Supernova are stellar explosions that take place when a star reaches its final stages of life. Research shows that they occur once every 3 centuries however the remnants of these explosions take a long time to reach us. Supernova can also take place if a star develops and iron core that is larger than the Chandrasekar mass (the largest mass that a star can reach before imploding). When these events do take place, they can be up to five billion times brighter than the sun!All in all, supernova are very dangerous, but very interesting scientific phenomena.

What are Supernova?

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A supernova (<a href="https://en.wikipedia.org/wiki/Help:IPA/English">/ˌ s uː p ər ˈ n oʊ v ə / </a> plural: supernovae <a href="https://en.wikipedia.org/wiki/Help:IPA/English">/ˌ s uː p ər ˈ n oʊ v iː / </a> or supernovas , abbreviations: SN and SNe ) is a powerful and luminous stellar <a href="https://en.wikipedia.org/wiki/Explosion">explosion </a>. This <a href="https://en.wikipedia.org/wiki/Transient_astronomical_event">transient astronomical event </a>occurs during the last <a href="https://en.wikipedia.org/wiki/Stellar_evolution">evolutionary stages </a>of a <a href="https://en.wikipedia.org/wiki/Massive_star">massive star </a>or when a <a href="https://en.wikipedia.org/wiki/White_dwarf">white dwarf </a>is triggered into runaway <a href="https://en.wikipedia.org/wiki/Nuclear_fusion">nuclear fusion </a>. The original object, called the progenitor , either collapses to a <a href="https://en.wikipedia.org/wiki/Neutron_star">neutron star </a>or <a href="https://en.wikipedia.org/wiki/Black_hole">black hole </a>, or is completely destroyed. The peak optical <a href="https://en.wikipedia.org/wiki/Luminosity">luminosity </a>of a supernova can be comparable to that of an entire <a href="https://en.wikipedia.org/wiki/Galaxy">galaxy </a>before fading over several weeks or months.

Supernovae are more energetic than <a href="https://en.wikipedia.org/wiki/Nova">novae </a>. In <a href="https://en.wikipedia.org/wiki/Latin_language">Latin </a>, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by <a href="https://en.wikipedia.org/wiki/Walter_Baade">Walter Baade </a>and <a href="https://en.wikipedia.org/wiki/Fritz_Zwicky">Fritz Zwicky </a>in <a href="https://en.wikipedia.org/wiki/1929">1929 </a>.

The most recent directly observed supernova in the <a href="https://en.wikipedia.org/wiki/Milky_Way">Milky Way </a>was <a href="https://en.wikipedia.org/wiki/Kepler%27s_Supernova">Kepler's Supernova </a>in 1604, but the <a href="https://en.wikipedia.org/wiki/Supernova_remnant">remnants </a>of more recent supernovae have been found. Observations of supernovae in other galaxies suggest they occur in the Milky Way on average about three times every century. These supernovae would almost certainly be observable with modern astronomical telescopes. The most recent naked-eye supernova was <a href="https://en.wikipedia.org/wiki/SN_1987A">SN 1987A </a>, the explosion of a <a href="https://en.wikipedia.org/wiki/Blue_supergiant_star">blue supergiant star </a>in the <a href="https://en.wikipedia.org/wiki/Large_Magellanic_Cloud">Large Magellanic Cloud </a>, a satellite of the Milky Way.

Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of <a href="https://en.wikipedia.org/wiki/Nuclear_fusion">nuclear fusion </a>in a <a href="https://en.wikipedia.org/wiki/Compact_star">degenerate star </a>such as a white dwarf, or the sudden <a href="https://en.wikipedia.org/wiki/Gravitational_collapse">gravitational collapse </a>of a massive star's <a href="https://en.wikipedia.org/wiki/Stellar_core">core </a>. In the first class of events, the object's temperature is raised enough to trigger <a href="https://en.wikipedia.org/wiki/Thermal_runaway">runaway </a>nuclear fusion, completely disrupting the star. Possible causes are an accumulation of material from a <a href="https://en.wikipedia.org/wiki/Binary_star">binary companion </a>through <a href="https://en.wikipedia.org/wiki/Accretion_(astrophysics)">accretion </a>, or a <a href="https://en.wikipedia.org/wiki/Stellar_merger">stellar merger </a>. In the massive star case, the core of a <a href="https://en.wikipedia.org/wiki/Stellar_evolution#Massive_stars">massive star </a>may undergo sudden collapse, releasing <a href="https://en.wikipedia.org/wiki/Gravitational_potential_energy">gravitational potential energy </a>as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics are established and accepted by the astronomical community.

Supernovae can expel several <a href="https://en.wikipedia.org/wiki/Solar_mass">solar masses </a>of material at speeds up to several percent of the <a href="https://en.wikipedia.org/wiki/Speed_of_light">speed of light </a>. This drives an expanding <a href="https://en.wikipedia.org/wiki/Shock_wave">shock wave </a>into the surrounding <a href="https://en.wikipedia.org/wiki/Interstellar_medium">interstellar medium </a>, sweeping up an expanding shell of gas and dust observed as a <a href="https://en.wikipedia.org/wiki/Supernova_remnant">supernova remnant </a>. Supernovae are a major source of <a href="https://en.wikipedia.org/wiki/Chemical_element">elements </a>in the interstellar medium from <a href="https://en.wikipedia.org/wiki/Oxygen">oxygen </a>to <a href="https://en.wikipedia.org/wiki/Rubidium">rubidium </a>. The expanding shock waves of supernovae can trigger the <a href="https://en.wikipedia.org/wiki/Star_formation">formation of new stars </a>. Supernova remnants might be a major source of <a href="https://en.wikipedia.org/wiki/Cosmic_ray">cosmic rays </a>. Supernovae might produce <a href="https://en.wikipedia.org/wiki/Gravitational_wave">gravitational waves </a>, though thus far, gravitational waves have been detected only from the mergers of black holes and neutron stars.

Compared to a star's entire history, the visual appearance of a supernova is very brief, sometimes spanning several months, so that the chances of observing one with the naked eye is roughly once in a lifetime. Only a tiny fraction of the 100 billion stars in a typical <a href="https://en.wikipedia.org/wiki/Galaxy">galaxy </a>have the capacity to become a supernova, restricted to either those having large mass or extraordinarily rare kinds of <a href="https://en.wikipedia.org/wiki/Binary_star">binary stars </a>containing <a href="https://en.wikipedia.org/wiki/White_dwarf">white dwarfs </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-1">[1] </a>

The earliest possible recorded supernova, known as HB9, could have been viewed and recorded by unknown prehistoric people of <a href="https://en.wikipedia.org/wiki/India">Indian </a>subcontinent, on a rock carving found in Burzahama region in Kashmir, dated to 4500± 1000 <a href="https://en.wikipedia.org/wiki/Before_Christ">BC </a> .<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-Joglekar-2">[2] </a> Later, <a href="https://en.wikipedia.org/wiki/SN_185">SN 185 </a>was viewed by <a href="https://en.wikipedia.org/wiki/Chinese_astronomy">Chinese astronomers </a>in 185 AD. The brightest recorded supernova was <a href="https://en.wikipedia.org/wiki/SN_1006">SN 1006 </a>, which occurred in 1006 AD in the constellation of <a href="https://en.wikipedia.org/wiki/Lupus_(constellation)">Lupus </a>, and was described by observers across China, Japan, Iraq, Egypt, and Europe.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-Murdin-3">[3] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-4">[4] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-winkler-5">[5] </a> The widely observed supernova <a href="https://en.wikipedia.org/wiki/SN_1054">SN 1054 </a>produced the <a href="https://en.wikipedia.org/wiki/Crab_Nebula">Crab Nebula </a>. Supernovae <a href="https://en.wikipedia.org/wiki/SN_1572">SN 1572 </a>and <a href="https://en.wikipedia.org/wiki/SN_1604">SN 1604 </a>, 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 <a href="https://en.wikipedia.org/wiki/Aristotle">Aristotelian </a>idea that the universe beyond the Moon and planets was static and unchanging.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-6">[6] </a> <a href="https://en.wikipedia.org/wiki/Johannes_Kepler">Johannes Kepler </a>began observing SN 1604 at its peak on October 17, 1604, and continued to make estimates of its brightness until it faded from naked eye view a year later.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-kepler-7">[7] </a> It was the second supernova to be observed in a generation (after SN 1572 seen by <a href="https://en.wikipedia.org/wiki/Tycho_Brahe">Tycho Brahe </a>in Cassiopeia).<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-history-8">[8] </a>

There is some evidence that the youngest galactic supernova, <a href="https://en.wikipedia.org/wiki/G1.9%2B0.3">G1.9+0.3 </a>, occurred in the late 19th century, considerably more recently than <a href="https://en.wikipedia.org/wiki/Cassiopeia_A">Cassiopeia A </a>from around 1680.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-chakraborti-9">[9] </a> Neither supernova was noted at the time. In the case of G1.9+0.3, high extinction along the plane of our galaxy could have dimmed the event sufficiently to go unnoticed. The situation for Cassiopeia A is less clear. Infrared <a href="https://en.wikipedia.org/wiki/Light_echo">light echos </a>have been detected showing that it was a type IIb supernova and was not in a region of especially high <a href="https://en.wikipedia.org/wiki/Extinction_(astronomy)">extinction </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-krause-10">[10] </a>

Observation and discovery of extragalactic supernovae are now far more common. The first such observation was of <a href="https://en.wikipedia.org/wiki/SN_1885A">SN 1885A </a>in the <a href="https://en.wikipedia.org/wiki/Andromeda_Galaxy">Andromeda Galaxy </a>. Today, amateur and professional astronomers are finding several hundred every year, some when near maximum brightness, others on old astronomical photographs or plates. American astronomers <a href="https://en.wikipedia.org/wiki/Rudolph_Minkowski">Rudolph Minkowski </a>and <a href="https://en.wikipedia.org/wiki/Fritz_Zwicky">Fritz Zwicky </a>developed the modern supernova classification scheme beginning in 1941.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-11">[11] </a> During the 1960s, astronomers found that the maximum intensities of supernovae could be used as <a href="https://en.wikipedia.org/wiki/Standard_candles">standard candles </a>, hence indicators of astronomical distances.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-12">[12] </a> Some of the most distant supernovae observed in 2003 appeared dimmer than expected. This supports the view that the expansion of the <a href="https://en.wikipedia.org/wiki/Accelerating_universe">universe is accelerating </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-Leibundgut-13">[13] </a> Techniques were developed for reconstructing supernovae events that have no written records of being observed. The date of the <a href="https://en.wikipedia.org/wiki/Cassiopeia_A">Cassiopeia A </a>supernova event was determined from <a href="https://en.wikipedia.org/wiki/Light_echo">light echoes </a>off <a href="https://en.wikipedia.org/wiki/Nebula">nebulae </a>,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-14">[14] </a> while the age of supernova remnant <a href="https://en.wikipedia.org/wiki/RX_J0852.0-4622">RX J0852.0-4622 </a>was estimated from temperature measurements<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-15">[15] </a> and the <a href="https://en.wikipedia.org/wiki/Gamma_ray">gamma ray </a>emissions from the radioactive decay of <a href="https://en.wikipedia.org/wiki/Titanium-44">titanium-44 </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-16">[16] </a>

The most luminous supernova ever recorded is <a href="https://en.wikipedia.org/wiki/ASASSN-15lh">ASASSN-15lh </a>, at a distance of 3.82 <a href="https://en.wikipedia.org/wiki/Light-year">gigalight-years </a>. It was first detected in June 2015 and peaked at 570 billion <a href="https://en.wikipedia.org/wiki/Solar_luminosity">L ☉ </a>, which is twice the <a href="https://en.wikipedia.org/wiki/Bolometric_luminosity">bolometric luminosity </a>of any other known supernova.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-18">[18] </a> However, the nature of this supernova continues to be debated and several alternative explanations have been suggested, e.g. tidal disruption of a star by a black hole.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-19">[19] </a>

Among the earliest detected since time of detonation, and for which the earliest spectra have been obtained (beginning at 6 hours after the actual explosion), is the type II <a href="https://en.wikipedia.org/wiki/SN_2013fs">SN 2013fs </a>(iPTF13dqy) which was recorded 3 hours after the supernova event on 6 October 2013 by the <a href="https://en.wikipedia.org/wiki/Palomar_Transient_Factory">Intermediate Palomar Transient Factory </a>(iPTF). The star is located in a <a href="https://en.wikipedia.org/wiki/Spiral_galaxy">spiral galaxy </a>named <a href="https://en.wikipedia.org/wiki/NGC_7610">NGC 7610 </a>, 160 million light-years away in the constellation of Pegasus.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-20">[20] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-21">[21] </a>

On 20 September 2016, amateur astronomer Victor Buso from <a href="https://en.wikipedia.org/wiki/Rosario">Rosario </a>, <a href="https://en.wikipedia.org/wiki/Argentina">Argentina </a>was testing his telescope.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-AS_Now-22">[22] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-NatBuso-23">[23] </a> When taking several photographs of galaxy <a href="https://en.wikipedia.org/wiki/NGC_613">NGC 613 </a>, Buso chanced upon a supernova that had just become visible on Earth. After examining the images, he contacted the Instituto de Astrofísica de La Plata. "It was the first time anyone had ever captured the initial moments of the 'shock breakout' from an optical supernova, one not associated with a gamma-ray or X-ray burst."<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-AS_Now-22">[22] </a> The odds of capturing such an event were put between one in ten million to one in a hundred million, according to astronomer Melina Bersten from the Instituto de Astrofísica. The supernova Buso observed was a type IIb made by a star twenty times the mass of the sun.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-AS_Now-22">[22] </a> Astronomer <a href="https://en.wikipedia.org/wiki/Alex_Filippenko">Alex Filippenko </a>, from the <a href="https://en.wikipedia.org/wiki/University_of_California">University of California </a>, remarked that professional astronomers had been searching for such an event for a long time. He stated: "Observations of stars in the first moments they begin exploding provide information that cannot be directly obtained in any other way."<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-AS_Now-22">[22] </a>

Early work on what was originally believed to be simply a new category of <a href="https://en.wikipedia.org/wiki/Nova">novae </a>was performed during the 1920s. These were variously called "upper-class Novae", "Hauptnovae", or "giant novae".<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-bode-24">[24] </a> The name "supernovae" is thought to have been coined by <a href="https://en.wikipedia.org/wiki/Walter_Baade">Walter Baade </a>and <a href="https://en.wikipedia.org/wiki/Fritz_Zwicky">Fritz Zwicky </a>in lectures at <a href="https://en.wikipedia.org/wiki/Caltech">Caltech </a>during 1931. It was used, as "super-Novae", in a journal paper published by <a href="https://en.wikipedia.org/wiki/Knut_Lundmark">Knut Lundmark </a>in 1933,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-baas33_1330-25">[25] </a> and in a 1934 paper by Baade and Zwicky.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-Baade1934-26">[26] </a> By 1938, the hyphen had been lost and the modern name was in use.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-27">[27] </a> Because supernovae are relatively rare events within a galaxy, occurring about three times a century in the Milky Way,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-reynolds-28">[28] </a> 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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-29">[29] </a> To use supernovae as <a href="https://en.wikipedia.org/wiki/Standard_candle">standard candles </a>for measuring distance, observation of their peak luminosity is required. It is therefore important to discover them well before they reach their maximum. <a href="https://en.wikipedia.org/wiki/Amateur_astronomy">Amateur astronomers </a>, who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an <a href="https://en.wikipedia.org/wiki/Optical_telescope">optical telescope </a>and comparing them to earlier photographs.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-30">[30] </a>

Toward the end of the 20th century, astronomers increasingly turned to computer-controlled telescopes and <a href="https://en.wikipedia.org/wiki/Charge-coupled_device">CCDs </a>for hunting supernovae. While such systems are popular with amateurs, there are also professional installations such as the <a href="https://en.wikipedia.org/wiki/Katzman_Automatic_Imaging_Telescope">Katzman Automatic Imaging Telescope </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-31">[31] </a> The <a href="https://en.wikipedia.org/wiki/Supernova_Early_Warning_System">Supernova Early Warning System </a>(SNEWS) project uses a network of <a href="https://en.wikipedia.org/wiki/Neutrino_detector">neutrino detectors </a>to give early warning of a supernova in the Milky Way galaxy.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-32">[32] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-33">[33] </a> <a href="https://en.wikipedia.org/wiki/Neutrino">Neutrinos </a>are <a href="https://en.wikipedia.org/wiki/Subatomic_particle">particles </a>that are produced in great quantities by a supernova, and they are not significantly absorbed by the interstellar gas and dust of the galactic disk.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-34">[34] </a>

Supernova searches fall into two classes: those focused on relatively nearby events and those looking farther away. Because of the <a href="https://en.wikipedia.org/wiki/Metric_expansion_of_space">expansion of the universe </a>, the distance to a remote object with a known <a href="https://en.wikipedia.org/wiki/Emission_spectrum">emission spectrum </a>can be estimated by measuring its <a href="https://en.wikipedia.org/wiki/Doppler_shift">Doppler shift </a>(or <a href="https://en.wikipedia.org/wiki/Redshift">redshift </a>); 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<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-35">[35] </a> —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 <a href="https://en.wikipedia.org/wiki/Hubble_diagram">Hubble diagrams </a>and make cosmological predictions. Supernova spectroscopy, used to study the physics and environments of supernovae, is more practical at low than at high redshift.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-36">[36] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-37">[37] </a> Low redshift observations also anchor the low-distance end of the <a href="https://en.wikipedia.org/wiki/Hubble_curve">Hubble curve </a>, which is a plot of distance versus redshift for visible galaxies.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-38">[38] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-39">[39] </a>

Supernova discoveries are reported to the <a href="https://en.wikipedia.org/wiki/International_Astronomical_Union">International Astronomical Union </a>'s <a href="https://en.wikipedia.org/wiki/Central_Bureau_for_Astronomical_Telegrams">Central Bureau for Astronomical Telegrams </a>, which sends out a circular with the name it assigns to that supernova. The name is formed from the prefix 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 2003C designates the third supernova reported in the year 2003.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-40">[40] </a> The last supernova of 2005, SN 2005nc, was the 367th (14 × 26 + 3 = 367). The suffix "nc" acts as a <a href="https://en.wikipedia.org/wiki/Bijective_numeration#The_bijective_base-26_system">bijective base-26 </a>encoding, with a = 1, b = 2, c = 3, ... z = 26. Since 2000, professional and amateur astronomers have been finding several hundred supernovae each year (572 in 2007, 261 in 2008, 390 in 2009; 231 in 2013).<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-41">[41] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-42">[42] </a>

Historical supernovae are known simply by the year they occurred: <a href="https://en.wikipedia.org/wiki/SN_185">SN 185 </a>, <a href="https://en.wikipedia.org/wiki/SN_1006">SN 1006 </a>, <a href="https://en.wikipedia.org/wiki/SN_1054">SN 1054 </a>, <a href="https://en.wikipedia.org/wiki/SN_1572">SN 1572 </a>(called Tycho's Nova ) and <a href="https://en.wikipedia.org/wiki/SN_1604">SN 1604 </a>(Kepler's Star ). Since 1885 the additional letter notation has been used, even if there was only one supernova discovered that year (e.g. <a href="https://en.wikipedia.org/wiki/SN_1885A">SN 1885A </a>, <a href="https://en.wikipedia.org/w/index.php?title=SN_1907A&action=edit&redlink=1">SN 1907A </a>, etc.)—this last happened with <a href="https://en.wikipedia.org/w/index.php?title=SN_1947A&action=edit&redlink=1">SN 1947A </a>. SN , for SuperNova, is a standard prefix. Until 1987, two-letter designations were rarely needed; since 1988, however, they have been needed every year. Since 2016, the increasing number of discoveries has regularly led to the additional use of three-digit designations.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-43">[43] </a>

Astronomers classify supernovae according to their <a href="https://en.wikipedia.org/wiki/Light_curve">light curves </a>and the <a href="https://en.wikipedia.org/wiki/Absorption_line">absorption lines </a>of different <a href="https://en.wikipedia.org/wiki/Chemical_element">chemical elements </a>that appear in their <a href="https://en.wikipedia.org/wiki/Astronomical_spectroscopy">spectra </a>. If a supernova's spectrum contains lines of <a href="https://en.wikipedia.org/wiki/Hydrogen">hydrogen </a>(known as the <a href="https://en.wikipedia.org/wiki/Balmer_series">Balmer series </a>in the visual portion of the spectrum) it is classified Type II ; otherwise it is Type I . In each of these two types there are subdivisions according to the presence of lines from other elements or the shape of the <a href="https://en.wikipedia.org/wiki/Light_curve">light curve </a>(a graph of the supernova's <a href="https://en.wikipedia.org/wiki/Apparent_magnitude">apparent magnitude </a>as a function of time).<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-types-45">[45] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-taxonomy-46">[46] </a>

Type I supernovae are subdivided on the basis of their spectra, with type Ia showing a strong <a href="https://en.wikipedia.org/wiki/Silicon">ionised silicon </a><a href="https://en.wikipedia.org/wiki/Absorption_line">absorption line </a>. Type I supernovae without this strong line are classified as type Ib and Ic, with type Ib showing strong neutral helium lines and type Ic lacking them. The light curves are all similar, although type Ia are generally brighter at peak luminosity, but the light curve is not important for classification of type I supernovae.

A small number of type Ia supernovae exhibit unusual features, such as non-standard luminosity or broadened light curves, and these are typically classified by referring to the earliest example showing similar features. For example, the sub-luminous <a href="https://en.wikipedia.org/wiki/SN_2008ha">SN 2008ha </a>is often referred to as <a href="https://en.wikipedia.org/wiki/SN_2002cx">SN 2002cx </a>-like or class Ia-2002cx.

A small proportion of type Ic supernovae show highly broadened and blended emission lines which are taken to indicate very high expansion velocities for the ejecta. These have been classified as type Ic-BL or Ic-bl.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-bianco-48">[48] </a>

The supernovae of <a href="https://en.wikipedia.org/wiki/Type_II_supernova">type II </a>can also be sub-divided based on their spectra. While most type II supernovae show very broad <a href="https://en.wikipedia.org/wiki/Emission_line">emission lines </a>which indicate expansion velocities of many thousands of <a href="https://en.wikipedia.org/wiki/Kilometres_per_second">kilometres per second </a>, some, such as <a href="https://en.wikipedia.org/wiki/SN_2005gl">SN 2005gl </a>, have relatively narrow features in their spectra. These are called type IIn, where the 'n' stands for 'narrow'.

A few supernovae, such as <a href="https://en.wikipedia.org/w/index.php?title=SN_1987K&action=edit&redlink=1">SN 1987K </a><a href="https://en.wikipedia.org/wiki/Supernova#cite_note-49">[49] </a> and <a href="https://en.wikipedia.org/wiki/SN_1993J">SN 1993J </a>, 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 <a href="https://en.wikipedia.org/wiki/Type_IIb_supernova">"type IIb" </a>is used to describe the combination of features normally associated with types II and Ib.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-taxonomy-46">[46] </a>

Type II supernovae with normal spectra dominated by broad hydrogen lines that remain for the life of the decline are classified on the basis of their light curves. The most common type shows a distinctive "plateau" in the light curve shortly after peak brightness where the visual luminosity stays relatively constant for several months before the decline resumes. These are called type II-P referring to the plateau. Less common are type II-L supernovae that lack a distinct plateau. The "L" signifies "linear" although the light curve is not actually a straight line.

Supernovae that do not fit into the normal classifications are designated peculiar, or 'pec'.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-taxonomy-46">[46] </a>

<a href="https://en.wikipedia.org/wiki/Fritz_Zwicky">Fritz Zwicky </a>defined additional supernovae types based on a very few examples that did not cleanly fit the parameters for type I or type II supernovae. <a href="https://en.wikipedia.org/wiki/SN_1961i">SN 1961i </a>in <a href="https://en.wikipedia.org/wiki/NGC_4303">NGC 4303 </a>was the prototype and only member of the type III supernova class, noted for its broad light curve maximum and broad hydrogen Balmer lines that were slow to develop in the spectrum. <a href="https://en.wikipedia.org/w/index.php?title=SN_1961f&action=edit&redlink=1">SN 1961f </a>in <a href="https://en.wikipedia.org/wiki/NGC_3003">NGC 3003 </a>was the prototype and only member of the type IV class, with a light curve similar to a type II-P supernova, with <a href="https://en.wikipedia.org/wiki/Hydrogen_absorption_line">hydrogen absorption lines </a>but weak <a href="https://en.wikipedia.org/wiki/Hydrogen_emission_line">hydrogen emission lines </a>. The type V class was coined for <a href="https://en.wikipedia.org/wiki/SN_1961V">SN 1961V </a>in <a href="https://en.wikipedia.org/wiki/NGC_1058">NGC 1058 </a>, an unusual faint supernova or <a href="https://en.wikipedia.org/wiki/Supernova_impostor">supernova impostor </a>with a slow rise to brightness, a maximum lasting many months, and an unusual emission spectrum. The similarity of SN 1961V to the <a href="https://en.wikipedia.org/wiki/Eta_Carinae">Eta Carinae </a>Great Outburst was noted.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-zwicky2-50">[50] </a> Supernovae in M101 (1909) and M83 (1923 and 1957) were also suggested as possible type IV or type V supernovae.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-zwicky-51">[51] </a>

These types would now all be treated as peculiar type II supernovae (IIpec), of which many more examples have been discovered, although it is still debated whether SN 1961V was a true supernova following an <a href="https://en.wikipedia.org/wiki/Luminous_blue_variable">LBV </a>outburst or an impostor.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-doggett-47">[47] </a>

Supernovae type codes, as described above, are <a href="https://en.wikipedia.org/wiki/Taxonomy_(general)">taxonomic </a> : the type number describes the light observed from the supernova, not necessarily its cause. For example, type Ia supernovae are produced by runaway fusion ignited on degenerate <a href="https://en.wikipedia.org/wiki/White_dwarf">white dwarf </a>progenitors, while the spectrally similar type Ib/c are produced from massive Wolf–Rayet progenitors by core collapse. The following summarises what is currently believed to be the most plausible explanations for supernovae.

A white dwarf star may accumulate sufficient material from a <a href="https://en.wikipedia.org/wiki/Binary_star">stellar companion </a>to raise its core temperature enough to <a href="https://en.wikipedia.org/wiki/Carbon_detonation">ignite </a><a href="https://en.wikipedia.org/wiki/Carbon_burning_process">carbon fusion </a>, at which point it undergoes <a href="https://en.wikipedia.org/wiki/Thermal_runaway">runaway </a>nuclear fusion, completely disrupting it. There are three avenues by which this detonation is theorised to happen: stable <a href="https://en.wikipedia.org/wiki/Accretion_(astrophysics)">accretion </a>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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-Piro2014-53">[53] </a> 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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-chen-54">[54] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-55">[55] </a>

There are several means by which a supernova of this type can form, but they share a common underlying mechanism. If a <a href="https://en.wikipedia.org/wiki/Carbon">carbon </a>-<a href="https://en.wikipedia.org/wiki/Oxygen">oxygen </a><a href="https://en.wikipedia.org/wiki/White_dwarf">white dwarf </a>accreted enough matter to reach the <a href="https://en.wikipedia.org/wiki/Chandrasekhar_limit">Chandrasekhar limit </a>of about 1.44 <a href="https://en.wikipedia.org/wiki/Solar_mass">solar masses </a>(<a href="https://en.wikipedia.org/wiki/Solar_mass">M ☉ </a>)<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-Mazzali2007-56">[56] </a> (for a non-rotating star), it would no longer be able to support the bulk of its mass through <a href="https://en.wikipedia.org/wiki/Electron_degeneracy_pressure">electron degeneracy pressure </a><a href="https://en.wikipedia.org/wiki/Supernova#cite_note-Chandrasekhar-57">[57] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-canal1997-58">[58] </a> and would begin to collapse. However, the current view is that this limit is not normally attained; increasing temperature and density inside the core <a href="https://en.wikipedia.org/wiki/Carbon_detonation">ignite </a><a href="https://en.wikipedia.org/wiki/Carbon_burning_process">carbon fusion </a>as the star approaches the limit (to within about 1%<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-59">[59] </a> ) before collapse is initiated.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-Mazzali2007-56">[56] </a> For a core primarily composed of oxygen, neon and magnesium, the collapsing white dwarf will typically form a <a href="https://en.wikipedia.org/wiki/Neutron_star">neutron star </a>. In this case, only a fraction of the star's mass will be ejected during the collapse.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-canal1997-58">[58] </a>

Within a few seconds, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (1–2× 1044  <a href="https://en.wikipedia.org/wiki/Joule">J </a> )<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-aaa270-60">[60] </a> to <a href="https://en.wikipedia.org/wiki/Gravitational_binding_energy">unbind </a>the star in a supernova.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-ropke-61">[61] </a> An outwardly expanding <a href="https://en.wikipedia.org/wiki/Shock_wave">shock wave </a>is generated, with matter reaching velocities on the order of 5,000–20,000 <a href="https://en.wikipedia.org/wiki/Kilometers_per_second">km/s </a>, or roughly 3% of the speed of light. There is also a significant increase in luminosity, reaching an <a href="https://en.wikipedia.org/wiki/Absolute_magnitude">absolute magnitude </a>of −19.3 (or 5 billion times brighter than the Sun), with little variation.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-explosion_model-62">[62] </a>

The model for the formation of this category of supernova is a close <a href="https://en.wikipedia.org/wiki/Binary_star">binary star </a>system. The larger of the two stars is the first to <a href="https://en.wikipedia.org/wiki/Stellar_evolution">evolve </a>off the <a href="https://en.wikipedia.org/wiki/Main_sequence">main sequence </a>, and it expands to form a <a href="https://en.wikipedia.org/wiki/Red_giant">red giant </a>. 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 <a href="https://en.wikipedia.org/wiki/Nuclear_fusion">nuclear fusion </a>. At this point, it becomes a white dwarf star, composed primarily of carbon and oxygen.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-63">[63] </a> 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. 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 <a href="https://en.wikipedia.org/wiki/Light_curve">light curve </a>—the graph of luminosity as a function of time—after the event. This luminosity is generated by the <a href="https://en.wikipedia.org/wiki/Radioactive_decay">radioactive decay </a>of <a href="https://en.wikipedia.org/wiki/Nickel">nickel </a>-56 through <a href="https://en.wikipedia.org/wiki/Cobalt">cobalt </a>-56 to <a href="https://en.wikipedia.org/wiki/Iron">iron </a>-56.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-explosion_model-62">[62] </a> The peak luminosity of the light curve is extremely consistent across normal type Ia supernovae, having a maximum <a href="https://en.wikipedia.org/wiki/Absolute_magnitude">absolute magnitude </a>of about −19.3. This is because type Ia supernovae arise from a consistent type of progenitor star by gradual mass acquisition, and explode when they acquire a consistent typical mass, giving rise to very similar supernova conditions and behavior. This allows them to be used as a secondary<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-64">[64] </a> <a href="https://en.wikipedia.org/wiki/Standard_candle">standard candle </a>to measure the distance to their host galaxies.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-65">[65] </a>

Another model for the formation of type Ia supernovae involves the merger of two white dwarf stars, with the combined mass momentarily exceeding the <a href="https://en.wikipedia.org/wiki/Chandrasekhar_limit">Chandrasekhar limit </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-66">[66] </a> There is much variation in this type of event,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-67">[67] </a> and, in many cases, there may be no supernova at all, in which case they will have a broader and less luminous light curve than the more normal SN type Ia.

Abnormally bright type Ia supernovae occur when the white dwarf already has a mass higher than the Chandrasekhar limit,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-68">[68] </a> possibly enhanced further by asymmetry,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-69">[69] </a> 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 .<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-wang-70">[70] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-foley-71">[71] </a> This type of supernova may not always completely destroy the white dwarf progenitor and could leave behind a <a href="https://en.wikipedia.org/wiki/Zombie_star">zombie star </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-mccully-72">[72] </a>

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 <a href="https://en.wikipedia.org/w/index.php?title=SN_2002ic&action=edit&redlink=1">SN 2002ic </a>and <a href="https://en.wikipedia.org/wiki/SN_2005gj">SN 2005gj </a>. These supernovae have been dubbed type Ia/IIn , type Ian , type IIa and type IIan .<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-73">[73] </a>

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 <a href="https://en.wikipedia.org/wiki/Black_hole">black hole </a>or <a href="https://en.wikipedia.org/wiki/Neutron_star">neutron star </a>with little radiated energy.

Core collapse can be caused by several different mechanisms: exceeding the <a href="https://en.wikipedia.org/wiki/Chandrasekhar_limit">Chandrasekhar limit </a>; <a href="https://en.wikipedia.org/wiki/Electron_capture">electron capture </a>; <a href="https://en.wikipedia.org/wiki/Pair-instability_supernova">pair-instability </a>; or <a href="https://en.wikipedia.org/wiki/Photodisintegration">photodisintegration </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-heger-74">[74] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-nomoto-75">[75] </a>

<ul><li>When a massive star develops an iron core larger than the Chandrasekhar mass it will no longer be able to support itself by <a href="https://en.wikipedia.org/wiki/Electron_degeneracy_pressure">electron degeneracy pressure </a>and will collapse further to a neutron star or black hole. </li><li>Electron capture by magnesium in a <a href="https://en.wikipedia.org/wiki/Degenerate_star">degenerate </a>O/Ne/Mg core causes <a href="https://en.wikipedia.org/wiki/Gravitational_collapse">gravitational collapse </a>followed by explosive oxygen fusion, with very similar results. </li><li>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. </li><li>A sufficiently large and hot <a href="https://en.wikipedia.org/wiki/Stellar_core">stellar core </a>may generate gamma-rays energetic enough to initiate photodisintegration directly, which will cause a complete collapse of the core. </li> </ul>

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 <a href="https://en.wikipedia.org/wiki/Metallicity">metallicity </a>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 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 <a href="https://en.wikipedia.org/wiki/Supergiant">supergiants </a>or <a href="https://en.wikipedia.org/wiki/Hypergiant">hypergiants </a>(including <a href="https://en.wikipedia.org/wiki/Luminous_blue_variable">LBVs </a>). The narrow spectral lines for which they are named occur because the supernova is expanding into a small dense cloud of circumstellar material.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-moriya-76">[76] </a> It appears that a significant proportion of supposed type IIn supernovae are <a href="https://en.wikipedia.org/wiki/Supernova_impostor">supernova impostors </a>, massive eruptions of <a href="https://en.wikipedia.org/wiki/Luminous_blue_variable">LBV </a>-like stars similar to the Great Eruption of <a href="https://en.wikipedia.org/wiki/Eta_Carinae">Eta Carinae </a>. 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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-smith-77">[77] </a>

When a stellar core is no longer supported against gravity, it collapses in on itself with velocities reaching 70,000 km/s (0.23<a href="https://en.wikipedia.org/wiki/Speed_of_light">c </a>),<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-grav_waves-78">[78] </a> resulting in a rapid increase in temperature and density. 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 <a href="https://en.wikipedia.org/wiki/Beta_decay">beta decay </a>, photodisintegration and electron capture, which causes a burst of <a href="https://en.wikipedia.org/wiki/Electron_neutrino">electron neutrinos </a>. As the density increases, <a href="https://en.wikipedia.org/wiki/Supernova_neutrinos">neutrino emission </a>is cut off as they become trapped in the core. The inner core eventually reaches typically 30 <a href="https://en.wikipedia.org/wiki/Kilometre">km </a>in diameter<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-WoosleyJanka-79">[79] </a> and a density comparable to that of an <a href="https://en.wikipedia.org/wiki/Atomic_nucleus">atomic nucleus </a>, and neutron <a href="https://en.wikipedia.org/wiki/Degeneracy_pressure">degeneracy pressure </a>tries to halt the collapse. If the core mass is more than about 15 <a href="https://en.wikipedia.org/wiki/Solar_mass">M ☉ </a>then neutron degeneracy is insufficient to stop the collapse and a black hole forms directly with no supernova.

In lower mass cores the collapse is stopped and the newly formed neutron core has an initial temperature of about 100 billion <a href="https://en.wikipedia.org/wiki/Kelvin">kelvin </a>, 6000 times the temperature of the sun's core.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-80">[80] </a> At this temperature, neutrino-antineutrino pairs of all <a href="https://en.wikipedia.org/wiki/Neutrino_oscillation">flavours </a>are efficiently formed by <a href="https://en.wikipedia.org/wiki/Thermal_radiation">thermal emission </a>. These thermal neutrinos are several times more abundant than the electron-capture neutrinos.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-81">[81] </a> About 1046 joules, 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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-WoosleyJanka-79">[79] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-barwick-82">[82] </a> The suddenly halted core collapse rebounds and produces a <a href="https://en.wikipedia.org/wiki/Shock_wave">shock wave </a>that stalls within milliseconds<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-83">[83] </a> in the outer core as energy is lost through the dissociation of heavy elements. A process that is not clearly understood is necessary to allow the outer layers of the core to reabsorb around 1044 joules<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-barwick-82">[82] </a> (1 <a href="https://en.wikipedia.org/wiki/Foe_(unit)">foe </a>) from the neutrino pulse, producing the visible brightness, although there are also other theories on how to power the explosion.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-WoosleyJanka-79">[79] </a>

Some material from the outer envelope falls back onto the neutron star, and, for cores beyond about 8 M ☉ , there is sufficient fallback to form a black hole. 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.

The collapse of a massive non-degenerate core will ignite further fusion. When the core collapse is initiated by pair instability, oxygen fusion begins and the collapse may be halted. For core masses of 40–60 M ☉ , the collapse halts and the star remains intact, but collapse will occur again when a larger core has formed. For cores of around 60–130 M ☉ , the fusion of oxygen and heavier elements is so energetic that the entire star is disrupted, causing a supernova. 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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-kasen-84">[84] </a>

Stars with initial masses less than about 8 M ☉ never develop a core large enough to collapse and they eventually lose their atmospheres to become white dwarfs. Stars with at least 9 M ☉ (possibly as much as 12 M ☉ <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-superagb-85">[85] </a> ) evolve in a complex fashion, progressively burning heavier elements at hotter temperatures in their cores.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-WoosleyJanka-79">[79] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-science304-86">[86] </a> The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-heger-74">[74] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-hinshaw-87">[87] </a> Although popularly described as an onion with an iron core, the least massive supernova progenitors only have <a href="https://en.wikipedia.org/wiki/Oxygen">oxygen </a>-<a href="https://en.wikipedia.org/wiki/Neon">neon </a>(-<a href="https://en.wikipedia.org/wiki/Magnesium">magnesium </a>) cores. These <a href="https://en.wikipedia.org/wiki/Super-AGB_star">super-AGB stars </a>may form the majority of core collapse supernovae, although less luminous and so less commonly observed than those from more massive progenitors.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-superagb-85">[85] </a>

If core collapse occurs during a <a href="https://en.wikipedia.org/wiki/Supergiant">supergiant </a>phase when the star still has a <a href="https://en.wikipedia.org/wiki/Hydrogen">hydrogen </a>envelope, the result is a type II supernova. The rate of mass loss for luminous stars depends on the <a href="https://en.wikipedia.org/wiki/Metallicity">metallicity </a>and <a href="https://en.wikipedia.org/wiki/Luminosity">luminosity </a>. 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, 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. At very low metallicity, stars of around 140–250 M ☉ 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 56 Ni and high luminosity.

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 <a href="https://en.wikipedia.org/wiki/Stellar_wind">stellar winds </a>or else from interaction with a companion.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-90">[90] </a> These stars are known as <a href="https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet_star">Wolf–Rayet stars </a>, 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 and 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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-eldridge-91">[91] </a> Since a supernova can occur whenever the mass of the star at the time of core collapse is low enough not to cause complete fallback to a black hole, any massive star may result in a supernova if it loses enough mass before core collapse occurs.

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 <a href="https://en.wikipedia.org/wiki/Gamma-ray_burst">gamma-ray bursts </a>(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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-92">[92] </a> The mechanism for producing this type of GRB is the jets produced by the magnetic field of the rapidly spinning <a href="https://en.wikipedia.org/wiki/Magnetar">magnetar </a>formed at the collapsing core of the star. The jets would also transfer energy into the expanding outer shell, producing a <a href="https://en.wikipedia.org/wiki/Super-luminous_supernova">super-luminous supernova </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-93">[93] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-94">[94] </a>

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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-95">[95] </a> As a result, very little material is ejected from the exploding star (c. 0.1 M ☉ ). In the most extreme cases, ultra-stripped supernovae can occur in naked metal cores, barely above the Chandrasekhar mass limit. SN 2005ek<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-96">[96] </a> 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.

In 1980 a "third type" of supernova was predicted by Ken'ichi Nomoto of the <a href="https://en.wikipedia.org/wiki/University_of_Tokyo">University of Tokyo </a>, called an <a href="https://en.wikipedia.org/wiki/Electron_capture">electron-capture </a>supernova. It would arise when a star "in the transitional range (~8 to 10 solar masses) between <a href="https://en.wikipedia.org/wiki/White_dwarf">white dwarf </a>formation and iron core-collapse supernovae", and with a <a href="https://en.wikipedia.org/wiki/Degenerate_matter">degenerate </a><a href="https://en.wikipedia.org/wiki/Oxygen">O </a>+<a href="https://en.wikipedia.org/wiki/Neon">Ne </a>+<a href="https://en.wikipedia.org/wiki/Magnesium">Mg </a>core,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-HowellEtAl2021-01a-97">[97] </a> imploded after its core ran out of nuclear fuel, causing gravity to compress the <a href="https://en.wikipedia.org/wiki/Electron">electrons </a>in the star's core into their <a href="https://en.wikipedia.org/wiki/Atomic_nucleus">atomic nuclei </a>,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-KeckObsNews2021-06-28-01a-98">[98] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-RTE2021-06-28-01a-99">[99] </a> leading to a supernova explosion and leaving behind a <a href="https://en.wikipedia.org/wiki/Neutron_star">neutron star </a>.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-heger-74">[74] </a> In June 2021 a paper in the journal <a href="https://en.wikipedia.org/wiki/Nature_Astronomy">Nature Astronomy </a> reported that the 2018 supernova <a href="https://en.wikipedia.org/wiki/SN_2018zd">SN 2018zd </a>(in the galaxy <a href="https://en.wikipedia.org/wiki/NGC_2146">NGC 2146 </a>, about 31 million light years from Earth) appeared to be the first observation of an electron-capture supernova<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-HowellEtAl2021-01a-97">[97] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-KeckObsNews2021-06-28-01a-98">[98] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-RTE2021-06-28-01a-99">[99] </a> The 1054 supernova explosion that created the famous <a href="https://en.wikipedia.org/wiki/Crab_Nebula">Crab Nebula </a>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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-KeckObsNews2021-06-28-01a-98">[98] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-RTE2021-06-28-01a-99">[99] </a>

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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-failed-100">[100] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-gerke-101">[101] </a> The red supergiant <a href="https://en.wikipedia.org/wiki/N6946-BH1">N6946-BH1 </a>in <a href="https://en.wikipedia.org/wiki/NGC_6946">NGC 6946 </a>underwent a modest outburst in March 2009, before fading from view. Only a faint <a href="https://en.wikipedia.org/wiki/Infrared">infrared </a>source remains at the star's location.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-adams-102">[102] </a>

A historic puzzle concerned the source of energy that can maintain the optical supernova glow for months. Although the energy that disrupts each type of supernovae is delivered promptly, the light curves are 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 1960s.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-103">[103] </a> It was not until <a href="https://en.wikipedia.org/wiki/SN_1987A">SN 1987A </a>that direct observation of gamma-ray lines unambiguously identified the major radioactive nuclei.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-S.M._Matz,_G.H._Share_1988-104">[104] </a>

It is now known by direct observation that much of the <a href="https://en.wikipedia.org/wiki/Light_curve">light curve </a>(the graph of luminosity as a function of time) after the occurrence of a <a href="https://en.wikipedia.org/wiki/Type_II_Supernova">type II Supernova </a>, such as SN 1987A, is explained by those predicted <a href="https://en.wikipedia.org/wiki/Radioactive_decay">radioactive decays </a>. 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 <a href="https://en.wikipedia.org/wiki/Radioactive_decay">radioactive decay </a>of 56 <a href="https://en.wikipedia.org/wiki/Nickel">Ni </a>through its daughters 56 <a href="https://en.wikipedia.org/wiki/Cobalt">Co </a>to 56 <a href="https://en.wikipedia.org/wiki/Iron">Fe </a>produces gamma-ray <a href="https://en.wikipedia.org/wiki/Photon">photons </a>, primarily of 847keV and 1238keV, that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times (several weeks) to late times (several months).<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-105">[105] </a> Energy for the peak of the light curve of SN1987A was provided by the decay of <a href="https://en.wikipedia.org/wiki/Isotopes_of_nickel">56 Ni </a>to 56 Co (half-life 6 days) while energy for the later light curve in particular fit very closely with the 77.3-day half-life of <a href="https://en.wikipedia.org/wiki/Isotopes_of_cobalt">56 Co </a>decaying to 56 Fe. Later measurements by space gamma-ray telescopes of the small fraction of the 56 Co and 57 Co gamma rays that escaped the <a href="https://en.wikipedia.org/wiki/SN_1987A">SN 1987A </a>remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power sources.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-S.M._Matz,_G.H._Share_1988-104">[104] </a>

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. 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-56 (half-life 6 days), which then decays to radioactive cobalt-56 (half-life 77 days). These radioisotopes excite the surrounding material to incandescence. Studies of cosmology today rely on 56 <a href="https://en.wikipedia.org/wiki/Nickel">Ni </a>radioactivity providing the energy for the optical brightness of supernovae of type Ia, which are the "standard candles" of cosmology but whose diagnostic 847keV and 1238keV gamma rays were first detected only in 2014.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-106">[106] </a> 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 <a href="https://en.wikipedia.org/wiki/Positron_emission">positron emission </a>becomes dominant from the remaining cobalt-56, 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-56. 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 <a href="https://en.wikipedia.org/wiki/Hypernova">hypernovae </a>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 <a href="https://en.wikipedia.org/wiki/Gamma-ray_burst">gamma-ray bursts </a>.

The light curves for type II supernovae are characterised by a much slower decline than type I, on the order of 0.05 <a href="https://en.wikipedia.org/wiki/Absolute_magnitude">magnitudes </a>per day,<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-107">[107] </a> 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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-doggett-47">[47] </a>

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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-doggett-47">[47] </a>

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.

<ul><li>a. <a href="https://en.wikipedia.org/wiki/Supernova#ref_a">^ </a> Faint types may be a distinct sub-class. Bright types may be a continuum from slightly over-luminous to hypernovae. </li><li>b. <a href="https://en.wikipedia.org/wiki/Supernova#ref_b">^ </a> These magnitudes are measured in the R band. Measurements in V or B bands are common and will be around half a magnitude brighter for supernovae. </li><li>c. <a href="https://en.wikipedia.org/wiki/Supernova#ref_c">^ </a> <a href="https://en.wikipedia.org/wiki/Order_of_magnitude">Order of magnitude </a>kinetic energy. Total electromagnetic radiated energy is usually lower, (theoretical) neutrino energy much higher. </li><li>d. <a href="https://en.wikipedia.org/wiki/Supernova#ref_d">^ </a> Probably a heterogeneous group, any of the other types embedded in nebulosity. </li> </ul>

A long-standing puzzle surrounding type II supernovae is why the remaining compact object receives a large velocity away from the epicentre;<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-111">[111] </a> <a href="https://en.wikipedia.org/wiki/Pulsar">pulsars </a>, 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. 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 remains a puzzle. Proposed explanations for this kick include convection in the collapsing star and jet production during <a href="https://en.wikipedia.org/wiki/Neutron_star_formation">neutron star formation </a>.

One possible explanation for this asymmetry is large-scale <a href="https://en.wikipedia.org/wiki/Convection">convection </a>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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-112">[112] </a>

Another possible explanation is that accretion of gas onto the central neutron star can create a <a href="https://en.wikipedia.org/wiki/Accretion_disk">disk </a>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 href="https://en.wikipedia.org/wiki/Supernova#cite_note-113">[113] </a> <a href="https://en.wikipedia.org/wiki/Supernova#cite_note-114">[114] </a> (A similar model is now favored for explaining long <a href="https://en.wikipedia.org/wiki/Gamma-ray_burst">gamma-ray bursts </a>.)

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.<a href="https://en.wikipedia.org/wiki/Supernova#cite_note-115">[115] </a>

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