Nova (plural novae) means "new" in Latin, referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super-" distinguishes supernovae from ordinary novae, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism. The word supernova was coined by Swiss astrophysicist and astronomer Fritz Zwicky,[4][5] and was first used in print in 1926.[6] Several types of supernovae exist. Types I and II can be triggered in one of two ways, either turning off or suddenly turning on the production of energy through nuclear fusion. After the core of an aging massive star ceases generating energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing gravitational potential energy that heats and expels the star's outer layers. Alternatively a white dwarf star may accumulate sufficient material from a stellar companion (either through accretion or via a merger) to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it. Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the Chandrasekhar limit, while accreting white dwarfs ignite as they approach this limit (roughly 1.38[7] times the solar mass). White dwarfs are also subject to a different, much smaller type of thermonuclear explosion fueled by hydrogen on their surfaces called a nova. Solitary stars with a mass below approximately 9 solar masses, such as the Sun, evolve into white dwarfs without ever becoming supernovae.
Although no supernova has been observed in the Milky Way since 1604, supernovae remnants indicate on average the event occurs about once every 50 years in the Milky Way.[8][9] They play a significant role in enriching the interstellar medium with higher mass elements.[10] Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.[11][12][13]
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Observation history
Main article: History of supernova observation
Since the development of the telescope the field of supernova discovery has extended to other galaxies, starting with the 1885 observation of supernova S Andromedae in the Andromeda galaxy. Supernovae provide important information on cosmological distances.[18] During the twentieth century successful models for each type of supernova were developed, and scientists' comprehension of the role of supernovae in the star formation process is growing[update]. American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941.[19]
In the 1960s astronomers found that the maximum intensities of supernova explosions could be used as standard candles, hence indicators of astronomical distances.[20] Some of the most distant supernovae recently observed appeared dimmer than expected. This supports the view that the expansion of the universe is accelerating.[21][22] Techniques were developed for reconstructing supernova explosions that have no written records of being observed. The date of the Cassiopeia A supernova event was determined from light echoes off nebulae,[23] while the age of supernova remnant RX J0852.0-4622 was estimated from temperature measurements[24] and the gamma ray emissions from the decay of titanium-44.[25] In 2009 nitrates were discovered in Antarctic ice deposits that matched the times of past supernova events.[26]
Discovery
Because supernovae are relatively rare events within a galaxy, occurring about once every 50 years in the Milky Way,[9] 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.[27] Most scientific interest in supernovae—as standard candles for measuring distance, for example—require an observation of their peak luminosity. It is therefore important to discover them well before they reach their maximum. Amateur astronomers, who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an optical telescope and comparing them to earlier photographs.[28]
Towards the end of the 20th century astronomers increasingly turned to computer-controlled telescopes and CCDs for hunting supernovae. While such systems are popular with amateurs, there are also professional installations such as the Katzman Automatic Imaging Telescope.[29] Recently the Supernova Early Warning System (SNEWS) project has begun using a network of neutrino detectors to give early warning of a supernova in the Milky Way galaxy.[30][31] Neutrinos are particles that are produced in great quantities by a supernova explosion,[32] and they are not significantly absorbed by the interstellar gas and dust of the galactic disk.
Supernova searches fall into two classes: those focused on relatively nearby events and those looking for explosions farther away. Because of the expansion of the universe, the distance to a remote object with a known emission spectrum can be estimated by measuring its Doppler shift (or redshift); on average, more distant objects recede with greater velocity than those nearby, and so have a higher redshift. Thus the search is split between high redshift and low redshift, with the boundary falling around a redshift range of z = 0.1–0.3[33]—where z is a dimensionless measure of the spectrum's frequency shift.
High redshift searches for supernovae usually involve the observation of supernova light curves. These are useful for standard or calibrated candles to generate Hubble diagrams and make cosmological predictions. Supernova spectroscopy, used to study the physics and environments of supernovae, is more practical at low than at high redshift.[34][35] Low redshift observations also anchor the low-distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies.[36][37] (See also Hubble's law).
Naming convention
Supernova discoveries are reported to the International Astronomical Union's Central Bureau for Astronomical Telegrams, which sends out a circular with the name it assigns to it. The name is the year of discovery, immediately followed by 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.[38] Since 2000, professional and amateur astronomers find several hundreds of supernovae each year (572 in 2007, 261 in 2008, 390 in 2009). For example, the last supernova of 2005 was SN 2005nc, indicating that it was the 367th[nb 1] supernova found in 2005.[39][40]
Historical supernovae are known simply by the year they occurred: SN 185, SN 1006, SN 1054, SN 1572 (Tycho's Nova) and SN 1604 (Kepler's Star). Since 1885 the letter notation has been used, even if there was only one supernova discovered that year (e.g. SN 1885A, 1907A, etc.)—this last happened with SN 1947A. "SN", for SuperNova, is a standard prefix. Until 1987, two-letter designations were rarely needed; since 1988, however, they have been needed every year.
Classification
As part of the attempt to understand supernovae, astronomers have classified them according to the absorption lines of different chemical elements that appear in their spectra. The first element for a division is the presence or absence of a line caused by hydrogen. If a supernova's spectrum contains a line of hydrogen (known as the Balmer series in the visual portion of the spectrum) it is classified Type II; otherwise it is Type I. Among those types, there are subdivisions according to the presence of lines from other elements and the shape of the light curve (a graph of the supernova's apparent magnitude as a function of time).[41]| Type | Characteristics |
|---|---|
| Type I | |
| Type Ia | Lacks hydrogen and presents a singly ionized silicon (Si II) line at 615.0 nm (nanometers), near peak light. |
| Type Ib | Non-ionized helium (He I) line at 587.6 nm and no strong silicon absorption feature near 615 nm. |
| Type Ic | Weak or no helium lines and no strong silicon absorption feature near 615 nm. |
| Type II | |
| Type IIP | Reaches a "plateau" in its light curve |
| Type IIL | Displays a "linear" decrease in its light curve (linear in magnitude versus time).[43] |
A few supernovae, such as SN 1987K and SN 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type IIb" is used to describe the combination of features normally associated with Types II and Ib.[42]
Current models
Type Ia
Main article: Type Ia supernova
There are several means by which a supernova of this type can form, but they share a common underlying mechanism. If a carbon-oxygen[nb 2] white dwarf accreted enough matter to reach the Chandrasekhar limit of about 1.38 solar masses[7] (for a non-rotating star), it would no longer be able to support the bulk of its plasma through electron degeneracy pressure[44][45] and would begin to collapse. However, the current view is that this limit is not normally attained; increasing temperature and density inside the core ignite carbon fusion as the star approaches the limit (to within about 1%[46]), before collapse is initiated.[7] Within a few seconds, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (1–2 × 1044 joules)[47] to unbind the star in a supernova explosion.[48] An outwardly expanding shock wave is generated, with matter reaching velocities on the order of 5,000–20,000 km/s, or roughly 3% of the speed of light. There is also a significant increase in luminosity, reaching an absolute magnitude of -19.3 (or 5 billion times brighter than the Sun), with little variation.[49]One model for the formation of this category of supernova is a close binary star system. The larger of the two stars is the first to evolve off the main sequence, and it expands to form a red giant.[50] The two stars now share a common envelope, causing their mutual orbit to shrink. The giant star then sheds most of its envelope, losing mass until it can no longer continue nuclear fusion. At this point it becomes a white dwarf star, composed primarily of carbon and oxygen.[51][52] Eventually the secondary star also evolves off the main sequence to form a red giant. Matter from the giant is accreted by the white dwarf, causing the latter to increase in mass.
Another model for the formation of a Type Ia explosion involves the merger of two white dwarf stars, with the combined mass momentarily exceeding the Chandrasekhar limit.[53] A white dwarf could also accrete matter from other types of companions, including a main sequence star (if the orbit is sufficiently close).
Type Ia supernovae follow a characteristic light curve—the graph of luminosity as a function of time—after the explosion. This luminosity is generated by the radioactive decay of nickel-56 through cobalt-56 to iron-56.[49] The peak luminosity of the light curve was believed to be consistent across Type Ia supernovae (the vast majority of which are initiated with a uniform mass via the accretion mechanism), having a maximum absolute magnitude of about -19.3. This would allow them to be used as a secondary[54] standard candle to measure the distance to their host galaxies.[55] However, recent discoveries reveal that there is some evolution in the average lightcurve width, and thus in the intrinsic luminosity of supernovae, although significant evolution is found only over a large redshift baseline.[56]
Type Ib and Ic
Main article: Type Ib and Ic supernovae
Type II
Main article: Type II supernova
When the core's supply of hydrogen is exhausted, this outward pressure is no longer created. The core begins to collapse, causing a rise in temperature and pressure which becomes great enough to ignite the helium and start a helium-to-carbon fusion cycle, creating sufficient outward pressure to halt the collapse. The core expands and cools slightly, with a hydrogen-fusion outer layer, and a hotter, higher pressure, helium-fusion center. (Other elements such as magnesium, sulfur and calcium are also created and in some cases burned in these further reactions.)
This process repeats several times; each time the core collapses, and the collapse is halted by the ignition of a further process involving more massive nuclei and higher temperatures and pressures. Each layer is prevented from collapse by the heat and outward pressure of the fusion process in the next layer inward; each layer also burns hotter and quicker than the previous one—the final burn of silicon to iron consumes its fuel in just a few days at most.[62] The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells.[63][64]
In the later stages increasingly heavier elements with higher binding energy undergo nuclear fusion. Fusion produces progressively less energy, and also at higher core energies photodisintegration and electron capture occur which cause further energy loss in the core, requiring a general acceleration of the fusion processes to maintain hydrostatic equilibrium.[62] This escalation culminates with the production of nickel-56, which is unable to produce energy through fusion (but does produce iron-56 through radioactive decay).[65] As a result, a nickel-iron core[66] builds up that cannot produce further outward pressure on the scale needed to support the rest of the structure. It can only support the overlaying mass of the star through the degeneracy pressure of electrons in the core. If the star is sufficiently large, then the iron-nickel core will eventually exceed the Chandrasekhar limit (1.38 solar masses), at which point this mechanism catastrophically fails. The forces holding atomic nuclei apart in the innermost layer of the core suddenly give way, the core implodes due to its own mass, and no further fusion process is available to ignite and prevent collapse this time.[44]
Core collapse
See also: Gravitational collapse
The core collapses in on itself with velocities reaching 70,000 km/s (0.23c),[67] resulting in a rapid increase in temperature and density. The energy loss processes operating in the core cease to be in equilibrium. Through photodisintegration, gamma rays decompose iron into helium nuclei and free neutrons, absorbing energy, while electrons and protons merge via electron capture, producing neutrons and electron neutrinos, which escape.In a typical Type II supernova the newly formed neutron core has an initial temperature of about 100 billion kelvin (100 GK), 6000 times the temperature of the sun's core. A further release of neutrinos carries away much of the thermal energy, allowing a stable neutron star to form (the neutrons would "boil away" if this cooling did not occur).[68] These 'thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos.[69] About 1046 joules of gravitational energy—approximately 10% of the star's rest mass—is converted into a ten-second burst of neutrinos, which is the main output of the event.[62][70] These carry away energy from the core and accelerate the collapse, while some neutrinos are absorbed by the star's outer layers and provide energy to the supernova explosion.[71]
The inner core eventually reaches typically 30 km diameter,[62] and a density comparable to that of an atomic nucleus, and further collapse is abruptly stopped by strong force interactions and by degeneracy pressure of neutrons. The infalling matter, suddenly halted, rebounds, producing a shock wave that propagates outward. Computer simulations indicate that this expanding shock does not directly cause the supernova explosion;[62] rather, it stalls within milliseconds[72] in the outer core as energy is lost through the dissociation of heavy elements, and a process that is not clearly understood[update] is necessary to allow the outer layers of the core to reabsorb around 1044 joules[nb 3] (1 foe) of energy, producing the visible explosion.[73] Current[update] research focuses upon a combination of neutrino reheating, rotational and magnetic effects as the basis for this process.[62]
Above 50 solar masses stars were believed to collapse directly into a black hole without forming a supernova explosion,[76] although uncertainties in models of supernova collapse make accurate calculation of these limits difficult. Above about 140 solar masses stars may become pair-instability supernovae that do not leave behind a black hole remnant.[77][78]
Light curves and unusual spectra
The plateau phase in Type II-P supernovae is due to a change in the opacity of the exterior layer. The shock wave ionizes the hydrogen in the outer envelope, which greatly increases the opacity. This prevents photons from the inner parts of the explosion from escaping. Once the hydrogen cools sufficiently to recombine, the outer layer becomes transparent.[79]
Of the Type II supernovae with unusual features in their spectra, Type IIn supernovae may be produced by the interaction of the ejecta with circumstellar material.[80] Type IIb supernovae are likely massive stars which have lost most, but not all, of their hydrogen envelopes through tidal stripping by a companion star. As the ejecta of a Type IIb expands, the hydrogen layer quickly becomes optically thin and reveals the deeper layers.[81]
The peak absolute magnitude of Type II supernovae varies from one to another, but they are dimmer than Type Ia.[82] For instance, the low-luminosity SN 1987A had a peak visual absolute magnitude of -15.5 (apparent magnitude +3 for a distance of 51 kpc), as compared to the standard -19.3 for Type Ia.
Pair-instability type
Main article: Pair-instability supernova
The core temperature of a star of over about 140 solar masses can become so high that photons convert spontaneously to electron-positron pairs, reducing the photon pressure supporting the star's outer layers and triggering a collapse that leads to a supernova explosion. This pair-instability supernova creates a larger quantity of elements heavier than helium ("metals") than other types of supernova and leaves no black hole as a remnant. Stars of this size can only form from interstellar gas with very low metal content, which is characteristic of the early universe before the first supernovae produced metals from the primordial hydrogen and helium. It is believed that supernova SN 2007bi was of this type; it was distinguished from other supernovae by very long duration—77 days to peak brightness, bright enough to observe for 555 days—and production of much more radioactive nickel. The pair-instability supernova was predicted by Gary S. Fraley in 1968.[83]Asymmetry
A long-standing puzzle surrounding Type II supernovae is why the compact object remaining after the explosion is given a large velocity away from the core.[84] (Neutron stars are observed, as pulsars, to have high velocities; black holes presumably do as well, but are far harder to observe in isolation.) The initial impetus can be substantial, propelling an object of more than a solar mass at a velocity of 500 km/s or greater. This displacement indicates an asymmetry in the explosion, but the mechanism by which this momentum is transferred to the compact object remains[update] a puzzle. Proposed explanations for this kick include convection in the collapsing star and jet production during neutron star formation.
Another possible explanation is that accretion of gas onto the central neutron star can create a disk that drives highly directional jets, propelling matter at a high velocity out of the star, and driving transverse shocks that completely disrupt the star. These jets might play a crucial role in the resulting supernova explosion.[88][89] (A similar model is now favored for explaining long gamma ray bursts.)
Initial asymmetries have also been confirmed in Type Ia supernova explosions through observation. This result may mean that the initial luminosity of this type of supernova depends on the viewing angle. However, the explosion becomes more symmetrical with the passage of time. Early asymmetries are detectable by measuring the polarization of the emitted light.[90]
Energy output
Because they have a similar functional model, Types Ib, Ic and various Types II supernovae are collectively called Core Collapse supernovae. A fundamental difference between Type Ia and Core Collapse supernovae is the source of energy for the radiation emitted near the peak of the light curve. The progenitors of Core Collapse supernovae are stars with extended envelopes that can attain a degree of transparency with relatively little expansion. Most of the energy powering the emission at peak light is derived from the shock wave that heats and ejects the envelope.[91]The progenitors of Type Ia supernovae, on the other hand, are compact objects, much smaller (but more massive) than the Sun, that must expand (and therefore cool) enormously before becoming transparent. Heat from the explosion is dissipated in the expansion and is not available for light production. The radiation emitted by Type Ia supernovae is thus entirely attributable to the decay of radionuclides produced in the explosion; principally nickel-56 (with a half-life of 6.1 days) and its daughter cobalt-56 (with a half-life of 77 days). Gamma rays emitted during this nuclear decay are absorbed by the ejected material, heating it to incandescence.
As the material ejected by a Core Collapse supernova expands and cools, radioactive decay eventually takes over as the main energy source for light emission in this case also. A bright Type Ia supernova may expel 0.5–1.0 solar masses of nickel-56,[92] while a Core Collapse supernova probably ejects closer to 0.1 solar mass of nickel-56.[93]
Interstellar impact
Source of heavy elements
Main article: Supernova nucleosynthesis
Supernovae are a key source of elements heavier than oxygen.[94] These elements are produced by nuclear fusion (for iron-56 and lighter elements), and by nucleosynthesis during the supernova explosion for elements heavier than iron.[95] Supernovae are the most likely, although not undisputed, candidate sites for the r-process, which is a rapid form of nucleosynthesis that occurs under conditions of high temperature and high density of neutrons. The reactions produce highly unstable nuclei that are rich in neutrons. These forms are unstable and rapidly beta decay into more stable forms.The r-process reaction, which is likely to occur in type II supernovae, produces about half of all the element abundance beyond iron, including plutonium, uranium and californium.[96][not in citation given] The only other major competing process for producing elements heavier than iron is the s-process in large, old red giant stars, which produces these elements much more slowly, and which cannot produce elements heavier than lead.[97]
Role in stellar evolution
Main article: Supernova remnant
The remnant of a supernova explosion consists of a compact object and a rapidly expanding shock wave of material. This cloud of material sweeps up the surrounding interstellar medium during a free expansion phase, which can last for up to two centuries. The wave then gradually undergoes a period of adiabatic expansion, and will slowly cool and mix with the surrounding interstellar medium over a period of about 10,000 years.[98]The Big Bang produced hydrogen, helium, and traces of lithium, while all heavier elements are synthesized in stars and supernovae. Supernovae tend to enrich the surrounding interstellar medium with metals—elements other than hydrogen and helium.
The kinetic energy of an expanding supernova remnant can trigger star formation due to compression of nearby, dense molecular clouds in space.[100] The increase in turbulent pressure can also prevent star formation if the cloud is unable to lose the excess energy.[11]
Evidence from daughter products of short-lived radioactive isotopes shows that a nearby supernova helped determine the composition of the Solar System 4.5 billion years ago, and may even have triggered the formation of this system.[101] Supernova production of heavy elements over astronomic periods of time ultimately made the chemistry of life on Earth possible.
Impact on Earth
Main article: Near-Earth supernova
A near-Earth supernova is a supernova close enough to the Earth to have noticeable effects on its biosphere. This would need to be nearer than about 100 to 3000 light-years away, depending upon type and energy—different figures have been suggested. Gamma rays from a supernova would induce a chemical reaction in the upper atmosphere converting molecular nitrogen into nitrogen oxides, depleting the ozone layer enough to expose the surface to harmful solar and cosmic radiation. This has been proposed as the cause of the Ordovician-Silurian extinction, which resulted in the death of nearly 60% of the oceanic life on Earth.[102] In 1996 it was theorized that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Iron-60 enrichment was later reported in deep-sea rock of the Pacific Ocean.[103][104][105] In 2009, elevated levels of nitrate ions were found in Antarctic ice, which coincided with the 1006 and 1054 supernovae. Gamma rays from these supernovae could have boosted levels of nitrogen oxides, which became trapped in the ice.[106]Type Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because these supernovae arise from dim, common white dwarf stars, it is likely that a supernova that can affect the Earth will occur unpredictably and in a star system that is not well studied. One theory suggests that a Type Ia supernova would have to be closer than a thousand parsecs (3300 light-years) to affect the Earth.[107] The closest known candidate is IK Pegasi (see below).[108] Recent estimates predict that a Type II supernova would have to be closer than eight parsecs (26 light-years) to destroy half of the Earth's ozone layer.[109]
Milky Way candidates
Main article: List of supernova candidates
The nearest supernova candidate is IK Pegasi (HR 8210), located at a distance of 150 light-years. This closely orbiting binary star system consists of a main sequence star and a white dwarf 31 million kilometres apart. The dwarf has an estimated mass 1.15 times that of the Sun.[122] It is thought that several million years will pass before the white dwarf can accrete the critical mass required to become a Type Ia supernova.[123][124]
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