Supernova

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A supernova (abbreviated SN, plural SNe after supernovae) is a star|stellar explosion that is more Energy|energetic than a nova . It is pronounced with the plural supernovae or supernovas . Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy , before fading from view over several weeks or months. During this short interval a supernova can radiate as much energy as the Sun is expected to emit over its entire life span.
The explosion expels much or all of a star's material
at a velocity of up to (10% of 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 .

Nova (plural novae ) means "new" in Latin language|Latin , referring to what appears to be a very bright new star shining in the celestial sphere ; the prefix "super-" distinguishes supernovae from ordinary novae, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931. Several types of supernovae exist. Types I and II can be triggered in one of two ways, either turning off or suddenly turning on the production of energy through nuclear fusion . After the core of an stellar evolution#Massive stars|aging massive star ceases generating energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole , releasing potential energy#Gravitational 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|stellar companion (either through Accretion (astrophysics)|accretion or via a merger) to raise its core temperature enough to Carbon detonation|ignite Carbon burning process|carbon fusion , at which point it undergoes Thermal runaway|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|fueled by hydrogen on their surfaces called a nova. Solitary stars (such as the Sun) with a mass below approximately 9 solar masses, evolve into white dwarfs without ever becoming supernovae.

Although no supernova has been observed in the Milky Way since SN 1604|1604 , supernovae remnants indicate that on average the event occurs about once every 50& nbsp;years in the Milky Way.
They play a significant role in enriching the interstellar medium with higher mass chemical element|elements .
Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.

Observation history

Hipparchus ' interest in the fixed stars may have been inspired by the observation of a supernova (according to Pliny).
The earliest recorded supernova, SN 185 , was viewed by Chinese astronomy|Chinese astronomer s in 185 AD. The brightest recorded supernova was the SN 1006 , which was described in detail by Chinese and Islamic astronomy|Islamic astronomers .
The widely observed supernova SN 1054 produced the Crab Nebula . Supernovae SN 1572 and SN 1604 , the latest to be observed with the naked eye in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotle|Aristotelian idea that the universe beyond the Moon and planets was immutable.
Johannes Kepler began observing SN 1604 on October 17, 1604.
It was the second supernova to be observed in a generation (after SN 1572 seen by Tycho Brahe in Cassiopeia).

Since the development of the telescope the field of supernova discovery has extended to other galaxies, starting with the 1885 observation of supernova S Andromedae in the Andromeda galaxy . Supernovae provide important information on cosmological distances.
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 . American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941.

In the 1960s astronomers found that the maximum intensities of supernova explosions could be used as standard candles , hence indicators of astronomical distances.
Some of the most distant supernovae recently observed appeared dimmer than expected. This supports the view that the expansion of the Accelerating universe|universe is accelerating .

Techniques were developed for reconstructing supernova explosions that have no written records of being observed. The date of the Cassiopeia A supernova event was determined from light echo es off nebula e,
while the age of supernova remnant RX J0852.0-4622 was estimated from temperature measurements
and the gamma ray emissions from the decay of titanium-44 .
In 2009 nitrate s were discovered in Antarctic ice deposits that matched the times of past supernova events.

Discovery

Early work on what was originally believed to be simply a new category of nova e was performed during the 1930s by Walter Baade and Fritz Zwicky at Mount Wilson Observatory. The name super-novae was first used during 1931 lectures held at Caltech by Baade and Zwicky, then used publicly in 1933 at a meeting of the American Physical Society . By 1938, the hyphen had been lost and the modern name was in use. Because supernovae are relatively rare events within a galaxy, occurring about once every 50& nbsp;years in the Milky Way, obtaining a good sample of supernovae to study requires regular monitoring of many galaxies.

Supernovae in other galaxies cannot be predicted with any meaningful accuracy. Normally, when they are discovered, they are already in progress.
Most scientific interest in supernovae—as standard candle s 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|Amateur astronomers , 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|CCDs for hunting supernovae. While such systems are popular with amateurs, there are also professional installations such as the Katzman Automatic Imaging Telescope .
Recently the Supernova Early Warning System (SNEWS) project has begun using a network of neutrino detector s to give early warning of a supernova in the Milky Way galaxy.

Neutrino s are Subatomic particle|particles that are produced in great quantities by a supernova explosion,
and they are not significantly absorbed 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|expansion of the universe , 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 & nbsp;=& nbsp;0.1–0.3
—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. Supernova spectroscopy, used to study the physics and environments of supernovae, is more practical at low than at high redshift.

Low redshift observations also anchor the low-distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies.

(See also Hubble's law ).

Naming convention

Supernova 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 that supernova. The name is the marker SN followed by the year of discovery, suffixed with a one or two-letter designation. The first 26 supernovae of the year are designated with a capital letter from A to Z . Afterward pairs of lower-case letters are used: aa , ab , and so on. Hence, for example, SN& nbsp;2003C designates the third supernova reported in the year 2003. The last supernova of 2005 was SN& nbsp;2005nc, indicating that it was the 367thThe value is obtained by converting the suffix "nc" from base (exponentiation)|base 26 , with a & nbsp;=& nbsp;1, b & nbsp;=& nbsp;2, c & nbsp;=3, & nbsp;... z & nbsp;=& nbsp;26. Thus nc& nbsp;= n & nbsp;& times;& nbsp;26& nbsp;+& nbsp; c & nbsp;= 14& nbsp;& times;& nbsp;26& nbsp;+& nbsp;3& nbsp;= 367. supernova found in 2005. Since the year 2000, professional and amateur astronomers find several hundreds of supernovae each year (572 in 2007, 261 in 2008, 390 in 2009).

Historical supernovae are known simply by the year they occurred: SN 185|SN& nbsp;185 , SN 1006|SN& nbsp;1006 , SN 1054|SN& nbsp;1054 , SN 1572|SN& nbsp;1572 (called ''Tycho's Nova ) and SN 1604|SN& nbsp;1604 ( Kepler's Star ). Since 1885 the additional letter notation has been used, even if there was only one supernova discovered that year (e.g. SN& nbsp;1885A, SN& nbsp;1907A, etc.) — this last happened with SN& nbsp;1947A. SN , for SuperNova, is a standard prefix. Until 1987, two-letter designations were rarely needed; since 1988, however, they have been needed every year.

Classification

As part of the attempt to understand supernovae, astronomers have classified them according to the absorption line s of different chemical elements that appear in their Astronomical spectroscopy|spectra . 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& nbsp;II ; otherwise it is Type& nbsp;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 as a function of time).

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first=M.	date=2002-02-12	title=Supernova Taxonomy	url=http:/ / rsd-www.nrl.navy.mil/ 7212/ montes/ snetax.html	US Naval Research Laboratory	accessdate=2006-11-09
Type& nbsp;I
Type& nbsp;Ia	ionized silicon (Si II) line at 615.0& nbsp; Nanometre
Type& nbsp;Ib	Non-ionized helium (He I) line at 587.6& nbsp;nm and no strong silicon absorption feature near 615& nbsp;nm.
Type& nbsp;Ic	Weak or no helium lines and no strong silicon absorption feature near 615& nbsp;nm.
Type& nbsp;II
Type& nbsp;IIP	Reaches a "plateau" in its light curve
Type& nbsp;IIL	Displays a "linear" decrease in its light curve (linear in magnitude versus time).Cite journal	first=J. B.	first2=D.	year=1985	title=A Comparative Study of Supernova Light Curves	journal= Astronomical Journal	pages=2303–2311	bibcode=1985AJ.....90.2303D	doi=10.1086/ 113934

The supernovae of Type& nbsp;II can also be sub-divided based on their spectra. While most Type& nbsp;II supernova show very broad emission line s which indicate expansion velocities of many thousands of kilometres per second , some have relatively narrow features. These are called Type& nbsp;IIn, where the 'n' stands for 'narrow'. Supernovae that do not fit into the normal classifications are designated peculiar, or 'pec'.

A few supernovae, such as SN 1987K|SN& nbsp;1987K and SN 1993J|SN& nbsp;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 supernova|"Type& nbsp;IIb" is used to describe the combination of features normally associated with Types& nbsp;II and Ib.

Current models

The type codes described above that astronomers give to supernovas are taxonomy|taxonomic in nature: the type number describes the light observed from the supernova, not necessarily its cause. The following summarize what astronomers currently believe are the most plausible explanations for supernovas.

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 For a core primarily composed of oxygen, neon and magnesium, the collapsing white dwarf will typically form a neutron star . In this case, only a fraction of the star's mass will be ejected during the collapse.
See: white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1.38& nbsp; solar mass es (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|ignite Carbon burning process|carbon fusion as the star approaches the limit (to within about 1%
), 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 & times; 10⁴⁴& nbsp; joule s)
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& nbsp; kilometers per second|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 .
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.

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.
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 was believed to be consistent across Type Ia supernovae (the vast majority of which are initiated with a uniform mass via the accretion mechanism), having a maximum absolute magnitude of about -19.3. This would allow them to be used as a secondary
standard candle to measure the distance to their host galaxy|galaxies .
However, recent discoveries reveal that there is some evolution in the average lightcurve width, and thus in the intrinsic luminosity of supernovae, although significant evolution is found only over a large redshift baseline.

Type Ib and Ic

supernova, shown in X-ray (left) and visible light (right) at the far upper end of the galaxy.

These supernovae, like those of Type& nbsp;II, are probably massive stars running out of fuel at their centers; however, the stars which become Types& nbsp;Ib and Ic supernovae have lost most of their outer (hydrogen) envelopes due to strong stellar wind s or else from interaction with a companion.
Type& nbsp;Ib supernovae are thought to result from the collapse of massive Wolf-Rayet star s. There is some evidence that a few percent of the Type& nbsp;Ic supernovae may be the producers of gamma ray bursts (GRB), though it is also believed that any hydrogen-stripped Type& nbsp;Ib or Ic supernova could produce a GRB, depending on 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 thermal 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|collapse , 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; 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 more quickly than the previous one—the final burn of silicon to iron consumes its fuel in a few weeks at most.
The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells.

In the later stages increasingly heavier elements with higher binding energy undergo nuclear fusion. Fusion produces progressively less energy, and also at higher core energies photodisintegration and electron capture occur which cause further energy loss in the core, requiring a general acceleration of the fusion processes to maintain hydrostatic equilibrium . This escalation culminates with the silicon burning process|production of nickel-56 , which is unable to produce energy through fusion (but does produce iron-56 through radioactive decay).
As a result, a nickel-iron core
builds up that cannot produce further outward pressure on the scale needed to support the rest of the structure. It can only support the overlying mass of the star through the degeneracy pressure of electron s in the core. If the star is sufficiently large, the iron-nickel core will eventually exceed the Chandrasekhar limit (1.38& nbsp;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)|implodes under its own gravity, and no further fusion process is available to ignite and prevent collapse this time.

Core collapse

The core collapses in on itself with velocities reaching 70,000& nbsp;km/s (0.23 Speed of light|c ),
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 ray s decompose iron into helium nuclei and free neutron s, absorbing energy, while electron s and proton s merge via electron capture , producing neutrons and electron neutrino s, which escape.

In a typical Type II supernova the newly formed neutron core has an initial temperature of about 100& nbsp;billion kelvin (100& nbsp;GK), 6000 times the temperature of the sun's core. A further release of neutrinos carries away much of the thermal energy, allowing a stable neutron star to form (the neutrons would "boil away" if this cooling did not occur).
These 'thermal' neutrinos form as neutrino-antineutrino pairs of all Neutrino oscillation|flavors , and total several times the number of electron-capture neutrinos.
About 10⁴⁶& nbsp;joules of gravitational energy—approximately 10% of the star's rest mass—is converted into a ten-second burst of neutrinos, which is the main output of the event.
These carry away energy from the core and accelerate the collapse, while some neutrinos are absorbed by the star's outer layers and provide energy to the supernova explosion.

The inner core eventually reaches typically 30& nbsp; kilometer|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 millisecond s
in the outer core as energy is lost through the dissociation of heavy elements, and a process that is is necessary to allow the outer layers of the core to reabsorb around 10⁴⁴& nbsp;joulesPer the American Physical Society Neutrino Study reference, Barwick, Beacom et al. (2004), roughly 99% of the gravitational potential energy is released as neutrinos of all flavors. The remaining 1% is equal to 10⁴⁴ J (1& nbsp; Foe (unit)|foe ) of energy, producing the visible explosion.
research focuses upon a combination of neutrino reheating, rotation al and magnetic field|magnetic effects as the basis for this process.

When the progenitor star is below about 20& nbsp;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 burst s, possibly producing a large burst of gamma ray s through a hypernova explosion.)
The theoretical limiting mass for this type of core collapse scenario was estimated around 40–50& nbsp;solar masses.

Above 50& nbsp;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. Above about 140 solar masses stars may become pair-instability supernova e that do not leave behind a black hole remnant.

Light curves and unusual spectra

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

The plateau phase in Type& nbsp;II-P supernovae is due to a change in the opacity (optics)|opacity of the exterior layer. The shock wave ionize s 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.

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

The peak absolute magnitude of Type II supernovae varies from one to another, but they are dimmer than Type Ia.
For instance, the low-luminosity SN 1987A had a peak visual absolute magnitude of -15.5 (apparent magnitude +3 for a distance of 51 kpc), as compared to the standard -19.3 for Type Ia.

Pair-instability type

The core temperature of a star of over about 140 solar masses can become so high that pair production|photons convert spontaneously to electron-positron pairs , reducing the radiation pressure|photon pressure supporting the star's outer layers and triggering a collapse that leads to a supernova explosion. This pair-instability supernova creates a larger quantity of elements heavier than helium (" metallicity|metals ") than other types of supernova and leaves no black hole as a remnant. Stars of this size can only form from interstellar gas with very low metal content, which is characteristic of the early universe before the first supernovae produced metals from the primordial hydrogen and helium. It is believed that supernova SN 2007bi was of this type; it was distinguished from other supernovae by very long duration—77 days to peak brightness, bright enough to observe for 555 days—and production of much more radioactive nickel. The pair-instability supernova was predicted by Gary S. Fraley in 1968.

Asymmetry

A long-standing puzzle surrounding Type II supernovae is why the compact object remaining after the explosion is given a large velocity away from the core.
(Neutron stars are observed, as pulsar s, to have high velocities; black holes presumably do as well, but are far harder to observe in isolation.) The initial impetus can be substantial, propelling an object of more than a solar mass at a velocity of 500& nbsp;km/s or greater. This displacement indicates an asymmetry in the explosion, but the mechanism by which this momentum is transferred to the compact object a puzzle. Proposed explanations for this kick include convection in the collapsing star and jet production during neutron star formation.

This neutron star is travelling at an estimated 375& nbsp;km/s.

One possible 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 possible explanation is that accretion of gas onto the central neutron star can create a accretion disk|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.

(A similar model is now favored for explaining long gamma ray bursts .)

Initial asymmetries have also been confirmed in Type& nbsp;Ia supernova explosions through observation. This result may mean that the initial luminosity of this type of supernova depends 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.

Energy output

Because they have a similar functional model, Types& nbsp;Ib, Ic and various Types& nbsp;II supernovae are collectively called Core Collapse supernovae. A fundamental difference between Type& nbsp;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 relatively little expansion. Most of the energy powering the emission at peak light is derived from the shock wave that heats and ejects the envelope.

The progenitors of Type& nbsp;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& nbsp;Ia supernovae is thus entirely attributable to the decay of radionuclide s produced in the explosion; principally nickel -56 (with a half-life of 6.1& nbsp;days) and its daughter cobalt -56 (with a half-life of 77& nbsp;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& nbsp;Ia supernova may expel 0.5–1.0& nbsp;solar masses of nickel-56,
while a Core Collapse supernova probably ejects closer to 0.1& nbsp;solar mass of nickel-56.

Interstellar impact

Source of heavy elements

Supernovae are a key source of chemical element|elements 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.
Supernovae 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|nuclei that are rich in neutron s. 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 and uranium .
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|adiabatic expansion , and will slowly cool and mix with the surrounding interstellar medium over a period of about 10,000& nbsp;years.

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 metallicity|metals —elements other than hydrogen and helium.

These injected elements ultimately enrich the molecular cloud s that are the sites of star formation.
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. 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 planet s 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 isotope s shows that a nearby supernova helped determine the composition of the Solar System 4.5& nbsp;billion years ago, and may even have triggered the formation of this system.
Supernova production of heavy elements over astronomic periods of time ultimately made the biochemistry|chemistry of life on Earth possible.

Effect on Earth

A near-Earth supernova is a supernova close enough to the Earth to have noticeable effects on its biosphere . Depending upon the type and energy of the supernova, it could be as far as 3000& nbsp; light-year s away. Gamma ray s from a supernova would induce a chemical reaction in the upper Earth's atmosphere|atmosphere converting molecular nitrogen into nitrogen oxide s, depleting the ozone layer enough to expose the surface to harmful solar radiation|solar and cosmic radiation . This has been proposed as the cause of the Ordovician–Silurian extinction , which resulted in the death of nearly 60% of the oceanic life on Earth.

In 1996 it was theorized that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata . Iron#Isotopes|Iron-60 enrichment was later reported in deep-sea rock of the Pacific Ocean .

In 2009, elevated levels of nitrate ions were found in Antarctic ice, which coincided with the 1006 and 1054 supernovae. Gamma rays from these supernovae could have boosted levels of nitrogen oxides, which became trapped in the ice.

Type& nbsp;Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because these supernovae arise from dim, common white dwarf stars, it is likely that a supernova that can affect the Earth will occur unpredictably and in a star system that is not well studied. One theory suggests that a Type& nbsp;Ia supernova would have to be closer than a thousand parsecs (3300& nbsp;light-years) to affect the Earth.
See section 4 in The closest known candidate is IK Pegasi (see below).
Recent estimates predict that a Type& nbsp;II supernova would have to be closer than eight parsec s (26& nbsp;light-years) to destroy half of the Earth's ozone layer.

Milky Way candidates

Several large stars within the Milky Way have been suggested as possible supernovae within the next million years. These include Rho Cassiopeiae ,
Eta Carinae ,

RS Ophiuchi ,

U Scorpii ,
VY Canis Majoris ,
Betelgeuse , Antares , and Spica .
Many Wolf–Rayet star s, such as Gamma Velorum ,
WR 104 ,
and those in the Quintuplet Cluster ,
are also considered possible precursor stars to a supernova explosion in the 'near' future.

The nearest supernova candidate is IK Pegasi (HR 8210), located at a distance of 150& nbsp;light-years. This closely orbiting binary star system consists of a main sequence star and a white dwarf 31& nbsp;million& nbsp;kilometres apart. The dwarf has an estimated mass 1.15 times that of the Sun.
It is thought that several million years will pass before the white dwarf can accrete the critical mass required to become a Type& nbsp;Ia supernova.

A popular-science account.

External links

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| title=List of Supernovae with IAU Designations
| url= http://www.cbat.eps.harvard.edu/lists/RecentSupernovae.html
| publisher= International Astronomical Union|IAU: Central Bureau for Astronomical Telegrams
| accessdate=2010-10-25

| last=Overbye | first=D.
| date=2008-05-21
| title=Scientists See Supernova in Action
| url= http://www.nytimes.com/2008/05/22/science/22nova.html
| work= The New York Times
| accessdate=2008-05-21

Category:Supernovae|
Category:Stellar phenomena
Category:Stellar evolution
Category:Standard candles
Category:Space plasmas
Category:Light sources

af:Supernova
ar:????? ????
az:Ifrat yeni ulduz
bn:?????????
be:????????? ?????
bg:?????????
bs:Supernova
ca:Supernova
cs:Supernova
da:Supernova
de:Supernova
et:Supernoova
el:?pe??a???fa?e?? ast??e?
es:Supernova
eo:Supernovao
eu:Supernoba
fa:?????????
fr:Supernova
fy:Supernova
ga:Ollnóva
gl:Supernova
ko:???
hi:???????
hr:Supernova
id:Supernova
ia:Supernova
is:Sprengistjarna
it:Supernova
he:????????
jv:Supernova
ka:??????? ??????????
ht:Sipènova
la:Supernova
lv:Parnova
lb:Supernova
lt:Supernova
hu:Szupernóva
mk:?????????
mg:Kinta-mazavabe
ml:???????????
mt:Supernova
mr:?????????
ms:Supernova
my:?????????
nl:Supernova
ja:???
no:Supernova
nn:Supernova
pl:Supernowa
pt:Supernova
ro:Supernova
ru:?????????? ??????
scn:Supernova
simple:Supernova
sk:Supernova
sl:Supernova
sr:Supernova
sh:Supernova
su:Supernova
fi:Supernova
sv:Supernova
tl:Supernoba
ta:???????? ???????? ????????
te:????? ????
th:?????????
tr:Süpernova
uk:???????
ur:??? ????
vi:Siêu tân tinh
war:Supernova
zh-yue:???
zh:???

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