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These types would now all be treated as peculiar Type II supernovae, of which many more examples have been discovered, although it is still debated whether SN V was a true supernova following an LBV outburst or an impostor. The type codes, described above given to supernovae, are taxonomic in nature: The following summarizes what is currently believed to be the most plausible explanations for supernovae.

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A white dwarf star may accumulate sufficient material from a stellar companion to raise its core temperature enough to ignite carbon fusion , at which point it undergoes runaway nuclear fusion, completely disrupting it. There are three avenues by which this detonation is theorized to happen: The dominant mechanism by which Type Ia supernovae are produced remains unclear. Some calibrations are required to compensate for the gradual change in properties or different frequencies of abnormal luminosity supernovae at high red shift, and for small variations in brightness identified by light curve shape or spectrum.

There are several means by which a supernova of this type can form, but they share a common underlying mechanism. If a carbon - oxygen [nb 2] white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1. The model for the formation of this category of supernova is a closed 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. Matter from the giant is accreted by the white dwarf, causing the latter to increase in mass. Despite widespread acceptance of the basic model, the exact details of initiation and of the heavy elements produced in the catastrophic event are still unclear.

Type Ia supernovae follow a characteristic light curve —the graph of luminosity as a function of time—after the event. This luminosity is generated by the radioactive decay of nickel through cobalt to iron This allows them to be used as a secondary [68] standard candle to measure the distance to their host galaxies. Another model for the formation of Type Ia supernovae involves the merger of two white dwarf stars, with the combined mass momentarily exceeding the Chandrasekhar limit.

Abnormally bright Type Ia supernovae are expected when the white dwarf already has a mass higher than the Chandrasekhar limit, [72] possibly enhanced further by asymmetry, [73] but the ejected material will have less than normal kinetic energy. There is no formal sub-classification for the non-standard Type Ia supernovae. It has been proposed that a group of sub-luminous supernovae that occur when helium accretes onto a white dwarf should be classified as Type Iax.

One specific type of non-standard Type Ia supernova develops hydrogen, and other, emission lines and gives the appearance of mixture between a normal Type Ia and a Type IIn supernova. Examples are SN ic and SN gj. Very massive stars can undergo core collapse when nuclear fusion becomes unable to sustain the core against its own gravity; passing this threshold is the cause of all types of supernova except Type Ia.

The collapse may cause violent expulsion of the outer layers of the star resulting in a supernova, or the release of gravitational potential energy may be insufficient and the star may collapse into a black hole or neutron star with little radiated energy. Core collapse can be caused by several different mechanisms: Electron-positron pair production in a large post-helium burning core removes thermodynamic support and causes initial collapse followed by runaway fusion, resulting in a pair-instability supernova.

A sufficiently large and hot stellar core may generate gamma-rays energetic enough to initiate photodisintegration directly, which will cause a complete collapse of the core. The table below lists the known reasons for core collapse in massive stars, the types of star that they occur in, their associated supernova type, and the remnant produced. The metallicity is the proportion of elements other than hydrogen or helium, as compared to the Sun.

The initial mass is the mass of the star prior to the supernova event, given in multiples of the Sun's mass, although the mass at the time of the supernova may be much lower. Type IIn supernovae are not listed in the table. They can potentially be produced by various types of core collapse in different progenitor stars, possibly even by Type Ia white dwarf ignitions, although it seems that most will be from iron core collapse in luminous supergiants or hypergiants including LBVs.

The narrow spectral lines for which they are named occur because the supernova is expanding into a small dense cloud of circumstellar material. In these events, material previously ejected from the star creates the narrow absorption lines and causes a shock wave through interaction with the newly ejected material. What follows next depends on the mass and structure of the collapsing core, with low mass degenerate cores forming neutron stars, higher mass degenerate cores mostly collapsing completely to black holes, and non-degenerate cores undergoing runaway fusion.

The initial collapse of degenerate cores is accelerated by beta decay , photodisintegration and electron capture, which causes a burst of electron neutrinos. As the density increases, neutrino emission is cut off as they become trapped in the core. In lower mass cores the collapse is stopped and the newly formed neutron core has an initial temperature of about billion kelvin , times the temperature of the sun's core. These thermal neutrinos are several times more abundant than the electron-capture neutrinos.

A process that is not clearly understood [update] is necessary to allow the outer layers of the core to reabsorb around 10 44 joules [85] 1 foe from the neutrino pulse, producing the visible brightness, although there are also other theories on how to power the explosion. This fallback will reduce the kinetic energy created and the mass of expelled radioactive material, but in some situations it may also generate relativistic jets that result in a gamma-ray burst or an exceptionally luminous supernova. Collapse of massive non-degenerate cores will ignite further fusion.

When the core collapse is initiated by pair instability, oxygen fusion begins and the collapse may be halted. At the upper end of the mass range, the supernova is unusually luminous and extremely long-lived due to many solar masses of ejected 56 Ni. For even larger core masses, the core temperature becomes high enough to allow photodisintegration and the core collapses completely into a black hole.

Stars with initial masses less than about eight times the sun never develop a core large enough to collapse and they eventually lose their atmospheres to become white dwarfs. These super AGB stars may form the majority of core collapse supernovae, although less luminous and so less commonly observed than those from more massive progenitors. If core collapse occurs during a supergiant phase when the star still has a hydrogen envelope, the result is a Type II supernova.

The rate of mass loss for luminous stars depends on the metallicity and luminosity. Extremely luminous stars at near solar metallicity will lose all their hydrogen before they reach core collapse and so will not form a Type II supernova. At low metallicity, all stars will reach core collapse with a hydrogen envelope but sufficiently massive stars collapse directly to a black hole without producing a visible supernova. Stars with an initial mass up to about 90 times the sun, or a little less at high metallicity, are expected to result in a Type II-P supernova which is the most commonly observed type.

At moderate to high metallicity, stars near the upper end of that mass range will have lost most of their hydrogen when core collapse occurs and the result will be a Type II-L supernova. These supernovae, like those of Type II, are massive stars that undergo core collapse. However the stars which become Types Ib and Ic supernovae have lost most of their outer hydrogen envelopes due to strong stellar winds or else from interaction with a companion. Binary models provide a better match for the observed supernovae, with the proviso that no suitable binary helium stars have ever been observed.

Type Ib supernovae are the more common and result from Wolf—Rayet stars of Type WC which still have helium in their atmospheres. For a narrow range of masses, stars evolve further before reaching core collapse to become WO stars with very little helium remaining and these are the progenitors of Type Ic supernovae. A few percent of the Type Ic supernovae are associated with gamma-ray bursts GRB , though it is also believed that any hydrogen-stripped Type Ib or Ic supernova could produce a GRB, depending on the circumstances of the geometry.

The jets would also transfer energy into the expanding outer shell, producing a super-luminous supernova. Ultra-stripped supernovae occur when the exploding star has been stripped almost all the way to the metal core, via mass transfer in a close binary. In the most extreme cases, ultra-stripped supernovae can occur in naked metal cores, barely above the Chandrasekhar mass limit.

SN ek [99] might be an observational example of an ultra-stripped supernova, giving rise to a relatively dim and fast decaying light curve. The nature of ultra-stripped supernovae can be both iron core-collapse and electron capture supernovae, depending on the mass of the collapsing core. The core collapse of some massive stars may not result in a visible supernova. The main model for this is a sufficiently massive core that the kinetic energy is insufficient to reverse the infall of the outer layers onto a black hole. These events are difficult to detect, but large surveys have detected possible candidates.

Only a faint infrared source remains at the star's location. A historic puzzle concerned the source of energy that can maintain the optical supernova glow for months.

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Although the energy that disrupts each type of supernovae is delivered promptly, the light curves are mostly dominated by subsequent radioactive heating of the rapidly expanding ejecta. Some have considered rotational energy from the central pulsar. The ejecta gases would dim quickly without some energy input to keep it hot. The intensely radioactive nature of the ejecta gases, which is now known to be correct for most supernovae, was first calculated on sound nucleosynthesis grounds in the late s.

It is now known by direct observation that much of the light curve the graph of luminosity as a function of time after the occurrence of a Type II Supernova , such as SN A, is explained by those predicted radioactive decays. Although the luminous emission consists of optical photons, it is the radioactive power absorbed by the ejected gases that keeps the remnant hot enough to radiate light. The radioactive decay of 56 Ni through its daughters 56 Co to 56 Fe produces gamma-ray photons , primarily of keV and keV, that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times several weeks to late times several months.

Later measurements by space gamma-ray telescopes of the small fraction of the 56 Co and 57 Co gamma rays that escaped the SN A remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power sources. The visual light curves of the different supernova types all depend at late times on radioactive heating, but they vary in shape and amplitude because of the underlying mechanisms, the way that visible radiation is produced, the epoch of its observation, and the transparency of the ejected material.

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The light curves can be significantly different at other wavelengths. For example, at ultraviolet wavelengths there is an early extremely luminous peak lasting only a few hours corresponding to the breakout of the shock launched by the initial event, but that breakout is hardly detectable optically.

The light curves for Type Ia are mostly very uniform, with a consistent maximum absolute magnitude and a relatively steep decline in luminosity. Their optical energy output is driven by radioactive decay of ejected nickel half life 6 days , which then decays to radioactive cobalt half life 77 days. These radioisotopes excite the surrounding material to incandescence. Studies of cosmology today rely on 56 Ni radioactivity providing the energy for the optical brightness of supernovae of Type Ia, which are the "standard candles" of cosmology but whose diagnostic keV and keV gamma rays were first detected only in The light curve continues to decline in the B band while it may show a small shoulder in the visual at about 40 days, but this is only a hint of a secondary maximum that occurs in the infra-red as certain ionised heavy elements recombine to produce infra-red radiation and the ejecta become transparent to it.

The visual light curve continues to decline at a rate slightly greater than the decay rate of the radioactive cobalt which has the longer half life and controls the later curve , because the ejected material becomes more diffuse and less able to convert the high energy radiation into visual radiation.

After several months, the light curve changes its decline rate again as positron emission becomes dominant from the remaining cobalt, although this portion of the light curve has been little-studied. Type Ib and Ic light curves are basically similar to Type Ia although with a lower average peak luminosity. The visual light output is again due to radioactive decay being converted into visual radiation, but there is a much lower mass of the created nickel The most luminous Type Ic supernovae are referred to as hypernovae and tend to have broadened light curves in addition to the increased peak luminosity.

The source of the extra energy is thought to be relativistic jets driven by the formation of a rotating black hole, which also produce gamma-ray bursts. The light curves for Type II supernovae are characterised by a much slower decline than Type I, on the order of 0. The visual light output is dominated by kinetic energy rather than radioactive decay for several months, due primarily to the existence of hydrogen in the ejecta from the atmosphere of the supergiant progenitor star.

In the initial destruction this hydrogen becomes heated and ionised.

The majority of Type II supernovae show a prolonged plateau in their light curves as this hydrogen recombines, emitting visible light and becoming more transparent. This is then followed by a declining light curve driven by radioactive decay although slower than in Type I supernovae, due to the efficiency of conversion into light by all the hydrogen.

In Type II-L the plateau is absent because the progenitor had relatively little hydrogen left in its atmosphere, sufficient to appear in the spectrum but insufficient to produce a noticeable plateau in the light output. In Type IIb supernovae the hydrogen atmosphere of the progenitor is so depleted thought to be due to tidal stripping by a companion star that the light curve is closer to a Type I supernova and the hydrogen even disappears from the spectrum after several weeks.

Type IIn supernovae are characterised by additional narrow spectral lines produced in a dense shell of circumstellar material. Their light curves are generally very broad and extended, occasionally also extremely luminous and referred to as a superluminous supernova. These light curves are produced by the highly efficient conversion of kinetic energy of the ejecta into electromagnetic radiation by interaction with the dense shell of material.

This only occurs when the material is sufficiently dense and compact, indicating that it has been produced by the progenitor star itself only shortly before the supernova occurs. Large numbers of supernovae have been catalogued and classified to provide distance candles and test models. Average characteristics vary somewhat with distance and type of host galaxy, but can broadly be specified for each supernova type.

A long-standing puzzle surrounding Type II supernovae is why the remaining compact object receives a large velocity away from the epicentre; [] pulsars , and thus neutron stars, are observed to have high velocities, and black holes presumably do as well, although they are far harder to observe in isolation. This indicates an expansion asymmetry, but the mechanism by which momentum is transferred to the compact object remains [update] a puzzle.

Proposed explanations for this kick include convection in the collapsing star and jet production during neutron star formation.


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One possible explanation for this asymmetry is a 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 expansion. Another possible explanation is that accretion of gas onto the central neutron star can create a 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.

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Initial asymmetries have also been confirmed in Type Ia supernovae through observation. This result may mean that the initial luminosity of this type of supernova depends on the viewing angle. However, the expansion becomes more symmetrical with the passage of time. Early asymmetries are detectable by measuring the polarization of the emitted light.

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Although we are used to thinking of supernovae primarily as luminous visible events, the electromagnetic radiation they release is almost a minor side-effect. Particularly in the case of core collapse supernovae, the emitted electromagnetic radiation is a tiny fraction of the total energy released during the event. There is a fundamental difference between the balance of energy production in the different types of supernova.

In Type Ia white dwarf detonations, most of the energy is directed into heavy element synthesis and the kinetic energy of the ejecta. Academic speed dating is a format employed by many leading research organizations, including universities worldwide, as an effective vehicle to promote interdisciplinary and collaborative research.

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