Editing Supernova remnant (section)

==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]]}}