Editing Crab Nebula (section)

===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 [[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 |author-link=Virginia Louise Trimble |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 |author-link=Virginia Louise Trimble |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>