves from Gravitational Collapse, section 3.1 | publisher = [Los Alamos National Laboratory | accessdate = 2006-12-09 --> Current research focusses upon rotational and magnetic field effects as the basis for this process.

]

When the progenitor star is below about 20 solar masses (depending on the strength of the explosion and the amount of material that falls back), the degenerate remnant of a core collapse is a neutron star. Above this mass the remnant collapses to form a black hole. (This type of collapse is one of many candidate explanations for gamma ray bursts—producing a large burst of gamma rays through a still theoretical hypernova explosion.){{cite news (ESO) | date=June 18, [ | url=http://www.eso.org/outreach/press-rel/pr-2003/pr-16-03.html | accessdate=2006-10-30 --> The theoretical limiting mass for this type of core collapse scenario was estimated around 40–50 solar masses.

Above 50 solar masses, stars were believed to collapse directly into a black hole without forming a supernova explosion, although uncertainties in models of supernova collapse make accurate calculation of these limits difficult. In fact recent evidence has shown stars in the range of about 140–250 solar masses, with a relatively low proportion of elements more massive than helium, may be capable of forming pair-instability supernovae without leaving behind a black hole remnant. This rare type of supernova is formed by an alternate mechanism (partially analogous to that of Type Ia explosions) that does not require an iron core. An example is the Type II supernova SN 2006gy, with an estimated 150 solar masses, that demonstrated the explosion of such a massive star differed fundamentally from previous theoretical predictions.{{cite web| last = Boen | first = Brooke | date = May 5, 2007 | url = http://www.nasa.gov/mission_pages/chandra/news/chandra_bright_supernova.html | title = NASA's Chandra Sees Brightest Supernova Ever | publisher = NASA | accessdate = 2007-08-09 -->{{cite news | first=Robert | last=Sanders | title=Largest, brightest supernova ever seen may be long-sought pair-instability supernova | publisher=University of California, Berkeley | date=May 7, [ | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/ | accessdate=2006-05-24 -->

Light curves and unusual spectra The light curves for Type II supernovae are distinguished by the presence of hydrogen Balmer series in the spectra. These light curves have an average decay rate of 0.008 absolute magnitude per day; much lower than the decay rate for Type I supernovae. Type II are sub-divided into two classes, depending on whether there is a plateau in their light curve (Type II-P) or a linear decay rate (Type II-L). The net decay rate is higher at 0.012 magnitudes per day for Type II-L compared to 0.0075 magnitudes per day for Type II-P. The difference in the shape of the light curves is believed to be caused, in the case of Type II-L supernovae, by the expulsion of most of the hydrogen envelope of the progenitor star.

The plateau phase in Type II-P supernovae is due to a change in the opacity (optics) of the exterior layer. The shock wave ionizes the hydrogen in the outer envelope, which greatly increases the opacity. This prevents photons from the inner parts of the explosion from escaping. Once the hydrogen cools sufficiently to recombine, the outer layer becomes transparent.{{cite web | url = http://cosmos.swin.edu.au/lookup.html?e=typeiisupernovalightcurves| title = Type II Supernova Light Curves | publisher = [Swinburne University of Technology | accessdate = 2007-03-17 -->

Of the Type II supernovae with unusual features in their spectra, Type IIn supernovae may be produced by the interaction of the ejecta with circumstellar material. Type IIb supernovae are likely massive stars which have lost most, but not all, of their hydrogen envelopes through tidal force by a companion star. As the ejecta of a Type IIb expands, the hydrogen layer quickly becomes optically thin and reveals the deeper layers.

Asymmetry A long-standing puzzle surrounding supernovae has been a need to explain why the compact object remaining after the explosion is given a large velocity away from the core.{{cite book | editor=P. Hoflich, P. Kumar, J. C. Wheeler| title=Cosmic explosions in three dimensions: asymmetries in supernovae and gamma-ray bursts | chapter=Neutron star kicks and supernova asymmetry | publisher=[Cambridge University Press | location=Cambridge | year=2004 | pages=276 | url=http://adsabs.harvard.edu/abs/2004cetd.conf..276L | accessdate = 2007-02-01 --> (Neutron stars are observed, as pulsars, to have high velocities; black holes presumably do as well, but are far harder to observe in isolation.) This kick can be substantial, propelling an object of more than a solar mass at a velocity of 500 km/s or greater. This displacement is believed to be caused by an asymmetry in the explosion, but the mechanism by which this momentum is transferred to the compact object has remained a puzzle. Some explanations for this kick include convection in the collapsing star and jet production during neutron star formation.

(blue) and optical (red) radiation from the Crab Nebula's core region. A pulsar near the center is propelling particles to almost the speed of light.{{cite web , | url = http://hubblesite.org/newscenter/archive/releases/2002/24/text/ | title = Space Movie Reveals Shocking Secrets of the Crab Pulsar | publisher = [NASA | accessdate = 2006-08-10 --> This neutron star is travelling at an estimated 375 km/s. NASA/CXC/HST/ASU/J. Hester et al. image credit.One explanation for the asymmetry in the explosion is large-scale convection above the core. The convection can create variations in the local abundances of elements, resulting in uneven nuclear burning during the collapse, bounce and resulting explosion.

Another explanation is that accretion of gas onto the central neutron star can create a accretion disk that drives highly directional jets, propelling matter at a high velocity out of the star, and driving transverse shocks that completely disrupt the star. These jets might play a crucial role in the resulting supernova explosion.{{cite news | title=Jets, Not Neutrinos, May Cause Supernova Explosions, Scientists Say| publisher=[McDonald Observatory | date=March 2, [ | url=http://mcdonaldobservatory.org/news/releases/2000/0302a.html | accessdate=2006-12-11 --> {{cite web | last = Foust | first = Jeff| date = January 9, [ | url = http://spaceflightnow.com/news/n0101/09supernova/ | title = Evidence presented for new supernova explosion model | publisher = Spaceflight Now | accessdate = 2006-12-13 --> (A similar model is now favored for explaining long gamma ray bursts.)

Initial asymmetries have also been confirmed in Type Ia supernova explosions through observation. This result may mean that the initial luminosity of this type of supernova may depend on the viewing angle. However, the explosion becomes more symmetrical with the passage of time. Early asymmetries are detectable by measuring the polarization of the emitted light.{{cite news (ESO) | date=August 6, [ | url=http://www.eso.org/outreach/press-rel/pr-2003/pr-23-03.html | accessdate=2006-12-11 -->

Type Ia versus core collapse Because they have a similar functional model, Types Ib, Ic and various Types II supernovae are collectively called Core Collapse supernovae. A fundamental difference between Type Ia and Core Collapse supernovae is the source of energy for the radiation emitted near the peak of the light curve. The progenitors of Core Collapse supernovae are stars with extended envelopes that can attain a degree of transparency with a relatively small amount of expansion. Most of the energy powering the emission at peak light is derived from the shock wave that heats and ejects the envelope.{{cite conference | first = B. | last = Leibundgut| title = Observations of Supernovae | booktitle = Proceedings of the NATO Advanced Study Institute on the Lives of the Neutron Stars | pages = 3 | publisher = Kluwer Academic | date = August 29 – [September 12, [ | location = Kemer, Turkey | url = http://adsabs.harvard.edu/abs/1995lns..conf....3L | accessdate = 2006-12-18 | id = ISBN 0-7923-324-6-6 -->

The progenitors of Type Ia supernovae, on the other hand, are compact objects, much smaller (but more massive) than the Sun, that must expand (and therefore cool) enormously before becoming transparent. Heat from the explosion is dissipated in the expansion and is not available for light production. The radiation emitted by Type Ia supernovae is thus entirely attributable to the decay of radionuclides produced in the explosion; principally nickel-56 (with a half-life of 6.1 days) and its daughter cobalt-56 (with a half-life of 77 days). Gamma rays emitted during this nuclear decay are absorbed by the ejected material, heating it to incandescence.

As the material ejected by a Core Collapse supernova expands and cools, radioactive decay eventually takes over as the main energy source for light emission in this case also. A bright Type Ia supernova may expel 0.5–1.0 solar masses of nickel-56, while a Core Collapse supernova probably ejects closer to 0.1 solar mass of nickel-56.

Interstellar impact Source of heavy elements Supernovae are a key source of chemical element heavier than oxygen. These elements are produced by nuclear fusion (for iron-56 and lighter elements), and by nucleosynthesis during the supernova explosion for elements heavier than iron. Supernova are the most likely, although not undisputed, candidate sites for the r-process, which is a rapid form of nucleosynthesis that occurs under conditions of high temperature and high density of neutrons. The reactions produce highly unstable atomic nucleus that are rich in neutrons. These forms are unstable and rapidly beta decay into more stable forms.

The r-process reaction, which is likely to occur in type II supernovae, produces about half of all the element abundance beyond iron, including plutonium, uranium and californium. The only other major competing process for producing elements heavier than iron is the s-process in large, old red giant stars, which produces these elements much more slowly, and which cannot produce elements heavier than lead.

Role in stellar evolution The remnant of a supernova explosion consists of a compact object and a rapidly expanding shock wave of material. This cloud of material sweeps up the surrounding interstellar medium during a free expansion phase, which can last for up to two centuries. The wave then gradually undergoes a period of adiabatic process, and will slowly cool and mix with the surrounding interstellar medium over a period of about 10,000 years.{{cite web | date = September 7, 2006| url = http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html | title = Introduction to Supernova Remnants | publisher = High Energy Astrophysics Science Archive Research Center, [NASA (HEASARC) | accessdate = 2006-10-20 -->

In standard astronomy, the Big Bang produced hydrogen, helium, and traces of lithium, while all heavier elements are synthesized in stars and supernovae. Supernovae tend to enrich the surrounding interstellar medium with metals, which for astronomers means all of the elements other than hydrogen and helium and is a different definition than that used in chemistry.

. NASA image.These injected elements ultimately enrich the molecular clouds that are the sites of star formation.{{cite web , | url = http://www.space.com/scienceastronomy/060619_mystery_monday.html | title = Explosive Debate: Supernova Dust Lost and Found | publisher = [space.com | accessdate = 2006-12-01 --> Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. Supernovae are the dominant mechanism for distributing these heavier elements, which are formed in a star during its period of nuclear fusion, throughout space. The different abundances of elements in the material that forms a star have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.

The kinetic energy of an expanding supernova remnant can trigger star formation due to compression of nearby, dense molecular clouds in space. The increase in turbulent pressure can also prevent star formation if the cloud is unable to lose the excess energy.

Evidence from daughter products of short-lived radioactive isotopes shows that a nearby supernova helped determine the composition of the Solar System 4.5 billion years ago, and may even have triggered the formation of this system.{{cite web , | url = http://www.psrd.hawaii.edu/May03/SolarSystemTrigger.html | title = Triggering the Formation of the Solar System | publisher = Planetary Science Research | accessdate = 2006-10-20 --> Supernova production of heavy elements over astronomic periods of time ultimately made the biochemistry on Earth possible.

Impact on Earth A near-Earth supernova is an explosion resulting from the death of a star that occurs close enough to the Earth (roughly fewer than 100 light-years away) to have noticeable effects on its biosphere. Gamma rays are responsible for most of the adverse effects a supernova can have on a living terrestrial planet. In Earth's case, gamma rays induce a chemical reaction in the upper Earth's atmosphere, converting molecular nitrogen into nitrogen oxides, depleting the ozone layer enough to expose the surface to harmful Sun and cosmic radiation. The gamma ray burst from a nearby supernova explosion has been proposed as the cause of the Ordovician-Silurian extinction events, which resulted in the death of nearly 60% of the oceanic life on Earth.

Speculation as to the effects of a nearby supernova on Earth often focuses on large stars as Type II supernova candidates. Several prominent stars within a few hundred light years from the Sun are candidates for becoming supernovae in as little as a millennium. One example is Betelgeuse, a red supergiant 427 light-years from Earth.{{cite web ], 2005| url = http://chandra.harvard.edu/resources/faq/sources/snr/snr-5.html | title = Supernova Remnants and Neutron Stars | publisher = [Harvard-Smithsonian Center for Astrophysics | accessdate = 2006-06-08 --> Though spectacular, these "predictable" supernovae are thought to have little potential to affect Earth.

Recent estimates predict that a Type II supernova would have to be closer than eight parsecs (26 light-years) to destroy half of the Earth's ozone layer.{{Cite journal | url=http://xxx.lanl.gov/abs/astro-ph/0211361| title=Ozone Depletion from Nearby Supernovae | first=Neil | last=Gehrels | coauthors=Claude M. Laird ''et al'' | journal=[Astrophysical Journal | date=March 10, [ | volume=585 | pages= 1169–1176 | accessdate = 2007-02-01 --> Such estimates are mostly concerned with atmospheric modeling and considered only the known radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud. Estimates of the rate of supernova occurrence within 10 parsecs of the Earth vary from once every 100 million years to once every one to ten billion years.

Type Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because Type Ia supernovae arise from dim, common white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably and take place in a star system that is not well studied. One theory suggests that a Type Ia supernova would have to be closer than a thousand parsecs (3300 light-years) to affect the Earth.{{cite web | url=http://www.tass-survey.org/richmond/answers/snrisks.txt| title=Will a Nearby Supernova Endanger Life on Earth? | first=Michael | last=Richmond | year=April 8, [ | format=TXT | accessdate=2006-03-30 --> The closest known candidate is IK Pegasi (see below).

In 1996, astronomers at the University of Illinois at Urbana-Champaign theorized that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Subsequently, Iron#Isotopes enrichment has been reported in deep-sea rock of the Pacific Ocean by researc image of the supernova remnant of Johannes Kepler Supernova, SN 1604. (Chandra X-ray Observatory)A supernova (plural: supernovae or supernovas) is a Astronomy#Stellar astronomy explosion that creates an extremely luminosity object. A supernova causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. During this short interval, a supernova can Radiation as much energy as the Sun would emit over 10 billion years. The explosion expels much or all of a star's material{{cite web | date = July 27, 2006| url = http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html | title = Introduction to Supernova Remnants | publisher = [NASA [Goddard Space Flight Center | accessdate = 2006-09-07 --> at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

Several types of supernovae exist that may be triggered in one of two ways, involving either turning off or suddenly turning on the production of energy through nuclear fusion. After the core of an stellar evolution #Massive stars star ceases to generate energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing potential energy #Gravitational potential energy that heats and expels the star's outer layers. Alternatively, a white dwarf star may accumulate sufficient material from a Binary star (usually through Accretion (astrophysics), rarely via a merger) to raise its core temperature enough to Carbon detonation Carbon burning process, at which point it undergoes Thermal runaway nuclear fusion, completely disrupting it. Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the Chandrasekhar limit, while accreting white dwarfs ignite as they approach this limit (roughly 1.38 times the [solar mass). White dwarfs are also subject to a different, much smaller type of thermonuclear explosion [CNO cycle on their surfaces called a [nova. Solitary stars with a mass below approximately nine [solar masses, such as the Sun itself, evolve into white dwarfs without ever becoming supernovae.

On average, supernovae occur about once every 50 years in a galaxy the size of the Milky Way{{cite news | date=January 4, [ | url=http://www.esa.int/SPECIALS/Integral/SEMACK0VRHE_0.html | accessdate=2007-02-02 --> and play a significant role in enriching the interstellar medium with heavy chemical element. Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.{{cite web | last = Allen | first = Jesse| date = February 02, [ | url = http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980202b.html | title = Supernova Effects | publisher = [NASA | accessdate = 2007-02-02 -->

Nova (plural novae) means "new" in Latin language, referring to what appears to be a very bright new star shining in the celestial sphere; the Prefix (linguistics) "super" distinguishes supernovae from ordinary novae, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism. According to Merriam-Webster's Collegiate Dictionary, the word supernova was first used in print in 1926.

Observation history is a pulsar wind nebula associated with the SN 1054.The earliest recorded supernova, SN 185, was viewed by China astronomers in 185. The widely observed supernova of SN 1054 produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the last to be observed in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotle idea that the world beyond the Moon and planets was immutable.{{cite conference | author=D. H. Clark, F. R. Stephenson| title = The Historical Supernovae | booktitle = Supernovae: A survey of current research; Proceedings of the Advanced Study Institute | pages = 355–370 | publisher = Dordrecht, D. Reidel Publishing Co. | date = June 29, [ | location = Cambridge, England | url = http://adsabs.harvard.edu/abs/1982sscr.conf..355C | accessdate = 2006-09-24 -->

Since the development of the telescope, the field of supernova discovery has enlarged to other galaxies, starting with the 1885 observation of supernova S Andromedae in the Andromeda galaxy. Supernovae provide important information on cosmological distances.{{cite web | last =van Zyl | first = Jan Eben| year = 2003 | url = http://www.aqua.co.za/assa_jhb/new/canopus/can2003/c039litu.htm | title = VARIABLE STARS VI | publisher = [Astronomical Society of Southern Africa | accessdate = 2006-09-27 --> During the twentieth century, successful models for each type of supernova were developed, and scientists' comprehension of the role of supernovae in the star formation process is growing.

Some of the most distant supernovae recently observed appeared dimmer than expected.This has provided evidence that the expansion of the Accelerating universe. {{cite news | title=Confirmation of the accelerated expansion of the Universe| publisher=[Centre National de la Recherche Scientifique | date=September 19, [ | url=http://www2.cnrs.fr/en/45.htm?&debut=160xt/ | accessdate=2006-11-03 -->

Discovery Because supernovae are relatively rare events, occurring about once every 50 years in a galaxy like the Milky Way, many galaxies must be monitored regularly in order to obtain a good sample of supernovae to study.

Supernovae in other galaxies cannot be predicted with any meaningful accuracy. When they are discovered, they are already in progress.{{cite web | last = Bishop | first = David| url = http://www.rochesterastronomy.org/snthemes/main/images/ | title = Latest Supernovae | publisher = Rochester's Astronomy Club | accessdate = 2006-11-28 --> Most scientific interest in supernovae—as standard candles for measuring distance, for example—require an observation of their peak luminosity. It is therefore important to discover them well before they reach their maximum. Amateur astronomy, who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an optical telescope and comparing them to earlier photographs.

Towards the end of the 20th century, astronomers increasingly turned to computer-controlled telescopes and charge-coupled device for hunting supernovae. While such systems are popular with amateurs, there are also larger installations like the Katzman Automatic Imaging Telescope.{{cite web | last=Evans | first=Robert O.| year=1993 | url = http://www.aavso.org/observing/programs/sn/ | title = Supernova Search Manual, 1993 | publisher = [American Association of Variable Star Observers (AAVSO) | accessdate = 2006-10-05 --> Recently, the Supernova Early Warning System (SNEWS) project has also begun using a network of neutrino detectors to give early warning of a supernova in the Milky Way galaxy. {{cite web | url = http://snews.bnl.gov/| title = SNWES: Supernova Early Warning System | publisher = [National Science Foundation | accessdate = 2006-11-28 --> A neutrino is a Subatomic particle that is produced in great quantities by a supernova explosion, and it is not obscured by the interstellar gas and dust of the galactic disk.

Supernova searches fall into two classes: those focused on relatively nearby events and those looking for explosions farther away. Because of the Metric expansion of space, the distance to a remote object with a known emission spectrum can be estimated by measuring its Doppler shift (or redshift); on average, more distant objects recede with greater velocity than those nearby, and so have a higher redshift. Thus the search is split between high redshift and low redshift, with the boundary falling around a redshift range of z = 0.1–0.3{{cite web| last = Frieman | first = Josh | year=2006 | url =http://sdssdp47.fnal.gov/sdsssn/sdsssn.html | title =SDSS Supernova Survey | publisher =SDSS | accessdate = 2006-08-10 -->—where z is a dimensionless measure of the spectrum's frequency shift.

High redshift searches for supernovae usually involve the observation of supernova light curves. These are useful for standard or calibrated candles to generate Hubble diagrams and make cosmological predictions. At low redshift, supernova spectroscopy is more practical than at high redshift, and this is used to study the physics and environments of supernovae.{{cite web | last = Perlmutter | first = Saul| url = http://www-supernova.lbl.gov/public/ | title = High Redshift Supernova Search | publisher = [Lawrence Berkeley National Laboratory | accessdate = 2006-10-09 --> Low redshift observations also anchor the low distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies. {{cite web | last = Aldering | first = Greg| date = December 1, [ | url = http://snfactory.lbl.gov/ | title = The Nearby Supernova Factory | publisher = [Lawrence Berkeley National Laboratory | accessdate = 2006-12-01 -->

Naming convention in the NGC 4526 galaxy (bright spot on the lower left). Image by NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search TeamSupernova discoveries are reported to the International Astronomical Union's Central Bureau for Astronomical Telegrams, which sends out a circular with the name it assigns to it. The name is formed by the year of discovery, immediately followed by a one or two-letter designation. The first 26 supernovae of the year get designated with an upper case letter from A to Z. Afterward, pairs of lower-case letters are used, starting with aa, ab, and so on.{{cite web | url = http://www.cfa.harvard.edu/iau/lists/RecentSupernovae.html| title = List of Recent Supernovae | publisher = [Harvard-Smithsonian Center for Astrophysics | accessdate = 2007-10-16 --> Professional and amateur astronomers find several hundred supernovae per year (in recent years: 367 in 2005 and 551 in 2006). For example, the last supernova of 2005 was SN 2005nc, indicating that it was the 367th supernova found in 2005.{{cite web | url = http://www.cfa.harvard.edu/iau/lists/Supernovae.html| title = List of Supernovae | publisher = [International Astronomical Union (IAU) Central Bureau for Astronomical Telegrams | accessdate = 2007-10-16 --> {{cite web | url = http://web.oapd.inaf.it/supern/snean.txt| title = The Padova-Asiago supernova catalogue | publisher = Astronomical Observatory of Padua | accessdate = 2006-11-28 -->

Historical supernovae are known simply by the year they occurred: SN 185, SN 1006, SN 1054, SN 1572 (Tycho's Nova), and SN 1604 (Kepler's Star). Beginning in 1885, the letter notation is used, even if there was only one supernova discovered that year (e.g. SN 1885A, 1907A, etc.)—this last happened with SN 1947A. The standard abbreviation "SN" is an optional prefix.

Classification As part of the attempt to understand supernovae, astronomers have classified them according to the absorption lines of different chemical elements that appear in their Astronomical spectroscopy. The first element for a division is the presence or absence of a line caused by hydrogen. If a supernova's spectrum contains a line of hydrogen (known as the Balmer series in the visual portion of the spectrum) it is classified Type II; otherwise it is Type I. Among those types, there are subdivisions according to the presence of lines from other elements and the shape of the light curve (a graph of the supernova's apparent magnitude versus time).{{cite conference | author=E. Cappellaro, M. Turatto| title=Supernova Types and Rates | booktitle=Influence of Binaries on Stellar Population Studies | publisher=Dordrecht: Kluwer Academic Publishers | date=August 08, [ | location=Brussels, Belgium | url=http://adsabs.harvard.edu/abs/2000astro.ph.12455C | accessdate=2006-09-15 --> {| class="wikitable"|+Supernova taxonomy{{cite web | last = Montes | first = M.| date = February 12, [ | url = http://rsd-www.nrl.navy.mil/7212/montes/snetax.html | title = Supernova Taxonomy | publisher = [Naval Research Laboratory | accessdate = 2006-11-09 --> !Type!Characteristics|-|colspan="2" style="background: #EEEEEE; text-align: center"|Type I|-|Type Ia supernova|Lacks hydrogen and presents a singly-ionization silicon (Si II) line at 615.0 1 E-9 m, near peak light.|-|Type Ib and Ic supernovae|Non-ionized helium (He I) line at 587.6 nm and no strong silicon absorption feature near 615 nm.]|Weak or no helium lines and no strong silicon absorption feature near 615 nm.|-|colspan="2" style="background: #EEEEEE; text-align: center"|Type II|-|Type II supernova|Reaches a "plateau" in its light curve|-|Type II supernova|Displays a "linear" decrease in its light curve (linear in magnitude versus time). |}

The supernovae of Type II can also be sub-divided based on their spectra. While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of kilometres per second, some have relatively narrow features. These are called Type IIn, where the "n" stands for "narrow".

A few supernovae, such as SN 1987K and SN 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type IIb" is used to describe the combination of features normally associated with Types II and Ib.

Current models Type Ia There are several means by which a supernova of this type can form, but they share a common underlying mechanism. If a carbon-oxygen white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1.38 solar masses (for a non-rotating star), it would no longer be able to support the bulk of its plasma through electron degeneracy pressure and would begin to collapse. However, the current view is that this limit is not normally attained; increasing temperature and density inside the core Carbon detonation Carbon burning process as the star approaches the limit (to within about 1%{{cite book | last = http://www.as.utexas.edu/~wheel/ J. Craig Wheeler | first = | title = Cosmic Catastrophes: Supernovae, Gamma-Ray Bursts, and Adventures in Hyperspace | publisher = [Cambridge University Press | date = 2000-01-15 | location = Cambridge, UK | pages = p. 96 | url = http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521857147 | isbn = 0521651956-->), before collapse is initiated. Within a few seconds, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (1–2 × 1044 [joules) to unbind the star in a supernova explosion. An outwardly expanding shock wave is generated, with matter reaching velocities on the order of 5,000–20,000 km/s, or roughly 3% of the speed of light. There is also a significant increase in luminosity, reaching an absolute magnitude of -19.3 (or 5 billion times brighter than the Sun), with little variation.

One model for the formation of this category of supernova is a close binary star system. The larger of the two stars is the first to evolve off the main sequence, and it expands to form a red giant.{{cite web| last = Richmond | first = Michael | url = http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html | title = Late stages of evolution for low-mass stars | publisher = Rochester Institute of Technology | accessdate = 2006-08-04 --> The two stars now share a common envelope, causing their mutual orbit to shrink. The giant star then sheds most of its envelope, losing mass until it can no longer continue [nuclear fusion. At this point it becomes a white dwarf star, composed primarily of carbon and oxygen. {{cite conference | first = B. | last = Paczynski| title = Common Envelope Binaries | booktitle = Structure and Evolution of Close Binary Systems | pages = 75–80 | publisher = Dordrecht, D. Reidel Publishing Co. | date = July 28 – [August 1, [ | location = Cambridge, England | url = http://adsabs.harvard.edu/abs/1976IAUS...73...75P | accessdate = 2007-01-08 --> {{cite web | author=K. A. Postnov, L. R. Yungelson| year = 2006 | url = http://relativity.livingreviews.org/open?pubNo=lrr-2006-6&page=articlesu8.html | title = The Evolution of Compact Binary Star Systems | publisher = Living Reviews in Relativity | accessdate = 2007-01-08 --> Eventually the secondary star also evolves off the main sequence to form a red giant. Matter from the giant is accreted by the white dwarf, causing the latter to increase in mass.

Another model for the formation of a Type Ia explosion involves the merger of two white dwarf stars, with the combined mass momentarily exceeding the Chandrasekhar limit.{{cite web | author=Staff| url =http://cosmos.swin.edu.au/entries/typeiasupernovaprogenitors/typeiasupernovaprogenitors.html?e=1| title =Type Ia Supernova Progenitors| publisher =Swinburne University | accessdate = 2007-05-20 --> A white dwarf could also accrete matter from other types of companions, including a main sequence star (if the orbit is sufficiently close).

Type Ia supernovae follow a characteristic light curve—the graph of luminosity as a function of time—after the explosion. This luminosity is generated by the radioactive decay of nickel-56 through cobalt-56 to iron-56. The peak luminosity of the light curve is consistent across Type Ia supernovae (the vast majority of which are initiated with a uniform mass via the accretion mechanism), allowing them to be used as a secondary standard candle to measure the distance to their hostgalaxy.

Type Ib and Ic These events, like supernovae of Type II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types Ib and Ic have lost most of their outer (hydrogen) envelopes due to strong stellar winds or else from interaction with a companion.{{cite conference | last = Pols | first = Onno| title = Close Binary Progenitors of Type Ib/Ic and IIb/II-L Supernovae | booktitle = Proceedings of the The Third Pacific Rim Conference on Recent Development on Binary Star Research | pages = 153–158 | date = October 26 – [November 1, [ | location = Chiang Mai, Thailand | url = http://adsabs.harvard.edu/abs/1997rdbs.conf..153P | accessdate = 2006-11-29 --> Type Ib supernovae are thought to be the result of the collapse of a massive Wolf-Rayet star. There is some evidence that a few percent of the Type Ic supernovae may be the progenitors of gamma ray bursts (GRB), though it is also believed that any hydrogen-stripped, Type Ib or Ic supernova could be a GRB, dependent upon the geometry of the explosion.

Type II

Stars with at least nine solar masses of material evolve in a complex fashion. In the core of the star, hydrogen is fused into helium and the energy released creates an outward pressure, which maintains the core in [hydrostatic equilibrium and prevents collapse.

When the core's supply of hydrogen is exhausted, this outward pressure is no longer created. The core begins to gravitational collapse inwardly, causing a rise in temperature and pressure which becomes great enough to ignite the helium and start a helium-to-carbon fusion cycle, creating sufficient outward pressure to halt the collapse. The core expands and cools slightly, with a hydrogen-fusion outer layer, and a hotter, higher pressure, helium-fusion center. (Other elements such as magnesium, sulfur and calcium are also created and in some cases burned in these further reactions.)

This process repeats several times, and each time the core collapses and the collapse is halted by the ignition of a further process involving more massive nuclei and higher temperatures and pressures. Each layer is prevented from collapse by the heat and outward pressure of the fusion process in the next layer inward; each layer also burns hotter and quicker than the previous one – the final burn of silicon to nickel consumes its fuel in around one day, or a few days. The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells.{{cite web | last = Richmond | first = Michael| url = http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html | title = Late stages of evolution for low-mass stars | publisher = [Rochester Institute of Technology | accessdate = 2006-08-04 --> {{cite web | last = Hinshaw | first = Gary| date = August 23, [ | url = http://map.gsfc.nasa.gov/m_uni/uni_101stars.html | title = The Life and Death of Stars | publisher = [NASA [Wilkinson Microwave Anisotropy Probe (WMAP) Mission | accessdate = 2006-09-01 -->

In the later stages, increasingly heavier elements undergo nuclear fusion, and the binding energy of the relevant nuclei increases. Fusion produces progressively lower levels of energy, and also at higher core energies photodisintegration and electron capture occur which cause energy loss in the core and a general acceleration of the fusion processes to maintain equilibrium. This escalation culminates with the silicon burning process, which is unable to produce energy through fusion (but does produce iron-56 through radioactive decay). As a result, a nickel-iron core{{cite web| last=Fleurot | first=Fabrice | year=1988 | url=http://nu.phys.laurentian.ca/~fleurot/evolution/ | title=Evolution of Massive Stars | publisher=Laurentian University | accessdate=2007-08-13 --> builds up that cannot produce any further outward pressure on a scale needed to support the rest of the structure. It can only support the overlaying mass of the star through the [degeneracy pressure of [electrons in the core. If the star is sufficiently large, then the iron-nickel core will eventually exceed the [Chandrasekhar limit (1.38 [solar masses), at which point this mechanism catastrophically fails. The forces holding atomic nuclei apart in the innermost layer of the core suddenly give way, the core [Implosion (mechanical process) due to its own mass, and no further fusion process can ignite or prevent collapse this time.

Core collapse The core collapses in on itself with velocities reaching 70,000 km/s (0.23Speed of light),{{cite web | author=C. L. Fryer, K. C. B. New| date = January 24, [ | url = http://relativity.livingreviews.org/Articles/lrr-2003-2/ | title = Gravitational Waves from Gravitational Collapse | publisher = [Max Planck Institute for Gravitational Physics | accessdate = 2006-12-14 --> resulting in a rapid increase in temperature and density. The energy loss processes operating in the core cease to be in equilibrium. Through photodisintegration, gamma rays decompose iron into helium nuclei and free neutrons, absorbing energy, whilst electrons and protons merge via electron capture, producing neutrons and electron neutrinos which escape. About 1046 joules of gravitational energy—about 10% of the star's rest mass—is converted into a ten-second burst of neutrinos; the main output of the event.{{cite web , | url = http://www.aps.org/policy/reports/multidivisional/neutrino/upload/Neutrino_Astrophysics_and_Cosmology_Working_Group.pdf | title = APS Neutrino Study: Report of the Neutrino Astrophysics and Cosmology Working Group | publisher = [American Physical Society | format=PDF | accessdate = 2006-12-12 --> These carry away energy from the core and accelerate the collapse, while some neutrinos are absorbed by the star's outer layers and begin the supernova explosion.

The inner core eventually reaches typically 30 km diameter, and a density comparable to that of an atomic nucleus, and further collapse is abruptly stopped by strong force interactions and by degeneracy pressure of neutrons. The infalling matter, suddenly halted, rebounds, producing a shock wave that propagates outward. Computer simulations indicate that this expanding shock does not directly cause the supernova explosion; rather, it stalls within millisecondshttp://adsabs.harvard.edu/abs/1990ApJ...364..222M in the outer core as energy is lost through the dissociation of heavy elements, and a process that is not clearly understood is necessary to allow the outer layers of the core to reabsorb around 1044 joules (1 Foe (unit of energy)) of energy, producing the visible explosion.{{cite web | author = C. L. Fryer, K. B. C. New| date = January 24, [ | url = http://relativity.livingreviews.org/open?pubNo=lrr-2003-2&page=articlesu6.html | title = Gravitational Waves from Gravitational Collapse, section 3.1 | publisher = [Los Alamos National Laboratory | accessdate = 2006-12-09 --> Current research focusses upon rotational and magnetic field effects as the basis for this process.

]

When the progenitor star is below about 20 solar masses (depending on the strength of the explosion and the amount of material that falls back), the degenerate remnant of a core collapse is a neutron star. Above this mass the remnant collapses to form a black hole. (This type of collapse is one of many candidate explanations for gamma ray bursts—producing a large burst of gamma rays through a still theoretical hypernova explosion.){{cite news (ESO) | date=June 18, [ | url=http://www.eso.org/outreach/press-rel/pr-2003/pr-16-03.html | accessdate=2006-10-30 --> The theoretical limiting mass for this type of core collapse scenario was estimated around 40–50 solar masses.

Above 50 solar masses, stars were believed to collapse directly into a black hole without forming a supernova explosion, although uncertainties in models of supernova collapse make accurate calculation of these limits difficult. In fact recent evidence has shown stars in the range of about 140–250 solar masses, with a relatively low proportion of elements more massive than helium, may be capable of forming pair-instability supernovae without leaving behind a black hole remnant. This rare type of supernova is formed by an alternate mechanism (partially analogous to that of Type Ia explosions) that does not require an iron core. An example is the Type II supernova SN 2006gy, with an estimated 150 solar masses, that demonstrated the explosion of such a massive star differed fundamentally from previous theoretical predictions.{{cite web| last = Boen | first = Brooke | date = May 5, 2007 | url = http://www.nasa.gov/mission_pages/chandra/news/chandra_bright_supernova.html | title = NASA's Chandra Sees Brightest Supernova Ever | publisher = NASA | accessdate = 2007-08-09 -->{{cite news | first=Robert | last=Sanders | title=Largest, brightest supernova ever seen may be long-sought pair-instability supernova | publisher=University of California, Berkeley | date=May 7, [ | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/ | accessdate=2006-05-24 -->

Light curves and unusual spectra The light curves for Type II supernovae are distinguished by the presence of hydrogen Balmer series in the spectra. These light curves have an average decay rate of 0.008 absolute magnitude per day; much lower than the decay rate for Type I supernovae. Type II are sub-divided into two classes, depending on whether there is a plateau in their light curve (Type II-P) or a linear decay rate (Type II-L). The net decay rate is higher at 0.012 magnitudes per day for Type II-L compared to 0.0075 magnitudes per day for Type II-P. The difference in the shape of the light curves is believed to be caused, in the case of Type II-L supernovae, by the expulsion of most of the hydrogen envelope of the progenitor star.

The plateau phase in Type II-P supernovae is due to a change in the opacity (optics) of the exterior layer. The shock wave ionizes the hydrogen in the outer envelope, which greatly increases the opacity. This prevents photons from the inner parts of the explosion from escaping. Once the hydrogen cools sufficiently to recombine, the outer layer becomes transparent.{{cite web | url = http://cosmos.swin.edu.au/lookup.html?e=typeiisupernovalightcurves| title = Type II Supernova Light Curves | publisher = [Swinburne University of Technology | accessdate = 2007-03-17 -->

Of the Type II supernovae with unusual features in their spectra, Type IIn supernovae may be produced by the interaction of the ejecta with circumstellar material. Type IIb supernovae are likely massive stars which have lost most, but not all, of their hydrogen envelopes through tidal force by a companion star. As the ejecta of a Type IIb expands, the hydrogen layer quickly becomes optically thin and reveals the deeper layers.

Asymmetry A long-standing puzzle surrounding supernovae has been a need to explain why the compact object remaining after the explosion is given a large velocity away from the core.{{cite book | editor=P. Hoflich, P. Kumar, J. C. Wheeler| title=Cosmic explosions in three dimensions: asymmetries in supernovae and gamma-ray bursts | chapter=Neutron star kicks and supernova asymmetry | publisher=[Cambridge University Press | location=Cambridge | year=2004 | pages=276 | url=http://adsabs.harvard.edu/abs/2004cetd.conf..276L | accessdate = 2007-02-01 --> (Neutron stars are observed, as pulsars, to have high velocities; black holes presumably do as well, but are far harder to observe in isolation.) This kick can be substantial, propelling an object of more than a solar mass at a velocity of 500 km/s or greater. This displacement is believed to be caused by an asymmetry in the explosion, but the mechanism by which this momentum is transferred to the compact object has remained a puzzle. Some explanations for this kick include convection in the collapsing star and jet production during neutron star formation.

(blue) and optical (red) radiation from the Crab Nebula's core region. A pulsar near the center is propelling particles to almost the speed of light.{{cite web , | url = http://hubblesite.org/newscenter/archive/releases/2002/24/text/ | title = Space Movie Reveals Shocking Secrets of the Crab Pulsar | publisher = [NASA | accessdate = 2006-08-10 --> This neutron star is travelling at an estimated 375 km/s. NASA/CXC/HST/ASU/J. Hester et al. image credit.One explanation for the asymmetry in the explosion is large-scale convection above the core. The convection can create variations in the local abundances of elements, resulting in uneven nuclear burning during the collapse, bounce and resulting explosion.

Another explanation is that accretion of gas onto the central neutron star can create a accretion disk that drives highly directional jets, propelling matter at a high velocity out of the star, and driving transverse shocks that completely disrupt the star. These jets might play a crucial role in the resulting supernova explosion.{{cite news | title=Jets, Not Neutrinos, May Cause Supernova Explosions, Scientists Say| publisher=[McDonald Observatory | date=March 2, [ | url=http://mcdonaldobservatory.org/news/releases/2000/0302a.html | accessdate=2006-12-11 --> {{cite web | last = Foust | first = Jeff| date = January 9, [ | url = http://spaceflightnow.com/news/n0101/09supernova/ | title = Evidence presented for new supernova explosion model | publisher = Spaceflight Now | accessdate = 2006-12-13 --> (A similar model is now favored for explaining long gamma ray bursts.)

Initial asymmetries have also been confirmed in Type Ia supernova explosions through observation. This result may mean that the initial luminosity of this type of supernova may depend on the viewing angle. However, the explosion becomes more symmetrical with the passage of time. Early asymmetries are detectable by measuring the polarization of the emitted light.{{cite news (ESO) | date=August 6, [ | url=http://www.eso.org/outreach/press-rel/pr-2003/pr-23-03.html | accessdate=2006-12-11 -->

Type Ia versus core collapse Because they have a similar functional model, Types Ib, Ic and various Types II supernovae are collectively called Core Collapse supernovae. A fundamental difference between Type Ia and Core Collapse supernovae is the source of energy for the radiation emitted near the peak of the light curve. The progenitors of Core Collapse supernovae are stars with extended envelopes that can attain a degree of transparency with a relatively small amount of expansion. Most of the energy powering the emission at peak light is derived from the shock wave that heats and ejects the envelope.{{cite conference | first = B. | last = Leibundgut| title = Observations of Supernovae | booktitle = Proceedings of the NATO Advanced Study Institute on the Lives of the Neutron Stars | pages = 3 | publisher = Kluwer Academic | date = August 29 – [September 12, [ | location = Kemer, Turkey | url = http://adsabs.harvard.edu/abs/1995lns..conf....3L | accessdate = 2006-12-18 | id = ISBN 0-7923-324-6-6 -->

The progenitors of Type Ia supernovae, on the other hand, are compact objects, much smaller (but more massive) than the Sun, that must expand (and therefore cool) enormously before becoming transparent. Heat from the explosion is dissipated in the expansion and is not available for light production. The radiation emitted by Type Ia supernovae is thus entirely attributable to the decay of radionuclides produced in the explosion; principally nickel-56 (with a half-life of 6.1 days) and its daughter cobalt-56 (with a half-life of 77 days). Gamma rays emitted during this nuclear decay are absorbed by the ejected material, heating it to incandescence.

As the material ejected by a Core Collapse supernova expands and cools, radioactive decay eventually takes over as the main energy source for light emission in this case also. A bright Type Ia supernova may expel 0.5–1.0 solar masses of nickel-56, while a Core Collapse supernova probably ejects closer to 0.1 solar mass of nickel-56.

Interstellar impact Source of heavy elements Supernovae are a key source of chemical element heavier than oxygen. These elements are produced by nuclear fusion (for iron-56 and lighter elements), and by nucleosynthesis during the supernova explosion for elements heavier than iron. Supernova are the most likely, although not undisputed, candidate sites for the r-process, which is a rapid form of nucleosynthesis that occurs under conditions of high temperature and high density of neutrons. The reactions produce highly unstable atomic nucleus that are rich in neutrons. These forms are unstable and rapidly beta decay into more stable forms.

The r-process reaction, which is likely to occur in type II supernovae, produces about half of all the element abundance beyond iron, including plutonium, uranium and californium. The only other major competing process for producing elements heavier than iron is the s-process in large, old red giant stars, which produces these elements much more slowly, and which cannot produce elements heavier than lead.

Role in stellar evolution The remnant of a supernova explosion consists of a compact object and a rapidly expanding shock wave of material. This cloud of material sweeps up the surrounding interstellar medium during a free expansion phase, which can last for up to two centuries. The wave then gradually undergoes a period of adiabatic process, and will slowly cool and mix with the surrounding interstellar medium over a period of about 10,000 years.{{cite web | date = September 7, 2006| url = http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html | title = Introduction to Supernova Remnants | publisher = High Energy Astrophysics Science Archive Research Center, [NASA (HEASARC) | accessdate = 2006-10-20 -->

In standard astronomy, the Big Bang produced hydrogen, helium, and traces of lithium, while all heavier elements are synthesized in stars and supernovae. Supernovae tend to enrich the surrounding interstellar medium with metals, which for astronomers means all of the elements other than hydrogen and helium and is a different definition than that used in chemistry.

. NASA image.These injected elements ultimately enrich the molecular clouds that are the sites of star formation.{{cite web , | url = http://www.space.com/scienceastronomy/060619_mystery_monday.html | title = Explosive Debate: Supernova Dust Lost and Found | publisher = [space.com | accessdate = 2006-12-01 --> Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. Supernovae are the dominant mechanism for distributing these heavier elements, which are formed in a star during its period of nuclear fusion, throughout space. The different abundances of elements in the material that forms a star have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.

The kinetic energy of an expanding supernova remnant can trigger star formation due to compression of nearby, dense molecular clouds in space. The increase in turbulent pressure can also prevent star formation if the cloud is unable to lose the excess energy.

Evidence from daughter products of short-lived radioactive isotopes shows that a nearby supernova helped determine the composition of the Solar System 4.5 billion years ago, and may even have triggered the formation of this system.{{cite web , | url = http://www.psrd.hawaii.edu/May03/SolarSystemTrigger.html | title = Triggering the Formation of the Solar System | publisher = Planetary Science Research | accessdate = 2006-10-20 --> Supernova production of heavy elements over astronomic periods of time ultimately made the biochemistry on Earth possible.

Impact on Earth A near-Earth supernova is an explosion resulting from the death of a star that occurs close enough to the Earth (roughly fewer than 100 light-years away) to have noticeable effects on its biosphere. Gamma rays are responsible for most of the adverse effects a supernova can have on a living terrestrial planet. In Earth's case, gamma rays induce a chemical reaction in the upper Earth's atmosphere, converting molecular nitrogen into nitrogen oxides, depleting the ozone layer enough to expose the surface to harmful Sun and cosmic radiation. The gamma ray burst from a nearby supernova explosion has been proposed as the cause of the Ordovician-Silurian extinction events, which resulted in the death of nearly 60% of the oceanic life on Earth.

Speculation as to the effects of a nearby supernova on Earth often focuses on large stars as Type II supernova candidates. Several prominent stars within a few hundred light years from the Sun are candidates for becoming supernovae in as little as a millennium. One example is Betelgeuse, a red supergiant 427 light-years from Earth.{{cite web ], 2005| url = http://chandra.harvard.edu/resources/faq/sources/snr/snr-5.html | title = Supernova Remnants and Neutron Stars | publisher = [Harvard-Smithsonian Center for Astrophysics | accessdate = 2006-06-08 --> Though spectacular, these "predictable" supernovae are thought to have little potential to affect Earth.

Recent estimates predict that a Type II supernova would have to be closer than eight parsecs (26 light-years) to destroy half of the Earth's ozone layer.{{Cite journal | url=http://xxx.lanl.gov/abs/astro-ph/0211361| title=Ozone Depletion from Nearby Supernovae | first=Neil | last=Gehrels | coauthors=Claude M. Laird ''et al'' | journal=[Astrophysical Journal | date=March 10, [ | volume=585 | pages= 1169–1176 | accessdate = 2007-02-01 --> Such estimates are mostly concerned with atmospheric modeling and considered only the known radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud. Estimates of the rate of supernova occurrence within 10 parsecs of the Earth vary from once every 100 million years to once every one to ten billion years.

Type Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because Type Ia supernovae arise from dim, common white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably and take place in a star system that is not well studied. One theory suggests that a Type Ia supernova would have to be closer than a thousand parsecs (3300 light-years) to affect the Earth.{{cite web | url=http://www.tass-survey.org/richmond/answers/snrisks.txt| title=Will a Nearby Supernova Endanger Life on Earth? | first=Michael | last=Richmond | year=April 8, [ | format=TXT | accessdate=2006-03-30 --> The closest known candidate is IK Pegasi (see below).

In 1996, astronomers at the University of Illinois at Urbana-Champaign theorized that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Subsequently, Iron#Isotopes enrichment has been reported in deep-sea rock of the Pacific Ocean by researc

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