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SupernovaA supernova (/ˌsuːpərnoʊvə/ plural: supernovae /ˌsuːpərnoʊviː/ or supernovas, abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stellar evolutionary stages of a star's life, either a massive star or a white dwarf, whose destruction is marked by one final, titanic explosion. This causes the sudden appearance of a "new" bright star, before slowly fading from sight over several weeks or months.
Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931.[1]
Only three Milky Way naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies using telescopes. The most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but two more recent supernova remnants have also been found. Statistical observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, and that any galactic supernova would almost certainly be observable with modern astronomical telescopes.
Supernovae may expel much, if not all, of the material away from a star[2] at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave[3] into the surrounding interstellar medium, and in turn, sweeping up an expanding shell of gas and dust, which is observed as a supernova remnant. Supernovae create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis.[4] Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements.[5] Furthermore, the expanding shock waves from supernovae can trigger the formation of new stars.[6][7] Supernova remnants are expected to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production was found only in a few of them so far.[8] They are also potentially strong galactic sources of gravitational waves.[9]
Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion, completely disrupting the star. In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time.
Due to the wide range of astrophysical consequences of these events, astronomers now deem supernova research, across the fields of stellar and galactic evolution, as an especially important area for investigation.
Observation history
Main article: History of supernova observation
The earliest recorded supernova, SN 185, was viewed by Chinese astronomers in 185 AD. The brightest recorded supernova was SN 1006, which occurred in 1006 AD and was described in detail by Chinese and Islamic astronomers.[10] The widely observed supernova SN 1054 produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the latest to be observed with the naked eye in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the universe beyond the Moon and planets was static and unchanging.[11] Johannes Kepler began observing SN 1604 at its peak on October 17, 1604, and continued to make estimates of its brightness until it faded from naked eye view a year later.[12] It was the second supernova to be observed in a generation (after SN 1572 seen by Tycho Brahe in Cassiopeia).[13]
There is some evidence that the youngest galactic supernova, G1.9+0.3, occurred in the late 19th century, considerably more recently than Cassiopeia A from around 1680.[14] Neither supernova was noted at the time. In the case of G1.9+0.3, high extinction along the plane of the galaxy could have dimmed the event sufficiently to go unnoticed. The situation for Cassiopeia A is less clear. Infrared light echos have been detected showing that it was a type IIb supernova and was not in a region of especially high extinction.[15]
Before the development of the telescope, there have only been five supernovae seen in the last millennium. Compared to a star's entire history, the visual appearance of a galactic supernova is very brief, perhaps spanning several months, so that the chances of observing one is roughly once in a lifetime. Only a tiny fraction of the 100 billion stars in a typical galaxy have the capacity to become a supernova, restricted to either having large enough mass or under extraordinarily rare kinds of binary star in configurations containing white dwarf stars.[16]
However, observation and discovery of extragalactic supernovae are now far more common; that started with SN 1885A in the Andromeda galaxy. Today, amateur and professional astronomers are finding several hundreds every year, some when near maximum brightness or others unrecognised on old astronomical photographs or plates. American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941.[17] During the 1960s, astronomers found that the maximum intensities of supernovae could be used as standard candles, hence indicators of astronomical distances.[18] Some of the most distant supernovae recently observed appeared dimmer than expected. This supports the view that the expansion of the universe is accelerating.[19] Techniques were developed for reconstructing supernovae events that have no written records of being observed. The date of the Cassiopeia A supernova event was determined from light echoes off nebulae,[20] while the age of supernova remnant RX J0852.0-4622 was estimated from temperature measurements[21] and the gamma ray emissions from the radioactive decay of titanium-44.[22]
The most luminous supernova ever recorded is ASASSN-15lh. It was first detected in June 2015 and peaked at 570 billion L☉, which is twice the bolometric luminosity of any other known supernova.[24] However, the nature of this supernova continues to be debated and several alternative explanations have been suggested, e.g. tidal disruption of a star by a black hole.[25]
Among the earliest detected since time of detonation, and for which the earliest spectra have been obtained (beginning at 6 hours after the actual explosion), is the Type II SN 2013fs (iPTF13dqy) which was recorded 3 hours after the supernova event on 6 October 2013 by the Intermediate Palomar Transient Factory (iPTF). The star is located in a spiral galaxy named NGC 7610, 160 million light years away in the constellation of Pegasus.[26][27]
On 20 September 2016, amateur astronomer Victor Buso from Rosario, Argentina was testing out his new 16 inch telescope.[28][29] When taking several twenty second exposures of galaxy NGC 613, Buso chanced upon a supernova that had just become visible on earth. After examining the images he contacted the Instituto de Astrofísica de La Plata. "It was the first time anyone had ever captured the initial moments of the “shock breakout” from an optical supernova, one not associated with a gamma-ray or X-ray burst."[28] The odds of capturing such an event were put between one in ten million to one in a hundred million, according to astronomer Melina Bersten from the Instituto de Astrofísica.
The supernova Buso observed was a Type IIb made by a star twenty times the mass of the sun.[28] Astronomer Alex Filippenko from the University of California remarked that professional astronomers had been searching for such an event for a long time. He stated: "Observations of stars in the first moments they begin exploding provide information that cannot be directly obtained in any other way."[28]
Discovery
Main article: History of supernova observation § Telescope observation
Early work on what was originally believed to be simply a new category of novae was performed during the 1930s by two astronomers named Walter Baade and Fritz Zwicky at Mount Wilson Observatory.[30] The name super-novae was first used during 1931 lectures held at Caltech by Baade and Zwicky, then used publicly in 1933 at a meeting of the American Physical Society.[1] By 1938, the hyphen had been lost and the modern name was in use.[31] Because supernovae are relatively rare events within a galaxy, occurring about three times a century in the Milky Way,[32] 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.[33] 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.[34]
Toward 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.[35] 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.[36][37] Neutrinos are particles that are produced in great quantities by a supernova,[38] 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 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[39]—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.[40][41] Low redshift observations also anchor the low-distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies.[42][43] (See also Hubble's law).
Effect 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. Depending upon the type and energy of the supernova, it could be as far as 3000 light-years away. 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 ultraviolet solar 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.[134] 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.[135][136][137] 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.[138]
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 in binary systems, it is likely that a supernova that can affect the Earth will occur unpredictably and in a star system that is not well studied. The closest known candidate is IK Pegasi (see below).[139] 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, and there are no such candidates closer than about 500 light years.[140] [/quote]
List of supernova remnantsList of supernovae
