Utente:Emc2 Deepwatcher/Sandbox3: differenze tra le versioni

Versione delle 20:24, 18 ott 2013

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<!---[[File:SOHO sungrazer with prominent tail.jpg|thumb|right|300px|Individuazione da parte della sonda [[Solar and Heliospheric Observatory|SOHO]] di una cometa radente di Kreutz, in caduta verso il [[Sole]], dalla coda molto pronunciata]]--->

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Le '''comete radenti di Kreutz''' (o '''Kreutz sungrazers''', pronuncia {{IPA|[ˈkrɔɪts]}} {{Link audio| Mgm_pronunciation_Kreutz_Sungrazers.ogg |ascolta}}) sono una famiglia di [[cometa radente|comete radenti]] caratterizzate da delle orbite che le portano estremamente vicine al [[Sole]] durante il [[perielio]]. Si ritiene che questa famiglia sia composta dai frammenti di un'unica grande cometa che si frammentò molti secoli fa, e traggono il nome dall'[[astronomo]] tedesco [[Heinrich Kreutz]], che per primo dimostrò il loro legame.<ref name=Sekanina2004>{{cita pubblicazione|cognome=Sekanina|nome=Zdenek|coautori=Paul W. Chodas|data=20|anno=05|mese=2004|titolo=Fragmentation Hierarchy of Bright Sungrazing Comets and the Birth and Orbital Evolution of the Kreutz System. I. Two-Superfragment Model|rivista= [[The Astrophysical Journal]]|volume=607|numero=1|pagine=620-639|doi=doi:10.1086/383466|url=http://iopscience.iop.org/0004-637X/607/1/620/|lingua=inglese|accesso=2013-10-18}}</ref>

! style="color:white" colspan="3" align="center" | [[Classificazione dei pianeti extrasolari|<span style="color:white;">Classificazione dei pianeti extrasolari</span>]]

|rowspan="7"|[[File:Hypothetical exoplanet.jpg|155px]]

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Molti dei membri di questa famiglia sono diventati [[grande cometa|grandi comete]], occasionalmente anche visibili in pieno giorno vicino al Sole. La più recente di queste è stata la [[Cometa Ikeya-Seki (C/1965 S1)|Cometa Ikeya-Seki]] nel 1965, probabilmente la più luminosa dell'ultimo millennio.<ref name=Sekanina2004/> Si è ipotizzato che un altro sciame di comete Kreutz molto luminose potrebbe cominciare ad arrivare nel Sistema Solare interno nei prossi anni o decenni.<ref name=Sekanina2007>{{cita pubblicazione

| width="10%" |'''Massa'''

|cognome=Sekanina|nome=Zdenek|coautori=Paul W. Chodas|data=01|anno=07|mese=2007|titolo=Fragmentation Hierarchy of Bright Sungrazing Comets and the Birth and Orbital Evolution of the Kreutz System. II. The Case for Cascading Fragmentation|rivista= [[The Astrophysical Journal]]|volume=663|numero=1|pagine=657|doi=doi:10.1086/517490|url=http://iopscience.iop.org/0004-637X/663/1/657/|lingua=en|accesso=2013-10-18}}</ref>

| colspan=2 width="90%" |[[Cometa extrasolare]] • [[Asteroide extrasolare]] • [[Pianeta mercuriano|Mercuriano]] • [[Pianeta subterrestre|Sub Terra]] • [[Pianeta di massa terrestre|Terra]] • [[Pianeta superterrestre|Super Terra]] • [[Mini Nettuno]] • [[Pianeta nettuniano|Nettuniano]] • [[Pianeta Gioviano|Gioviano]] • [[Pianeta supergioviano|Supergioviano]] • [[Sub-nana bruna]] • [[Nana bruna]]

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Dopo il lancio della sonda [[Solar and Heliospheric Observatory|SOHO]] nel 1995 sono stati scoperti centinaia di membri minori della famiglia, alcuni grandi solo pochi metri. Nessuno di questi frammenti è mai sopravvissuto al passaggio al perielio, in quanto solo comete radenti molto più grandi, come la [[Grande Cometa del 1843]] e [[C/2011 W3 (Lovejoy)]] possiedono abbastanza massa per evitare la completa evaporazione. Astronomi amatori hanno scoperto con successo centinaia di questi membri più piccoli della famiglia, grazie ai dati disponibili in tempo rale su Internet.<ref name=Sekanina2007/>

|rowspan=2 style="border-top: 1px solid #aaa" |'''Composizione'''

| width="12%" style="border-top: 1px solid #aaa" |'''[[Pianeta terrestre|Pianeti terrestri]]'''

| width="88%" style="border-top: 1px solid #aaa" |[[Pianeta di silicio]] • [[Pianeta oceanico]] • [[Pianeta di carbonio]] • [[Pianeta di ferro]] • [[Pianeta senza nucleo]]

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| style="border-top: 1px solid #aaa" |'''Pianeti giganti'''

| style="border-top: 1px solid #aaa" |[[Gigante gassoso]] • [[Gigante di ghiaccio]] • [[Gigante di carbonio]] • [[Pianeta di elio]] • [[Pianeta ctonio]]

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==Scoperta e osservazioni storiche==

| style="border-top: 1px solid #aaa" |'''Configurazione orbitale'''

[[File:Great Comet of 1843.jpg|thumb|150px|left|Illustrazione della [[Grande Cometa del 1843]], vista dalla [[Tasmania]]]]

| colspan=2 style="border-top: 1px solid #aaa" |[[Pianeta extrasolare]] • [[Luna extrasolare]] • [[Pianeta circumbinario]] • [[Pianeta doppio]] • [[Pianeti delle pulsar]] • [[Giove eccentrico|Gioviano eccentrico]] • [[Pianeta interstellare]] • [[Cometa interstellare]] • [[Pianeta extragalattico]]

The first comet whose orbit had been found to take it extremely close to the Sun was the [[C/1680 V1|Great Comet of 1680]]. This comet was found to have passed just 200,000&nbsp;km (0.0013&nbsp;[[Astronomical unit|AU]]) above the sun's surface, equivalent to about half the distance between the [[Earth]] and the [[Moon]].<ref name=Marsden1967>{{cite journal|last=Marsden|first=Brian G.|title=The sungrazing comet group|journal=The Astronomical Journal|volume=72|issue=9|pages=1170–1183|bibcode=1967AJ.....72.1170M|year=1967|doi=10.1086/110396}}</ref> It thus became the first known [[sungrazing comet]]. Its perihelion distance was just 1.3 [[Solar radius|solar radii]].

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Astronomers at the time, including [[Edmond Halley]], speculated that this comet was a return of a bright comet seen close to the Sun in the sky in 1106.<ref name=Marsden1967/> 163 years later, the [[Great Comet of 1843]] appeared and also passed extremely close to the Sun. Despite orbital calculations showing that it had a period of several centuries, some astronomers wondered if it was a return of the 1680 comet.<ref name=Marsden1967/> A bright comet seen in 1880 was found to be travelling on an almost identical orbit to that of 1843, as was the subsequent [[Great Comet of 1882]]. Some astronomers suggested that perhaps they were all one comet, whose orbital period was somehow being drastically shortened at each perihelion passage, perhaps by retardation by some dense material surrounding the Sun.<ref name=Marsden1967/>

| style="border-top: 1px solid #aaa" |'''Distanza orbitale'''

| colspan=2 style="border-top: 1px solid #aaa" |[[Pianeta di lava]] • [[Pianeta nettuniano caldo|Nettuniano caldo]] • [[Pianeta gioviano caldo|Gioviano caldo]] • [[Zona abitabile]] • [[Pianeta ghiacciato]] • [[Classificazione di Sudarsky]]

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An alternative suggestion was that the comets were all fragments of an earlier sun-grazing comet.<ref name=Sekanina2004/> This idea was first proposed in 1880, and its plausibility was amply demonstrated when the Great Comet of 1882 broke up into several fragments after its perihelion passage.<ref name=Kreutz>{{cite journal|last=Kreutz|first=Heinrich Carl Friedrich|title=Untersuchungen über das cometensystem 1843 I, 1880 I und 1882 II|publisher=Kiel, Druck von C. Schaidt, C. F. Mohr nachfl., 1888–91|year=1888|bibcode=1888QB4.K5ns36.....|volume=5ns3|journal=Kiel}}</ref> In 1888, Heinrich Kreutz published a paper showing that the comets of 1843 (C/1843 D1, the Great March Comet), 1880 (C/1880 C1, the Great Southern Comet), and 1882 (C/1882 R1, Great September Comet) were probably fragments of a giant comet that had broken up several orbits before.<ref name=Sekanina2004/> The comet of 1680 proved to be unrelated to this family of comets.

! style="font-size: 90%;color:white" colspan=3 | <div style="text-align:center">[[File:Crystal Project konquest.png|15px|link=progetto:Astronomia]] <span style="color:white;">Questo box:&nbsp;&nbsp;&nbsp;&nbsp; [[template:Formazione stellare|<span style="color:white;">'''vedi'''</span>]] · [[template_talk:Formazione stellare|<span style="color:white;">'''disc.'''</span>]] · <span class="plainlinks">[{{fullurl:Template:Formazione stellare|action=edit}} <span style="color:white;">'''mod.'''</span>]</span></span></div>

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After another Kreutz Sungrazer was seen in 1887 (C/1887 B1, the [[Great Southern Comet of 1887]]), the next one did not appear until 1945.<ref name=Sekanina2001/> Two further sungrazers appeared in the 1960s, [[Comet Pereyra]] in 1963 and [[Comet Ikeya–Seki]], which became extremely bright in 1965, and broke into three pieces after its perihelion.<ref name=Sekanina2007/> The appearance of two Kreutz Sungrazers in quick succession inspired further study of the dynamics of the group.<ref name=Sekanina2001>{{cite journal|last=Sekanina|first=Zdeněk|year=2001|title=Kreutz sungrazers: the ultimate case of cometary fragmentation and disintegration?|journal=Publications of the Astronomical Institute of the [[Academy of Sciences of the Czech Republic]]|issue=89|pages=78–93|url=http://www.asu.cas.cz/~had/sekanina.ps|format=PS}}</ref>

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== Notable members ==

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The brightest members of the Kreutz Sungrazers have been spectacular, easily visible in the daytime sky. The three most impressive have been the [[Great Comet of 1843]], the [[Great Comet of 1882]] and [[Comet Ikeya–Seki]]. Another notable Kreutz Sungrazer was the Eclipse Comet of 1882 (see further below).<ref name=Sekanina2004/>

! style="color:white" colspan="3" align="center" | [[Pianeta extrasolare|<span style="color:white;">Pianeti extrasolari</span>]]

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=== The Great Comet of 1843 ===

| width="17%" |'''Formazione'''

{{main|Great Comet of 1843}}

| width="83%" |[[Disco protoplanetario]] • [[Planetesimo]] • [[Protopianeta]] • [[Sistema planetario]]

The Great Comet of 1843 was first noticed in early February of that year, just over three weeks before its perihelion passage. By February 27 it was easily visible in the daytime sky,<ref name=Hubbard1849/> and observers described seeing a tail 2–3° long stretching away from the sun before being lost in the glare of the sky. After its perihelion passage, it reappeared in the morning sky,<ref name=Hubbard1849/> and developed an extremely long tail. It extended about 45° across the sky on March 11 and was more than 2° wide;<ref name=Obs1843>{{cite journal|year=1843|journal=Monthly Notices of the Royal Astronomical Society|title=Observations of the great comet of 1843|volume=6|issue=2|pages=3–6|bibcode=1843MNRAS...6....3.}}</ref> the tail was calculated to be more than 300 million kilometers (2&nbsp;AU) long. This held the record for the longest measured cometary tail until 2000, when [[Comet Hyakutake]]'s tail was found to stretch to some 550 million kilometers in length. (The Earth–Sun distance—1&nbsp;AU—is only 150 million kilometers.)<ref name=Jones2000>{{cite journal|last=Jones|year=2000|journal=Nature|first=Geraint H.|coauthors=Balogh, André; Horbury, Timothy S.|title=Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures|volume=404|issue=6778|pages=574–576|bibcode=2000Natur.404..574J|doi=10.1038/35007011|pmid=10766233}}</ref>

|rowspan=5 bgcolor="#003399"|[[File:Icy Exoplanet.jpg|180px]]

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The comet was very prominent throughout early March, before fading away to almost below [[naked eye]] visibility by the beginning of April.<ref name=Obs1843/> It was last detected on April 20. This comet apparently made a substantial impression on the public, inspiring in some a fear that judgement day was imminent.<ref name=Hubbard1849>{{cite journal|last=Hubbard|year=1849|journal=The Astronomical Journal|first=J.S.|title=On the orbit of Great comet of 1843|volume=1|issue=2|pages=10–13|bibcode=1849AJ......1...10H|doi=10.1086/100004}}</ref>

| style="border-top: 1px solid #aaa" |'''Abitabilità'''

| style="border-top: 1px solid #aaa" |[[Zona abitabile]] • [[Analogo terrestre]] • [[Planetary Habitability Laboratory]] • [[Earth Similarity Index|ESI]]

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=== The Eclipse Comet of 1882 ===

| style="border-top: 1px solid #aaa" |'''[[Metodi di individuazione di pianeti extrasolari|Metodi di scoperta]]'''

A party of observers gathered in [[Egypt]] to watch a [[Solar eclipse of May 17, 1882|solar eclipse in May 1882]] were greatly surprised when they observed a bright streak near to the Sun once totality began. By a remarkable coincidence, the eclipse had coincided with the perihelion passage of a Kreutz comet. The comet would otherwise have gone unnoticed — its sighting during the eclipse was the only observation of it. Photographs of the eclipse revealed that the comet had moved noticeably during the 1m50s eclipse, as would be expected for a comet racing past the Sun at almost 500&nbsp;km/s. The comet is sometimes referred to as ''Tewfik'', after [[Tewfik Pasha]], the [[Khedive of Egypt]] at the time.<ref name=Marsden1967/>

| style="border-top: 1px solid #aaa" |[[Metodi di individuazione di pianeti extrasolari#Direct Imaging|Direct Imaging]] • [[Metodo del transito|Transito]] • [[Metodo delle velocità radiali|Velocità radiali]] • [[Metodo delle microlenti gravitazionali|Microlensing]] • [[Metodi di individuazione di pianeti extrasolari#Variazioni degli intervalli di emissioni di una Pulsar|Pulsar timing]] • [[Metodi di individuazione di pianeti extrasolari#Astrometria|Astrometria]] • [[Polarimetria]] • [[Relativistic beaming]]

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=== The Great Comet of 1882 ===

| style="border-top: 1px solid #aaa" |'''Liste'''

{{main|Great Comet of 1882}}

| style="border-top: 1px solid #aaa" |[[Pianeti extrasolari confermati|Pianeti confermati]] • [[Stelle con pianeti extrasolari confermati|Stelle con pianeti confermati]] • [[Lista dei pianeti extrasolari non confermati o controversi|Controversi e non confermati]] • [[Lista dei pianeti scoperti dalla missione Kepler|Pianeti Kepler]] • [[Lista dei pianeti scoperti dal progetto SuperWASP|Pianeti SuperWASP]] • [[Lista dei pianeti scoperti dal progetto HATNet|Pianeti HATNet]] • [[Pianeti extrasolari notevoli|Scoperte notevoli e miliari]] • [[Lista dei pianeti potenzialmente abitabili|Potenzialmente abitabili]]

[[File:Great Comet of 1882.jpg|thumb|right|250px|Photograph of the Great Comet of 1882, as seen from South Africa]]

The [[Great Comet of 1882]] was discovered independently by many observers, as it was already easily visible to the naked eye when it appeared in early September 1882, just a few days before perihelion. It grew rapidly brighter and was eventually so bright it was visible in the daytime for two days (16–17 September), even through light cloud.<ref name=Obs1883>{{cite journal|year=1883|journal=Monthly Notices of the Royal Astronomical Society|title=The comets of 1882|volume=43|issue=2|pages=203–209|bibcode=1883MNRAS..43R.203.}}</ref>

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After its perihelion passage, the comet remained bright for several weeks. During October, its nucleus was seen to fragment into first two and then four pieces. Some observers also reported seeing diffuse patches of light several degrees away from the nucleus. The rate of separation of the fragments of the nucleus was such that they will return about a century apart, between 670 and 960 years after the break-up.<ref name=Sekanina2007/>

|colspan=2 style="border-top: 1px solid #aaa"|'''[[Classificazione dei pianeti extrasolari]]''' • '''[[Nomenclatura dei pianeti extrasolari]]'''

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=== Comet Ikeya–Seki ===

! style="font-size: 90%;color:white" colspan=3 | <div style="text-align:center">[[File:Crystal Project konquest.png|15px|link=progetto:Astronomia]] <span style="color:white;">Questo box:&nbsp;&nbsp;&nbsp;&nbsp; [[template:Formazione stellare|<span style="color:white;">'''vedi'''</span>]] · [[template_talk:Formazione stellare|<span style="color:white;">'''disc.'''</span>]] · <span class="plainlinks">[{{fullurl:Template:Formazione stellare|action=edit}} <span style="color:white;">'''mod.'''</span>]</span></span></div>

{{main|Comet Ikeya–Seki}}

|}

Comet Ikeya–Seki is the most recent very bright Kreutz sungrazer. It was discovered independently by two [[Japan]]ese amateur astronomers on September 18, 1965, within 15 minutes of each other, and quickly recognised as a Kreutz Sungrazer.<ref name=Marsden1967/> It brightened rapidly over the following four weeks as it approached the sun, and reached [[apparent magnitude]] 2 by October 15. Its perihelion passage occurred on October 21, and observers across the world easily saw it in the daytime sky.<ref name=Marsden1967/> A few hours before perihelion passage on October 21 it had a visible magnitude from −10 to −11, comparable to the [[lunar phase|first quarter]] of the [[Moon]] and brighter than any other comet seen since 1106. A day after perihelion its magnitude decreased to just −4.<ref name=Opik1966>{{cite journal|last=Opik|first=E.J.|title=Sungrazing comets and tidal disruption|journal=The Irish Astronomical Journal|volume=7|issue=5|pages=141–161|bibcode=1966IrAJ....7..141O|year=1966}}</ref>

Japanese astronomers using a [[coronagraph]] saw the comet break into three pieces 30 minutes before perihelion. When the comet reappeared in the morning sky in early November, two of these nuclei were definitely detected with the third suspected. The comet developed a very prominent tail, about 25° in length, before fading throughout November. It was last detected in January 1966.<ref name=Hirayama1965>{{cite journal|last=Hirayama|first=T.|coauthors=Moriyama, F.|title=Observations of Comet Ikeya–Seki (1965f)|journal=Publications of the Astronomical Society of Japan|volume=17|pages=433–436|bibcode=1965PASJ...17..433H|year=1965}}</ref>

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== Dynamical history and evolution ==

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[[File:Kreutz Group fragmentation hierarchy en.svg|thumb|right|350px|Approximate relationship of the largest members of the Kreutz Sungrazers. Note that the perihelion passage at which fragmentations occurred may not be well established]]

! style="color:white" colspan="4" align="center" | Ricerca dei [[Pianeta extrasolare|<span style="color:white;">pianeti extrasolari</span>]]

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A study by [[Brian Marsden]] in 1967 was the first attempt to trace back the orbital history of the group to identify the progenitor comet.<ref name=Marsden1967/><ref name=Sekanina2001/> All known members of the group up until 1965 had almost identical [[orbital inclination]]s at about 144°, as well as very similar values for the [[longitude]] of [[perihelion]] at 280–282°, with a couple of outlying points probably due to uncertain orbital calculations. A greater range of values existed for the [[argument of periapsis|argument of perihelion]] and [[longitude of the ascending node]].<ref name=Sekanina2001/>

| width="11%" rowspan=3 style="border-top: 1px solid #aaa" |'''[[Osservatorio astronomico|Ricerca da Terra]]'''

| style="border-top: 1px solid #aaa" |'''[[Metodo delle velocità radiali|Velocità radiali]]'''

| style="border-top: 1px solid #aaa" |[[Anglo-Australian Planet Search|AAPS]] • [[Lick–Carnegie Exoplanet Survey|LCES]] • [[High Accuracy Radial Velocity Planet Searcher|HARPS]] e [[HARPS-N]], parte del [[Geneva Extrasolar Planet Search|GEPS]] • [[Magellan Planet Search Program|MPSP]] • [[East-Asian Planet Search Network|EAPSNet]] • [[HIRES]] • [[MARVELS]] • [[Automated Planet Finder|APF]]

|bgcolor="#003399" rowspan="6"|[[File:Telescope Kepler-NASA.jpeg|215px]]

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Marsden found that the Kreutz Sungrazers could be split into two groups, with slightly different orbital elements, implying that the family resulted from fragmentations at more than one perihelion.<ref name=Marsden1967/> Tracing back the orbits of Ikeya–Seki and the Great Comet of 1882, Marsden found that at their previous perihelion passage, the difference between their orbital elements was of the same order of magnitude as the difference between the elements of the fragments of Ikeya–Seki after it broke up.<ref name=Marsden1989>{{cite journal|last=Marsden|first=B.G.|title=The sungrazing comet group. II|journal=The Astronomical Journal|volume=98|issue=6|pages=2306–2321|bibcode=1989AJ.....98.2306M|year=1989|doi=10.1086/115301}}</ref> This meant it was realistic to presume that they were two parts of the same comet which had broken up one orbit ago. By far the best candidate for the progenitor comet was that seen in 1106 ([[Great Comet of 1106]]): Ikeya–Seki's derived orbital period gave a previous perihelion almost exactly at the right time, and while the Great Comet of 1882's derived orbit implied a previous perihelion a few decades later, it would only require a small error in the orbital elements to bring it into agreement.<ref name=Marsden1967/>

| style="border-top: 1px solid #aaa" |'''[[Metodo del transito|Transito]]'''

| style="border-top: 1px solid #aaa" |[[Progetto HATNet|HATNet]] • [[Progetto MEarth|MEarth]] • [[Progetto APACHE|APACHE]] • [[Progetto SuperWASP|SuperWASP]] • [[Trans-Atlantic Exoplanet Survey|TrES]] • [[Telescopio XO|XO]]

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The sungrazing comets of 1668, 1689, 1702 and 1945 seem to be closely related to those of 1882 and 1965,<ref name=Marsden1967/> although their orbits are not well enough determined to establish whether they broke off from the parent comet in 1106, or the previous perihelion passage before that, some time in the 3–5th centuries AD.<ref name=Sekanina2007/> This subgroup of comets is known as Subgroup II.<ref name=Sekanina2004/> [[Comet White–Ortiz–Bolelli]], which was seen in 1970,<ref name=Sekanina2002/> is more closely related to this group than Subgroup I, but appears to have broken off during the previous orbit to the other fragments.<ref name=Sekanina2004/>

| style="border-top: 1px solid #aaa" |'''[[Metodo delle microlenti gravitazionali|Microlensing]]'''

| style="border-top: 1px solid #aaa" |[[Microlensing Observations in Astrophysics|MOA]] • [[Optical Gravitational Lensing Experiment|OGLE]] • [[MicroFUN]]

|- style="font-size: 90%"

The sungrazing comets observed in 1843 (Great Comet of 1843) and 1963 ([[Comet Pereyra]]) seem to be closely related and belong to the subgroup I, although when their orbits are traced back to one previous perihelion, the differences between the orbital elements are still rather large, probably implying that they broke apart from each other one revolution before that.<ref name=Marsden1989/> They may not be related to the comet of 1106, but rather a comet that returned about 50 years before that.<ref name=Sekanina2004/> Subgroup I also includes comets seen in 1695, 1880 (Great Southern Comet of 1880) and in 1887 (Great Southern Comet of 1887), as well as the vast majority of comets detected by SOHO mission (see below).<ref name=Sekanina2004/>

| rowspan=3 style="border-top: 1px solid #aaa" |'''[[Telescopio spaziale|Ricerca dallo spazio]]'''

| width="12%" style="border-top: 1px solid #aaa" |'''Completate'''

| width="88%" style="border-top: 1px solid #aaa" |[[EPOXI]] <small>(2005)</small> • [[SWEEPS]] <small>(2006)</small> • [[CoRoT]] <small>(2006-2013)</small>

|- style="font-size: 90%"

The distinction between the two sub-groups is thought to imply that they result from two separate parent comets, which themselves were once part of a 'grandparent' comet which fragmented several orbits previously.<ref name=Sekanina2004/> One possible candidate for the grandparent is a comet observed by [[Aristotle]] and [[Ephorus]] in 371 BC. Ephorus claimed to have seen this comet break into two. However modern astronomers are skeptical of the claims of Ephorus, because they were not confirmed by other sources.<ref name=Sekanina2007/> Instead comets that arrived between 3rd and 5th centuries AD (comets of 214, 426 and 467) are considered as possible progenerators of the Kreutz family.<ref name=Sekanina2007/> The original comet must certainly have been very large indeed, perhaps as large as 100&nbsp;km across (for comparison, the nucleus of [[Comet Hale–Bopp]] was about 40&nbsp;km across).<ref name=Sekanina2004/>

| style="border-top: 1px solid #aaa" |'''In corso'''

| style="border-top: 1px solid #aaa" |[[Missione Kepler]] <small>(2009-)</small> • [[Satellite Gaia|GAIA]] <small>(2013-)</small>

|- style="font-size: 90%"

Although its orbit is rather different from those of the main two groups, it is possible that the comet of 1680 is also related to the Kreutz Sungrazers via a fragmentation many orbits ago.<ref name=Sekanina2007/>

| style="border-top: 1px solid #aaa" |'''Programmate'''

| style="border-top: 1px solid #aaa" |[[Transiting Exoplanet Survey Satellite|TESS]] <small>(2017)</small> • Proposte ([[Missione Plato|Plato]] • [[EChO]] • [[Exoplanetary Circumstellar Environments and Disk Explorer|EXCEDE]] • [[New Worlds Mission|New Worlds]]) • Cancellate ([[Missione Darwin|Darwin]] • [[Terrestrial Planet Finder|TPE]] • [[Missione Eddington|Eddington]] • [[Space Interferometry Mission|SIM]])

|-bgcolor="#003399"

The Kreutz Sungrazers are probably not a unique phenomenon. Studies have shown that for comets with high orbital inclinations and perihelion distances of less than about 2&nbsp;AU, the cumulative effect of gravitational perturbations tends to result in sungrazing orbits.<ref name=Bailey1992>{{cite journal|last=Bailey|first=M. E.|coauthors=Chambers, J. E.; Hahn, G.|year=1992|title=Origin of sungrazers – A frequent cometary end-state'|journal=Astronomy and Astrophysics|volume=257|pages=315–322|bibcode=1992A&A...257..315B}}</ref> One study has estimated that Comet Hale–Bopp has about a 15% chance of eventually becoming a sun-grazing comet.<ref name=Bailey1996>{{cite journal|last=Bailey|first=M. E.|coauthors=Emel'yanenko, V.V.; Hahn, G.; et al.|year=1996|title=Orbital evolution of Comet 1995 O1 Hale–Bopp|journal=Monthly Notices of the Royal Astronomical Society|volume=281|pages=916–924|bibcode = 1996MNRAS.281..916B }}</ref>

! style="font-size: 90%;color:white" colspan=4 | <div style="text-align:center">[[File:Crystal Project konquest.png|15px|link=progetto:Astronomia]] <span style="color:white;">Questo box:&nbsp;&nbsp;&nbsp;&nbsp; [[template:Formazione stellare|<span style="color:white;">'''vedi'''</span>]] · [[template_talk:Formazione stellare|<span style="color:white;">'''disc.'''</span>]] · <span class="plainlinks">[{{fullurl:Template:Formazione stellare|action=edit}} <span style="color:white;">'''mod.'''</span>]</span></span></div>

|}

== Recent observations ==

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[[File:Two Kreutz Sungrazers imaged by SOHO.jpg|thumb|right|300px|[[Solar and Heliospheric Observatory|SOHO]] image of two Kreutz comets on their way towards the sun]]

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[[Categoria:Template di navigazione - Astronomia|Formazione stellare]]

Until recently, it would have been possible for even a very bright member of the Kreutz Sungrazers to pass through the inner solar system unnoticed, if its perihelion had occurred between about May and August.<ref name=Sekanina2004/> At this time of year, as seen from [[Earth]], the comet would approach and recede almost directly behind the sun, and could only become visible extremely close to the sun if it became very bright. Only a remarkable coincidence between the perihelion passage of the Eclipse Comet of 1882 and a total solar eclipse allowed its discovery.<ref name=Sekanina2004>{{cite journal|last=Sekanina|first=Zdeněk|coauthors=and Chodas, Paul W.|title=Fragmentation hierarchy of bright sungrazing comets and the birth and orbital evolution of the kreutz system. I. Two-superfragment model|journal=The Astrophysical Journal|year=2004|volume=607|issue=1|pages=620–639|doi=10.1086/383466|id={{hdl|2014/39288}}|bibcode=2004ApJ...607..620S}}</ref>

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However, during the 1980s, two Sun-observing satellites serendipitously discovered several new members of the Kreutz family, and since the launch of the [[Solar and Heliospheric Observatory|SOHO]] Sun-observing satellite in 1995, it has been possible to observe comets very close to the sun at any time of year.<ref name=Sekanina2007/> The satellite provides a constant view of the immediate solar vicinity, and SOHO has now discovered hundreds of new sungrazing comets, some just a few metres across. About 83% of the sungrazers found by SOHO are members of the Kreutz group, with the other being referred to as 'non-Kreutz' or 'sporadic' sungrazers (Meyer, Marsden, and Kracht1&2 families).<ref name="fullist">{{cite web|url=http://www.ast.cam.ac.uk/~jds/klist.htm|title=Full list of SOHO and STEREO comets|publisher=British Astronomical Association and Society for Popular Astronomy|date=October 2008|accessdate=2008-11-07}}</ref> Apart from Comet Lovejoy, none of the sungrazers seen by SOHO has survived its perihelion passage; some may have plunged into the Sun itself, but most are likely to have simply evaporated away completely.<ref name=Sekanina2007>{{cite journal|last=Sekanina|first=Zdeněk|coauthors=Chodas, Paul W.|title=Fragmentation Hierarchy of Bright Sungrazing Comets and the Birth and Orbital Evolution of the Kreutz System. II. The Case for Cascading Fragmentation|journal=The Astrophysical Journal|volume=663|issue=1|year=2007|pages=657–676|doi=10.1086/517490|id={{hdl|2014/40925}}|bibcode=2007ApJ...663..657S}}</ref>

More than 75% of the SOHO sungrazers have been discovered by amateur astronomers analysing SOHO's observations via the [[Internet]]. Some amateurs have managed remarkable numbers of discoveries, with Rainer Kracht of [[Germany]] having chalked up 211 identifications, Michael Oates of the [[United Kingdom]] making 144, and Zhou Bo of [[China]] 97 identifications, discoveries.<ref>{{cite web|url=http://home.earthlink.net/~tonyhoffman/SOHOleaderboard.htm|title= SOHO Comet Discoverers' Totals|publisher=EarthLink|accessdate=2008-07-02}}</ref> As of October 2008, 1500 Kreutz Sungrazers have been identified.<ref name=Sekanina2007/>

SOHO observations have shown that Sungrazers frequently arrive in pairs separated by a few hours. These pairs are too frequent to occur by chance, and cannot be due to break-ups on the previous orbit, because the fragments would have separated by a much greater distance.<ref name=Sekanina2007/> Instead, it is thought that the pairs result from fragmentations far away from the perihelion. Many comets have been observed to fragment far from perihelion, and it seems that in the case of the Kreutz Sungrazers, an initial fragmentation near perihelion can be followed by an ongoing 'cascade' of break-ups throughout the rest of the orbit.<ref name=Sekanina2007/><ref name=Bailey1992/>

The number of Subgroup I Kreutz comets discovered is about four times the number of Subgroup II members. This suggests that the 'grandparent' comet split into parent comets of unequal size.<ref name=Sekanina2007/>

== Future ==

Dynamically, the Kreutz Sungrazers might continue to be recognised as a distinct family for many thousands of years yet. Eventually their orbits will be dispersed by gravitational perturbations, although depending on the rate of fragmentation of the constituent parts, the group might be completely destroyed before it is gravitationally dispersed.<ref name=Bailey1992/> The continuing discovery of large numbers of the smaller members of the family by SOHO will undoubtedly lead to a greater understanding of how comets break up to form families.<ref name=Sekanina2007/>

It is not possible to estimate the chances of another very bright Kreutz comet arriving in the near future, but given that at least 10 have reached naked eye visibility over the last 200 years, another great comet from the Kreutz family seems almost certain to arrive at some point.<ref name=Sekanina2002>{{cite journal|last=Sekaina|first=Zdeněk|coauthors=and Chodas, Paul W.|title=Fragmentation of major sungrazing comets C/1970 K1, C/1880 C1, AND C/1843 D1|journal=The Astrophysical Journal|year=2002|volume=581|issue=2|pages=1389–1398|doi=10.1086/344261|bibcode=2002ApJ...581.1389S}}</ref> [[Comet White–Ortiz–Bolelli]] in 1970 reached an apparent magnitude of 1. In December 2011, Kreutz sungrazer [[C/2011 W3 (Lovejoy)]] survived its perihelion passage and had an apparent magnitude of −3.

== Further reading ==

* Marsden B. G. (1989), ''The Sungrazing Comets Revisited'', Asteroids, comets, meteors III, Proceedings of meeting (AMC 89), Uppsala: Universitet, 1990, eds C. I. Lagerkvist, H. Rickman, B. A. Lindblad., p.&nbsp;393

* {{cite journal|doi=10.5140/JASS.2007.24.3.227|last=Lee|first=Sugeun|coauthors=Yi, Yu; Kim, Yong Ha; Brandt, John C.|year=2007|title=Distribution of Perihelia for SOHO Sungrazing Comets and the Prospective Groups|journal=Journal of Astronomy and Space Sciences|volume=24|issue=3|pages=227–234|bibcode=2007JASS...24..227L}}

== External links ==

{{Commons category}}

* [http://sungrazer.nrl.navy.mil Sungrazer Project Web Site]

* [http://www.seds.org/~spider/spider/Comets/kreutz-g.html SEDS Kreutz group page]

* [http://cometography.com/sungrazers/sungrazer.html Cometography sungrazers page]

* [http://umbra.nascom.nasa.gov/comets/comet_release.html NASA press release about two Sungrazers seen by SOHO]

* [http://www.ast.cam.ac.uk/~jds/klist.htm Complete list of SOHO comets]

* [http://sohowww.nascom.nasa.gov/data/realtime-images.html Real time SOHO data]

{{Comete}}

[[Categoria:Comete]]

[[Categoria:Comete radenti Kreutz]]

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==Note==

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