Editing Supernova (section)

===Asymmetry===
[[Image:Chandra-crab.jpg|thumb|The [[pulsar]] in the [[Crab Nebula]] is travelling at 375 km/s relative to the nebula.<ref>
{{Cite journal
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A long-standing puzzle surrounding type II supernovae is why the remaining compact object receives a large velocity away from the epicentre;<ref name="Höflich-2004">
{{Cite book |first=Dong |last=Lai |editor-last1=Höflich |editor-first1=P. A. |title=Cosmic explosions in three dimensions: Asymmetries in supernovae and gamma-ray bursts |editor-last2=Kumar |editor-first2=P. |editor-last3=Wheeler |editor-first3=J. Craig |date=2004 |publisher=[[Cambridge University Press]] |isbn=0-521-84286-7 |page=276 |chapter=Neutron star kicks and supernova asymmetry |bibcode=2004cetd.conf..276L |arxiv=astro-ph/0312542}}</ref> [[pulsar]]s, and thus neutron stars, are observed to have high [[Peculiar velocity|peculiar velocities]], and black holes presumably do as well, although they are far harder to observe in isolation. The initial impetus can be substantial, propelling an object of more than a solar mass at a velocity of 500&nbsp;km/s or greater. This indicates an expansion asymmetry, but the mechanism by which momentum is transferred to the compact object {{As of|2023|alt=remains}} a puzzle. Proposed explanations for this kick include convection in the collapsing star, asymmetric ejection of matter during [[neutron star formation]], and asymmetrical [[neutrino]] emissions.<ref name="Höflich-2004"/><ref name="Janka-2022">{{Cite journal |last1=Janka |first1=Hans-Thomas |last2=Wongwathanarat |first2=Annop |last3=Kramer |first3=Michael |date=1 February 2022 |title=Supernova Fallback as Origin of Neutron Star Spins and Spin-kick Alignment |doi-access=free |journal=The Astrophysical Journal |volume=926 |issue=1 |pages=9 |doi=10.3847/1538-4357/ac403c |bibcode=2022ApJ...926....9J |arxiv=2104.07493 |issn=0004-637X}}</ref>

One possible explanation for this asymmetry is large-scale [[convection]] above the core. The convection can create radial variations in density giving rise to variations in the amount of energy absorbed from neutrino outflow.<ref name="Janka-2007"/> However analysis of this mechanism predicts only modest momentum transfer.<ref name=Fryer-2004>
{{Cite journal
 |last=Fryer |first=C. L.
 |date=2004
 |title=Neutron Star Kicks from Asymmetric Collapse
 |journal=[[Astrophysical Journal]]
 |volume=601 |issue=2 |pages=L175–L178
 |arxiv=astro-ph/0312265
 |bibcode=2004ApJ...601L.175F
 |doi=10.1086/382044
|s2cid=1473584
 }}</ref> Another possible explanation is that accretion of gas onto the central neutron star can create a [[accretion disk|disk]] that drives highly directional jets, propelling matter at a high velocity out of the star, and driving transverse shocks that completely disrupt the star. These jets might play a crucial role in the resulting supernova.<ref>
{{cite journal
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 |year=2014
 |title=Implications of turbulence for jets in core-collapse supernova explosions
 |journal=The Astrophysical Journal
 |volume=806 |issue=1 |page=28
 |arxiv=1412.4984
 |doi=10.1088/0004-637X/806/1/28
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|s2cid=119002386
 }}</ref><ref>
{{cite journal
 |last1=Khokhlov |first1=A. M.
 |last2=Höflich |first2=P. A.
 |last3=Oran |first3=E. S.
 |last4=Wheeler |first4=J. Craig
 |last5=Wang |first5=L.
 |last6=Chtchelkanova |first6=A. Yu.
 |year=1999
 |title=Jet-induced Explosions of Core Collapse Supernovae
 |journal=The Astrophysical Journal
 |volume=524 |issue=2 |pages=L107
 |arxiv=astro-ph/9904419
 |bibcode=1999ApJ...524L.107K
 |doi=10.1086/312305
|s2cid=37572204
 }}</ref> (A similar model is used for explaining long gamma-ray bursts.) The dominant mechanism may depend upon the mass of the progenitor star.<ref name="Janka-2022"/>

Initial asymmetries have also been confirmed in type Ia supernovae through observation. This result may mean that the initial luminosity of this type of supernova depends on the viewing angle. However, the expansion becomes more symmetrical with the passage of time. Early asymmetries are detectable by measuring the polarisation of the emitted light.<ref>
{{cite journal
 |last1=Wang |first1=L.
 |last2=Baade |first2=D.
 |last3=Höflich |first3=P. A.
 |last4=Khokhlov |first4=A. M.
 |last5=Wheeler |first5=J. C.
 |last6=Kasen |first6=D.
 |last7=Nugent |first7=P. E.
 |last8=Perlmutter |first8=S. A.
 |last9=Fransson |first9=C.
 |last10=Lundqvist |first10=P.
 |year=2003
 |title=Spectropolarimetry of SN 2001el in NGC 1448: Asphericity of a Normal Type Ia Supernova
 |journal=The Astrophysical Journal
 |volume=591 |issue=2 |pages=1110–1128
 |arxiv=astro-ph/0303397
 |bibcode=2003ApJ...591.1110W 
 |doi=10.1086/375444
|s2cid=2923640
 }}</ref>