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Supernova

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For other uses, see Supernova (disambiguation).
Multiwavelength X-ray image of the remnant of Kepler's Supernova, SN 1604. (Chandra X-ray Observatory)

A supernova (plural: supernovae) is a stellar explosion which produces an extremely luminous object that is initially made of plasma—an ionized state of matter. A supernova may briefly out-shine its entire host galaxy before fading from view over several weeks or months. It would take about 10 billion years for the Sun to produce the energy output from the supernova explosion of a massive star.[1] The explosion expels much or all of a star's material[2] at a velocity up to a tenth the speed of light, driving a shock wave into the surrounding interstellar gas. This shock sweeps up an expanding shell of gas and dust called a supernova remnant.

There are several types of supernovae and at least two possible routes to their formation. A massive star may cease to generate energy from the nuclear fusion of atoms in its core, and collapse under the force of its own gravity to form a neutron star or black hole. Alternatively, a white dwarf star may accumulate material from a companion star (either through accretion or a collision) until it nears the Chandrasekhar limit of roughly 1.4 times the mass of the Sun, then undergoes runaway nuclear fusion in its interior, completely disrupting it. This second type of supernova is distinct from a surface thermonuclear explosion on a white dwarf, which is called a nova. Solitary stars with a mass below approximately 8 solar masses, such as the Sun itself, will evolve into white dwarfs without ever becoming supernovae.

A supernova is a relatively rare event that occurs, on average, only once every 50 years in a galaxy the size of the Milky Way. However they play a significant role in enriching the interstellar medium with heavy elements, and the expanding shock waves from these explosions can trigger the formation of new stars.[3]

"Nova" is Latin for "new", referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super" distinguishes this from an ordinary nova, which also involves a star increasing in brightness, though to a lesser extent and through a different mechanism.

Observation history

Main article: History of supernova observation
The Crab Nebula is a pulsar wind nebula associated with the 1054 supernova.

The earliest recorded supernova, SN 185, was viewed by Chinese astronomers in AD 185. A widely-observed supernova, SN 1054, produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the last to be observed in the Milky Way galaxy, had notable impacts on the development of astronomy in Europe.[4]

Since the development of the telescope, the field of supernova discovery has expanded to other galaxies, starting with the 1885 observation of supernova S Andromedae in the Andromeda galaxy. These events provide important information on cosmological distances.[5] During the twentieth century, successful supernova models for each type of supernovae were developed, and the role of supernova in the star formation process is now increasingly understood.

Most recently it has been discovered that some of the most distant supernovae appeared dimmer than expected. This has provided evidence that the expansion of the universe may be accelerating.[6][7]

Discovery

Supernova explosions are relatively rare events, occuring about once every 50 years in a galaxy like the Milky Way.[8] Thus, in order to obtain a good sample of supernovae to study, a large number of galaxies must be observed on a regular basis.

The explosion of supernovae in other galaxies cannot be predicted with any meaningful accuracy. When they are discovered, they are already in progress.[9] Most uses for supernovae—as standard candles for measuring distance, for instance—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.

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 larger installations like the Katzman Automatic Imaging Telescope.[10] Recently, the Supernova Early Warning System (SNEWS) project has also begun using a network of neutrino detectors to give early warning of a supernova in the Milky Way galaxy.[11][12] A neutrino is a particle that is produced in great quantities by a supernova explosion,[13] and it is not obscured by the interstellar gas and dust of the galactic disk.

Supernova searches fall into two regimes: those relatively nearby and those at a more extreme distance. Due to the expansion of the universe, the distance of a remote object can be estimated by measuring the Doppler shift (or redshift) of its spectrum; on average, more distant objects will be receding with greater velocity than those nearby, and so will have a higher redshift. Thus the search is split between high redshift and low redshift, with the boundary falling somewhere around a redshift of z = 0.2—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. At low redshift, supernova spectroscopy is more practical than at high redshift, and this information can be used to study the physics and environments of supernovae.[14][15] Low redshift observations also anchor the low distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies.[16][17]

See also: Hubble's law

Naming convention

SN 1994D in the NGC 4526 galaxy (bright spot on the lower left). Image by NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team

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 formed by the year of discovery, immediately followed by a one or two-letter designation. The first 26 supernovae of the year get an upper case letter from A to Z. Afterward, pairs of lower-case letters are used, starting with aa, ab, and so on.[18] Professional and amateur astronomers find several hundred supernovae per year—341 in 2005 and 529 in 2006. For example, the last supernova of 2005 was SN 2005nc, indicating that it was the 341st supernova found in 2005.[19][20]

Four historical supernovae are known simply by the year they occurred: SN 1006, 1054, 1572 (Tycho's Nova), and 1604 (Kepler's Star). Beginning in 1885, the letter notation is used, even if there was only one supernova that year (e.g. SN 1885A, 1907A, etc.) —this last happened with SN 1947A. The standard abbreviation "SN" is an optional prefix.

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 versus time).[21]

Supernova taxonomy[22]
Type Characteristics
Type I
Type Ia Lacks hydrogen and present a singly-ionized silicon (Si II) line at 615.0 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 their light curve
Type IIL Displays a "linear" decrease in their light curve, where it is "linear" in magnitude versus time, or exponential in luminosity versus time.[23]

The supernovae of Type II can also be sub-divided based on their spectra. While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of kilometres per second, some have relatively narrow features. These are called Type IIn, where the "n" stands for "narrow".[22]

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.[22]

Current models

Type Ia

The most commonly accepted theory of this type of supernovae is that they are the result of a carbon-oxygen white dwarf accreting matter from a nearby companion star, typically a red giant.[24]

The progenitor binary system consists of main sequence stars, with the primary possessing more mass than the secondary. Being greater in mass, the primary is the first of the pair to evolve onto the asymptotic giant branch, where the star's envelope expands considerably. If the two stars share a common envelope then the system can lose significant amounts of mass, and the angular momentum, orbital radius and period will all be reduced. After the primary has degenerated into a white dwarf, the secondary star later evolves in a red giant and the stage is set for mass accretion onto the primary. During this final shared-envelope phase, the two stars spiral in closer together as angular momentum is lost. The resulting orbit can have a period as brief as a few hours.[25][26]

If the accretion continues long enough, the white dwarf may eventually approach the Chandrasekhar limit of 1.44 solar masses; the maximum mass that can be supported by electron degeneracy pressure.[27] Beyond this limit the white dwarf would collapse to form a neutron star.[28]

The current view is that this limit is never actually attained, however, so that collapse is never initiated. Instead, the increase in pressure due to the elevated mass of the white dwarf raises the temperature near the core, and a period of convection ensues lasting approximately 1,000 years[29] begins. At some point in this simmering phase, a deflagration flame front is born, powered by carbon fusion. (The details of the ignition—the location and number of points where the flame begins—is still unknown.[30]) Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon.[31]

Once fusion has begun, the temperature of the white dwarf starts to rise. Normally a typical main sequence star would expand and cool in order to counter-balance an increase in thermal energy. However, degeneracy pressure is independent of temperature, so the white dwarf is unable to regulate the burning process in the manner of normal stars. The flame accelerates dramatically, through the Rayleigh-Taylor instability and interactions with turbulence. It is still a matter of considerable debate as to whether this flame transitions from a subsonic deflagration into a supersonic detonation.[29][32]

Regardless of the exact details of nuclear burning, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf is burned into heavier elements within a period of only a few seconds,[31] raising the internal temperature to billions of degrees.

This energy release from thermonuclear burning (≈1046 joules)[31] is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy that they are all able to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5-20,000 km/s, or roughly 3% of the speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visual absolute magnitude of Type Ia supernovae is Mv = -19.3 (≈ 5 billion times brighter than Sol), with little variation.[29] Whether or not the supernova remnant remains bound to its companion depends on the amount of mass ejected. As a general rule, the system will remain bound if the remnant is heavier than one half of the original total system mass. If not, the companion will evolve into a runaway star.[33]

Multiwavelength X-ray image of SN 1572 or Tycho's Nova (NASA/CXC/Rutgers/J.Warren & J.Hughes et al.)

The theory of this type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not approach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a hydrogen fusion surface explosion that does not disrupt the star.[29]

Formation

Unlike the other types of supernovae, Type Ia supernovae generally occur in all types of galaxies, including ellipticals. They show no preference for regions of current stellar formation.[34] As white dwarf stars form at the end of a star's main sequence evolutionary period, such a long-lived star system may have wandered far from the region where it originally formed. Thereafter a close binary system may spend another million years in the mass transfer stage (possibly forming persistent nova outbursts) before the conditions are ripe for a Type Ia supernova to occur.[35]

A second possible, but much less likely, mechanism for triggering a Type Ia supernova is the merger of two white dwarfs.[36] In such a case, the total mass would not be constrained by the Chandrasekhar limit. This is one of several explanations proposed for the anomalously massive (2 solar mass) progenitor of the "Champagne Supernova" (SN 2003fg or SNLS-03D3bb).[37][38]

Collisions of solitary stars within our galaxy are thought to occur only once every 107–1013 years; far less frequently than the appearance of novae.[39] However, collisions occur with greater frequency in the dense core regions of globular clusters.[40] (C.f. blue stragglers.) A likely scenario is a collision with a binary star system, or between two binary systems containing white dwarfs. This collision can leave behind a close binary system of two white dwarfs. Their orbit decays and they merge together through their shared envelope.[41]

Light curve

This plot of luminosity (relative to the Sun) versus time shows the characteristic light curve for a Type Ia supernova. The peak is primarily due to the decay of Nickel (Ni), while the later stage is powered by Cobalt (Co).

Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion. Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star, heavy elements synthesized during the explosion, most prominently iron-group elements. The radioactive decay of Nickel-56 through Cobalt-56 to Iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.[29]

The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as a secondary[42] standard candle in extragalactic astronomy.[43] The cause of this uniformity in the luminosity curve is still an open question. In 1998, observations of Type Ia supernovae indicated the unexpected result that the universe seems to undergo an accelerating expansion.[6][7]

Type Ib and Ic

Main article: Type Ib and Ic supernovae

These events, like supernovae of Type II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types Ib and Ic have lost most of their outer envelopes due to strong stellar winds or else from interaction with a companion.[44] Type Ib supernovae are thought to be the result of the collapse of a massive Wolf-Rayet star. There is some evidence that a few percent of the Type Ic supernovae may be the progenitors of gamma ray bursts (GRB), though it is also believed that any Hydrogen-stripped core-collapse supernova (Type Ib, Ic) could be a GRB, dependent upon the geometry of the explosion.[45]

Type II

Stars far more massive than the sun evolve in much more complex fashions. In the core of the sun, hydrogen is fused into helium, releasing energy which heats the sun's core, and providing pressure which supports the sun's layers against collapse (see hydrostatic equilibrium). The helium produced in the core accumulates there since temperatures in the core are not yet high enough to cause it to fuse. Eventually, as the hydrogen at the core is exhausted, fusion begins to slow down and gravity begins to cause the core to contract. This contraction raises the temperature high enough to initiate a shorter phase of helium fusion, which accounts for less than 10% of the star's total lifetime. In stars with less than eight solar masses, the carbon produced by helium fusion does not fuse, and the star gradually cools to become a white dwarf.[46][47] White dwarf stars, if they have a near companion, may then become Type Ia supernovae.

A much larger star, however, is massive enough to create temperatures and pressures needed to cause the carbon in the core to begin to fuse once the star contracts at the end of the helium-burning stage. The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center, with an outermost layer of hydrogen gas, surrounding a layer of hydrogen fusing into helium, surrounding a layer of helium fusing into carbon (via the triple-alpha process), surrounding layers that fuse to progressively heavier elements. As a star this massive evolves, it undergoes repeated stages where fusion in the core stops, and the core collapses until the pressure and temperature is sufficient to begin the next stage of fusion, reigniting to halt collapse.[46][47]

Core collapse

The factor limiting this process is the amount of energy that is released through fusion, which is dependent on the binding energy of these atomic nuclei. Each additional step produces progressively heavier nuclei, which release progressively less energy when fusing, until iron is produced. As iron and nickel have the highest binding energy per nucleon of all the elements,[48] iron cannot produce energy when fused, and an iron core grows.[47] This iron core is under huge gravitational pressure. As there is no fusion to further raise the star's temperature to support it against collapse, it is supported only by degeneracy pressure of electrons. When the core's size exceeds the Chandrasekhar limit, degeneracy pressure can no longer support it, and catastrophic collapse ensues.[27]

The outer part of the core reaches velocities of up to 70,000 km/s (0.23c) as it collapses toward the center of the star.[49] The rapidly shrinking core heats up, producing high energy gamma rays which decompose iron nuclei into helium nuclei and free neutrons (via photodissociation). As the core's density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and neutrinos. The neutrinos escape from the core, carrying away energy and further accelerating the collapse, which proceeds in milliseconds as the core detaches from the outer layers of the star. Some of these neutrinos are absorbed by the star's outer layers, beginning the supernova explosion.[50]

For Type II supernovae, the collapse is eventually halted by short-range repulsive neutron-neutron interactions mediated by the strong force, as well as by degeneracy pressure of neutrons, at a density comparable to that of an atomic nucleus. Once collapse stops, the infalling matter rebounds, producing a shock wave that propagates outward. The energy from this shock dissociates heavy elements within the core. This reduces the energy of the shock, which can stall the explosion within the outer core.[51]

The core collapse phase is known to be so dense and energetic that only neutrinos are able to escape. Most of the gravitational potential energy of the collapse gets converted to a ten second neutrino burst, releasing about 1046 joules (100 foes).[52] Of this energy, about 1044 Joule (1 foe) is reabsorbed by the star producing an explosion.[a] This energy revives the stalled shock, which blows off the rest of the star's material.[51] The neutrinos produced by a supernova have been actually observed in the case of Supernova 1987A leading astronomers to conclude that the core collapse picture is basically correct.

Within a massive, evolved star (a) the onion-layered shells of elements undergo fusion, forming an iron core (b) that reaches Chandrasekhar-mass and starts to collapse. The inner part of the core is compressed into neutrons (c), causing infalling material to bounce (d) and form an outward-propagating shock front (red). The shock starts to stall (e), but it is re-invigorated by neutrino interaction. The surrounding material is blasted away (f), leaving only a degenerate remnant.

When the progenitor star is below about 20 solar masses (depending on the strength of the explosion and the amount of material that falls back), the degenerate remnant of a core collapse is a neutron star.[49] Above this mass the remnant collapses to form a black hole.[47][53] The theoretical limiting mass for this type of core collapse scenario is about 40–50 solar masses. Above that mass, a star is believed to collapse directly into a black hole without forming a supernova explosion.[54]

Type II and theoretical models

The energy per particle in a supernova is typically one to one hundred and fifty picojoules (tens to hundreds of MeV).[55] The per-particle energy involved in a supernova is small enough that the predictions gained from the Standard Model of particle physics are likely to be basically correct, but the high densities may include corrections to the Standard Model.[56] In particular, Earth-based particle accelerators can produce particle interactions which are of much higher energy than are found in supernovae,[57] but these experiments involve individual particles interacting with individual particles, and it is likely that the high densities within the supernova will produce novel effects. The interactions between neutrinos and the other particles in the supernova take place with the weak nuclear force which is believed to be well understood. However, the interactions between the protons and neutrons involve the strong nuclear force which is much less well understood.

The major unsolved problem with Type II supernovae is that it is not understood how the burst of neutrinos transfers its energy to the rest of the star producing the shock wave which causes the star to explode. From the above discussion, only one percent of the energy needs to be transferred to produce an explosion, but getting that one percent of transfer has proven very difficult. In the 1990s, one model for doing this involved convective overturn, which suggests that convection, either from neutrinos from below, or infalling matter from above, completes the process of destroying the progenitor star. Heavier elements than iron are formed during this explosion by neutron capture, and from the pressure of the neutrinos pressing into the boundary of the "neutrinosphere", seeding the surrounding space with a cloud of gas and dust which is richer in heavy elements than the material from which the star originally formed.[58]

Neutrino physics, which is modeled by the Standard Model, is crucial to the understanding of this process.[56] The other crucial area of investigation is the hydrodynamics of the plasma that makes up the dying star; how it behaves during the core collapse determines when and how the "shock wave" forms and when and how it "stalls" and is reenergized.[59] Computer models have been very successful at calculating the behavior of Type II supernovae once the shock has been formed. By ignoring the first second of the explosion, and assuming that an explosion is started, astrophysicists have been able to make detailed predictions about the elements produced by the supernova and of the expected light curve from the supernova.[60][61][62] However some aspects of the supernova light curves remain unexplained.

Large-scale, multi-dimensional computing simulations are being planned that will model supernovae explosions in much greater detail, which could help explain many of the observables.[63]

Light curves and unusual spectra

This graph of the luminosity (relative to the Sun) as a function of time shows the characterisic shapes of the light curves for a Type II-L and II-P supernova.

The light curves for Type II supernovae are distinguished by the presence of hydrogen Balmer absorption lines in the spectra. These light curves have an average decay rate of 0.008 magnitudes per day; much lower than the decay rate for Type I supernovae. Type II are sub-divided into two classes, depending on whether there is a plateau in their light curve (Type II-P) or a linear decay rate (Type II-L). The net decay rate is lower at 0.012 magnitudes per day for Type II-L compared to 0.0075 magnitudes per day for Type II-P. The difference in the shape of the light curves is believed to be caused, in the case of Type II-L supernovae, by the expulsion of most of the hydrogen envelope of the progenitor star.[23]

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.[64]

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.[65] 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.[66]

Hypernovae (Collapsars)

Main article: Hypernova

The core collapse of sufficiently massive stars may not be halted. Degeneracy pressure and repulsive neutron-neutron interactions can only support a neutron star whose mass does not exceed the Tolman-Oppenheimer-Volkoff limit of very roughly 4 solar masses.[67] Above this limit, the core collapses to directly form a black hole,[54] perhaps producing a (still theoretical) hypernova explosion. In the proposed hypernova mechanism (known as a collapsar) two extremely energetic jets of plasma are emitted from the star's rotational poles at nearly light speed. These jets emit intense gamma rays, and are one of many candidate explanations for gamma ray bursts.[68]

Asymmetry

A long-standing puzzle surrounding supernovae has been a need to explain why the compact object remaining after the explosion is given a large velocity away from the core.[69] (Neutron stars are observed, as pulsars, to have high velocities; black holes presumably do as well, but are far harder to observe in isolation.) This kick can be substantial, propelling an object of more than a solar mass at a velocity of 500 km/s or greater. This displacement is believed to be caused by an asymmetry in the explosion, but the mechanism by which this momentum is transferred to the compact object has remained a puzzle. Some explanations for this kick include convection in the collapsing star and jet production during neutron star formation.

This composite image shows X-ray (blue) and optical (red) radiation from the Crab Nebula's core region. A pulsar near the center is propelling particles to almost the speed of light.[70] This neutron star is travelling at an estimated 375 km/s.[71] NASA/CXC/HST/ASU/J. Hester et al. image credit.

One explanation for the asymmetry in the explosion is large-scale convection above the core. The convection can create variations in the local abundances of elements, resulting in uneven nuclear burning during the collapse, bounce and resulting explosion.[72]

Another 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.[73][74] (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 may depend on the viewing angle. However the explosion becomes more symmetrical with the passage of time. Early asymmetries may be detectable by measuring slight differences in the polarization of the emitted light.[75]

Type I versus Type II

A fundamental difference between Type I and Type II supernovae is the source of energy for the radiation emitted near the peak of the light curve. The progenitors of Type II supernovae are stars with extended envelopes that can attain a degree of transparency with a relatively small amount of expansion. Most of the energy powering the emission at peak light is derived from the shock wave that heats and ejects the envelope.[76]

The progenitors of Type I 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 I 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 Type II 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,[77] while a Type Ib, Ic or Type II supernova probably ejects closer to 0.1 solar mass of Nickel-56.[78]

Interstellar impact

Source of heavy elements

Main article: Supernova nucleosynthesis

Supernovae are a key source of elements heavier than oxygen. These elements are produced by fusion (for iron-56 and lighter elements), and by nucleosynthesis during the supernova explosion for elements heavier than iron. The synthesis of heavy nuclei within a supernova occurs a result of 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.

Th r-process reaction in supernovae produces about half of all the element abundance beyond iron, including plutonium, uranium and californium.[79] The only 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.[80]

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.[81]

In standard astronomy, 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, which for astronomers means all of the elements other than hydrogen and helium and is a different definition than that used in chemistry.

Supernova remnant N 63A lies within a clumpy region of gas and dust in the Large Magellanic Cloud. NASA image.

These injected elements ultimately enrich the molecular clouds that are the sites of star formation.[82] Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. Supernovae are the dominant mechanism for distributing these heavier elements, which are formed in a star during its period of nuclear fusion, throughout space. The different abundances of elements in the material that forms a star have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.

The kinetic energy of an expanding supernova remnant can trigger star formation due to compression of nearby, dense molecular clouds in space. The increase in turbulent pressure can also prevent star formation if the cloud is unable to lose the excess energy.[83]

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.[84] Supernova production of heavy elements over astronomic periods of time ultimately made the chemistry of life on Earth possible.

Impact on Earth

A near-Earth supernova is an explosion resulting from the death of a star that occurs close enough to the Earth (roughly fewer than 100 light-years away) to have noticeable effects on its biosphere. Gamma rays are responsible for most of the adverse effects a supernova can have on a living terrestrial planet. In Earth's case, gamma rays 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. The gamma ray burst from a nearby supernova explosion has been proposed as the cause of the end Ordovician extinction, which resulted in the death of nearly 60% of the oceanic life on Earth. [85]

Speculation as to the effects of a nearby supernova on Earth often focuses on large stars, such as Betelgeuse, a red supergiant 427 light-years from Earth which is a Type II supernova candidate. Several prominent stars within a few light centuries from the Sun are candidates for becoming supernovae in as little as a millennium.[86] Though spectacular, these "predictable" supernovae are thought to have little potential to affect Earth. Type Ia supernovae, though, are thought to be potentially the most dangerous if they occur close enough to the Earth. Because Type Ia supernovae arise from dim, common white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably and take place 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.[87]

Recent estimates predict that a Type II supernova would have to be closer than eight parsecs (twenty-six light years) to destroy half of the Earth's ozone layer.[88] Such estimates are mostly concerned with atmospheric modeling and considered only the known radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud. Estimates of the rate of supernova occurrence within 10 parsecs of the Earth vary from once every 100 million years [89] to once every one to ten billion years.[90]

In 1996, astronomers at the University of Illinois at Urbana-Champaign theorized that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Subsequently, iron-60 enrichment has been reported in deep-sea rock of the Pacific Ocean by researchers from the Technical University of Munich.[91][92][93]

Milky Way candidates

Several large stars within the Milky Way have been suggested as possible supernovae within the next few thousand to hundred million years. These include Rho Cassiopeiae,[94] Eta Carinae,[95][96] RS Ophiuchi,[97][98] the Kitt Peak Downes star KPD1930+2752,[99] HD 179821,[100][101] IRC+10420,[102] VY Canis Majoris,[103] Betelgeuse, Antares, and Spica.[86]

Many Wolf-Rayet stars, such as Gamma Velorum,[104] WR 104,[105] and those in the Quintuplet Cluster,[106] are also considered possible precursor stars to a supernova explosion in the 'near' future.

The nearest supernova candidate is IK Pegasi (HR 8210), located at a distance of only 150 light years. This closely-orbiting binary star system consists of a red giant and a white dwarf, separated by only 31 million km. The dwarf has an estimated mass equal to 1.15 times that of the Sun.[107] 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.[108] [109]

See also

Notes

  1. ^ Per the APS Neutrino Study reference,[52] roughly 99% of the gravitational potential energy is released as neutrinos of all flavors. The remaining 1% is equal to 1044 J

References

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Further reading

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