Editing Supernova remnant

{{Short description|Remnants of an exploded star}}
[[Image:Crab Nebula.jpg|thumb|300px|right|[[SN 1054]] remnant (''[[Crab Nebula]]'').]]
A '''supernova remnant''' ('''SNR''') is the structure resulting from the explosion of a [[star]] in a [[supernova]].  The supernova remnant is bounded by an expanding [[shock wave]], and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.

There are two common routes to a supernova: either a massive star may run out of fuel, ceasing to generate fusion energy in its core, and collapsing inward under the force of its own gravity to form a [[neutron star]] or a [[black hole]]; or a [[white dwarf]] star may [[accretion (astrophysics)|accrete]] material from a companion star until it reaches a critical mass and undergoes a [[thermonuclear explosion]].

In either case, the resulting supernova explosion expels much or all of the stellar material with velocities as much as 10% the speed of light (or approximately 30,000&nbsp;km/s) and a strong [[shock wave]] forms ahead of the ejecta. That heats the upstream [[Plasma (physics)|plasma]] up to temperatures well above millions of&nbsp;K. The shock continuously slows down over time as it sweeps up the ambient medium, but it can expand over hundreds or thousands of years and over tens of [[parsec]]s before its speed falls below the local sound speed.

One of the best observed young supernova remnants was formed by [[SN 1987A]], a supernova in the [[Large Magellanic Cloud]] that was observed in February 1987. Other well-known supernova remnants include the [[Crab Nebula]]; Tycho, the remnant of [[SN 1572]], named after [[Tycho Brahe]] who recorded the brightness of its original explosion; and Kepler, the remnant of [[SN 1604]], named after [[Johannes Kepler]]. The youngest known remnant in the [[Milky Way]] is [[Supernova remnant G1.9+0.3|G1.9+0.3]], discovered in the [[Galactic Center]].<ref>[http://chandra.harvard.edu/press/08_releases/press_051408.html Discovery of most recent supernova in our galaxy] May 14, 2008</ref>

==Stages==
An SNR passes through the following stages as it expands:<ref>{{Cite journal|title = Supernova Remnants at High Energy|date = 2008|first = Stephen P.|last = Reynolds|journal = Annual Review of Astronomy and Astrophysics|issue = 46|doi = 10.1146/annurev.astro.46.060407.145237|pages = 89–126|bibcode = 2008ARA&A..46...89R|volume = 46 }}</ref>

# Free expansion of the ejecta, until they sweep up their own weight in circumstellar or [[interstellar medium]].  This can last tens to a few hundred years depending on the density of the surrounding gas.
# Sweeping up of a shell of shocked circumstellar and interstellar gas. This begins the Sedov-Taylor phase, which can be well modeled by a self-similar analytic solution (see [[Blast wave#Astronomy|blast wave]]).  Strong [[X-ray]] emission traces the strong shock waves and hot shocked gas.
# Cooling of the shell, to form a thin (<&nbsp;1&nbsp;[[Parsec|pc]]), dense (1&nbsp;to 100&nbsp;million atoms per cubic metre) shell surrounding the hot (few million kelvin) interior. This is the pressure-driven snowplow phase.  The shell can be clearly seen in optical emission from recombining ionized [[hydrogen]] and ionized [[oxygen]] atoms.
# Cooling of the interior.  The dense shell continues to expand from its own momentum.  This stage is best seen in the radio emission from neutral hydrogen atoms.
# Merging with the surrounding interstellar medium.  When the supernova remnant slows to the speed of the random velocities in the surrounding medium, after roughly 30,000 years, it will merge into the general turbulent flow, contributing its remaining kinetic energy to the turbulence.
{{Multiple image|direction=horizontal|align=center|footer_align=center|width=300|image1=15-044a-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg|image2=15-044b-SuperNovaRemnant-PlanetFormation-SOFIA-20150319.jpg|footer=Supernova remnant ejecta producing [[Nebular hypothesis|planet-forming material]]}}

==Types of supernova remnant==
There are three types of supernova remnant:
* Shell-like, such as [[Cassiopeia A]]
* Composite, in which a shell contains a central [[pulsar wind nebula]], such as G11.2-0.3 or G21.5-0.9.
* Mixed-morphology (also called "thermal composite") remnants, in which central thermal X-ray emission is seen, enclosed by a radio shell. The thermal X-rays are primarily from swept-up interstellar material, rather than supernova ejecta. Examples of this class include the SNRs W28 and W44. (Confusingly, W44 additionally contains a [[pulsar]] and pulsar wind nebula; so it is simultaneously both a "classic" composite and a thermal composite.)
{{multiple image|align=center|caption_align=center|header=Supernova remnants|image1=PIA22564-SupernovaRemnant-HBH3-20180802.jpg |caption1=HBH 3 ([[Spitzer Space Telescope]]; August 2, 2018) |width1=300 |image2=PIA22569-SuperNovaRemnant-G54.1+0.3-20181116.jpg |caption2=G54.1+0.3 (November 16, 2018) |width2=233}}
<!---[[File:PIA22564-SupernovaRemnant-HBH3-20180802.jpg|thumb|center|500px|Supernova remnant HBH 3 ([[Spitzer Space Telescope]]; August 2, 2018)]]--->

Remnants which could only be created by significantly higher ejection energies than a standard supernova are called ''hypernova remnants'', after the high-energy [[hypernova]] explosion that is assumed to have created them.<ref>{{cite journal |doi=10.1086/318420 |title=A Critical Examination of Hypernova Remnant Candidates in M101. I. MF 83 |journal=The Astrophysical Journal |volume=547 |issue=2 |pages=754–764 |year=2001 |last1=Lai |first1=Shih-Ping |last2=Chu |first2=You-Hua |last3=Chen |first3=C.-H. Rosie |last4=Ciardullo |first4=Robin |last5=Grebel |first5=Eva K. |bibcode=2001ApJ...547..754L |arxiv=astro-ph/0009238 |s2cid=14620463 }}</ref>

==Origin of cosmic rays==
Supernova remnants are considered the major source of [[galactic cosmic ray]]s.<ref name="Koyama">{{cite journal | display-authors=4 | author=K. Koyama | author2=R. Petre | author3=E.V. Gotthelf| author4=U. Hwang | author5=M. Matsuura| author6=M. Ozaki | author7=S. S. Holt | title=Evidence for shock acceleration of high-energy electrons in the supernova remnant SN1006 | journal=Nature | date=1995 | volume=378 | pages=255–258 | doi= 10.1038/378255a0 |bibcode = 1995Natur.378..255K | issue=6554| s2cid=4257238 | url=https://zenodo.org/record/1233170 }}</ref><ref>{{cite news | title=Supernova produces cosmic rays | work=[[BBC News]] | date=November 4, 2004 | url=http://news.bbc.co.uk/2/hi/science/nature/3981619.stm | access-date=2006-11-28 }}</ref><ref>{{cite web | url = http://imagine.gsfc.nasa.gov/docs/features/topics/snr_group/cosmic_rays.html | archive-url = https://web.archive.org/web/19990221015927/http://imagine.gsfc.nasa.gov/docs/features/topics/snr_group/cosmic_rays.html | url-status = dead | archive-date = 1999-02-21 | title = SNR and Cosmic Ray Acceleration | publisher = NASA Goddard Space Flight Center | access-date = 2007-02-08 }}</ref> 
The connection between cosmic rays and supernovas was first suggested by [[Walter Baade]] and [[Fritz Zwicky]] in 1934.  
[[Vitaly Ginzburg]] and Sergei Syrovatskii in 1964 remarked that if the efficiency of cosmic ray acceleration 
in supernova remnants is about 10 percent, the cosmic ray losses of the Milky Way are compensated.
This hypothesis is supported by a specific mechanism called "shock wave acceleration" based on [[Enrico Fermi]]'s ideas, which is still under development.<ref>{{cite journal | author = S.P. Reynolds | title = Particle acceleration in supernova-remnant shocks | journal=Astrophysics and Space Science | date = 2011 | volume=336 | pages=257–262 | doi= 10.1007/s10509-010-0559-8 |bibcode = 2011Ap&SS.336..257R | issue = 1 | arxiv = 1012.1306 | s2cid = 118735190 }}</ref>

In 1949, Fermi proposed a model for the acceleration of cosmic rays through particle collisions with magnetic clouds in the [[interstellar medium]].<ref>{{cite journal | author = E. Fermi | title = On the Origin of the Cosmic Radiation | journal=Physical Review | date = 1949 | volume=75 | pages=1169–1174 | doi= 10.1103/PhysRev.75.1169 |bibcode = 1949PhRv...75.1169F | issue = 8 }}</ref> This process, known as the "Second Order [[Fermi acceleration|Fermi Mechanism]]", increases particle energy during head-on collisions, resulting in a steady gain in energy. A later model to produce Fermi Acceleration was generated by a powerful shock front moving through space. Particles that repeatedly cross the front of the shock can gain significant increases in energy. This became known as the "First Order Fermi Mechanism".<ref name="uhecr">{{cite web | url = http://www.cosmic-ray.org/reading/uhecr.html | title = Ultra-High Energy Cosmic Rays | publisher = [[University of Utah]] | access-date = 2006-08-10 }}</ref>

Supernova remnants can provide the energetic shock fronts required to generate ultra-high energy cosmic rays. Observation of the [[SN 1006]] remnant in the X-ray has shown [[synchrotron emission]] consistent with it being a source of cosmic rays.<ref name="Koyama" /> However, for energies higher than about 10<sup>18</sup> eV a different mechanism is required as supernova remnants cannot provide sufficient energy.<ref name="uhecr" />

It is still unclear whether supernova remnants accelerate cosmic rays up to PeV energies. The future telescope [[Cherenkov Telescope Array|CTA]] will help to answer this question.

==See also==
{{cmn|colwidth=30em|
* [[List of supernova remnants]]
* [[Local Bubble]]
* [[Nova remnant]]
* [[Planetary nebula]]
* [[Sigma-D relation]]
* [[Superbubble]]
}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category|Supernova remnants}}
* [https://sne.space/?event=snr List of All Known Galactic and Extragalactic Supernovae] on the Open Supernova Catalog (these are not supernova remnants yet)
* [http://www.mrao.cam.ac.uk/surveys/snrs/ Galactic SNR Catalogue] (D. A. Green, University of Cambridge)
* Chandra observations of supernova remnants: [http://hea-www.cfa.harvard.edu/ChandraSNR/ catalog], [http://chandra.harvard.edu/photo/category/snr.html photo album], [https://web.archive.org/web/20020918225120/http://www.astro.psu.edu/users/green/Main/main5.html selected picks]
* [http://www.ipac.caltech.edu/2mass/gallery/images_snrs.html 2MASS images of Supernova Remnants]
* [http://agile.gsfc.nasa.gov/docs/objects/snrs/snrstext.html NASA: Introduction to Supernova Remnants] {{Webarchive|url=https://web.archive.org/web/20070311020249/http://agile.gsfc.nasa.gov/docs/objects/snrs/snrstext.html |date=2007-03-11 }}
* [http://imagine.gsfc.nasa.gov/docs/science/know_l2/supernova_remnants.html NASA's Imagine: Supernova Remnants]
* [http://www.universetoday.com/am/publish/afterlife_supernova.html Afterlife of a Supernova] on UniverseToday.com
* [http://xstructure.inr.ac.ru/x-bin/theme3.py?level=2&index1=59244 Supernova remnant] on arxiv.org
* [http://messier.seds.org/snr.html Supernova Remnants], SEDS

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