Supernova

A supernova (plural: supernovae) represents a powerful and highly luminous stellar explosion. This phenomenon typically manifests during the terminal evolutionary phases of a massive star or when a white dwarf undergoes uncontrolled nuclear fusion. The initial celestial body, termed the progenitor, either collapses into a neutron star or a black hole, or is entirely disintegrated, resulting in the formation of a diffuse nebula. The maximum optical brightness of a supernova can rival that of an entire galaxy, subsequently diminishing over a period of weeks to months.

A supernova (pl.: supernovae) is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.

Within our galaxy, supernovae are projected to occur approximately once every 61 years on average; however, the most recent observation was Kepler's Supernova in 1604. SN 1987A was observed in 1987 within the Large Magellanic Cloud, a satellite galaxy of the Milky Way. Annually, several thousand supernovae are routinely detected in remote galaxies.

Theoretical investigations suggest that the majority of supernovae originate from one of two fundamental mechanisms: either the abrupt re-ignition of nuclear fusion within a white dwarf or the sudden gravitational collapse of a massive star's core.

• During the re-ignition of a white dwarf, the star's internal temperature escalates sufficiently to initiate uncontrolled nuclear fusion, leading to its complete disruption. Potential triggers for this process include the accretion of material from a binary companion or a stellar merger event.
• Regarding the abrupt implosion of a massive star, its core undergoes a sudden collapse when it can no longer generate adequate energy through fusion to counterbalance its immense gravitational force. This critical event is inevitable once the star commences iron fusion, though it may also occur during an earlier phase of metal fusion.

Supernovae are capable of ejecting several solar masses of material at velocities reaching several percent of the speed of light. This expulsion generates an expanding shock wave that propagates into the ambient interstellar medium, accumulating an expanding shell of gas and dust, which is subsequently observed as a supernova remnant. These events constitute a primary source of elements, ranging from oxygen to rubidium, within the interstellar medium. Furthermore, the expanding shock waves from supernovae can induce the genesis of new stars and are a significant origin of cosmic rays. They may also be responsible for generating gravitational waves.

Occurrence

Tycho's Supernova in 1572 and Kepler's Supernova in 1604 represent the earliest supernovae subjected to systematic astronomical investigation. Both events occurred within the Milky Way galaxy and were discernible to the unaided eye. Historical analyses indicate that, excluding telescopic discoveries, fewer than ten supernovae have been observed over the past two millennia.

Contemporary observations of supernova remnants within the Milky Way, combined with investigations of supernovae in external galaxies, imply that these energetic stellar explosions manifest in our galaxy at an average rate of approximately 1.6 to 4.6 events per century.

In 1987, supernova SN 1987A emerged in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, situated in a region of the sky conducive to observation. Extensive astronomical studies were conducted on SN 1987A, encompassing the sole measurements of astronomical neutrinos originating from a source other than the Sun. This event was subsequently attributed to the explosion of a blue supergiant star.

Etymology

The term supernova possesses two plural forms: supernovae or supernovas, and is frequently abbreviated as SN or SNe. Its etymology traces back to the Latin word nova, signifying 'new', which historically described the appearance of a transient, bright new star. The prefix "super-" serves to differentiate supernovae from conventional novae, which exhibit significantly lower luminosity. Walter Baade and Fritz Zwicky are credited with coining the word supernova, introducing it in astrophysics lectures in 1931. Its inaugural appearance in a journal article occurred the subsequent year in a publication by Knut Lundmark, who may have independently originated the term.

Observation History

Relative to a star's complete evolutionary lifespan, the visual manifestation of a supernova is exceedingly transient, occasionally persisting for several months; consequently, the probability of observing such an event with the unaided eye is approximately once per lifetime. Merely a minuscule proportion of the 100 billion stars within a characteristic galaxy possess the potential to evolve into a supernova, with this capability confined to high-mass stars and those residing in uncommon binary star systems that include at least one white dwarf.

Early Observations

A rock carving discovered in Kashmir's Burzahama region, provisionally dated to approximately 4500±1000 BC, potentially depicts nova HB9, representing the earliest among numerous purported yet unconfirmed prehistoric observations of supernovae. In contrast, SN 1006, observed in AD 1006 within the constellation of Lupus, stands as the first supernova extensively documented, with accounts originating from observers in China, Japan, Iraq, Egypt, and Europe.

Chinese astronomers documented supernova SN 1054 in AD 1054, an event that subsequently formed the Crab Nebula. Supernovae SN 1572 and SN 1604, which represent the most recent naked-eye observations of supernovae within the Milky Way, significantly impacted European astronomical thought. These events provided crucial evidence to refute the prevailing Aristotelian cosmology, which posited an immutable and static celestial sphere beyond the Moon and planets. Johannes Kepler commenced his observations of SN 1604 at its peak intensity on October 17, 1604, meticulously estimating its brightness until it became imperceptible to the unaided eye approximately one year later. This supernova marked the second such event witnessed within a single generation, following Tycho Brahe's observation of SN 1572 in the constellation Cassiopeia.

Evidence suggests that G1.9+0.3, identified as the youngest known supernova within our galaxy, transpired in the late 19th century, a considerably more recent occurrence than Cassiopeia A, which dates to approximately 1680. Remarkably, neither of these significant events was recorded by contemporary observers. For G1.9+0.3, substantial extinction caused by dust within the galactic disk's plane likely attenuated its luminosity, rendering it undetectable. The absence of observation for Cassiopeia A remains less explicable, particularly since detected infrared light echoes indicate that its location was not characterized by unusually high extinction.

Telescopic Discoveries

The advent of the astronomical telescope facilitated the detection and study of supernovae that were both fainter and more distant. The initial such observation involved SN 1885A within the Andromeda Galaxy. Subsequently, a decade later, SN 1895A and SN 1895B were identified in NGC 4424 and NGC 5253, respectively. During the 1920s, preliminary research focused on what was initially perceived as a distinct class of novae, referred to by terms such as "upper-class Novae," "Hauptnovae," or "giant novae." The term "supernovae" is widely attributed to Walter Baade and Fritz Zwicky, who reportedly coined it during lectures at Caltech in 1931. It subsequently appeared in print as "super-Novae" in a 1933 journal article by Knut Lundmark and in a 1934 publication by Baade and Zwicky. By 1938, the hyphenated form had been abandoned, and the contemporary nomenclature was firmly established.

Rudolph Minkowski and Fritz Zwicky initiated the development of the contemporary supernova classification system in 1941. By the 1960s, astronomers recognized that the peak luminosities of supernovae could serve as "standard candles," thereby functioning as reliable indicators of cosmic distances. Observations in 2003 revealed that some of the most remote supernovae appeared less luminous than anticipated, providing corroborating evidence for the hypothesis of an accelerating universal expansion. Furthermore, methodologies were devised to reconstruct supernova events for which no historical observational records exist. For instance, the precise date of the Cassiopeia A supernova was ascertained through the analysis of light echoes reflecting off surrounding nebulae. Similarly, the age of the supernova remnant RX J0852.0-4622 was estimated by combining temperature measurements with the gamma-ray emissions resulting from the radioactive decay of ⁴⁴
Ti.

ASASSN-15lh, situated approximately 3.82 gigalight-years away, holds the distinction of being the most luminous supernova ever documented. Initially detected in June 2015, its peak luminosity reached an extraordinary 570 billion L_☉, a value twice that of any other previously identified supernova's bolometric luminosity. The precise astrophysical nature of ASASSN-15lh remains a subject of ongoing debate, with various alternative hypotheses proposed, including the tidal disruption of a star by a black hole.

Supernova SN 2013fs was observed merely three hours post-detonation on October 6, 2013, by the Intermediate Palomar Transient Factory. This observation represents one of the earliest detections of a supernova following its explosion, and notably, it is the earliest instance for which spectral data were acquired, commencing six hours after the actual event. The progenitor star is situated within the spiral galaxy NGC 7610, located approximately 160 million light-years distant in the constellation Pegasus.

Supernova SN 2016gkg was discovered by amateur astronomer Victor Buso from Rosario, Argentina, on September 20, 2016. This event marked the inaugural observation of the initial "shock breakout" from an optical supernova. The progenitor star has since been identified in Hubble Space Telescope images captured prior to its catastrophic collapse. Astronomer Alex Filippenko remarked, "Observations of stars in the first moments they begin exploding provide information that cannot be directly obtained in any other way."

Supernova Discovery Programs

Given the relative infrequency of supernovae within a galaxy, with an approximate occurrence rate of three per century in the Milky Way, securing a comprehensive sample for study necessitates consistent surveillance of numerous galaxies. Currently, amateur and professional astronomers collectively identify approximately two thousand supernovae annually; some are detected near their peak luminosity, while others are identified retrospectively from archival astronomical photographs or plates. Supernovae in other galaxies cannot be predicted with substantial precision; typically, their discovery occurs after the event has already commenced. For supernovae to serve as reliable standard candles in cosmic distance measurement, observing their peak luminosity is essential. Consequently, early detection, significantly preceding their maximum brightness, is crucial. Amateur astronomers, whose numbers significantly exceed those of professionals, have been instrumental in supernova discovery, often by scrutinizing nearby galaxies with optical telescopes and cross-referencing observations with historical photographic records.

By the late 20th century, astronomers progressively adopted computer-controlled telescopes and Charge-Coupled Devices (CCDs) for supernova detection. Although these systems are widely utilized by amateur astronomers, professional observatories also employ them, exemplified by installations like the Katzman Automatic Imaging Telescope. The Supernova Early Warning System (SNEWS) project employs a network of neutrino detectors to provide advance notification of a supernova within the Milky Way galaxy. Neutrinos are subatomic particles copiously generated during a supernova event, and they are not substantially attenuated by the interstellar gas and dust within the galactic disk.

Supernova searches are categorized into two primary types: those concentrating on proximate occurrences and those investigating more distant phenomena. Owing to cosmic expansion, the distance to a remote celestial object possessing a known emission spectrum can be ascertained through the measurement of its Doppler shift, commonly referred to as redshift. Statistically, more remote objects exhibit higher recession velocities than closer ones, consequently displaying greater redshifts. Therefore, search efforts are bifurcated into high-redshift and low-redshift categories, with the demarcation typically situated within a redshift range of z=0.1–0.3, where 'z' denotes a dimensionless metric of the spectral frequency shift.

High-redshift searches for supernovae typically entail the observation of supernova light curves. These curves are instrumental for utilizing supernovae as standard or calibrated candles, facilitating the construction of Hubble diagrams and enabling cosmological predictions. Supernova spectroscopy, employed to investigate the astrophysical properties and surrounding environments of supernovae, proves more feasible at low redshifts compared to high redshifts. Furthermore, low-redshift observations establish the foundational, near-distance segment of the Hubble curve, which graphically represents the relationship between distance and redshift for observable galaxies.

With the rapid expansion of survey programs leading to an escalating number of detected supernovae, comprehensive datasets of observations, including light decay curves, astrometry, pre-supernova imagery, and spectroscopy, have been compiled. For instance, the Pantheon dataset, compiled in 2018, encompassed detailed information for 1048 supernovae. By 2021, this dataset had been augmented to include 1701 light curves corresponding to 1550 supernovae, sourced from 18 distinct surveys, representing a 50% increase in less than three years.

Supernova Naming Convention

Supernova discoveries are formally communicated to the International Astronomical Union's Central Bureau for Astronomical Telegrams, which subsequently issues a circular detailing the assigned nomenclature for each event. This designation comprises the prefix SN, succeeded by the year of discovery, and further appended with a one- or two-letter identifier. The initial 26 supernovae identified within a given year are denoted by a single capital letter, ranging from A to Z. Subsequently, pairs of lowercase letters are employed sequentially, such as aa, ab, and so forth. For instance, SN 2003C signifies the third supernova officially reported in 2003. The final supernova of 2005, SN 2005nc, represented the 367th discovery that year (calculated as 14 × 26 + 3 = 367). Since the year 2000, both professional and amateur astronomers have collectively identified several hundred supernovae annually, with specific counts including 572 in 2007, 261 in 2008, 390 in 2009, and 231 in 2013.

Historically, supernovae have been designated by their discovery year, with examples including SN 185, SN 1006, SN 1054, SN 1572 (known as Tycho's Nova), and SN 1604 (Kepler's Star). Since 1885, an additional letter notation has been employed, even when only a single supernova was identified in a given year (e.g., SN 1885A, SN 1907A); the most recent instance of this single-letter usage was SN 1947A. The prefix SN, denoting SuperNova, is standard. Prior to 1987, two-letter designations were infrequently required; however, since 1988, they have become an annual necessity. The escalating rate of discoveries since 2016 has consistently necessitated the adoption of three-letter designations. Following 'zz', the sequence progresses to 'aaa', 'aab', 'aac', and so forth. For instance, the final supernova recorded in the Asiago Supernova Catalogue, upon its termination on December 31, 2017, was designated SN 2017jzp.

Classification

Supernovae are categorized by astronomers based on their light curves and the absorption lines of various chemical elements present in their spectra. A supernova's classification is Type II if its spectrum exhibits hydrogen lines (specifically, the Balmer series in the visual spectrum); otherwise, it is designated Type I. Both primary types are further subdivided based on the presence of spectral lines from other elements or the morphology of their light curves, which plot the supernova's apparent magnitude over time.

Type I Supernovae

Type I supernovae are spectrally subdivided, with Type Ia characterized by a prominent ionized silicon absorption line. Type I supernovae lacking this distinct line are categorized as Type Ib or Type Ic; Type Ib exhibits strong neutral helium lines, whereas Type Ic does not. Historically, the light curves of Type I supernovae were considered broadly similar, precluding meaningful differentiation based solely on their morphology. Although light curve variations have been investigated, classification remains primarily based on spectral characteristics rather than light curve morphology.

A limited subset of Type Ia supernovae display anomalous characteristics, including non-standard luminosity or broadened light curves. These are typically classified by referencing the earliest observed example exhibiting similar features. For instance, the sub-luminous SN 2008ha is frequently designated as SN 2002cx-like or belonging to class Ia-2002cx.

A minor fraction of Type Ic supernovae exhibit highly broadened and blended emission lines, which are interpreted as indicative of exceptionally high ejecta expansion velocities. Such events are classified as Type Ic-BL or Ic-bl.

Calcium-rich supernovae constitute a rare class of rapidly evolving supernovae, distinguished by unusually strong calcium lines in their spectra. Theoretical models propose their origin involves the accretion of material from a helium-rich companion star, as opposed to a hydrogen-rich one. Despite spectral similarities to Type Ib supernovae due to the presence of helium lines, they are hypothesized to originate from distinct progenitor systems.

The Type Ien classification has been introduced to account for observations of supernova SN 2021yfj. Prior to its explosion, the progenitor star, having shed its outer envelopes of hydrogen, helium, and carbon, ejected an anomalous, concealed layer composed of silicon, sulfur, and argon—elements not commonly observed in stellar demise. During the event, material from the stellar core impacted this gaseous shell, with the resultant collision heat causing the silicon and sulfur layer to incandesce. This explosion demonstrated that stars can undergo extensive mass loss yet still generate luminous explosions detectable across vast cosmic distances. This discovery furnished direct empirical evidence for the long-hypothesized, yet observationally challenging, internal structure of massive stars. Within this classification, 'e' denotes the internal structural position of the silicon/sulfur layer, and 'n' indicates narrow emission lines.

Type II Supernovae

Type II supernovae are also amenable to spectral sub-classification. Although the majority of Type II supernovae exhibit very broad emission lines, signifying expansion velocities of several thousand kilometers per second, a subset, exemplified by SN 2005gl, displays comparatively narrow spectral features. These are designated Type IIn, with the 'n' denoting 'narrow'.

A limited number of supernovae, including SN 1987K and SN 1993J, exhibit a transitional classification: initially displaying hydrogen lines, they subsequently become dominated by helium lines over a period spanning weeks to months. The designation "Type IIb" characterizes this amalgamation of features typically associated with both Type II and Type Ib supernovae.

Type II supernovae, characterized by normal spectra dominated by broad hydrogen lines that persist throughout their decline phase, are categorized according to their light curve morphology. The predominant subtype exhibits a characteristic "plateau" phase shortly after peak brightness, during which the visual luminosity remains comparatively stable for several months prior to the resumption of its luminosity decay. These are designated Type II-P, indicating the presence of this plateau. Conversely, Type II-L supernovae, which are less frequent, do not display a prominent plateau. While the "L" denotes "linear," the light curve itself does not strictly follow a linear trajectory.

Supernovae that deviate from standard classification criteria are categorized as peculiar, often abbreviated as "pec."

Historical Supernova Types III, IV, and V

Fritz Zwicky established supplementary supernova classifications, derived from a limited number of instances that did not precisely conform to the established criteria for Type I or Type II supernovae. SN 1961i, located in NGC 4303, served as the sole prototype for the Type III supernova category, distinguished by its expansive light curve maximum and broad hydrogen Balmer lines that exhibited delayed spectral evolution. Similarly, SN 1961f in NGC 3003 represented the singular prototype for the Type IV class, characterized by a light curve resembling a Type II-P supernova, featuring hydrogen absorption lines alongside faint hydrogen emission lines. The Type V classification was introduced for SN 1961V in NGC 1058, an anomalous, dim supernova or potential supernova impostor, notable for its gradual luminosity increase, a prolonged maximum extending over several months, and an atypical emission spectrum. A notable resemblance was observed between SN 1961V and the Great Outburst of Eta Carinae. Additionally, supernovae observed in M101 (1909) and M83 (1923 and 1957) were proposed as potential candidates for Type IV or Type V supernovae.

Presently, all these classifications are subsumed under the category of peculiar Type II supernovae (IIpec), numerous additional examples of which have since been identified. However, the precise nature of SN 1961V remains a subject of ongoing discussion, specifically whether it constituted a genuine supernova subsequent to a Luminous Blue Variable (LBV) outburst or merely an impostor event.

Contemporary Models

Supernova classification codes are fundamentally taxonomic, meaning the assigned type is derived from the observed light characteristics of the supernova, rather than its underlying physical cause. For instance, Type Ia supernovae originate from runaway thermonuclear fusion initiated on degenerate white dwarf progenitors. Conversely, the spectrally analogous Type Ib/c supernovae result from the core collapse of massive, stripped progenitor stars.

Thermonuclear Runaway

A white dwarf star can accrete an adequate quantity of material from a stellar companion, thereby elevating its core temperature sufficiently to initiate carbon fusion. Upon reaching this threshold, it experiences a thermonuclear runaway, leading to its complete disruption. Three primary theoretical pathways are proposed for this detonation: stable accretion of matter from a companion star, the catastrophic collision of two white dwarfs, or accretion-induced ignition within a shell that subsequently triggers core detonation. The precise dominant mechanism responsible for Type Ia supernova production, however, remains unresolved. Notwithstanding this ambiguity regarding their formation, Type Ia supernovae exhibit remarkably uniform properties, rendering them invaluable as standard candles for measuring intergalactic distances. Nevertheless, certain calibrations are necessary to account for subtle evolutionary changes in their characteristics, varying frequencies of anomalous luminosity supernovae at high redshift, and minor intrinsic brightness variations discernible through light curve morphology or spectral analysis.

Canonical Type Ia Supernovae

While multiple pathways can lead to the formation of this supernova type, they converge upon a shared fundamental mechanism. Should a carbon-oxygen white dwarf accrete sufficient material to attain the Chandrasekhar limit, approximately 1.44 solar masses for a non-rotating star, it would subsequently lose its capacity to sustain the majority of its mass via electron degeneracy pressure, thereby initiating gravitational collapse. Nevertheless, contemporary understanding suggests that this limit is typically not reached; instead, escalating temperature and density within the core trigger carbon fusion as the star approaches the limit (within approximately 1%) prior to the onset of collapse. Conversely, a collapsing white dwarf with a core predominantly composed of oxygen, neon, and magnesium will generally evolve into a neutron star. Under such circumstances, only a partial fraction of the star's total mass is expelled during the collapse event.

Immediately following the collapse initiation, a significant portion of the white dwarf's mass undergoes nuclear fusion, generating sufficient energy (1–2×10⁴⁴ J) to gravitationally unbind the star, resulting in a supernova. This event produces an outwardly propagating shock wave, propelling material at velocities ranging from 5,000 to 20,000 km/s, approximately 3% of the speed of light. Concurrently, a substantial and remarkably consistent increase in luminosity occurs, achieving an absolute magnitude of −19.3, which is equivalent to five billion times the Sun's brightness.

The prevailing model for the genesis of this supernova class involves a close binary star system. Initially, the more massive primary star evolves off the main sequence, expanding into a red giant. Subsequently, both stars share a common envelope, leading to a reduction in their orbital separation. The giant star then expels the majority of its outer layers, shedding mass until nuclear fusion ceases. Consequently, it transforms into a white dwarf, predominantly composed of carbon and oxygen. Eventually, the secondary star similarly evolves off the main sequence, becoming a red giant. Material from this giant is subsequently accreted by the white dwarf, leading to a gradual increase in its mass. However, the precise mechanisms of initiation and the specific heavy elements generated during this catastrophic event are not yet fully understood.

Following the event, Type Ia supernovae exhibit a characteristic light curve, which plots luminosity against time. This luminosity originates from the radioactive decay chain of ⁵⁶
Ni, transitioning through ⁵⁶
Co to ultimately form ⁵⁶
Fe. The peak luminosity of these light curves demonstrates remarkable consistency among normal Type Ia supernovae, typically reaching a maximum absolute magnitude of approximately −19.3. This consistency stems from the fact that standard Type Ia supernovae originate from a uniform class of progenitor stars, which undergo gradual mass accretion and detonate upon reaching a consistent critical mass, thereby producing highly similar supernova conditions and observational characteristics. Consequently, they serve as crucial secondary standard candles for determining distances to their host galaxies.

An alternative model for Type Ia supernova formation posits the merger of two white dwarf stars, where their combined mass temporarily surpasses the Chandrasekhar limit. This scenario is often termed the double-degenerate model, given that both constituent stars are degenerate white dwarfs. However, the diverse combinations of mass and chemical composition within such binary pairs introduce significant variability in these events; in many instances, a supernova may not occur, or if it does, it will exhibit a less luminous light curve compared to typical Type Ia supernovae.

Non-Standard Type Ia

Abnormally luminous Type Ia supernovae arise when the white dwarf progenitor already possesses a mass exceeding the Chandrasekhar limit, potentially augmented by asymmetry, yet the ejected material exhibits lower than typical kinetic energy. Such a super-Chandrasekhar-mass configuration can be sustained, for instance, by differential rotation within the star.

Currently, no formal sub-classification exists for non-standard Type Ia supernovae. However, a proposal suggests classifying a group of sub-luminous supernovae, which result from helium accretion onto a white dwarf, as Type Iax. These particular supernovae may not invariably lead to the complete destruction of the white dwarf progenitor, potentially leaving behind a remnant often referred to as a 'zombie star'.

A distinct class of supernovae, originating from exploding white dwarfs akin to Type Ia events, exhibits hydrogen lines in their spectra, likely due to the presence of a hydrogen-rich circumstellar envelope surrounding the white dwarf. These events have been designated by various names, including Type Ia/IIn, Type Ian, Type IIa, and Type IIan.

The quadruple star system HD 74438, located within the open cluster IC 2391 in the Vela constellation, is predicted to evolve into a non-standard Type Ia supernova.

Core Collapse

Extremely massive stars undergo core collapse when their nuclear fusion processes can no longer counteract the immense force of their own gravity; this critical threshold is the fundamental cause of all supernova types except Type Ia. This collapse can trigger a violent expulsion of the star's outer layers, culminating in a supernova. Conversely, if the gravitational potential energy released is insufficient, the star may instead directly collapse into a black hole or neutron star, emitting minimal radiation.

Core collapse can be initiated by various mechanisms, including exceeding the Chandrasekhar limit, electron capture, pair-instability, or photodisintegration.

• Upon developing an iron core that surpasses the Chandrasekhar mass, a massive star loses its ability to sustain itself through electron degeneracy pressure, leading to a subsequent collapse into either a neutron star or a black hole.
• In a degenerate oxygen-neon-magnesium (O/Ne/Mg) core, typically found in progenitor stars with 8–10 solar masses, electron capture by magnesium removes structural support, precipitating gravitational collapse and subsequent explosive oxygen fusion, yielding outcomes highly similar to other core collapse events.
• The formation of electron-positron pairs within a substantial post-helium burning core diminishes thermodynamic support, triggering an initial collapse that culminates in runaway fusion and a pair-instability supernova.
• A stellar core of adequate size and temperature can produce gamma-rays with sufficient energy to directly induce photodisintegration, leading to a complete core collapse.

This section describes the established causes of core collapse in massive stars, the stellar classifications in which these events manifest, their corresponding supernova types, and the resulting stellar remnants. Metallicity is defined as the fractional abundance of elements other than hydrogen or helium, relative to the Sun's composition. The initial mass refers to the star's mass before the supernova event, expressed in solar masses, though the mass at the time of the supernova can be considerably reduced.

Type IIn supernovae are not included in the aforementioned categorization. These events can arise from diverse core collapse mechanisms in various progenitor stars, potentially even from Type Ia white dwarf ignitions; however, the majority are believed to originate from iron core collapse within luminous supergiants or hypergiants, including Luminous Blue Variables (LBVs). Their characteristic narrow spectral lines result from the supernova's expansion into a compact, dense cloud of circumstellar material. A substantial fraction of purported Type IIn supernovae are, in fact, supernova impostors, which are massive eruptions of LBV-like stars, analogous to the Great Eruption of Eta Carinae. During such impostor events, previously expelled stellar material generates the narrow absorption lines and induces a shock wave through interaction with newly ejected matter.

Detailed Process

Once a stellar core loses its gravitational support, it undergoes an inward collapse at velocities up to 70,000 km/s (0.23c), leading to a swift escalation in temperature and density. The subsequent evolution is contingent upon the mass and internal structure of the collapsing core: low-mass degenerate cores typically form neutron stars, higher-mass degenerate cores predominantly collapse entirely into black holes, and non-degenerate cores experience runaway fusion.

The initial collapse of degenerate cores is exacerbated by beta decay, photodisintegration, and electron capture, which collectively generate a burst of electron neutrinos. As the core density intensifies, neutrino emission ceases due to their confinement within the core. The inner core ultimately attains a typical diameter of 30 km and a density akin to that of an atomic nucleus, at which point neutron degeneracy pressure attempts to arrest the collapse. However, if the core mass exceeds approximately 15 solar masses, neutron degeneracy proves inadequate to prevent the collapse, resulting in the direct formation of a black hole without a supernova event.

In cores of lower mass, the collapse is arrested, and the nascent neutron core exhibits an initial temperature of approximately 100 billion kelvins, which is 6,000 times hotter than the Sun's core. At this extreme temperature, neutrino-antineutrino pairs of all flavors are efficiently generated via thermal emission. These thermally produced neutrinos are significantly more numerous than those originating from electron capture. Approximately 10⁴⁶ joules, representing about 10% of the star's rest mass, is transformed into a ten-second neutrino burst, constituting the primary energy output of the event. The abruptly halted core collapse subsequently rebounds, generating a shock wave that rapidly stalls in the outer core within milliseconds due to energy dissipation through the dissociation of heavy elements. A mechanism, not yet fully comprehended, is required for the outer core layers to reabsorb approximately 10⁴⁴ joules (1 foe) from this neutrino pulse, thereby producing the observable brightness, although alternative theories for powering the explosion also exist.

A portion of the outer stellar envelope accretes back onto the neutron star. For stellar cores exceeding approximately 8 M_☉, this accretion is substantial enough to facilitate the formation of a black hole. Such fallback diminishes both the generated kinetic energy and the quantity of ejected radioactive material; however, under specific circumstances, it can also induce the formation of relativistic jets, leading to either a gamma-ray burst or an exceptionally luminous supernova.

The gravitational collapse of a massive, non-degenerate stellar core triggers subsequent nuclear fusion reactions. If core collapse is initiated by pair instability—a process where photons convert into electron-positron pairs, consequently reducing radiation pressure—oxygen fusion commences, potentially arresting the collapse. Cores with masses ranging from 40 to 60 M_☉ experience a temporary halt in collapse, preserving the star's integrity; however, a subsequent collapse will ensue once a more massive core develops. Conversely, for cores approximately 60–130 M_☉, the fusion of oxygen and heavier elements generates such immense energy that it completely disrupts the star, culminating in a supernova. Supernovae originating from the upper limit of this mass range exhibit exceptional luminosity and extended duration, attributed to the ejection of numerous solar masses of ⁵⁶
Ni. With even greater core masses, the internal temperature reaches a threshold sufficient for photodisintegration, leading to the complete collapse of the core into a black hole.

Type II

Stars possessing initial masses below approximately 8 M_☉ do not form a core substantial enough for gravitational collapse; instead, they ultimately shed their outer envelopes to become white dwarfs. Conversely, stars with initial masses of at least 9 M_☉ (potentially up to 12 M_☉) undergo intricate evolutionary pathways, characterized by the sequential fusion of progressively heavier elements at increasing temperatures within their cores. This process results in a stellar structure resembling an onion, where the fusion of lighter elements takes place in progressively larger, outer shells. While commonly depicted as having an iron core, the least massive progenitors of supernovae actually possess oxygen-neon(-magnesium) cores. These super-AGB stars are hypothesized to constitute the predominant source of core-collapse supernovae, despite being less luminous and consequently less frequently observed compared to those originating from more massive progenitors.

Should core collapse transpire during a star's supergiant phase, while it still retains a hydrogen envelope, the outcome is classified as a Type II supernova. The stellar mass-loss rate for luminous stars is contingent upon their metallicity and intrinsic luminosity. Exceptionally luminous stars with near-solar metallicity will completely shed their hydrogen envelopes prior to core collapse, thereby precluding the formation of a Type II supernova. Conversely, in environments of low metallicity, all stars will undergo core collapse while still possessing a hydrogen envelope; however, sufficiently massive stars in this category will collapse directly into a black hole without generating a visible supernova.

Stars with an initial mass up to approximately 90 solar masses, or slightly less in high-metallicity environments, typically produce a Type II-P supernova, representing the most frequently observed subtype. In moderate to high metallicity regimes, stars approaching the upper limit of this mass range will have largely depleted their hydrogen envelopes by the time core collapse ensues, resulting in a Type II-L supernova. Under conditions of very low metallicity, stars with masses around 140–250 M_☉ will undergo core collapse via pair instability, retaining both a hydrogen atmosphere and an oxygen core. This process yields a supernova exhibiting Type II characteristics, yet distinguished by a substantial mass of ejected ⁵⁶
Ni and exceptionally high luminosity.

Type Ib and Ic

Type Ib and Ic supernovae, akin to Type II events, originate from massive stars experiencing core collapse. However, distinguishing them from Type II progenitors, the stars that evolve into Type Ib and Type Ic supernovae have shed the majority of their outer hydrogen envelopes, either through powerful stellar winds or via interaction with a binary companion. Such stars are identified as Wolf–Rayet stars, typically found in environments of moderate to high metallicity where continuum-driven winds facilitate substantial mass-loss rates. Nevertheless, observational data for Type Ib/c supernovae do not consistently align with the observed or predicted prevalence of Wolf–Rayet stars. Alternative hypotheses for this class of core-collapse supernovae propose stars whose hydrogen envelopes were removed through binary interactions. While binary models offer a more consistent explanation for the observed supernovae, this is contingent on the caveat that no appropriate binary helium stars have yet been directly observed.

Type Ib supernovae, which are more prevalent than Type Ic, originate from Wolf–Rayet stars of the WC type that retain helium in their atmospheres. Conversely, Type Ic supernovae are believed to stem from WO stars, which represent a more advanced evolutionary stage for a specific mass range, characterized by minimal residual helium prior to core collapse.

While a small percentage of Type Ic supernovae are linked to gamma-ray bursts (GRBs), it is hypothesized that any hydrogen-stripped Type Ib or Ic supernova could generate a GRB, contingent on geometric conditions. The underlying mechanism for these GRBs involves jets emanating from the magnetic field of a rapidly rotating magnetar, which forms within the collapsing stellar core. These jets would also impart energy to the expanding outer shell, potentially resulting in a super-luminous supernova.

Ultra-stripped supernovae arise when a star, through mass transfer in a close binary system, has been almost entirely denuded down to its metallic core. Consequently, the amount of material ejected during the explosion is minimal, approximately 0.1 M_☉. In the most extreme scenarios, these supernovae can originate from bare metal cores that barely exceed the Chandrasekhar mass limit. SN 2005ek has been proposed as the initial observational instance of an ultra-stripped supernova, characterized by a comparatively faint and rapidly decaying light curve. The classification of ultra-stripped supernovae can encompass both iron core-collapse and electron-capture types, determined by the mass of the collapsing core. These events are thought to be linked to the second supernova explosion within a binary system, potentially leading to the formation of compact double neutron star systems.

In 2022, a research team, spearheaded by astronomers from the Weizmann Institute of Science, announced the discovery of the first supernova explosion providing direct evidence of a Wolf-Rayet progenitor star. This event, designated SN 2019hgp, was classified as a Type Icn supernova and notably marked the initial detection of neon within such an explosion.

Electron-Capture Supernovae

In 1980, Ken'ichi Nomoto of the University of Tokyo theorized a "third type" of supernova, termed an electron-capture supernova. This phenomenon was predicted to occur when a star, possessing a degenerate oxygen-neon-magnesium core and falling within the transitional mass range (approximately 8 to 10 solar masses) between white dwarf formation and iron core-collapse supernovae, undergoes implosion after exhausting its nuclear fuel. This implosion would result from gravity compressing electrons within the stellar core into atomic nuclei, culminating in a supernova explosion and the formation of a neutron star. In June 2021, a publication in the journal Nature Astronomy presented findings suggesting that the 2018 supernova SN 2018zd, located in the galaxy NGC 2146 approximately 31 million light-years from Earth, represented the first observed instance of an electron-capture supernova. The 1054 supernova event responsible for the Crab Nebula in our galaxy had previously been considered the most probable candidate for an electron-capture supernova, and the 2021 research further supports this hypothesis.

Failed Supernovae

The core collapse of certain massive stars may not culminate in a discernible supernova. This outcome typically occurs when the initial core collapse cannot be counteracted by the explosive mechanism, often due to an excessively massive core. Such events pose detection challenges; however, extensive astronomical surveys have identified potential candidates. For instance, the red supergiant N6946-BH1 within NGC 6946 experienced a moderate outburst in March 2009, subsequently diminishing from visibility, with only a faint infrared source persisting at its original position.

Light Curves

Supernova ejecta would rapidly diminish in luminosity without a continuous energy input to sustain their high temperature. The origin of this energy, capable of maintaining the optical supernova glow for several months, initially presented a significant astrophysical enigma. While some theories proposed rotational energy from a central pulsar as the source, it is now understood that although the initial power for each supernova type is delivered promptly, their light curves are predominantly shaped by the subsequent radioactive heating of the rapidly expanding material. The highly radioactive composition of these ejected gases was first theoretically established based on nucleosynthesis principles in the late 1960s, a hypothesis subsequently validated for the majority of supernovae. Definitive identification of the primary radioactive nuclei through direct observation of gamma-ray lines was not achieved until SN 1987A.

Direct observational evidence confirms that the light curve (luminosity over time) following a Type II Supernova, exemplified by SN 1987A, is largely attributable to predicted radioactive decays. While the observed luminosity originates from optical photons, the sustained radiation is powered by the absorption of radioactive energy within the ejected gases, maintaining the remnant's temperature. The radioactive decay chain from ⁵⁶
Ni, through its daughter isotope ⁵⁶
Co, to ⁵⁶
Fe, generates gamma-ray photons, predominantly at energies of 847 keV and 1,238 keV. These photons are absorbed, becoming the primary source of heating and, consequently, the luminosity of the ejecta during intermediate (several weeks) to late (several months) phases. For SN1987A, the light curve's peak energy originated from the decay of ⁵⁶
Ni to ⁵⁶
Co (with a 6-day half-life), whereas the energy for the subsequent light curve phases closely correlated with the 77.3-day half-life of §6667§
Co decaying to §7778§
Fe. Subsequent observations by space-based gamma-ray telescopes, detecting the small fraction of §8889§
Co and §99
100§
Co gamma rays that escaped the SN 1987A remnant unabsorbed, validated prior predictions identifying these two radioactive nuclei as the primary energy sources.

While the late-time decay phase of visual light curves for all supernova types relies on radioactive heating, their morphology and amplitude diverge due to variations in underlying mechanisms, the production pathways of visible radiation, the observational epoch, and the opacity of the ejected material. Furthermore, light curves exhibit substantial differences when observed at alternative wavelengths. For instance, in the ultraviolet spectrum, an exceptionally luminous early peak, persisting for only a few hours, signifies the shock breakout initiated by the progenitor event; however, this phenomenon is barely discernible in optical observations.

Type Ia supernova light curves typically exhibit remarkable uniformity, characterized by a consistent maximum absolute magnitude and a comparatively rapid decrease in luminosity. The optical energy emitted by these supernovae is primarily powered by the radioactive decay of ejected ⁵⁶
Ni (with a 6-day half-life), which subsequently decays into radioactive ⁵⁶
Co (with a 77-day half-life). These radioisotopes induce incandescence in the surrounding material. Contemporary cosmological research utilizes the radioactivity of ⁵⁶
Ni as the energy source for the optical luminosity of Type Ia supernovae, which serve as cosmological "standard candles"; however, their characteristic 847 keV and 1,238 keV gamma rays were not detected until 2014. The initial phases of the light curve exhibit a steep decline, corresponding to a reduction in the effective photospheric size and the depletion of trapped electromagnetic radiation. While the light curve continues its decline in the B band, a subtle shoulder may appear in the visual spectrum around 40 days, indicating a secondary maximum observed in the infrared as specific ionized heavy elements recombine, generating infrared radiation to which the ejecta become transparent. The visual light curve's decline rate subsequently exceeds that of the radioactive cobalt decay (which, possessing a longer half-life, governs the later curve), attributed to the increasing diffusion of the ejected material, thereby diminishing its efficiency in converting high-energy radiation into visible light. Following several months, the light curve's decline rate undergoes another alteration as positron emission from the residual ⁵⁶
Co becomes the predominant factor, although this specific segment of the light curve remains largely unexplored.

Type Ib and Ic light curves exhibit similarities to Type Ia, albeit with a reduced average peak luminosity. The visual light emission in these types is again attributed to the conversion of radioactive decay into visual radiation; however, a significantly smaller mass of created ⁵⁶
Ni is present. Peak luminosity demonstrates considerable variability, with some Type Ib/c supernovae occasionally displaying luminosities orders of magnitude above or below the typical range. The most luminous Type Ic supernovae are designated as hypernovae, characterized by broadened light curves in addition to their elevated peak luminosity. The additional energy source for these events is hypothesized to be relativistic jets, driven by the formation of a rotating black hole, which are also responsible for generating gamma-ray bursts.

Type II supernovae light curves are distinguished by a substantially slower decline rate compared to Type I, approximately 0.05 magnitudes per day, excluding the plateau phase. For several months, the visual light output is predominantly governed by kinetic energy rather than radioactive decay, primarily due to the presence of hydrogen in the ejecta originating from the supergiant progenitor star's atmosphere. During the initial destructive phase, this hydrogen becomes heated and ionized. The majority of Type II supernovae display a prolonged plateau in their light curves as this hydrogen recombines, emitting visible light and increasing in transparency. This phase is subsequently followed by a declining light curve, powered by radioactive decay, which is slower than in Type I supernovae due to the efficient conversion into light facilitated by the abundant hydrogen.

In Type II-L supernovae, the plateau phase is absent because the progenitor star retained relatively little hydrogen in its atmosphere, sufficient for spectroscopic detection but insufficient to generate a discernible plateau in the light output. For Type IIb supernovae, the progenitor's hydrogen atmosphere is so significantly depleted (a condition often attributed to tidal stripping by a companion star) that the resulting light curve more closely resembles that of a Type I supernova, with hydrogen spectral lines even vanishing after several weeks.

Type IIn supernovae are identified by additional narrow spectral lines, which originate from a dense shell of circumstellar material. Their light curves are typically very broad and extended, occasionally reaching extreme luminosities, leading to their classification as superluminous supernovae. These distinctive light curves result from the highly efficient conversion of the ejecta's kinetic energy into electromagnetic radiation through interaction with the dense circumstellar shell. This phenomenon occurs only when the material is sufficiently dense and compact, indicating its production by the progenitor star itself shortly before the supernova event.

Numerous supernovae have been cataloged and classified to serve as distance indicators and to validate astrophysical models. While average characteristics exhibit some variation with distance and the type of host galaxy, they can generally be specified for each supernova classification.

Notes:

Asymmetry

A persistent enigma concerning Type II supernovae is the substantial velocity imparted to the remnant compact object, propelling it away from the explosion's epicenter. Pulsars, which are neutron stars, are observed to possess high peculiar velocities, and black holes are presumed to exhibit similar behavior, despite the considerable difficulty in observing them in isolation. This initial impulse can be significant, accelerating an object exceeding one solar mass to velocities of 500 km/s or greater. Such observations imply an expansion asymmetry, yet the precise mechanism by which momentum is transferred to the compact object remains unresolved. Proposed explanations for this "kick" include convection within the collapsing star, asymmetric ejection of matter during neutron star formation, and asymmetrical neutrino emissions.

One potential explanation for this asymmetry involves large-scale convection occurring above the stellar core. Such convection could induce radial density variations, leading to differential absorption of energy from the neutrino outflow. However, analyses of this mechanism typically predict only modest momentum transfer. An alternative hypothesis suggests that the accretion of gas onto the central neutron star might form a disk capable of driving highly directional jets. These jets could propel matter at high velocities out of the star and induce transverse shocks that completely disrupt the stellar structure. These jets may play a critical role in the resulting supernova, a model also employed to explain long gamma-ray bursts. The dominant mechanism may ultimately depend on the progenitor star's initial mass.

Observations have confirmed initial asymmetries in Type Ia supernovae. This finding suggests that the initial luminosity of these supernovae might be dependent on the viewing angle. Nevertheless, the expansion progressively achieves greater symmetry over time. Early asymmetries can be identified through measurements of the emitted light's polarization.

Energy Output

While supernovae are predominantly recognized for their luminosity, the electromagnetic radiation they emit constitutes a relatively minor byproduct. Specifically, for core-collapse supernovae, the electromagnetic radiation released represents only a minuscule fraction of the total energy generated during the event.

A fundamental distinction exists in the energy production balance across various supernova types. For Type Ia white dwarf detonations, the majority of energy is channeled into the synthesis of heavy elements and the kinetic energy of the ejected material. Conversely, in core-collapse supernovae, the overwhelming proportion of energy is directed towards neutrino emission; although a fraction of this energy seemingly drives the observed stellar disruption, over 99% of these neutrinos depart the star within the initial minutes subsequent to the onset of collapse.

Standard Type Ia supernovae obtain their energy from the uncontrolled nuclear fusion within a carbon-oxygen white dwarf. While the precise energetic mechanisms remain incompletely elucidated, the outcome involves the expulsion of the entire mass of the progenitor star with substantial kinetic energy. Approximately half a solar mass of this ejected material consists of ⁵⁶
Ni, produced through silicon burning. This ⁵⁶
Ni is radioactive and undergoes beta-plus decay, emitting gamma rays, to form ⁵⁶
Co, with a half-life of six days. Subsequently, ⁵⁶
Co decays via the beta-plus (positron) pathway, with a half-life of 77 days, into stable ⁵⁶
Fe. These two sequential radioactive decay processes are responsible for the electromagnetic radiation observed from Type Ia supernovae. Coupled with the evolving transparency of the ejected matter, these processes generate the characteristic rapidly declining light curve.

On average, core-collapse supernovae appear visually fainter than Type Ia supernovae; however, their total energy release is considerably greater, as detailed in the subsequent table.

In certain core-collapse supernovae, material fallback onto a black hole propels relativistic jets, which can generate a transient, energetic, and directional burst of gamma rays, concurrently transferring significant additional energy into the ejected matter. This mechanism represents a potential scenario for generating high-luminosity supernovae and is hypothesized to underlie Type Ic hypernovae and long-duration gamma-ray bursts. Should these relativistic jets be too ephemeral or fail to traverse the stellar envelope, a low-luminosity gamma-ray burst might ensue, potentially resulting in a sub-luminous supernova.

When a supernova detonates within a compact, dense cloud of circumstellar material, it generates a shock wave capable of efficiently converting a substantial proportion of its kinetic energy into electromagnetic radiation. Despite a normal initial energy output, the resultant supernova exhibits high luminosity and an extended duration, as its emission is not solely dependent on exponential radioactive decay. Such events are believed to be responsible for Type IIn hypernovae.

Although pair-instability supernovae are classified as core-collapse events, exhibiting spectra and light curves analogous to Type II-P supernovae, their post-core-collapse behavior resembles that of a massive Type Ia supernova, characterized by uncontrolled fusion of carbon, oxygen, and silicon. The total energy liberated by the most massive of these events is comparable to other core-collapse supernovae; however, neutrino production is hypothesized to be minimal, leading to exceptionally high kinetic and electromagnetic energy release. The stellar cores involved are considerably larger than any white dwarf, and the quantity of radioactive nickel and other heavy elements ejected can exceed that of other supernovae by orders of magnitude, resulting in correspondingly elevated visual luminosity.

Progenitor

The classification of supernovae is intricately linked to the characteristics of the progenitor star at the moment of collapse. The incidence of each supernova type is influenced by the star's metallicity, as this factor impacts the intensity of stellar winds and, consequently, the rate of stellar mass loss.

Type Ia supernovae originate from white dwarf stars within binary systems and are observed across all galaxy morphologies. In contrast, core-collapse supernovae are exclusively detected in galaxies exhibiting ongoing or recent star formation, a consequence of their progenitors being short-lived massive stars. These events predominantly manifest in Sc-type spiral galaxies, but are also prevalent in the spiral arms of other galactic types and in irregular galaxies, particularly those undergoing starburst activity.

Type Ib and Ic supernovae are theorized to result from the core collapse of massive stars that have shed their outer hydrogen and helium envelopes, a process attributed to either powerful stellar winds or mass transfer to a companion star. These events typically manifest in active star-forming regions and are exceedingly uncommon in elliptical galaxies. Similarly, the precursor stars of Type IIn supernovae exhibit substantial mass loss rates immediately preceding their explosive demise. Observations indicate that Type Ic supernovae frequently occur in environments characterized by elevated metallicity and higher-than-average star-formation rates within their host galaxies. Progenitor characteristics for the principal core-collapse supernova types, alongside their approximate observed frequencies in the local universe, are commonly presented in a tabular format.

Significant challenges persist in harmonizing theoretical stellar evolution models with observational data concerning the precursors to core-collapse supernovae. While red supergiants constitute the primary progenitors for the majority of core-collapse supernovae, observed examples are restricted to relatively modest masses and luminosities, specifically below approximately 18 M_☉ and 100,000 L_☉, respectively. A substantial proportion of Type II supernova progenitors remain undetected, implying they are considerably fainter and presumably less massive. This inconsistency has been termed the red supergiant problem, a concept first articulated and named by Stephen Smartt in 2009. Subsequent to conducting a volume-limited supernova survey, Smartt et al. determined the lower and upper mass thresholds for the formation of Type II-P supernovae to be 8.5+1
−1.5 M_☉ and 16.5±1.5 M_☉, respectively. Although the lower limit aligns with the anticipated maximum masses for white dwarf progenitors, the upper limit diverges from the observed massive star populations within the Local Group. The maximum mass for red supergiants capable of producing a visible supernova explosion has been calculated as §3738§+4
−2 M_☉.

It is hypothesized that red supergiants exceeding a certain mass threshold do not undergo supernova explosions, but rather revert to hotter evolutionary stages. Multiple confirmed progenitors of Type IIb supernovae include K and G supergiants, along with a single A supergiant. Yellow hypergiants or Luminous Blue Variables (LBVs) are posited as progenitors for Type IIb supernovae, with nearly all observable Type IIb supernovae exhibiting such precursor stars.

Blue supergiants constitute a surprisingly significant fraction of confirmed supernova progenitors, a phenomenon partially attributable to their high luminosity and consequent ease of detection, in stark contrast to the complete absence of clearly identified Wolf–Rayet progenitors to date. Astrophysical models have encountered difficulties in elucidating the mechanism by which blue supergiants shed sufficient mass to trigger a supernova event without transitioning into an alternative evolutionary phase. Nevertheless, one investigation has proposed a potential pathway for low-luminosity, post-red supergiant luminous blue variables to undergo collapse, most probably manifesting as a Type IIn supernova. Several instances of hot, luminous progenitors for Type IIn supernovae have been observed: SN 2005gy and SN 2010jl both appeared to be massive, luminous stars, though their considerable distances complicate detailed study; furthermore, SN 2009ip possessed a highly luminous progenitor, likely an LBV, yet the precise classification of this peculiar supernova remains a subject of debate.

The progenitors of Type Ib/c supernovae remain unobserved, with their potential luminosity often constrained to levels below those of established WC stars. Given the extreme rarity and visual faintness of WO stars, it is challenging to ascertain whether these progenitors are genuinely absent or simply await detection. Highly luminous progenitors have not been definitively identified, even though numerous supernovae have occurred at distances that would permit clear imaging of such precursors. Population modeling suggests that the observed Type Ib/c supernovae could originate from a combination of single massive stars and stripped-envelope stars within interacting binary systems. The persistent absence of unequivocal progenitor detection for typical Type Ib and Ic supernovae might indicate that most massive stars undergo direct collapse into black holes without a supernova outburst. Consequently, the majority of these supernovae are likely generated by lower-mass, low-luminosity helium stars in binary systems. A minority of these events would stem from rapidly rotating massive stars, probably correlating with the highly energetic Type Ic-BL phenomena linked to long-duration gamma-ray bursts.

External Impact

Supernova events synthesize heavier elements, which are subsequently dispersed throughout the ambient interstellar medium. The expanding shockwave emanating from a supernova is capable of initiating star formation. Furthermore, supernova explosions are responsible for the generation of galactic cosmic rays.

Source of Heavy Elements

Supernovae constitute a primary source of elements within the interstellar medium, ranging from oxygen to rubidium; however, the theoretical abundances of elements generated or observed in their spectra vary considerably across different supernova types. Type Ia supernovae primarily synthesize silicon and iron-peak elements, including metals such as nickel and iron. In contrast, core-collapse supernovae expel substantially smaller quantities of iron-peak elements compared to Type Ia supernovae, but larger masses of light alpha elements like oxygen and neon, as well as elements heavier than zinc. This latter characteristic is particularly pronounced in electron-capture supernovae. The predominant material ejected by Type II supernovae consists of hydrogen and helium. The synthesis of heavy elements occurs through several mechanisms: nuclear fusion for nuclei up to ³⁴
S; silicon photodisintegration rearrangement and quasiequilibrium during silicon burning for nuclei between ³⁶
Ar and ⁵⁶
Ni; and the rapid capture of neutrons (r-process) during the supernova's collapse for elements exceeding iron in atomic mass. The r-process generates highly unstable, neutron-rich nuclei that undergo rapid beta decay to achieve more stable configurations. Within supernovae, r-process reactions account for approximately half of all isotopes of elements beyond iron, although neutron star mergers might represent the principal astrophysical source for many of these elements.

In the contemporary universe, aged asymptotic giant branch (AGB) stars serve as the primary contributors of dust, originating from oxides, carbon, and s-process elements. Nevertheless, during the early universe, prior to the formation of AGB stars, supernovae likely constituted the principal source of cosmic dust.

Role in Stellar Evolution

Supernova remnants frequently comprise a compact object and a swiftly expanding shockwave of ejected material. This material cloud accretes ambient interstellar medium during a free expansion phase, which may persist for as long as two centuries. Subsequently, the wave progressively enters a period of adiabatic expansion, gradually cooling and intermixing with the surrounding interstellar medium over approximately 10,000 years.

The Big Bang generated hydrogen, helium, and trace amounts of lithium, while all elements with greater atomic mass are synthesized within stars, during supernovae, and through neutron star collisions, with neutron star collisions being an indirect consequence of supernovae. Supernovae typically enrich the ambient interstellar medium with elements beyond hydrogen and helium, collectively termed "metals" by astronomers. These expelled elements subsequently enhance the composition of molecular clouds, which serve as stellar nurseries. Consequently, successive stellar generations exhibit progressively distinct compositions, evolving from an almost pristine hydrogen-helium mixture to one increasingly abundant in metals. Supernovae represent the primary mechanism for disseminating these heavier elements, originally forged within stars through nuclear fusion. Variations in elemental abundances within a star's progenitor material significantly impact its stellar evolution and can influence the potential for orbiting planets; specifically, a higher metallicity correlates with the formation of more numerous giant planets.

The kinetic energy emanating from an expanding supernova remnant is capable of initiating star formation through the compression of proximate, dense molecular clouds. Conversely, an elevation in turbulent pressure may inhibit stellar genesis if the cloud cannot dissipate its surplus energy.

Isotopic analysis of daughter products from short-lived radioactive isotopes indicates that a proximate supernova contributed to the elemental composition of the Solar System approximately 4.5 billion years ago, potentially even instigating its formation.

Fast radio bursts (FRBs) are characterized as intense, transient radio pulses, generally persisting for durations not exceeding milliseconds. Numerous hypotheses have been advanced to explain these phenomena; among the foremost contenders are magnetars, which originate from core-collapse supernovae.

Cosmic Rays

Supernova remnants are hypothesized to accelerate a substantial proportion of galactic primary cosmic rays; however, direct empirical evidence for such cosmic ray generation has been observed in only a limited number of these remnants. Gamma-ray emissions resulting from pion decay have been identified originating from the supernova remnants IC 443 and W44. These emissions are generated when high-energy protons from the remnant collide with interstellar matter.

Gravitational Waves

Supernovae represent potentially potent galactic sources of gravitational waves, though none have been definitively detected to date. The sole gravitational wave events observed thus far originate from the mergers of black holes and neutron stars, which are themselves likely remnants of supernovae. Analogous to neutrino emissions, gravitational waves generated by a core-collapse supernova are anticipated to precede the electromagnetic radiation produced by the ensuing explosion. Therefore, these waves could furnish insights into the core-collapse mechanism that are otherwise unobtainable. The majority of gravitational-wave signals predicted by supernova models are ephemeral, enduring for less than a second, which renders their detection challenging. Leveraging the detection of a neutrino signal could offer a temporal trigger, enabling the identification of a specific window for gravitational wave searches and thereby aiding in their differentiation from ambient noise.

Terrestrial Impact

A near-Earth supernova refers to an event sufficiently proximate to Earth to exert discernible effects on its biosphere. The critical distance, contingent on the supernova's type and energy output, could extend up to 3,000 light-years. In 1996, a hypothesis emerged suggesting that vestiges of ancient supernovae might be discernible on Earth through metallic isotope signatures within geological strata. Subsequent investigations reported an enrichment of Iron-60 in deep-sea sediments from the Pacific Ocean. By 2009, elevated concentrations of nitrate ions were identified in Antarctic ice cores, correlating temporally with the 1006 and 1054 supernovae events. Gamma rays emitted from these supernovae are posited to have augmented atmospheric nitrogen oxide levels, which subsequently became sequestered within the ice.

Historically, proximate supernova occurrences may have exerted an influence on terrestrial biodiversity. Geological evidence indicates that such supernova events resulted in an elevated flux of cosmic rays, which, in turn, contributed to a cooler global climate. This amplified temperature differential between the planetary poles and the equator generated more vigorous winds, intensified oceanic mixing, and facilitated the advection of nutrients into the shallow waters of continental shelves. These conditions collectively fostered enhanced biodiversity.

Type Ia supernovae are considered potentially the most hazardous if they occur sufficiently close to Earth. Given their origin from faint, prevalent white dwarf stars within binary systems, a Type Ia supernova capable of impacting Earth is likely to manifest unpredictably from a poorly characterized stellar system. IK Pegasi (HR 8210), situated approximately 150 light-years distant, represents the nearest identified candidate; however, current observations indicate that the white dwarf may require up to 1.9 billion years to accumulate the requisite critical mass for a Type Ia supernova event.

A 2003 assessment indicated that a Type II supernova would need to be within 8 parsecs (26 light-years) to deplete half of Earth's ozone layer; currently, no candidates are known within approximately 500 light-years.

Milky Way Candidates

The subsequent supernova within the Milky Way is anticipated to be detectable, irrespective of its galactic location. This event will most probably result from the collapse of a typical red supergiant, with a high likelihood that its progenitor has already been documented in infrared surveys like 2MASS. A lesser probability exists for the next core-collapse supernova to originate from an alternative massive star type, such as a yellow hypergiant, luminous blue variable, or Wolf–Rayet star. The probability of the next supernova being a Type Ia, stemming from a white dwarf, is estimated at approximately one-third that of a core-collapse supernova. While a Type Ia event would also be observable regardless of its position, its progenitor is less likely to have been previously identified. The precise morphology of a Type Ia progenitor system remains undefined, rendering their detection challenging beyond a few parsecs. The estimated total supernova rate in the Milky Way ranges from 2 to 12 per century, despite no direct observations having occurred for several centuries.

Statistically, Type II-P supernovae represent the most prevalent category of core-collapse events, with red supergiants serving as their progenitors. Distinguishing between red supergiants nearing the terminal stages of core heavy element fusion and those with millions of years remaining poses a significant challenge. The most massive red supergiants undergo atmospheric mass loss, transitioning into Wolf–Rayet stars prior to core collapse. All Wolf–Rayet stars conclude their evolutionary phase within approximately one million years; however, pinpointing those imminently approaching core collapse remains problematic. A specific class, the WO Wolf–Rayet stars, are projected to explode within a few thousand years, having demonstrably depleted their core helium. Only eight such stars have been identified globally, with merely four located within the Milky Way galaxy.

Several proximate or well-documented stars have been designated as potential core-collapse supernova candidates. These include the high-mass blue stars Spica, Rigel, and Deneb; the red supergiants Betelgeuse, Antares, and VV Cephei A; the yellow hypergiant Rho Cassiopeiae; the luminous blue variable Eta Carinae, which previously exhibited a supernova impostor event; and both stellar components (a blue supergiant and a Wolf–Rayet star) of the Regor or Gamma Velorum system. Mimosa, Acrux, and Hadar (or Beta Centauri), three luminous stellar systems situated in the southern constellations of Crux and Centaurus, respectively, each comprise blue stars possessing sufficient mass to undergo supernova explosions. Additionally, certain other objects, such as WR 104, have garnered attention as improbable yet conceivable progenitors for gamma-ray bursts.

Identifying Type Ia supernova candidates is considerably more speculative. While any binary system featuring an accreting white dwarf could potentially generate a supernova, the precise underlying mechanism and temporal progression remain subjects of ongoing debate. These systems are inherently dim and challenging to detect; however, novae and recurrent novae represent such systems that conspicuously announce their presence. U Scorpii serves as a notable illustration.

Collapsar – A star that has experienced gravitational collapse.

• Collapsar – Star that has undergone gravitational collapse
• Kilonova – A stellar event resulting from the merger of neutron stars.
• List of supernova candidates
• List of supernova remnants
• List of supernovae
• Quark-nova – A hypothetical explosive event involving a neutron star.
• Superluminous supernova – A supernova exhibiting luminosity at least ten times greater than that of a typical supernova.
• Supernovae in fiction
• Timeline of white dwarfs, neutron stars, and supernovae – A chronological compilation of advancements in scientific understanding and observational records.

References

• Branch, D., & Wheeler, J. C. (2017). Supernova Explosions. Springer, p. 721. ISBN 978-3-662-55052-6.Iliadis, C. (2015). Nuclear Physics of Stars (2nd ed.). Weinheim: Wiley-VCH. doi:10.1002/9783527692668. ISBN 978-3-527-69266-8.Takahashi, K., Sato, K., Burrows, A., & Thompson, T. A. (2003). Supernova Neutrinos, Neutrino Oscillations, and the Mass of the Progenitor Star. Physical Review D, 68(11), 77–81. arXiv:hep-ph/0306056. Bibcode:2003PhRvD..68k3009T. doi:10.1103/PhysRevD.68.113009. S2CID 119390151.Woosley, S. E., & Janka, H.-T. (2005). The Physics of Core-Collapse Supernovae. Nature Physics, §34§(3), 147–154. arXiv:astro-ph/0601261. Bibcode:2005NatPh...1..147W. CiteSeerX 10.1.1.336.2176. doi:10.1038/nphys172. S2CID 118974639.Tsvetkov, D. Yu.; Pavlyuk, N. N.; Bartunov, O. S.; Pskovskii, Y. P. "Sternberg Astronomical Institute Supernova Catalogue" from the Sternberg Astronomical Institute, Moscow University, offers a searchable database.
  • Tsvetkov, D. Yu.; Pavlyuk, N. N.; Bartunov, O. S.; Pskovskii, Y. P. "Sternberg Astronomical Institute Supernova Catalogue". Sternberg Astronomical Institute, Moscow University. Retrieved 28 November 2006.The Open Supernova Catalog, hosted on GitHub and updated on 6 October 2022, provides an open-access collection of supernova light curves and spectra.The IAU: Central Bureau for Astronomical Telegrams maintains a list of supernovae with IAU designations.Source: TORIma Academy Archive