Supernova impostors are stellar explosions that appear at first to be a supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae analogs, and giant eruptions of luminous blue variables (LBV).
Supernova impostors appear as remarkably faint supernovae of spectral type IIn—which have hydrogen in their spectrum and narrow spectral lines that indicate relatively low gas speeds. These impostors exceed their pre-outburst states by several magnitudes, with typical peak absolute visual magnitudes of −11 to −14, making these outbursts as bright as the most luminous stars. The trigger mechanism of these outbursts remains unexplained, though it is thought to be caused by violating the classical Eddington luminosity limit, initiating severe mass loss. If the ratio of radiated energy to kinetic energy is near unity, as in Eta Carinae, then we might expect an ejected mass of about 0.16 solar masses.
Possible examples of supernova impostors include the Great Eruption of Eta Carinae, P Cygni, SN 1961V, SN 1954J, SN 1997bs, SN 2008S in NGC 6946, and SN 2010dn where detections of the surviving progenitor stars are claimed.
One supernova impostor that made news after the fact was the one observed on October 20, 2004, in the galaxy UGC 4904 by Japanese amateur astronomer Koichi Itagaki. This LBV star exploded just two years later, on October 11, 2006, as supernova SN 2006jc.
Apparent magnitude (m) is a measure of the relative brightness of a star or other astronomical objects as seen by an observer. An object's apparent magnitude depends on its intrinsic luminosity, its distance, and any extinction of the object's light by interstellar dust along the line of sight to the observer.
The magnitude scale is an inverse logarithmic relation, where a difference of 1.0 in magnitude corresponds to a brightness ratio of 5√100 or about 2.512. The brighter an object appears, the lower its magnitude. For example, a star of apparent magnitude 2.0 is 2.512 times brighter than a star of apparent magnitude 3.0. The brightest astronomical objects have negative apparent magnitudes: for example, Venus at −4.2 or Sirius at −1.46. The faintest naked-eye stars visible on the darkest night have apparent magnitudes of about +6.5. Apparent magnitudes range from −26.7 (the Sun) to fainter than +30 (such as the faintest objects detected in deep Hubble Space Telescope images).Measurement of the apparent magnitude of celestial objects is termed photometry. It is often quantified at ultraviolet, visible, and infrared wavelengths, as measured through standard passband filters corresponding to various adopted photometric systems such as the UBV system or the Strömgren uvbyβ system. In accepted astronomical notation, apparent magnitude may be denoted as mV, as in "mV = 15" to describe a 15th-magnitude object, where "V" coresponds to the visual filter band. In amateur astronomy, apparent magnitude is often understood as meaning apparent visual magnitude (v), defined as the brightness of a star or other astronomical source across the visible part of the electromagnetic spectrum when viewed by the human eye.Absolute magnitude rather than apparent brightness is a measure of its true intrinsic luminosity, and is expressed on the same inverse logarithmic scale. Absolute magnitude is defined as the apparent magnitude that a star or object would have if it were observed from a standard reference distance of 10 parsecs.Eta Carinae
Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around 7,500 light-years (2,300 parsecs) distant in the constellation Carina. Previously a 4th-magnitude star, it brightened in 1837 to become brighter than Rigel marking the start of the Great Eruption. Eta Carinae became the second-brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility after 1856. In a smaller eruption, it reached 6th magnitude in 1892 before fading again. It has brightened consistently since about 1940, becoming brighter than magnitude 4.5 by 2014. Eta Carinae is circumpolar from latitudes south of latitude 30°S, and it is never visible north of about latitude 30°N.
The two main stars of the Eta Carinae system have an eccentric orbit with a period of 5.54 years. The primary is a peculiar star similar to a luminous blue variable (LBV) that was initially 150–250 M☉ of which it has lost at least 30 M☉ already, and is expected to explode as a supernova in the astronomically near future. This is the only star known to produce ultraviolet laser emission. The secondary star is hot and also highly luminous, probably of spectral class O, around 30–80 times as massive as the Sun. The system is heavily obscured by the Homunculus Nebula, material ejected from the primary during the Great Eruption. It is a member of the Trumpler 16 open cluster within the much larger Carina Nebula.
Although unrelated to the star and nebula, the weak Eta Carinids meteor shower has a radiant very close to Eta Carinae.IPTF14hls
iPTF14hls is an unusual supernova star that has erupted continuously for about 1,000 days before becoming a remnant nebula. It had previously erupted in 1954. None of the theories nor proposed hypotheses fully explain all the aspects of the object.Kilonova
A kilonova (also called a macronova or r-process supernova) is a transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge into each other. Kilonovae are thought to emit short gamma-ray bursts and strong electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected fairly isotropically during the merger process.The term kilonova was introduced by Metzger et al. in 2010 to characterize the peak brightness, which they showed reaches 1000 times that of a classical nova. They are 1⁄10 to 1⁄100 the brightness of a typical supernova, the self-detonation of a massive star.The first kilonova to be found was detected as a short gamma-ray burst, SGRB 130603B, by instruments on board the Swift Gamma-Ray Burst Explorer and KONUS/WIND spacecrafts and then observed using the Hubble Space Telescope 9 and 30 days after burst.In October 2018, astronomers reported that GRB 150101B, a gamma-ray burst event detected in 2015, may be analogous to the historic GW170817, a gravitational wave event detected in 2017, and associated with the merger of two neutron stars. The similarities between the two events, in terms of gamma ray, optical and x-ray emissions, as well as to the nature of the associated host galaxies, are considered "striking", and this remarkable resemblance suggests the two separate and independent events may both be the result of the merger of neutron stars, and both may be a hitherto-unknown class of kilonova transients. Kilonova events, therefore, may be more diverse and common in the universe than previously understood, according to the researchers.Leo Minor
Leo Minor is a small and faint constellation in the northern celestial hemisphere. Its name is Latin for "the smaller lion", in contrast to Leo, the larger lion. It lies between the larger and more recognizable Ursa Major to the north and Leo to the south. Leo Minor was not regarded as a separate constellation by classical astronomers; it was designated by Johannes Hevelius in 1687.There are 37 stars brighter than apparent magnitude 6.5 in the constellation; three are brighter than magnitude 4.5. 46 Leonis Minoris, an orange giant of magnitude 3.8, is located some 95 light-years from Earth. At magnitude 4.4, Beta Leonis Minoris is the second-brightest star and the only one in the constellation with a Bayer designation. It is a binary star, the brighter component of which is an orange giant and the fainter a yellow-white main sequence star. The third-brightest star is 21 Leonis Minoris, a rapidly rotating white main-sequence star of average magnitude 4.5. The constellation also includes two stars with planetary systems, two pairs of interacting galaxies, and the unique deep-sky object Hanny's Voorwerp.List of stellar explosion types
Stellar explosion can refer to
type Ia supernova
Type Ib and Ic supernovae
Type II supernova
Supernova impostor, stellar explosions that appear similar to supernova, but do not destroy their progenitor stars
Luminous red nova, an explosion thought to be caused by stellar collision
solar flares are a minor type of stellar explosion
Tidal disruption event, the pulling apart of a star by tidal forcesList of supernovae
This is a list of supernovae that are of historical significance. These include supernovae that were observed prior to the availability of photography, and individual events that have been the subject of a scientific paper that contributed to supernova theory.NGC 3184
NGC 3184 is a spiral galaxy approximately 40 million light-years away in the constellation Ursa Major. It has two HII regions named NGC 3180 and NGC 3181.NGC 3184 houses a high abundance of heavy elements and (SN 1999gi) that was a magnitude 14 Type II supernova detected on December 9, 1999. Other supernovae in NGC 3184 include 1921B (mag 13.5), 1921C (mag 11) and 1937F (mag 13.5).NGC 4242
NGC 4242 is a spiral galaxy in the northern constellation of Canes Venatici. The galaxy is about 18 million light years (5.5 megaparsecs) away. It was discovered on 10 April 1788 by William Herschel, and it was described as "very faint, considerably large, irregular, round, very gradually brighter in the middle, resolvable" by John Louis Emil Dreyer, the compiler of the New General Catalogue.NGC 4242's galaxy morphological type is SABdm. This means that it is an intermediate spiral galaxy, with loosely wound spiral arms and is generally irregular in appearance. It was photographed by the Hubble Space Telescope in 2017. The image shows an asymmetric center and a small galactic bar. NGC 4242 has a relatively low surface brightness and rate of star formation. NGC 4242 may be a satellite galaxy of Messier 106 and is a member of the Canes II Group.SN 2002bu was detected in NGC 4242, brightening to its peak magnitude of 15.5 in 2002. It was originally classified as a type II supernova, but it may be a supernova impostor, like SN 2008S.NGC 7259
NGC 7259 is a spiral galaxy approximately 66 million light-years away from Earth in the constellation of Piscis Austrinus. It was discovered by John Herschel on September 28, 1834.Nova
A nova (plural novae or novas) or classical nova (CN, plural CNe, or Q) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star, that slowly fades over several weeks or many months.
Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve a white dwarf in a close binary system. The main sub-classes of novae are classical novae, recurrent novae (RNe), and dwarf novae. They are all considered to be cataclysmic variable stars.
Classical nova eruptions are the most common type of nova. They are likely created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star. When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. This atmosphere is mostly hydrogen and is thermally heated by the hot white dwarf, which eventually reaches a critical temperature causing rapid runaway ignition by fusion.
From the dramatic and sudden energies created, the now hydrogen-burnt atmosphere is then dramatically expelled into interstellar space, and its brightened envelope is seen as the visible light created from the nova event, and previously was mistaken as a "new" star. A few novae produce short-lived nova remnants, lasting for perhaps several centuries. Recurrent nova processes are the same as the classical nova, except that the fusion ignition may be repetitive because the companion star can again feed the dense atmosphere of the white dwarf.
Novae most often occur in the sky along the path of the Milky Way, especially near the observed galactic centre in Sagittarius; however, they can appear anywhere in the sky. They occur far more frequently than galactic supernovae, averaging about ten per year. Most are found telescopically, perhaps only one every year to eighteen months reaching naked-eye visibility. Novae reaching first or second magnitude occur only several times per century. The last bright nova was V1369 Centauri reaching 3.3 magnitude on 14 December 2013.Outline of astronomy
The following outline is provided as an overview of and topical guide to astronomy:
Astronomy – studies the universe beyond Earth, including its formation and development, and the evolution, physics, chemistry, meteorology, and motion of celestial objects (such as galaxies, planets, etc.) and phenomena that originate outside the atmosphere of Earth (such as the cosmic background radiation).Pulsational pair-instability supernova
A pulsational pair-instability supernova is a supernova impostor event that generally occurs in stars at around 100 to 130 solar mass (M☉), as opposed to a typical pair-instability supernova which occurs in stars of 130 to 250 M☉. Like pair-instability supernovae, pulsational pair-instability supernovae are caused by draining of a star's energy in the production of electron-positron pairs but, whereas a pair-instability supernova completely disrupts the star in a massive supernova, the star's pulsational pair-instability eruption sheds 10–25 M☉. This generally shrinks it down to a mass of less than 100 M☉, too small for electron-positron pair creation, where it then undergoes a core-collapse supernova or hypernova. It is possible that this is what occurred during the 1843 eruption of the primary star of the Eta Carinae star system although there is no substantial evidence supporting this.SN 1961V
SN 1961V was an abnormal, supernova-like event that was a potential supernova impostor. The potential impostor nature of SN 1961V was first identified by Fritz Zwicky in 1964. SN 1961V occurred in galaxy NGC 1058, about 9.3 Mpc away (about 30 million light years). Unlike many supernovae, the progenitor star is tentatively known: an extremely large, very bright blue star, similar to Eta Carinae. Mass estimates of the precursor star were as high as 2000 times the mass of the sun, but these are likely to be extreme overestimates. If SN 1961V was not a supernova then it was most likely an extremely large outburst by a luminous blue variable star.
The remnant of SN 1961V's explosion is expanding at 2,000 km/s, which is much slower than the majority of supernovae. The profile of its light curve, with a gradual climb to maximum brightness, was unusual when compared to a supernova. This unusual light curve led to suspicion that it was some other event. If the star survived this would identify SN 1961V as an impostor supernova rather than a true supernova. Attempts to determine if the progenitor star is still present have been extensive and have included use of both the Hubble Space Telescope and the Spitzer Space Telescope. These attempts have been hampered by the cloud of debris from the event, which have left the area obscured.
Christopher Kochanek at Ohio State University has compared the event to the confirmed supernova SN 2005gl and suggested that the low velocity of expansion can be explained by a pre-supernova mass loss event similar to that which was observed to occur in SN 2005gl. The analysis by Kochanek's group strongly suggests that SN 1961V was indeed a genuine supernova. Almost at the same moment, another team detected a highly luminous blue star, labeled Object 7, remaining at the site of the supernova, although they couldn't rule out this being a surviving companion of the exploded star.SN 2006jc
SN 2006jc was a supernova that was detected on October 9, 2006 in the galaxy UGC 4904, which is about 77 million light-years away in the constellation Lynx. It was first seen by Japanese amateur astronomer Koichi Itagaki, American amateur Tim Puckett and Italian amateur Roberto Gorelli. Two years earlier, the progenitor star produced a supernova impostor that was detected by Itagaki. This outburst was apparently the progenitor star shedding its outer layers. When the star exploded in 2006, the shockwave hit the material blown off in 2004, heating it to millions of degrees and emitting copious amounts of X-rays.SN 2009ip
SN 2009ip was a supernova discovered in 2009 in the spiral galaxy NGC 7259 in the constellation of Piscis Austrinus. Since the brightness waned after days post-discovery, it was redesignated as Luminous blue variable (LBV) Supernova impostor.During the following years several luminous outbursts were detected from the SN 2009ip. In September 2012 SN 2009ip was classified as a young type IIn supernova.Supernova
A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a transient astronomical event that occurs during the last stages of the life of a massive star or white dwarf, whose destruction is marked by a titanic explosion. This causes the sudden appearance of a "new" star, which then fades over several weeks or months or years.
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.
Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been observed in other galaxies. The most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but the remnants of recent supernovae have also been found. Observations of supernovae in other galaxies suggest they occur in the Milky Way on average about three times every century. These supernovae would almost certainly be observable with modern astronomical telescopes.
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 mechanics have been established and accepted by most astronomers for some time.
Supernovae can expel several solar masses of material at speeds up to several percent of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, sweeping up an expanding shell of gas and dust observed as a supernova remnant. Supernovae are a major source of elements in the interstellar medium from oxygen through to rubidium. The expanding shock waves of supernova can trigger the formation of new stars. Supernova remnants might be a major source of cosmic rays. Supernovae might produce strong gravitational waves, though, thus far, the gravitational waves detected have been from the merger of black holes and neutron stars.UGC 4904
UGC 4904 is a barred spiral galaxy in the constellation Lynx, located about 77 million light-years from Earth. On October 20, 2004, a supernova impostor was observed by Japanese amateur astronomer Koichi Itagaki within the galaxy. This same star may have transitioned from a LBV star to a Wolf–Rayet star shortly before it was observed as blowing up as hypernova SN 2006jc on October 11, 2006.
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