a supernova (the bright dot slightly above the galactic center) rapidly brightens, then fades more slowly. Supernova type codes, as summarised in the table above, are
taxonomic: the type number is based on the light observed from the supernova, not necessarily its cause. For example, Type Ia supernovae are produced by runaway fusion ignited on degenerate white dwarf progenitors, while the spectrally similar Type Ib/c are produced from massive stripped progenitor stars by core collapse.
Thermal runaway 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 theorised to happen: stable
accretion of material from a companion, the collision of two white dwarfs, or accretion that causes ignition in a shell that then ignites the core. The dominant mechanism by which Type Ia supernovae are produced remains unclear. Despite this uncertainty in how Type Ia supernovae are produced, Type Ia supernovae have very uniform properties and are useful
standard candles over intergalactic distances. Some calibrations are required to compensate for the gradual change in properties or different frequencies of abnormal luminosity supernovae at high redshift, and for small variations in brightness identified by light curve shape or spectrum.
Normal 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 1.44
solar masses (for a non-rotating star), then it would no longer be able to support the bulk of its mass 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 ignite carbon fusion as the star approaches the limit (to within about 1%) before collapse is initiated. to
unbind the star in a supernova. 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. The model for the formation of this category of supernova is a close binary star system. The more massive 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. The exact details of initiation and of the heavy elements produced in the catastrophic event remain unclear. Type Ia supernovae produce 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 through to . standard candle to measure the distance to their host galaxies. A second 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. This is sometimes referred to as the double-degenerate model, as both stars are degenerate white dwarfs. Due to the possible combinations of mass and chemical composition of the pair there is much variation in this type of event, and, in many cases, there may be no supernova at all, in which case they will have a less luminous light curve than the more normal SN Type Ia.
Non-standard Type Ia Abnormally bright Type Ia supernovae occur when the white dwarf already has a mass higher than the Chandrasekhar limit, possibly enhanced further by asymmetry, but the ejected material will have less than normal kinetic energy. This super-Chandrasekhar-mass scenario can occur, for example, when the extra mass is supported by
differential rotation. There is no formal sub-classification for 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. This type of supernova may not always completely destroy the white dwarf progenitor and could leave behind a
zombie star. One specific type of supernova originates from exploding white dwarfs, like Type Ia, but contains hydrogen lines in their spectra, possibly because the white dwarf is surrounded by an envelope of hydrogen-rich
circumstellar material. These supernovae have been dubbed
Type Ia/IIn,
Type Ian,
Type IIa and
Type IIan. The quadruple star
HD 74438, belonging to the open cluster
IC 2391 the
Vela constellation, has been predicted to become a non-standard Type Ia supernova.
Core collapse 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. However, if the release of gravitational potential energy is insufficient, the star may instead collapse into a
black hole or
neutron star with little radiated energy. • When a massive star develops an iron core larger than the Chandrasekhar mass it will no longer be able to support itself by
electron degeneracy pressure and will collapse further to a neutron star or black hole. • Electron capture by magnesium in a
degenerate O/Ne/Mg core (8–10 solar mass progenitor star) removes support and causes
gravitational collapse followed by explosive oxygen fusion, with very similar results. • 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 stars in which they occur, 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. It appears that a significant proportion of supposed Type IIn supernovae are supernova impostors, massive eruptions of LBV-like stars similar to the Great Eruption of
Eta Carinae. 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.
Detailed process . The surrounding material is blasted away (f), leaving only a degenerate remnant. resulting in a rapid increase in temperature and density. What follows 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. The inner core eventually reaches typically 30
km in diameter At this temperature, neutrino-antineutrino pairs of all
flavours are efficiently formed by
thermal emission. These thermal neutrinos are several times more abundant than the electron-capture neutrinos. About , 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. The suddenly halted core collapse rebounds and produces a shock wave that stalls in the outer core within milliseconds as energy is lost through the dissociation of heavy elements. A process that is is necessary to allow the outer layers of the core to reabsorb around The collapse of a massive non-degenerate core will ignite further fusion.) evolve in a complex fashion, progressively burning heavier elements at hotter temperatures in their cores. The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells. Although popularly described as an onion with an iron core, the least massive supernova progenitors only have oxygen-
neon(-
magnesium) cores. 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. 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 supernova of Type II. At very low metallicity, stars of around will reach core collapse by pair instability while they still have a hydrogen atmosphere and an oxygen core and the result will be a supernova with Type II characteristics but a very large mass of ejected and high luminosity.
Type Ib and Ic (left) and visible light (right), with the brighter SN 2007uy closer to the centre These supernovae, like those of Type II, are massive stars that undergo core collapse. Unlike the progenitors of Type II supernovae, the stars which become Type Ib and Type Ic supernovae have lost most of their outer (hydrogen) envelopes due to strong
stellar winds or else from interaction with a companion. These stars are known as
Wolf–Rayet stars, and they occur at moderate to high metallicity where continuum driven winds cause sufficiently high mass-loss rates. Observations of Type Ib/c supernova do not match the observed or expected occurrence of Wolf–Rayet stars. Alternate explanations for this type of core collapse supernova involve stars stripped of their hydrogen by binary interactions. 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 more common than Type Ic and they 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 mechanism for producing this type of GRB is the jets produced by the magnetic field of the rapidly spinning
magnetar formed at the collapsing core of the star. 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. As a result, very little material is ejected from the exploding star (c. ). In the most extreme cases, ultra-stripped supernovae can occur in naked metal cores, barely above the Chandrasekhar mass limit. SN 2005ek might be the first 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. Ultra-stripped supernovae are believed to be associated with the second supernova explosion in a binary system, producing for example a tight double neutron star system. In 2022 a team of astronomers led by researchers from the
Weizmann Institute of Science reported the first supernova explosion showing direct evidence for a Wolf-Rayet progenitor star.
SN 2019hgp was a Type Icn supernova and is also the first in which the element neon has been detected.
Electron-capture supernovae In 1980, a "third type" of supernova was predicted by
Ken'ichi Nomoto of the
University of Tokyo, called an electron-capture supernova. It would arise when a star "in the transitional range (~8 to 10 solar masses) between white dwarf formation and iron core-collapse supernovae", and with a
degenerate O+Ne+Mg core, The 1054 supernova explosion that created the Crab Nebula in our galaxy had been thought to be the best candidate for an electron-capture supernova, and the 2021 paper makes it more likely that this was correct. The red supergiant
N6946-BH1 in
NGC 6946 underwent a modest outburst in March 2009, before fading from view. Only a faint
infrared source remains at the star's location. Modjaz et al. for Type Ic and IIb; and Nyholm et al. for Type IIn. The gases ejected by a supernova would dim quickly without some energy input to keep them hot. The source of this energy—which can maintain the optical supernova glow for months—was, at first, a puzzle. Some considered rotational energy from the central pulsar as a source. Although the energy that initially powers each type of supernovae is delivered promptly, the light curves are dominated by subsequent radioactive heating of the rapidly expanding ejecta. The intensely radioactive nature of the ejecta gases was first calculated on sound nucleosynthesis grounds in the late 1960s, and this has since been demonstrated as correct for most supernovae. It was not until
SN 1987A that direct observation of gamma-ray lines unambiguously identified the major radioactive nuclei. 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 1987A, is explained by those predicted radioactive decays. Energy for the peak of the light curve of SN1987A was provided by the decay of Isotopes of nickel| to (half-life 6 days) while energy for the later light curve in particular fit very closely with the 77.3-day half-life of Isotopes of cobalt| decaying to . Later measurements by space gamma-ray telescopes of the small fraction of the and gamma rays that escaped the SN 1987A remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power sources. 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 (half-life 6 days), which then decays to radioactive (half-life 77 days). These radioisotopes excite the surrounding material to incandescence. The initial phases of the light curve decline steeply as the effective size of the
photosphere decreases and trapped electromagnetic radiation is depleted. 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 from the remaining becomes dominant, although this portion of the light curve has been little-studied. Type Ib and Ic light curves are 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 . The peak luminosity varies considerably and there are even occasional Type Ib/c supernovae orders of magnitude more and less luminous than the norm. 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 0.05 magnitudes per day, excluding the plateau phase. 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. 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. Notes:
Asymmetry in the
Crab Nebula is travelling at 375 km/s relative to the nebula. 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
peculiar velocities, and black holes presumably do as well, although they 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 km/s or greater. This indicates an expansion asymmetry, but the mechanism by which momentum is transferred to the compact object a puzzle. Proposed explanations for this kick include convection in the collapsing star, asymmetric ejection of matter during
neutron star formation, and asymmetrical
neutrino emissions. One possible explanation for this asymmetry is large-scale
convection above the core. The convection can create radial variations in density giving rise to variations in the amount of energy absorbed from neutrino outflow. 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. (A similar model is used for explaining long gamma-ray bursts.) The dominant mechanism may depend upon the mass of the progenitor star.
Energy output and the
kinetic energy of the ejecta. In core collapse supernovae, the vast majority of the energy is directed into
neutrino emission, and while some of this apparently powers the observed destruction, 99%+ of the neutrinos escape the star in the first few minutes following the start of the collapse. If the relativistic jets are too brief and fail to penetrate the stellar envelope then a low-luminosity gamma-ray burst may be produced and the supernova may be sub-luminous. When a supernova occurs inside a small dense cloud of circumstellar material, it will produce a shock wave that can efficiently convert a high fraction of the kinetic energy into electromagnetic radiation. Even though the initial energy was entirely normal the resulting supernova will have high luminosity and extended duration since it does not rely on exponential radioactive decay. This type of event may cause Type IIn hypernovae. Although pair-instability supernovae are core collapse supernovae with spectra and light curves similar to Type II-P, the nature after core collapse is more like that of a giant Type Ia with runaway fusion of carbon, oxygen and silicon. The total energy released by the highest-mass events is comparable to other core collapse supernovae but neutrino production is thought to be very low, hence the kinetic and electromagnetic energy released is very high. The cores of these stars are much larger than any white dwarf and the amount of radioactive nickel and other heavy elements ejected from their cores can be orders of magnitude higher, with consequently high visual luminosity.
Progenitor The supernova classification type is closely tied to the type of progenitor star at the time of the collapse. The occurrence of each type of supernova depends on the star's metallicity, since this affects the strength of the stellar wind and thereby the rate at which the star loses mass. Type Ia supernovae are produced from white dwarf stars in binary star systems and occur in all
galaxy types. Core collapse supernovae are only found in galaxies undergoing current or very recent star formation, since they result from short-lived massive stars. They are most commonly found in type Sc spirals, but also in the arms of other spiral galaxies and in
irregular galaxies, especially
starburst galaxies. Type Ib and Ic supernovae are hypothesised to have been produced by core collapse of massive stars that have lost their outer layer of hydrogen and helium, either via strong stellar winds or mass transfer to a companion. Type Ic supernovae have been observed to occur in regions that are more metal-rich and have higher star-formation rates than average for their host galaxies. The table shows the progenitor for the main types of core collapse supernova, and the approximate proportions that have been observed in the local neighbourhood. There are a number of difficulties reconciling modelled and observed stellar evolution leading up to core collapse supernovae. Red supergiants are the progenitors for the vast majority of core collapse supernovae, and these have been observed but only at relatively low masses and luminosities, below about and , respectively. Most progenitors of Type II supernovae are not detected and must be considerably fainter, and presumably less massive. This discrepancy has been referred to as the
red supergiant problem. The upper limit for red supergiants that produce a visible supernova explosion has been calculated at . It is thought that higher mass red supergiants do not explode as supernovae, but instead evolve back towards hotter temperatures. Several progenitors of Type IIb supernovae have been confirmed, and these were K and G supergiants, plus one A supergiant. Blue supergiants form an unexpectedly high proportion of confirmed supernova progenitors, partly due to their high luminosity and easy detection, while not a single Wolf–Rayet progenitor has yet been clearly identified. Several examples of hot luminous progenitors of Type IIn supernovae have been detected:
SN 2005gy and
SN 2010jl were both apparently massive luminous stars, but are very distant; and
SN 2009ip had a highly luminous progenitor likely to have been an LBV, but is a peculiar supernova whose exact nature is disputed. Population modelling shows that the observed Type Ib/c supernovae could be reproduced by a mixture of single massive stars and stripped-envelope stars from interacting binary systems. The continued lack of unambiguous detection of progenitors for normal Type Ib and Ic supernovae may be due to most massive stars collapsing directly to a black hole
without a supernova outburst. Most of these supernovae are then produced from lower-mass low-luminosity helium stars in binary systems. A small number would be from rapidly rotating massive stars, likely corresponding to the highly energetic Type Ic-BL events that are associated with long-duration gamma-ray bursts. ==External impact==