Editing Crab Nebula (section)

==Central star==
{{Main|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 [[Fermi Gamma-ray Space Telescope]] 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=[[The Astrophysical Journal]] |volume=96 |page=199 |date=September 1942 |doi=10.1086/144447 |author-link=Rudolph Minkowski |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 (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 (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 [[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 [[pulsar]]s to be discovered.<ref name=Green03/>

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 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 [[neutron star]]s, whose powerful [[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]]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=[[The Astrophysical Journal]] |volume=283 |pages=279–285 |date=1984 |doi=10.1086/162304 |bibcode=1984ApJ...283..279H}}</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 rate of [[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 148,000 times greater than that of the Sun.<ref>{{cite book |last1=Carroll |first1=B. W. |last2=Ostlie |first2=D. A. |name-list-style=amp |title=An Introduction To Modern Astrophysics |edition=2nd |publisher=[[Cambridge University Press]] |page=532 |date=2017 |isbn=978-1-108-42216-1}}</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=[[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 [[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/>