View source for Astronomy:Crab Nebula

{{Short description|Supernova remnant in the constellation Taurus}}
{{Sky|5|34|31.97|+|22|00|52.1|6500}}
{{Infobox nebula
 | name = Crab Nebula
 | image = [[File:Crab Nebula.jpg|260px]]
 | caption = [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] mosaic image assembled from 24 individual [[Engineering:Wide Field and Planetary Camera 2|Wide Field and Planetary Camera 2]] exposures taken in October 1999, January 2000, and December 2000
 | type = supernova remnant
 | epoch = [[Astronomy:J2000.0|J2000.0]]
 | ra = {{RA|05|34|31.94}}<ref name="simbad">{{cite simbad |title=M 1 |access-date=12 February 2012}}</ref>
 | dec = {{DEC|+22|00|52.2}}<ref name="simbad"/>
 | dist_ly = {{val|6500|1600}}
 | dist_pc = {{val|2000|500}}<ref name="Kaplanetal2008">{{cite journal |last1=Kaplan |first1=David L. |last2=Chatterjee |first2=S. |last3=Gaensler |first3=B. M. |last4=Anderson |first4=J. |display-authors=1 |title=A Precise Proper Motion for the Crab Pulsar, and the Difficulty of Testing Spin-Kick Alignment for Young Neutron Stars |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=677 |issue=2 |pages=1201–1215 |date=2008 |arxiv=0801.1142 |doi=10.1086/529026 |bibcode=2008ApJ...677.1201K|s2cid=17840947 }}</ref>
 | appmag_v = 8.4<ref>{{cite web |url=http://messier.seds.org/m/m001.html |title=Messier 1 |access-date=27 April 2022 |website=SEDS Messier Catalog}}</ref>
 | size_v = 420″ × 290″<ref name="Trimble1973"/>{{Ref label|A|a|none}}
 | constellation = [[Astronomy:Taurus (constellation)|Taurus]]
 | radius_ly = ~5.5
 | radius_pc = ~1.7<ref name="Hester2008">{{Cite journal |last1=Hester |first1=J. J. |title=The Crab Nebula: An Astrophysical Chimera |journal=Annual Review of Astronomy and Astrophysics |volume=46 |pages=127–155 |date=2008 |doi=10.1146/annurev.astro.45.051806.110608 |bibcode=2008ARA&A..46..127H}}</ref>
 | absmag_v = {{val|p=−|3.1|0.5}}{{Ref label|B|b|none}}
 | notes = [[Astronomy:Optical pulsar|Optical pulsar]]
 | names = Messier 1, NGC 1952, Taurus A, [[Astronomy:Sharpless catalog|Sh]]2-244<ref name="simbad"/>
}}

The '''Crab Nebula''' (catalogue designations '''M1''', '''NGC 1952''', '''Taurus A''') is a [[Astronomy:Supernova remnant|supernova remnant]] and [[Astronomy:Pulsar wind nebula|pulsar wind nebula]] in the [[Astronomy:Constellation|constellation]] of [[Astronomy:Taurus (constellation)|Taurus]]. The common name comes from a drawing that somewhat resembled a crab with arms produced by William Parsons, 3rd Earl of Rosse, in 1842 or 1843 using a {{convert|36|in|cm|adj=on}} [[Astronomy:Telescope|telescope]].<ref name=Ridpath>{{Cite web |last=Ridpath |first=Ian |title=Lord Rosse and the Crab Nebula |url=http://www.ianridpath.com/startales/rosse-crab.html |access-date=6 September 2023 |website=Star Tales}}</ref> The [[Astronomy:Nebula|nebula]] was discovered by English astronomer John Bevis in 1731. It corresponds with [[Astronomy:SN 1054|a bright supernova]] recorded by [[Astronomy:Chinese astronomy|Chinese astronomers]] in 1054 as a [[Astronomy:Guest star|guest star]]. The nebula was the first astronomical object identified that corresponds with a historically-observed supernova explosion.<ref>{{Cite web |last=Garner |first=Rob |date=6 October 2017 |title=Messier 1 (The Crab Nebula) |url=http://www.nasa.gov/feature/goddard/2017/messier-1-the-crab-nebula |access-date=27 April 2022 |website=NASA}}</ref>

At an [[Astronomy:Apparent magnitude|apparent magnitude]] of 8.4, comparable to that of [[Astronomy:Titan (moon)|Saturn's moon Titan]], it is not visible to the naked eye but can be made out using binoculars under favourable conditions. The nebula lies in the [[Astronomy:Perseus Arm|Perseus Arm]] of the [[Astronomy:Milky Way|Milky Way]] galaxy, at a distance of about {{convert|6500|ly|kpc|lk=on|order=flip}} from Earth. It has a diameter of {{convert|11|ly|pc|order=flip}}, corresponding to an apparent diameter of some 7&nbsp;[[Arcminute|arcminute]]s, and is expanding at a rate of about {{convert|1500|km/s}}, or 0.5% of the [[Physics:Speed of light|speed of light]].

At the center of the nebula lies the [[Astronomy:Crab Pulsar|Crab Pulsar]], a [[Astronomy:Neutron star|neutron star]] {{convert|28|-|30|km}} across with a spin rate of 30.2&nbsp;times per second, which emits pulses of [[Physics:Radiation|radiation]] from [[Physics:Gamma ray|gamma ray]]s to [[Radio wave|radio wave]]s. At [[Physics:X-ray|X-ray]] and gamma ray [[Physics:Photon energy|energies]] above 30 [[Physics:KeV|keV]], the Crab Nebula is generally the brightest persistent gamma-ray source in the sky, with measured flux extending to above 10 TeV. The nebula's radiation allows detailed study of celestial bodies that [[Astronomy:Occultation|occult]] it. In the 1950s and 1960s, the Sun's [[Physics:Solar corona|corona]] was mapped from observations of the Crab Nebula's radio waves passing through it, and in 2003, the thickness of the atmosphere of Saturn's moon Titan was measured as it blocked out X-rays from the nebula.

==Observational history==
{{further|Astronomy:SN 1054}}
The earliest recorded documentation of observation of astronomical object SN 1054 was as it was occurring in 1054, by Chinese astrononomers and Japanese observers, hence its numerical identification. Modern understanding that the Crab Nebula was created by a supernova traces back to 1921, when [[Biography:Carl Otto Lampland|Carl Otto Lampland]] announced he had seen changes in the nebula's structure.{{Ref label|D|d|none}}<ref name=lampland/> This 
eventually led to the conclusion that the creation of the Crab Nebula corresponds to the bright [[Astronomy:SN 1054|SN 1054]] supernova recorded by medieval astronomers in AD 1054.<ref>{{cite web |url=http://bc.ub.leidenuniv.nl/bc/tentoonstelling/Jan_Oort/object7.htm#7 |title=A short biography of Jan Hendrik Oort: 7. Crab Nebula |publisher=Leiden University Library |first1=J. |last1=Katgert-Merkelijn |first2=J. |last2=Damen |name-list-style=amp |date=2000 |access-date=9 March 2015  |archive-url=https://web.archive.org/web/20140904090251/http://bc.ub.leidenuniv.nl/bc/tentoonstelling/Jan_Oort/object7.htm#7 |archive-date=4 September 2014}}</ref>

===First identification===
[[File:M1rosse.jpg|thumb|left|upright|Reproduction of the first depiction of the nebula by Lord Rosse (1844) (colour-inverted to appear white-on-black)]]
[[File:The Crab Nebula M1 Goran Nilsson & The Liverpool Telescope.jpg|thumb|[[Physics:H-alpha|HaRGB]] image of the Crab Nebula from the Liverpool Telescope, exposures totalling 1.4 hours.]]
[[File:Stack 252frames 8064s ps-.jpg|thumb|The Crab Nebula M1]]
The Crab Nebula was first identified in 1731 by John Bevis.<ref name=Barrow2008>{{cite book |last1=Barrow |first1=John D. |title=Cosmic Imagery: Key Images in the History of Science |publisher=Random House |page=45 |date=2008 |url=https://books.google.com/books?id=uRg6iN10JCIC&pg=PA45 |isbn=978-0-224-07523-7}}</ref> The nebula was independently rediscovered in 1758 by [[Biography:Charles Messier|Charles Messier]] as he was observing a bright [[Astronomy:Comet|comet]].<ref name=Barrow2008/> Messier catalogued it as the first entry in his catalogue of comet-like objects;<ref name=Barrow2008/> in 1757, Alexis Clairaut reexamined the calculations of Edmund Halley and predicted the return of [[Astronomy:Halley's Comet|Halley's Comet]] in late 1758. The exact time of the comet's return required the consideration of perturbations to its orbit caused by planets in the Solar System such as Jupiter, which Clairaut and his two colleagues Jérôme Lalande and Nicole-Reine Lepaute carried out more precisely than Halley, finding that the comet should appear in the [[Astronomy:Constellation|constellation]] of [[Astronomy:Taurus (constellation)|Taurus]]. It was in searching in vain for the comet that [[Biography:Charles Messier|Charles Messier]] found the Crab Nebula, which he at first thought to be Halley's comet.<ref name=Pugh2011>{{cite book |last1=Pugh |first1=Philip |title=Observing the Messier Objects with a Small Telescope: In the Footsteps of a Great Observer |publisher=Springer Science |pages=8–10 |date=November 2011 |url=https://books.google.com/books?id=jDbjx-FSv5cC&pg=PA10 |isbn=978-0-387-85357-4}}</ref> After some observation, noticing that the object that he was observing was not moving across the sky, Messier concluded that the object was not a comet. Messier then realised the usefulness of compiling a catalogue of celestial objects of a cloudy nature, but fixed in the sky, to avoid incorrectly cataloguing them as comets. This realization led him to compile the "[[Astronomy:Messier object|Messier catalogue]]".<ref name=Pugh2011/>

William Herschel observed the Crab Nebula numerous times between 1783 and 1809, but it is not known whether he was aware of its existence in 1783, or if he discovered it independently of Messier and Bevis. After several observations, he concluded that it was composed of a group of stars.<ref name="Mayall"/> William Parsons, 3rd Earl of Rosse observed the nebula at Birr Castle in the early 1840s using a {{convert|36|in|m|1|adj=on}} telescope, and made a drawing of it that showed it with arms like those of a crab.<ref name=Ridpath/> He observed it again later, in 1848, using a {{convert|72|in|m|1|adj=on}} telescope but could not confirm the supposed resemblance, but the name stuck nevertheless.<ref>{{cite journal |last=Parsons |first=William |title=Observations on Some of the Nebulae |journal=Philosophical Transactions of the Royal Society of London |volume=134 |date=1844 |at=fig. 81, plate xviii, p. 321 |url=https://archive.org/stream/jstor-108366/108366#page/n5/mode/2up |jstor=108366 |doi=10.1098/rstl.1844.0012|s2cid=186212669 }}</ref><ref>{{cite book |last=Jones |first=Kenneth Glyn |title=The Search for the Nebulae |publisher=Alpha Academic |date=1975 |isbn=978-0-905193-01-4 |url=https://books.google.com/books?id=vGW1AAAAIAAJ}}</ref>

===Connection to SN 1054===
[[File:A whole new view of the Crab Nebula.jpg|thumb|The nebula is seen in the visible spectrum at 550&nbsp;nm (green light).]]

The Crab Nebula was the first astronomical object recognized as being connected to a supernova explosion.<ref name="Mayall"/> In the early twentieth 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 become visible on Earth about 900&nbsp;years before. 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 astronomers on 4 July 1054, and probably also by Japanese observers.<ref name="Mayall">{{cite journal |last=Mayall |first=Nicholas Ulrich |title=The Crab Nebula, a Probable Supernova |journal=Astronomical Society of the Pacific Leaflets |volume=3 |issue=119 |page=145 |date=1939 |bibcode=1939ASPL....3..145M}}</ref><ref>{{cite book |url=https://books.google.com/books?id=r47qBwAAQBAJ&pg=PA197 |page=197 |title=A History of Astronomy: from 1890 to the Present |author=David Leverington |publisher=Springer Science & Business Media |year=2012|isbn=978-1-4471-2124-4 }}</ref><ref name="Lundmark">{{cite journal |last=Lundmark |first=Knut |author-link=Knut Lundmark |title=Suspected New Stars Recorded in Old Chronicles and Among Recent Meridian Observations |journal=Publications of the Astronomical Society of the Pacific |volume=33 |issue=195 |pages=225–238 |date=1921 |bibcode=1921PASP...33..225L |jstor=40668518 |doi=10.1086/123101|s2cid=120275870 |url=https://zenodo.org/record/1431279 }}</ref>

In 1913, when Vesto Slipher registered his [[Physics:Spectroscopy|spectroscopy]] study of the sky, the Crab Nebula was again one of the first objects to be studied. Changes in the cloud, suggesting its small extent, were discovered by Carl Lampland in 1921.<ref name=lampland>{{cite journal |title=Observed Changes in the Structure of the "Crab" Nebula (N. G. C. 1952) |journal=Publications of the Astronomical Society of the Pacific |last=Lampland |first=C. O. |volume=33 |issue=192 |pages=79–84 |date=1921 |bibcode=1921PASP...33...79L |jstor=40710638 <!--|alternate url=https://archive.org/details/jstor-40710638--> |doi=10.1086/123039|s2cid=122115955 }}</ref> That same year, John Charles Duncan demonstrated that the remnant was expanding,<ref name=duncan>{{cite journal |last=Duncan |first=John Charles |title=Changes Observed in the Crab Nebula in Taurus |journal=Proceedings of the National Academy of Sciences |volume=7 |issue=6 |pages=179–181 |date=1921 |bibcode=1921PNAS....7..179D |doi=10.1073/pnas.7.6.179 |pmid=16586833 |pmc=1084821|doi-access=free }}</ref> while Knut Lundmark noted its proximity to the guest star of 1054.<ref name="Lundmark"/><ref name=Srinivasan1997>{{cite book |last=Srinivasan |first=G. |chapter=Neutron Stars |title=Stellar Remnants |publisher=Springer Science |series=Lecture Notes 1995, Swiss Society for Astrophysics and Astronomy |page=108 |date=1997 |isbn=978-3-540-61520-0 |chapter-url=https://books.google.com/books?id=79OhUC30PkkC&pg=PA108}}</ref>

In 1928, [[Biography:Edwin Hubble|Edwin Hubble]] proposed associating the cloud with the star of 1054, an idea that remained controversial until the nature of supernovae was understood, and it was [[Biography:Nicholas Mayall|Nicholas Mayall]] who indicated that the star of 1054 was undoubtedly the supernova whose explosion produced the Crab Nebula. The search for historical supernovae started at that moment: seven other historical sightings have been found by comparing modern observations of supernova remnants with astronomical documents of past centuries.{{Citation needed|date=July 2023}}

After the original connection to Chinese observations, in 1934 connections were made to a 13th-century Japanese reference to a "[[Astronomy:Guest star|guest star]]" in Meigetsuki a few weeks before the Chinese reference.<ref name="Usui2007">{{cite web|title=Why and how did a Japanese poet record the Supernova of AD 1054?|url=http://homepage3.nifty.com/silver-moon/teika/teika-e.htm|last=Usui|first=Tadashi|date=11 January 2007|archive-url=https://web.archive.org/web/20160303193701/http://homepage3.nifty.com/silver-moon/teika/teika-e.htm|archive-date=3 March 2016|access-date=4 March 2015}}</ref><ref>{{cite book|author=Fujiwara no Sadaie|title=Meigetsuki|date=c. 1200|trans-title=Record of the Clear Moon|author-link=Fujiwara no Teika}}</ref><ref>{{Cite journal|title=2003JAHH....6...46S Page 46|bibcode=2003JAHH....6...46S|last1=Stephenson|first1=F. Richard|last2=Green|first2=David A.|journal=Journal of Astronomical History and Heritage|year=2003|volume=6|issue=1|page=46|doi=10.3724/SP.J.1440-2807.2003.01.05 |s2cid=128868531 }}</ref> The event was long considered unrecorded in Islamic astronomy,<ref>{{cite journal|last=Gingerich|first=Owen|author-link=Owen Gingerich|date=April 1986|title=Islamic astronomy|url=http://www.as.utexas.edu/astronomy/education/spring05/bromm/readings/islam.pdf|journal=Scientific American|volume=254|issue=10|page=74|bibcode=1986SciAm.254d..74G|doi=10.1038/scientificamerican0486-74}}</ref> but in 1978 a reference was found in a 13th-century copy made by Ibn Abi Usaibia of a work by Ibn Butlan, a Nestorian Christian physician active in Baghdad at the time of the supernova.<ref name="Usaybia">{{cite book|author=Ibn Abi Usaibia|title=Lives of the Physicians|date=1971|others=Kopf, Lothar (trans.)|chapter=Chapter 10: On the Classes of Physicians of Iraq, al-Jazirah and Diyar Bekr|author-link=Ibn Abi Usaibia|orig-date=1245–1246|chapter-url=http://www.tertullian.org/fathers/ibn_abi_usaibia_02.htm#CHAPTER_X}}</ref><ref name="Green03">{{cite book|last1=Green|first1=David A.|last2=Stephenson|first2=F. Richard|chapter=Historical Supernovae |title=Supernovae and Gamma-Ray Bursters |date=2003|journal=Supernovae and Gamma Ray Bursters|publisher=Springer|isbn=978-3-540-44053-6|editor1-last=Weiler|editor1-first=K. W.|series=Lecture Notes in Physics|volume=598|location=Berlin|pages=7–19|arxiv=astro-ph/0301603|bibcode=2003LNP...598....7G|doi=10.1007/3-540-45863-8_2|s2cid=17099919|name-list-style=amp}}</ref>

Given its great distance, the daytime "guest star" observed by the Chinese could only have been a [[Astronomy:Supernova|supernova]]—a massive, exploding star, having exhausted its supply of energy from [[Physics:Nuclear fusion|nuclear fusion]] and collapsed in on itself.<ref>{{Cite book|last=Tao|first=Li|title=Xu Zizhi Tongjian Changbian|publisher=Zhonghua Book Company|year=2004|volume=176|location=Beijing|page=4263|language=zh|quote=己丑，客星出天关之东南可数寸。嘉祐元年三月乃没。}}</ref><ref>{{Cite book|title=Song Huiyao|language=zh|quote=嘉佑元年三月，司天监言：'客星没，客去之兆也'。初，至和元年五月，晨出东方，守天关。昼如太白，芒角四出，色赤白，凡见二十三日。}}</ref> Recent analysis of historical records have found that the supernova that created the Crab Nebula probably appeared in April or early May, rising to its maximum brightness of between [[Astronomy:Apparent magnitude|apparent magnitude]] −7 and −4.5 (brighter even than Venus' −4.2 and everything in the night sky except the [[Astronomy:Moon|Moon]]) by July. The supernova was visible to the [[Astronomy:Naked eye|naked eye]] for about two years after its first observation.<ref name="Collinsetal1999">{{cite journal |last1=Collins |first1=George W. II |last2=Claspy |first2=William P. |last3=Martin |first3=John C. |display-authors=1 |title=A Reinterpretation of Historical References to the Supernova of A.D. 1054 |journal=Publications of the Astronomical Society of the Pacific |volume=111 |issue=761 |pages=871–880 |date=1999 |bibcode=1999PASP..111..871C |doi=10.1086/316401 |arxiv=astro-ph/9904285|s2cid=14452581 }}</ref>

===Crab Pulsar===
{{Main|Astronomy:Crab Pulsar}}
[[File:Chandra-crab.jpg|thumb|Image combining optical data from [[Astronomy:Hubble Space Telescope|Hubble]] (in red) and X-ray images from Chandra X-ray Observatory (in blue).]]

In the 1960s, because of the prediction and discovery of [[Astronomy:Pulsar|pulsar]]s, the Crab Nebula again became a major center of interest. It was then that Franco Pacini predicted the existence of the [[Astronomy:Crab Pulsar|Crab Pulsar]] for the first time, which would explain the brightness of the cloud. The star was discovered shortly afterwards in 1968<ref>{{cite journal |last=Setti |first=Giancarlo |url=https://baas.aas.org/obituaries/franco-pacini-1939-2012/ |archive-url=https://web.archive.org/web/20200206201015/https://baas.aas.org/obituaries/franco-pacini-1939-2012/ |archive-date=6 February 2020 |title=Franco Pacini (1939–2012) |journal=Bulletin of the American Astronomical Society |volume=44 |page=011 |date=2012 |doi=10.3847/BAASOBIT2012011  |doi-broken-date=1 August 2023 |doi-access=free}}</ref> by Dr Susan Jocelyn Bell. The discovery of the Crab Pulsar, and the knowledge of its exact age (almost to the day) allows for the verification of basic physical properties of these objects, such as characteristic age and spin-down luminosity, the orders of magnitude involved (notably the strength of the [[Physics:Magnetic field|magnetic field]]), along with various aspects related to the dynamics of the remnant. The role of this supernova to the scientific understanding of supernova remnants was crucial, as no other historical supernova created a pulsar whose precise age is known for certain. The only possible exception to this rule would be [[Astronomy:SN 1181|SN&nbsp;1181]], whose supposed remnant [[Astronomy:3C 58|3C]] [[Astronomy:SN 1181|&nbsp;]]58 is home to a pulsar, but its identification using Chinese observations from 1181 is contested.<ref>{{cite journal |last=Bietenholz |first=M. F. |title=Radio Images of 3C 58: Expansion and Motion of Its Wisp |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=645 |issue=2 |pages=1180–1187 |date=July 2006 |doi=10.1086/504584 |bibcode=2006ApJ...645.1180B |arxiv=astro-ph/0603197|s2cid=16820726 }}</ref>

The inner part of the Crab Nebula is dominated by a pulsar wind nebula enveloping the pulsar. Some sources consider the Crab Nebula to be an example of both a pulsar wind nebula as well as a supernova remnant,<ref>{{Cite journal |last1=Gaensler|first1=Bryan M.|last2=Slane|first2=Patrick O.|date=18 August 2006|title=The Evolution and Structure of Pulsar Wind Nebulae|journal=Annual Review of Astronomy and Astrophysics |volume=44|issue=1|pages=17–47 |doi=10.1146/annurev.astro.44.051905.092528|arxiv=astro-ph/0601081 |bibcode=2006ARA&A..44...17G|s2cid=10699344|issn=0066-4146}}</ref><ref>{{Cite web|title=X-RAY UNIVERSE :: Make a Pulsar: Crab Nebula in 3D |url=https://chandra.cfa.harvard.edu/deadstar/crab.html|access-date=31 October 2020|website=chandra.cfa.harvard.edu|quote=the [Crab] nebula is not a classic supernova remnant, as once commonly thought, but that the system is better classified as a pulsar wind nebula}}</ref><ref name="sao20161104">{{cite web |url=https://www.cfa.harvard.edu/news/su201643 |title=Pulsar Wind Nebulae |publisher=Smithsonian Astrophysical Observatory |date=4 November 2016 |access-date=26 March 2017}}</ref> while others separate the two phenomena based on the different sources of energy production and behaviour.<ref name="Hester2008"/>

===Source of high-energy gamma rays===
The Crab Nebula was the first astrophysical object confirmed to emit gamma rays in the very-high-energy (VHE) band above 100 GeV in energy.  The VHE detection was carried out in 1989 by the Whipple Observatory 10m Gamma-Ray telescope,<ref>{{Cite book |last=Gibbs |first=Kenneth Gerard |title=THE APPLICATION OF IMAGING TO THE ATMOSPHERIC CERENKOV TECHNIQUE: OBSERVATIONS OF THE CRAB NEBULA |publisher=The University of Arizona |year=1987 |url=https://repository.arizona.edu/handle/10150/184235}}</ref><ref>{{Cite journal |last=Weekes |first=T.C. |date=1989 |title=Observation of TeV Gamma Rays from the Crab Nebula Using the Atmospheric Cerenkov Imaging Technique |bibcode=1989ApJ...342..379W |journal=Astrophysical Journal |volume=342 |page=379 |doi=10.1086/167599 |s2cid=119424766 |url=http://mural.maynoothuniversity.ie/12618/1/Cawley_ObservationTeV_1989.pdf }}</ref> which opened the VHE gamma-ray window and led to the detection of numerous VHE sources since then.

In 2019 the Crab Nebula was observed to emit [[Physics:Ultra-high-energy gamma ray|gamma rays]] in excess of 100&nbsp;[[Physics:TeV|TeV]], making it the first identified source beyond 100&nbsp;TeV.<ref name="Amenomori2019">{{cite journal |title=First Detection of Photons with Energy Beyond 100 TeV from an Astrophysical Source |journal=Physical Review Letters |first=M. |last=Amenomori |display-authors=0 |volume=123 |issue=5 |page=051101 |date=June 2019 |arxiv=1906.05521 |doi=10.1103/PhysRevLett.123.051101 |pmid=31491288 |bibcode=2019PhRvL.123e1101A |s2cid=189762075}}</ref>

==Physical parameters==
[[File:Filaments in the Crab Nebula.jpg|thumb|left|[[Astronomy:Hubble Space Telescope|Hubble]] image of a small region of the Crab Nebula, showing Rayleigh–Taylor instabilities in its intricate filamentary structure.]]

In [[Physics:Visible light|visible light]], the Crab Nebula consists of a broadly [[Oval|oval]]-shaped mass of filaments, about 6&nbsp;[[Arcminute|arcminute]]s long and 4&nbsp;arcminutes wide (by comparison, the [[Astronomy:Full moon|full moon]] is 30&nbsp;arcminutes across) surrounding a diffuse blue central region. In three dimensions, the nebula is thought to be shaped either like an oblate spheroid (estimated as {{cvt|1380|pc|ly|disp=x|/}} away) or a prolate spheroid (estimated as {{cvt|2020|pc|ly|disp=x|/}} away).<ref name="Trimble1973"/> The filaments are the remnants of the progenitor star's atmosphere, and consist largely of ionised [[Chemistry:Helium|helium]] and [[Chemistry:Hydrogen|hydrogen]], along with [[Chemistry:Carbon|carbon]], [[Chemistry:Oxygen|oxygen]], [[Chemistry:Nitrogen|nitrogen]], [[Chemistry:Iron|iron]], [[Chemistry:Neon|neon]] and [[Chemistry:Sulfur|sulfur]]. The filaments' temperatures are typically between 11,000 and 18,000&nbsp;[[Kelvin|K]], and their densities are about 1,300&nbsp;particles per cm<sup>3</sup>.<ref name="Fesenetal1982">{{cite journal |last1=Fesen |first1=R. A. |last2=Kirshner |first2=R. P. |name-list-style=amp |title=The Crab Nebula. I – Spectrophotometry of the filaments |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=258 |issue=1 |pages=1–10 |date=1982 |bibcode=1982ApJ...258....1F |doi=10.1086/160043}}</ref>

In 1953, [[Biography:Iosif Shklovsky|Iosif Shklovsky]] proposed that the diffuse blue region is predominantly produced by [[Physics:Synchrotron radiation|synchrotron radiation]], which is radiation given off by the curving motion of [[Physics:Electron|electron]]s in a magnetic field. The radiation corresponded to electrons moving at speeds up to half the [[Physics:Speed of light|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 |page=983 |date=1953 |bibcode=1957SvA.....1..690S}}</ref> Three years later the theory was confirmed by observations. In the 1960s it was found that the source of the curved paths of the electrons was the strong [[Physics:Magnetic field|magnetic field]] produced by a neutron star at the centre of the nebula.<ref>{{cite journal |last=Burn |first=B. J. |title=A synchrotron model for the continuum spectrum of the Crab Nebula |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |volume=165 |issue=4 |pages=421–429 |date=1973 |bibcode=1973MNRAS.165..421B |doi=10.1093/mnras/165.4.421}}</ref>

===Distance===
<!-- Deleted image removed: [[File:M1knpo4m1973.jpg|thumb|View of M1 from a 4-meter telescope, recording on chemical film (1973)]] -->
Even though the Crab Nebula is the focus of much attention among astronomers, its distance remains an open question, owing to uncertainties in every method used to estimate its distance. In 2008, the consensus was that its distance from Earth is {{cvt|2.0|±|0.5|kpc|ly}}.<ref name="Kaplanetal2008"/> Along its longest visible dimension, it thus measures about {{cvt|4.1|±|1|pc|ly|0}} across.{{Ref label|C|c|none}}

The Crab Nebula currently is expanding outward at about {{cvt|1500|km/s}}.<ref name="Bietneholz">{{cite journal |last1=Bietenholz |first1=M. F. |last2=Kronberg |first2=P. P. |last3=Hogg |first3=D. E. |last4=Wilson |first4=A. S. |display-authors=1 |title=The expansion of the Crab Nebula |journal=The Astrophysical Journal Letters |volume=373 |page=L59–L62 |date=1991 |bibcode=1991ApJ...373L..59B |doi=10.1086/186051}}</ref> Images taken several years apart reveal the slow expansion of the nebula,<ref>{{Cite APOD |title=Animation showing expansion from 1973 to 2001 |date=27 December 2001 |access-date=10 March 2010}}</ref> and by comparing this angular expansion with its [[Astronomy:Redshift|spectroscopically]] determined expansion velocity, the nebula's distance can be estimated. In 1973, an analysis of many methods used to compute the distance to the nebula had reached a conclusion of about {{cvt|6300|ly|kpc|order=flip}}, consistent with the currently cited value.<ref name="Trimble1973">{{cite journal |last1=Trimble |first1=Virginia Louise |title=The Distance to the Crab Nebula and NP 0532 |journal=Publications of the Astronomical Society of the Pacific |volume=85 |issue=507 |pages=579–585 |date=1973 |bibcode=1973PASP...85..579T |jstor=40675440 |doi=10.1086/129507|s2cid=122277030 }}</ref>
<!--why do we cite a 1973 estimate, without error bars, on top of the current estimate? -->

Tracing back its expansion (assuming a constant decrease of expansion speed due to the nebula's mass) yielded a date for the creation of the nebula several decades after 1054, implying that its outward velocity has decelerated less than assumed since the supernova explosion.<ref name="Trimble1968">{{cite journal |last1=Trimble |first1=Virginia Louise |title=Motions and Structure of the Filamentary Envelope of the Crab Nebula |journal=Astronomical Journal |volume=73 |page=535 |date=1968 |bibcode=1968AJ.....73..535T |doi=10.1086/110658 |s2cid=120669550 |url=https://thesis.library.caltech.edu/1562/1/Trimble_vl_1968.pdf}}</ref> This reduced deceleration 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 outward.<ref name="Bejgeretal2003">{{cite journal |last1=Bejger |first1=M. |last2=Haensel |first2=P. |name-list-style=amp |title=Accelerated expansion of the Crab Nebula and evaluation of its neutron-star parameters |journal=Astronomy and Astrophysics |volume=405 |issue=2 |pages=747–751 |date=2003 |bibcode=2003A&A...405..747B |doi=10.1051/0004-6361:20030642 |arxiv=astro-ph/0301071|s2cid=10254761 }}</ref><ref>{{cite web |url=http://www.astronomy.com/news/2007/06/crab-nebula-exploded-in-1054 |title=Crab Nebula exploded in 1054 |work=Astronomy.com |date=8 June 2007 |access-date=10 September 2014}}</ref>

===Mass===
Estimates of the total mass of the nebula are important for estimating the mass of the supernova's progenitor star. The amount of matter contained in the Crab Nebula's filaments (ejecta mass of ionized and neutral gas; mostly [[Chemistry:Helium|helium]]<ref name="Greenetal2004">{{cite journal |last1=Green |first1=D. A. |last2=Tuffs |first2=R. J. |last3=Popescu |first3=C. C. |display-authors=1 |title=Far-infrared and submillimetre observations of the Crab nebula |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |volume=355 |issue=4 |pages=1315–1326 |date=2004 |bibcode=2004MNRAS.355.1315G |doi=10.1111/j.1365-2966.2004.08414.x |arxiv=astro-ph/0409469|s2cid=6914133 }}</ref>) is estimated to be {{val|4.6|1.8|ul=solar mass}}.<ref name="Fesenetal1997">{{cite journal |last1=Fesen |first1=Robert A. |last2=Shull |first2=J. Michael |last3=Hurford |first3=Alan P. |display-authors=1 |title=An Optical Study of the Circumstellar Environment Around the Crab Nebula |journal=[[Astronomy:The Astronomical Journal|The Astronomical Journal]] |volume=113 |pages=354–363 |date=1997 |bibcode=1997AJ....113..354F |doi=10.1086/118258 |hdl=2060/19970022615}}</ref>

===Helium-rich torus===
One of the many nebular components (or anomalies) of the Crab Nebula is a helium-rich [[Torus|torus]] which is visible as an east–west band crossing the pulsar region. The torus composes about 25% of the visible ejecta. However, it is suggested by calculation that about 95% of the torus is helium. As yet, there has been no plausible explanation put forth for the structure of the torus.<ref name="MacAlpineetal2007"/>

==Central star==
{{Main|Astronomy:Crab Pulsar}}
[[File:M1.gif|thumb|upright=1.35|left|Slow-motion video of the Crab Pulsar, taken with OES Single-Photon-Camera.]]
[[File:The Crab Nebula - A Flickering X-ray Candle.ogv|thumb|Data from orbiting observatories show unexpected variations in the Crab Nebula's X-ray output, likely tied to the environment around its central neutron star.]]
[[File:NASA's Fermi Spots 'Superflares' in the Crab Nebula.ogv|thumb|NASA's [[Astronomy:Fermi Gamma-ray Space Telescope|Fermi]] spots 'superflares' in the Crab Nebula.]]

At the center of the Crab Nebula are two faint stars, one of which is the star responsible for the existence of the nebula. It was identified as such in 1942, when Rudolf Minkowski found that its optical spectrum was extremely unusual.<ref>{{cite journal |last=Minkowski |first=Rudolph |title=The Crab Nebula |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=96 |page=199 |date=September 1942 |doi=10.1086/144447 |bibcode=1942ApJ....96..199M}}</ref> The region around the star was found to be a strong source of radio waves in 1949<ref>{{cite journal |last1=Bolton |first1=John G. |author-link=John Gatenby Bolton |last2=Stanley |first2=G. J. |last3=Slee |first3=O. B. |display-authors=1 |title=Positions of three discrete sources of Galactic radio frequency radiation |journal=Nature |volume=164 |issue=4159 |pages=101–102 |date=1949 |doi=10.1038/164101b0 |bibcode=1949Natur.164..101B|s2cid=4073162 }}</ref> and X-rays in 1963,<ref name="Bowyer">{{cite journal |last1=Bowyer |first1=S. |last2=Byram |first2=E. T. |last3=Chubb |first3=T. A. |last4=Friedman |first4=H. |display-authors=1 |title=Lunar Occultation of X-ray Emission from the Crab Nebula |journal=Science |volume=146 |issue=3646 |pages=912–917 |date=1964 |doi=10.1126/science.146.3646.912 |pmid=17777056 |bibcode=1964Sci...146..912B|s2cid=12749817 }}</ref> and was identified as one of the brightest objects in the sky in [[Physics:Gamma ray|gamma ray]]s in 1967.<ref>{{cite journal |last1=Haymes |first1=R. C. |last2=Ellis |first2=D. V. |last3=Fishman |first3=G. J. |last4=Kurfess |first4=J. D. |last5=Tucker |first5=W. H. |display-authors=1 |title=Observation of Gamma Radiation from the Crab Nebula |journal=The Astrophysical Journal Letters |volume=151 |pages=L9 |date=1968 |doi=10.1086/180129 |bibcode=1968ApJ...151L...9H}}</ref> Then, in 1968, the star was found to be emitting its radiation in rapid pulses, becoming one of the first [[Astronomy:Pulsar|pulsar]]s to be discovered.<ref name=Green03/>

Pulsars are sources of powerful [[Physics:Electromagnetic radiation|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 who identified the first one considered the possibility that it could be a signal from an advanced civilization.<ref>{{cite journal |last=Del Puerto |first=C. |title=Pulsars In The Headlines |journal=EAS Publications Series |volume=16 |pages=115–119 |date=2005 |doi=10.1051/eas:2005070 |bibcode=2005EAS....16..115D}}</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.<ref name=LaViolette>{{cite book |last1=LaViolette |first1=Paul A. |title=Decoding the Message of the Pulsars: Intelligent Communication from the Galaxy |publisher=Bear & Co. |page=73 |date=April 2006 |isbn=978-1-59143-062-9 |url=https://books.google.com/books?id=Iv1W3njGhrIC&pg=PA73}}</ref> They now are understood to be rapidly rotating [[Astronomy:Neutron star|neutron star]]s, whose powerful [[Physics:Magnetic field|magnetic field]]s concentrates their radiation emissions into narrow beams.<ref name=LaViolette135>{{cite book |last1=LaViolette |first1=Paul A. |title=Decoding the Message of the Pulsars: Intelligent Communication from the Galaxy |publisher=Bear & Co. |page=135 |date=April 2006 |isbn=978-1-59143-062-9 |url=https://books.google.com/books?id=Iv1W3njGhrIC&pg=PA135}}</ref>

The Crab Pulsar is believed to be about {{cvt|28|-|30|km}} in diameter;<ref name="Bejgeretal2002">{{cite journal |last1=Bejger |first1=M. |last2=Haensel |first2=P. |name-list-style=amp |title=Moments of inertia for neutron and strange stars: Limits derived for the Crab pulsar |journal=Astronomy and Astrophysics |volume=396 |issue=3 |pages=917–921 |date=2002 |doi=10.1051/0004-6361:20021241 |bibcode=2002A&A...396..917B |arxiv=astro-ph/0209151|s2cid=13946022 }}</ref> it emits pulses of radiation every 33&nbsp;[[Millisecond|millisecond]]s.<ref>{{cite journal |last1=Harnden |first1=F. R. |last2=Seward |first2=F. D. |name-list-style=amp |title=Einstein observations of the Crab nebula pulsar |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=283 |pages=279–285 |date=1984 |doi=10.1086/162304 |bibcode=1984ApJ...283..279H}}</ref> Pulses are emitted at [[Wavelength|wavelength]]s across the [[Physics:Electromagnetic spectrum|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 [[Physics:Energy|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 [[Astronomy:Luminosity|luminosity]] about 75,000 times greater than that of the Sun.<ref>{{cite book |last=Kaufmann |first=W. J. |title=Universe |edition=4th |publisher=W. H. Freeman |page=428 |date=1996 |isbn=978-0-7167-2379-0}}</ref>

The pulsar's extreme energy output creates an 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 Nebula show changes over timescales of only a few days.<ref name=Hester996>{{cite journal |last1=Hester |first1=J. Jeff |last2=Scowen |first2=P. A. |last3=Sankrit |first3=R. |last4=Michel |first4=F. C. |last5=Graham |first5=J. R. |last6=Watson |first6=A. |last7=Gallagher |first7=J. S. |display-authors=1 |title=The Extremely Dynamic Structure of the Inner Crab Nebula |journal=[[Organization:Bulletin of the American Astronomical Society|Bulletin of the American Astronomical Society]] |volume=28 |issue=2 |page=950 |date=1996 |bibcode=1996BAAS...28..950H}}</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 [[Physics: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.<ref name=Hester996/>

==Progenitor star==
[[File:Changes in the Crab Nebula.jpg|thumb|left|This sequence of [[Astronomy:Hubble Space Telescope|Hubble]] images shows features in the inner Crab Nebula changing over a period of four months.]]

The star that exploded as a supernova is referred to as the supernova's ''progenitor star''. Two types of stars explode as supernovae: [[Astronomy:White dwarf|white dwarf]]s and massive stars. In the so-called [[Astronomy:Type Ia supernova|Type Ia supernova]]e, gases falling onto a 'dead' white dwarf raise its mass until it nears a critical level, the [[Astronomy:Chandrasekhar limit|Chandrasekhar limit]], resulting in a [[Astronomy:Thermal runaway|runaway nuclear fusion explosion]] that obliterates the star; in [[Astronomy:Type Ib and Ic supernovae|Type Ib/c]] and [[Astronomy:Type II supernova|Type II]] supernovae, the progenitor star is a massive star whose core runs out of fuel to power its [[Physics:Nuclear fusion|nuclear fusion]] reactions and collapses in on itself, releasing gravitational potential energy in a form that blows away the star's outer layers. Type Ia supernovae do not produce pulsars,<ref name="Pasachoff">{{cite book|last1=Pasachoff|first1=Jay M.|url=https://books.google.com/books?id=yQMlAAAAQBAJ&pg=PA357|title=The Cosmos: Astronomy in the New Millennium|last2=Filippenko|first2=Alex|date=August 2013|publisher=Cambridge University Press|isbn=978-1-107-27695-6|page=357|name-list-style=amp}}</ref> so the pulsar in the Crab Nebula shows it must have formed in a core-collapse supernova.<ref name=Maoz>{{cite book |last1=Maoz |first1=Dan |title=Astrophysics in a Nutshell |publisher=Princeton University Press |page=90 |date=December 2011 |isbn=978-1-4008-3934-6 |url=https://books.google.com/books?id=yWidGMhCNX8C&pg=PA90}}</ref>

Theoretical models of supernova explosions suggest that the star that exploded to produce the Crab Nebula must have had a [[Physics:Mass|mass]] of between {{solar mass|9 and 11|link=y}}.<ref name="MacAlpineetal2007">{{cite journal |last1=MacAlpine |first1=Gordon M. |last2=Ecklund |first2=Tait C. |last3=Lester |first3=William R. |last4=Vanderveer |first4=Steven J. |last5=Strolger |first5=Louis-Gregory |display-authors=1 |title=A Spectroscopic Study of Nuclear Processing and the Production of Anomalously Strong Lines in the Crab Nebula |journal=[[Astronomy:The Astronomical Journal|The Astronomical Journal]] |volume=133 |issue=1 |pages=81–88 |date=2007 |bibcode=2007AJ....133...81M |doi=10.1086/509504 |arxiv=astro-ph/0609803|s2cid=18599459 }}</ref><ref name="Nomoto1985">{{cite book |last1=Nomoto |first1=K. |chapter=Evolutionary models of the Crab Nebula's progenitor |title=The Crab Nebula and Related Supernova Remnants: Proceedings of a Workshop Held at George Mason University, Fairfax, Virginia, October 11–12, 1984 |journal=The Crab Nebula and Related Supernova Remnants |publisher={{wipe|Cambridge University Press}} |pages=97–113 |date=January 1985 |bibcode=1985cnrs.work...97N |isbn=0-521-30530-6}}</ref> Stars with masses lower than {{solar mass|8}} are thought to be too small to produce supernova explosions, and end their lives by producing a [[Astronomy:Planetary nebula|planetary nebula]] instead, while a star heavier than {{solar mass|12}} would have produced a nebula with a different chemical composition from that observed in the Crab Nebula.<ref name="Davidsonetal1985">{{cite journal |last1=Davidson |first1=K. |last2=Fesen |first2=R. A. |name-list-style=amp |title=Recent developments concerning the Crab Nebula |journal=[[Physics:Annual Review of Astronomy and Astrophysics|Annual Review of Astronomy and Astrophysics]] |volume=23 |issue=507 |pages=119–146 |date=1985 |bibcode=1985ARA&A..23..119D |doi=10.1146/annurev.aa.23.090185.001003}}</ref> Recent studies, however, suggest the progenitor could have been a [[Astronomy:Asymptotic giant branch#Super-AGB stars|super-asymptotic giant branch]] star in the {{solar mass|8 to 10}} range that would have exploded in an [[Astronomy:Supernova#Core collapse|electron-capture supernova]].<ref name="Tominaga2013">{{cite journal |last1=Tominaga |first1=N. |last2=Blinnikov |first2=S. I. |last3=Nomoto |first3=Ken'Ichi |display-authors=1 |title=Supernova explosions of super-asymptotic giant branch stars: multicolor light curves of electron-capture supernovae |journal=The Astrophysical Journal Letters |volume=771 |issue=1 |page=L12 |bibcode=2013ApJ...771L..12T |doi=10.1088/2041-8205/771/1/L12 |arxiv=1305.6813 |year=2013|s2cid=118860608 }}</ref> In June 2021 a paper in the journal ''[[Astronomy:Nature Astronomy|Nature Astronomy]]'' reported that the 2018 supernova SN 2018zd (in the galaxy [[Astronomy:NGC 2146|NGC 2146]], about 31 million light-years from Earth) appeared to be the first observation of an electron-capture supernova<ref name=HowellEtAl2021-01a>{{cite journal |vauthors=Hiramatsu D, Howell D, Van S |display-authors=0|date=28 June 2021 |title=The electron-capture origin of supernova 2018zd |url= https://www.nature.com/articles/s41550-021-01384-2|journal=Nat Astron |volume=5 |issue=9 |pages=903–910 |doi=10.1038/s41550-021-01384-2|arxiv=2011.02176|bibcode=2021NatAs...5..903H |s2cid=226246044 }}</ref><ref name=KeckObsNews2021-06-28-01a>{{cite web |url=https://keckobservatory.org/electron-capture-supernova |title=New, Third Type Of Supernova Observed |date=28 June 2021 |website=[[Astronomy:W. M. Keck Observatory|W. M. Keck Observatory]]}}</ref><ref name=RTE2021-06-28-01a>{{cite news|url=https://www.rte.ie/news/newslens/2021/0628/1231824-supernova-crab-nebula/
|title=Astronomers discover new type of supernova|publisher=RTE News |agency=PA|date=28 June 2021|access-date=1 July 2021}}</ref> The 1054 supernova explosion that created the Crab Nebula had been thought to be the best candidate for an electron-capture supernova, and the 2021 paper makes it more likely that this was correct.<ref name=KeckObsNews2021-06-28-01a/><ref name=RTE2021-06-28-01a/>

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 name="Fesenetal1997"/> 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 {{solar mass|1–5}}, with {{solar mass|2–3}} being the generally accepted value.<ref name="Davidsonetal1985"/> The neutron star mass is estimated to be between {{solar mass|1.4 and 2}}.

The predominant theory to account for the missing mass of the Crab Nebula is that a substantial proportion of the mass of the progenitor was carried away before the supernova explosion in a fast [[Astronomy:Stellar wind|stellar wind]], a phenomenon commonly seen in [[Astronomy:Wolf–Rayet star|Wolf–Rayet star]]s. However, this would have created a shell around the nebula. Although attempts have been made at several wavelengths to observe a shell, none has yet been found.<ref>{{cite journal |last1=Frail |first1=D. A. |last2=Kassim |first2=N. E. |last3=Cornwell |first3=T. J. |last4=Goss |first4=W. M. |display-authors=1 |title=Does the Crab Have a Shell? |journal=The Astrophysical Journal Letters |volume=454 |issue=2 |pages=L129–L132 |date=1995 |doi=10.1086/309794 |bibcode=1995ApJ...454L.129F |arxiv=astro-ph/9509135|s2cid=14787898 }}</ref>

==Transits by Solar System bodies==
[[File:Titan transits Crab Nebula 2003.jpg|thumb|[[Astronomy:Chandra X-ray Observatory|Chandra]] image showing Saturn's moon Titan transiting the nebula.]]

The Crab Nebula lies roughly 1.5 degrees away from the [[Astronomy:Ecliptic|ecliptic]]—the plane of Earth's orbit around the Sun. This means that the Moon—and occasionally, planets—can [[Astronomy:Astronomical transit|transit]] or [[Astronomy:Occultation|occult]] the nebula. Although the Sun does not transit the nebula, its [[Physics:Solar corona|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===
Lunar transits have been used to map X-ray emissions from the nebula. Before the launch of X-ray-observing satellites, such as the [[Astronomy:Chandra X-ray Observatory|Chandra X-ray Observatory]], X-ray observations generally had quite low [[Angular resolution|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>{{cite journal |last1=Palmieri |first1=T. M. |last2=Seward |first2=F. D. |last3=Toor |first3=A. |last4=van Flandern |first4=T. C. |display-authors=1 |title=Spatial distribution of X-rays in the Crab Nebula |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=202 |pages=494–497 |date=1975 |doi=10.1086/153998 |bibcode=1975ApJ...202..494P}}</ref> When X-rays were first observed from the Crab Nebula, a lunar occultation was used to determine the exact location of their source.<ref name="Bowyer"/>

===Solar===
The Sun's [[Physics:Solar corona|corona]] passes in front of the Crab Nebula every June. Variations in the radio waves received from the Crab Nebula 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>{{cite journal |last=Erickson |first=W. C. |title=The Radio-Wave Scattering Properties of the Solar Corona |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=139 |page=1290 |date=1964 |doi=10.1086/147865 |bibcode=1964ApJ...139.1290E}}</ref>

===Other objects===
Very rarely, [[Astronomy:Saturn|Saturn]] transits the Crab Nebula. Its transit on 4 January 2003 ([[UTC]]) was the first since 31 December 1295 ([[Old Style and New Style dates|O.S.]]); another will not occur until 5 August 2267. Researchers used the Chandra X-ray Observatory to observe Saturn's moon [[Astronomy: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 {{cvt|880|km}}.<ref>{{cite journal |last1=Mori |first1=K. |last2=Tsunemi |first2=H. |last3=Katayama |first3=H. |last4=Burrows |first4=D. N. |last5=Garmire |first5=G. P. |last6=Metzger |first6=A. E. |display-authors=1 |title=An X-Ray Measurement of Titan's Atmospheric Extent from Its Transit of the Crab Nebula |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |volume=607 |issue=2 |pages=1065–1069 |date=2004 |doi=10.1086/383521 |bibcode=2004ApJ...607.1065M |arxiv=astro-ph/0403283|s2cid=8836905 }} 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.

==Gallery==
{{wide image|Crab Nebula in Multiple Wavelengths.png|500px|The Crab Nebula seen in [[Astronomy:Radio astronomy|radio]], [[Astronomy:Infrared astronomy|infrared]], [[Physics:Visible light|visible light]], [[Astronomy:Ultraviolet astronomy|ultraviolet]], [[Astronomy:X-ray astronomy|X-rays]] and [[Astronomy:Gamma-ray astronomy|gamma-rays]] (8 March 2015)}}
{{wide image|PIA21474-CrabNebula-5Observatories-Text.jpg|500px|The Crab Nebula – five observatories (10 May 2017)}}
[[File:PIA21474-CrabNebula-5Observatories-Animation.gif|thumb|500px|center|The Crab Nebula – five observatories (animation; 10 May 2017)]]
{{wide image|Crab Nebula imaged using James Webb Space Telescope.png|500px|Crab Nebula imaged using [[Astronomy:James Webb Space Telescope|James Webb Space Telescope]] in [[Astronomy:Infrared astronomy|infrared]] via its NIRCam (Near-Infrared Camera) and [[Engineering:Mid-Infrared Instrument|MIRI]] (Mid-Infrared Instrument). (30 October 2023)}}

==See also==
* [[Astronomy:Lists of nebulae|Lists of nebulae]]
* [[Astronomy:Messier object#Messier objects|List of Messier objects]]
* [[Astronomy:Southern Crab Nebula|Southern Crab Nebula]], so named for its resemblance to the Crab Nebula, but visible from the southern hemisphere
* [[Astronomy:Galactic anticenter|Galactic anticenter]]

==Notes==
{{notelist}}
{{refbegin}}
<ol type="a">
<li>{{Note label|A|a|none}}Size as measured on a very deep plate taken by Sidney van den Bergh in late 1969.<ref name="Trimble1973"/><ref name="Bergh1970">{{cite journal |last=van den Bergh |first=Sidney |title=A Jetlike Structure Associated with the Crab Nebula |journal=The Astrophysical Journal Letters |volume=160 |page=L27 |date=1970 |bibcode=1970ApJ...160L..27V |doi=10.1086/180516}}</ref>
</li>
<li>{{Note label|B|b|none}}Apparent magnitude of 8.4—[[Physics:Distance modulus|distance modulus]] of {{val|11.5|0.5}} = {{val|p=−|3.1|0.5}}</li>
<li>{{Note label|C|b|none}}distance × tan ( diameter_angle = 420″ ) = {{val|4.1|1.0}} pc diameter = {{val|13|3}}-light-year diameter</li>
<li>{{Note label|D|d|none}}The nature of nebula at the time was unknown.</li>
</ol>
{{refend}}

==References==
{{Reflist}}

==External links==
{{Commons|Messier 1}}
* {{WikiSky|name=Crab Nebula}}
* [http://www.mrao.cam.ac.uk/surveys/snrs/snrs.G184.6-5.8.html Crab Nebula] in the [[Organization:University of Cambridge|University of Cambridge]] Catalogue of Galactic Supernova Remnants
* [http://messier.seds.org/m/m001.html Crab Nebula] in the SEDS Messier index
* [http://chandra.harvard.edu/xray_sources/crab/crab.html Crab Nebula] in the [[Astronomy:Chandra X-ray Observatory|Chandra X-ray Observatory]] Field Guide series
* [http://www.spacetelescope.org/images/?search=crab+nebula Crab Nebula images] by the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]]
* [https://www.astrobin.com/full/327338/0/ Animation of expansion from 2008–2017] by Detlef Hartmann
* [https://www.staracle.com/astronomy/deepsky.php?Messier=1 The Crab Nebula in the Staracle Messier catalog]

{{Catalogs|M=1|NGC=1952}}
{{Messier objects}}

[[Category:Crab Nebula| ]]
[[Category:Messier objects]]
[[Category:NGC objects]]
[[Category:Perseus Arm]]
[[Category:Pulsar wind nebulae]]
[[Category:Sharpless objects]]
[[Category:Supernova remnants]]
[[Category:Taurus (constellation)]]
[[Category:3C objects|144]]
[[Category:4C objects|21.19]]

{{Sourceattribution|Crab Nebula}}