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{{ Diffuse nebula |<!-- put name or coordinates into the following URL -->
| name = [http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=Crab+Nebula Crab Nebula]
| image = [[Image:Crab Nebula.jpg|250px]]
M1, the Crab Nebula. Courtesy of [[NASA]]/[[ESA]]
| type = [[Supernova_remnant|Supernova Remnant]]
| epoch = [[J2000.0]]
| ra = {{RA|05|34|31.97}}<ref name="simbad">{{cite web
| title=SIMBAD Astronomical Database
| work=Results for NGC 1952
| url=http://simbad.u-strasbg.fr/Simbad
| accessdate=2006-12-25}}</ref>
| dec = {{DEC|+22|00|52.1}}<ref name="simbad" />
| dist_ly = 6,300 [[light year|ly]]
| appmag_v = +8.4
| size_v = 6 × 4 [[Minute of arc|arcmin]]
| constellation = [[Taurus (constellation)|Taurus]]
| radius_ly = 3 ly
| absmag_v = −3
| notes = Optical [[pulsar]]
| names = M 1,<ref name="simbad" /> NGC 1952<ref name="simbad" />
}}
The '''Crab Nebula''' (catalogue designations [[Messier object|M]] 1, [[New General Catalogue|NGC 1952]], Taurus A) is a [[supernova remnant]] in the [[constellation]] of [[Taurus (constellation)|Taurus]]. The [[nebula]] was first observed in 1731 by [[John Bevis]]. It is the remnant of a supernova that was recorded, as a star visible in daylight, by [[Chinese astronomy|Chinese]] and [[Islamic astronomy|Arab]] astronomers [[SN 1054|in 1054]]. Located at a distance of about 6,300 [[light year]]s (2 [[parsec|kpc]]) from [[Earth]], the [[nebula]] has a diameter of 11 ly (3.4 pc) and is expanding at a rate of about 1,500 [[kilometre]]s per [[second]].
The nebula contains a [[pulsar]] in its centre which rotates thirty times per second, emitting pulses of [[radiation]], from [[gamma ray]]s to [[radio wave]]s. The nebula was the first astronomical object identified with a historical supernova explosion.
The nebula acts as a source of radiation for studying celestial bodies that [[Occultation|occult]] it. In the [[1950]]s and [[1960]]s, the [[Sun]]'s [[corona]] was mapped from observations of the Crab's radio waves passing through it, and more recently, the thickness of the atmosphere of [[Saturn]]'s moon [[Titan (moon)|Titan]] was measured as it blocked out [[X-ray]]s from the nebula.
==Origins==
{{main|SN 1054}}
First observed in 1731 by [[John Bevis]], the nebula was independently rediscovered in 1758 by [[Charles Messier]] as he was observing a bright [[comet]]. Messier catalogued it as the first entry in his [[Messier Catalogue|catalogue]] of comet-like objects. The [[William Parsons, 3rd Earl of Rosse|Earl of Rosse]] observed the nebula at [[Birr Castle]] in the [[1840s]], and referred to the object as the Crab Nebula because a drawing he made of it looked like a [[crab]].<ref>Glyn Jones K. (1976), ''The Search for the Nebulae'', Journal of the History of Astronomy, v. 7, p.67</ref>
In the early 20th century, the analysis of early [[astrophotography|photographs]] of the nebula taken several years apart revealed that it was expanding. Tracing the expansion back revealed that the nebula must have formed about 900 years ago. Historical records revealed that a new star bright enough to be seen in the daytime had been recorded in the same part of the sky by Chinese and Arab astronomers in 1054<ref name="Lundmark">Lundmark K. (1921), ''[http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1921PASP...33..225L&link_type=ARTICLE&db_key=AST Suspected New Stars Recorded in Old Chronicles and Among Recent Meridian Observations']', Publications of the Astronomical Society of the Pacific, v. 33, p.225</ref><ref name="Mayall">Mayall N.U. (1939), ''[http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1939ASPL....3..145M&link_type=ARTICLE&db_key=AST The Crab Nebula, a Probable Supernova]'', Astronomical Society of the Pacific Leaflets, v. 3, p.145</ref> Given its great distance, the daytime "guest star" observed by the Chinese and Arabs could only have been a [[supernova]]—a massive, exploding star, having exhausted its supply of energy from [[nuclear fusion]] and collapsed in on itself.
Recent analyses of historical records have found that the supernova that created the Crab Nebula probably occurred in April or early May, rising to its maximum brightness of between [[apparent magnitude]] −7 and −4.5 (brighter than everything in the night sky except the [[Moon]]) by July. The supernova was visible to the [[naked eye]] for about two years after its first observation.<ref>Collins G.W., Claspy W.P., Martin J.C. (1999), ''Reinterpretation of Historical References to the Supernova of A.D. 1054'', Publications of the Astronomical Society of the Pacific, v. 111, p. 871</ref> Thanks to the recorded observations of Far Eastern and Middle Eastern astronomers of 1054, Crab Nebula became the first astronomical object recognized as being connected to a supernova explosion.<ref name="Mayall">Mayall N.U. (1939), ''[http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1939ASPL....3..145M&link_type=ARTICLE&db_key=AST The Crab Nebula, a Probable Supernova]'', Astronomical Society of the Pacific Leaflets, v. 3, p.145</ref>
==Physical conditions==
[[Image:Chandra-crab.jpg|thumb|left|220px|The Crab Pulsar. This image combines optical data from [[Hubble Space Telescope|Hubble]] (in red) and [[X-ray astronomy|X-ray]] images from [[Chandra X-ray Observatory]] (in blue).]]
In [[visible light]], the Crab Nebula consists of a broadly [[oval]]-shaped mass of filaments, about 6 [[arcminute]]s long and 4 arcminutes wide, surrounding a diffuse blue central region (by comparison, the [[full moon]] is 30 arcminutes across). The filaments are the remnants of the progenitor star's atmosphere, and consist largely of [[ionisation|ionised]] [[helium]] and [[hydrogen]], along with [[carbon]], [[oxygen]], [[nitrogen]], [[iron]], [[neon]] and [[sulphur]]. The filaments' temperatures are typically between 11,000 and 18,000 [[Kelvin|K]], and their densities are about 1,300 particles per [[cm³]].<ref>Fesen R.A., Kirshner R.P. (1982), ''The Crab Nebula. I - Spectrophotometry of the filaments'', Astrophysical Journal, v. 258, p. 1-10</ref>
In 1953 [[Iosif Shklovsky]] proposed that the diffuse blue region is predominantly produced by [[synchrotron radiation]], which is radiation given off by the curving of [[electron]]s moving at speeds up to half the [[speed of light]].<ref>{{cite journal | last=Shklovskii | first=Iosif | title=On the Nature of the Crab Nebula’s Optical Emission | journal=Doklady Akademii Nauk SSSR | volume=90 | year=1953 | pages=983}}</ref> Three years later the theory was confirmed by observations. In the 1960s it was found that the source of the electron curved paths was the strong [[magnetic field]] produced by a neutron star at the centre of the nebula.<ref>Burn B.J. (1973), ''A synchrotron model for the continuum spectrum of the Crab Nebula'', Monthly Notices of the Royal Astronomical Society, v. 165, p. 421 (1973)</ref>
The Crab Nebula is currently expanding outwards at about 1,500 km/s.<ref name="Bietneholz">Bietenholz M.F., Kronberg P.P., Hogg D.E., Wilson A.S. (1991), ''The expansion of the Crab Nebula'', Astrophysical Journal Letters, vol. 373, p. L59-L62</ref> Images taken several years apart reveal the slow expansion of the nebula, and by comparing this angular expansion with its [[redshift|spectroscopically]]-determined expansion velocity, the nebula's distance can be estimated. Modern observations give a distance to the nebula of about 6,300 ly,<ref>Trimble, V. (1973), ''The Distance to the Crab Nebula and NP 0532'', Publications of the Astronomical Society of the Pacific, v. 85, p. 579</ref> meaning that it is about 11 ly in length.
Tracing back its expansion consistently yields a date for the creation of the nebula several decades after 1054, implying that its outward velocity has accelerated since the supernova explosion.<ref>Trimble V. (1968), ''Motions and Structure of the Filamentary Envelope of the Crab Nebula'', Astronomical Journal, v. 73, p. 535</ref> This acceleration is believed to be caused by energy from the pulsar that feeds into the nebula's magnetic field, which expands and forces the nebula's filaments outwards.<ref>Bejger M., Haensel P. (2003), ''Accelerated expansion of the Crab Nebula and evaluation of its neutron-star parameters'', Astronomy and Astrophysics, v.405, p.747-751</ref>
Estimates of the total mass of the nebula are important for estimating the mass of the supernova's progenitor star. Estimates of the amount of matter contained in the filaments of the Crab Nebula range from about 1–5 [[solar mass]]es<ref>Fesen R.A., Shull J.M., Hurford A.P. (1997), ''An Optical Study of the Circumstellar Environment Around the Crab Nebula'', Astronomical Journal v.113, p. 354-363</ref>; although other estimates based on the investigation of the [[Crab Pulsar]] yield different numbers{{Fact|date=April 2007}}.
==Central star==
{{main|Crab Pulsar}}
[[Image:Changes in the Crab Nebula.jpg|thumb|right|250px|This sequence of [[Hubble Space Telescope]] images shows features in the inner Crab Nebula changing over a period of four months. Credit: [[NASA]]/[[ESA]].]]
At the centre of the Crab Nebula are two faint stars, one of which is the star responsible for existence of the nebula. It was identified as such in [[1942]], when [[Rudolf Minkowski]] found that its optical spectrum was extremely unusual.<ref>Minkowski R. (1942), ''The Crab Nebula'', Astrophysical Journal, v. 96, p.199</ref> The region around the star was found to be a strong source of radio waves in 1949<ref>Bolton J.G., Stanley G.J., Slee O.B. (1949), ''Positions of three discrete sources of Galactic radio frequency radiation'', Nature, v. 164, p. 101</ref> and X-rays in 1963,<ref name="Bowyer">Bowyer S., Byram E.T., Chubb T.A., Friedman H. (1964), ''Lunar Occulation of X-ray Emission from the Crab Nebula'', Science, v. 146, pp. 912-917</ref> and was identified as one of the brightest objects in the sky in [[gamma ray]]s in [[1967]].<ref>Haymes R.C., Ellis D.V., Fishman G.J., Kurfess J.D., Tucker, W.H. (1968), ''Observation of Gamma Radiation from the Crab Nebula'', Astrophysical Journal, v. 151, p.L9</ref> Then, in 1968, the star was found to be emitting its radiation in rapid pulses, becoming one of the first [[pulsar]]s to be discovered.
Pulsars are sources of powerful [[electromagnetic radiation]], emitted in short and extremely regular pulses many times a second. They were a great mystery when discovered in 1967, and the team which identified the first one considered the possibility that it could be a signal from an advanced civilization.<ref>Del Puerto C. (2005), ''Pulsars In The Headlines'', EAS Publications Series, v. 16, pp.115-119</ref> However, the discovery of a pulsating radio source in the centre of the Crab Nebula was strong evidence that pulsars were formed by supernova explosions. They are now understood to be rapidly rotating [[neutron star]]s, whose powerful [[magnetic field]] concentrates their radiation emissions into narrow beams.
The Crab Pulsar is believed to be about 28-30 km in diameter<ref>M. Bejger and P. Haensel (2002), ''Moments of inertia for neutron and strange stars: Limits derived for the Crab pulsar'', Astronomy and Astrophysics , v. 396, p. 917–921</ref>; it emits pulses of radiation every 33 [[millisecond]]s.<ref>Harnden F.R., Seward F.D. (1984), ''Einstein observations of the Crab nebula pulsar'', Astrophysical Journal, v. 283, p. 279-285</ref> Pulses are emitted at [[wavelength]]s across the [[electromagnetic spectrum]], from radio waves to X-rays. Like all isolated pulsars, its period is slowing very gradually. Occasionally, its rotational period shows sharp changes, known as 'glitches', which are believed to be caused by a sudden realignment inside the neutron star. The [[energy]] released as the pulsar slows down is enormous, and it powers the emission of the synchrotron radiation of the Crab Nebula, which has a total [[luminosity]] about 75,000 times greater than that of the Sun.<ref>Kaufmann W.J. (1996), ''Universe'' 4th edition, Freeman press, p. 428</ref>
The pulsar's extreme energy output creates a unusually dynamic region at the centre of the Crab Nebula. While most astronomical objects evolve so slowly that changes are visible only over timescales of many years, the inner parts of the Crab show changes over timescales of only a few days.<ref>Hester J.J., Scowen P.A., Sankrit R., Michel F.C., Graham J.R., Watson A., Gallagher J.S. (1996), ''The Extremely Dynamic Structure of the Inner Crab Nebula'', Bulletin of the American Astronomical Society, Vol. 28, p.950</ref> The most dynamic feature in the inner part of the nebula is the point where the pulsar's equatorial wind slams into the bulk of the nebula, forming a [[shock wave|shock front]]. The shape and position of this feature shifts rapidly, with the equatorial wind appearing as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar to well out into the main body of the nebula.
==Progenitor star==
[[Image:Crab_3.6_5.8_8.0_microns_spitzer.png|thumb|250px|left|The Crab Nebula seen in [[infrared]] by the [[Spitzer Space Telescope]].]]
The star that exploded as a supernova is referred to as the supernova's ''progenitor star''. Two types of star explode as supernovae: [[white dwarf]]s and massive stars. In the so-called [[Type Ia supernova]]e, gases falling onto a white dwarf raise its mass until it nears a critical level, the [[Chandrasekhar limit]], resulting in an explosion; in [[Type Ib and Ic supernovae|Type Ib/c]] and [[Supernova#Type II|Type II]] supernovae, the progenitor star is a massive star which runs out of fuel to power its [[nuclear fusion]] reactions and collapses in on itself, reaching such phenomenal [[temperature]]s that it explodes. The presence of a pulsar in the Crab means it must have formed in a core-collapse supernova; Type Ia supernovae do not produce pulsars.
Theoretical models of supernova explosions suggest that the star that exploded to produce the Crab Nebula must have had a [[mass]] of between 8 and 12 [[solar mass]]es. Stars with masses lower than 8 solar masses are thought to be too small to produce supernova explosions, and end their lives by producing a [[planetary nebula]] instead, while a star heavier than 12 solar masses would have produced a nebula with a different chemical composition to that observed in the Crab.<ref name="Davidson">Davidson K., Fesen R.A. (1985), ''Recent developments concerning the Crab Nebula'', Annual Review of Astronomy and Astrophysics, v. 23, p. 119-146</ref>
A significant problem in studies of the Crab Nebula is that the combined mass of the nebula and the pulsar add up to considerably less than the predicted mass of the progenitor star, and the question of where the 'missing mass' is remains unresolved.<ref>Fesen R.A., Shull J.M., Hurford A.P. (1997), ''An Optical Study of the Circumstellar Environment Around the Crab Nebula'', Astronomical Journal v.113, p. 354-363</ref> Estimates of the mass of the nebula are made by measuring the total amount of light emitted, and calculating the mass required, given the measured temperature and density of the nebula. Estimates range from about 1–5 solar masses, with 2–3 solar masses being the generally accepted value.<ref name="Davidson" /> The neutron star mass is estimated to be between 1.4 and 2 solar masses.
The predominant theory to account for the missing mass of the Crab is that a substantial proportion of the mass of the progenitor was carried away before the supernova explosion in a fast [[stellar wind]]. However, this would have created a shell around the nebula. Although attempts have been made at several different wavelengths to observe a shell, none has yet been found.<ref>Frail D.A., Kassim N.E., Cornwell T.J., Goss W.M. (1995), ''Does the Crab Have a Shell?'', Astrophysical Journal, v. 454, p. L129–L132</ref>
==Transits by solar system bodies==
[[Image:Filaments in the Crab Nebula.jpg|thumb|right|250px|[[Hubble Space Telescope]] image of a small region of the Crab Nebula, showing its intricate filamentary structure. Credit: [[NASA]]/[[ESA]].]]
The Crab Nebula lies roughly 1½ ° away from the [[ecliptic]]—the plane of Earth's orbit around the Sun. This means that the [[Moon]] — and occasionally, [[planet]]s — can [[Astronomical transit|transit]] or [[occultation|occult]] the nebula. Although the Sun does not transit the nebula, its [[corona]] passes in front of it. These transits and occultations can be used to analyse both the nebula and the object passing in front of it, by observing how radiation from the nebula is altered by the transiting body.
Lunar transits have been used to map X-ray emissions from the nebula. Before the launch of X-ray-observing satellites, such as the [[Chandra X-ray Observatory]], X-ray observations generally had quite low [[angular resolution]], but when the Moon passes in front of the nebula, its position is very accurately known, and so the variations in the nebula's brightness can be used to create maps of X-ray emission.<ref>Palmieri T.M., Seward F.D., Toor A., van Flandern T.C. (1975), ''Spatial distribution of X-rays in the Crab Nebula'', Astrophysical Journal, v. 202, p. 494-497</ref> When X-rays were first observed from the Crab, a lunar occultation was used to determine the exact ___location of their source.<ref name="Bowyer" />
The Sun's corona passes in front of the Crab every June. Variations in the radio waves received from the Crab at this time can be used to infer details about the corona's density and structure. Early observations established that the corona extended out to much greater distances than had previously been thought; later observations found that the corona contained substantial density variations.<ref>Erickson W.C. (1964), ''The Radio-Wave Scattering Properties of the Solar Corona'', Astrophysical Journal, v. 139, p.1290</ref>
Very rarely, [[Saturn]] transits the Crab Nebula. Its transit in 2003 was the first since [[1296]]; another will not occur until [[2267]]. Observers used the Chandra X-ray Observatory to observe Saturn's moon [[Titan (moon)|Titan]] as it crossed the nebula, and found that Titan's X-ray 'shadow' was larger than its solid surface, due to absorption of X-rays in its atmosphere. These observations showed that the thickness of Titan's atmosphere is 880 km.<ref>Mori K., Tsunemi H., Katayama H., Burrows D.N., Garmire G.P., Metzger A.E. (2004), ''An X-Ray Measurement of Titan's Atmospheric Extent from Its Transit of the Crab Nebula'', Astrophysical Journal, v. 607, pp. 1065-1069. Chandra images used by Mori et al can be viewed [http://chandra.harvard.edu/photo/2004/titan/ here].</ref> The transit of Saturn itself could not be observed, because Chandra was passing through the [[Van Allen belts]] at the time.
==In fiction==
: ''See [[Nebulae in fiction#Crab Nebula|Crab Nebula in fiction]].''
==References==
<div class="references-small">
<references/>
</div>
==External links==
{{commons}}
*[http://www.spacetelescope.org/images/archive/freesearch/crab+nebula/viewall/1 Crab Nebula at ESA/Hubble]
* [http://www.seds.org/messier/m/m001.html Messier 1], [[SEDS]] Messier pages
* [http://chandra.harvard.edu/photo/0052/ Images of the Crab] from the [[Chandra X-ray Observatory]]
* [http://chandra.harvard.edu/xray_sources/crab/crab.html Chandra page about the nebula]
* [http://hubblesite.org/newscenter/newsdesk/archive/releases/1996/22/ Images of the Crab] from the [[Hubble Space Telescope]]
*[http://www.seds.org/messier/more/m001_rosse.html Lord Rosse's drawings of M1, the Crab Nebula] from [[SEDS]]
* [http://www.nightskyinfo.com/archive/m1_supernova_remnant NightSkyInfo.com - M1, the Crab Nebula]
[[Category:Supernova remnants]]
[[Category:Messier objects|Crab Nebula]]
[[Category:NGC objects]]
[[Category:Taurus constellation]]
{{Link FA|es}}
{{Link FA|pl}}
[[bg:Ракообразна мъглявина]]
[[cs:Krabí mlhovina]]
[[co:M1]]
[[da:Krabbetågen]]
[[de:Krebsnebel]]
[[es:Nebulosa del Cangrejo]]
[[eo:M1]]
[[eu:Karramarroaren nebulosa]]
[[fr:Nébuleuse du Crabe]]
[[ko:게 성운]]
[[hr:Messier 1]]
[[io:Kankro-nebuloso]]
[[it:Nebulosa del Granchio]]
[[lt:Krabo ūkas]]
[[hu:Rák-köd]]
[[nl:Krabnevel]]
[[ja:かに星雲]]
[[no:Krabbetåken]]
[[nov:Krabe-nebula]]
[[pl:Mgławica Kraba]]
[[pt:Nebulosa do Caranguejo]]
[[ru:Крабовидная туманность]]
[[sk:Krabia hmlovina]]
[[sl:Rakovica (meglica)]]
[[sr:Рак (маглина)]]
[[fi:Rapusumu]]
[[sv:Krabbnebulosan]]
[[uk:Крабоподібна туманність]]
[[zh:蟹狀星雲]]
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