Editing Nebula (section)

==Types==

<gallery mode="packed" class="center">
File:Hubble Sees a Stellar "Sneezing Fit" (11467249715).jpg|[[Herbig–Haro object|Herbig–Haro]] [[HH 161]] and [[HH 164]].<ref>{{cite news|title=A stellar sneezing fit|url=http://www.spacetelescope.org/images/potw1350a/|access-date=16 December 2013|newspaper=ESA/Hubble Picture of the Week}}</ref>
File:Omega Nebula.jpg|The [[Omega Nebula]], an example of an [[emission nebula]]
File:Horsehead-Hubble.jpg|The [[Horsehead Nebula]], an example of a [[dark nebula]].
File:NGC6543.jpg|The [[Cat's Eye Nebula]], an example of a [[planetary nebula]].
File:PIA04533.jpg|The [[Red Rectangle Nebula]], an example of a [[protoplanetary nebula]].
File:SNR 0509.jpg|The delicate shell of [[SNR 0509-67.5|SNR B0509-67.5]]
File:Tycho xrayonly.jpg|[[SN 1572|Tycho Supernova remnant in X-ray light]]
File:Southern Ring Nebula by Webb Telescope (2022).jpg|[[NGC 3132|Southern Ring Nebula]], Planetary Nebula
File:Webb captures detailed beauty of Ring Nebula (NIRCam and MIRI images) (weic2320a).jpg|[[Ring Nebula]] in the northern constellation of [[Lyra]]
</gallery>

===Classical types===
Objects named nebulae belong to four major groups.  Before their nature was understood, [[galaxy|galaxies]] ("spiral nebulae") and [[star cluster]]s too distant to be resolved as stars were also classified as nebulae, but no longer are.
* [[H II region]]s, large diffuse nebulae containing ionized hydrogen
* [[Planetary nebula]]e
* [[Supernova remnant]]s (e.g., Crab Nebula)
* [[Dark nebula]]e
Not all cloud-like structures are nebulae; [[Herbig–Haro object]]s are an example.

===Flux Nebulae===
{{Main|Integrated Flux Nebula}}
{{excerpt|Integrated Flux Nebula}}

===Diffuse nebulae===

[[Image: Carina Nebula by ESO.jpg|thumb|The Carina Nebula is an example of a diffuse nebula]]

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.<ref name="Messier">{{cite web|url=http://messier.seds.org/diffuse.html|publisher=SEDS|title=The Messier Catalog: Diffuse Nebulae|access-date=2007-06-12|archive-url=https://web.archive.org/web/19961225125109/http://messier.seds.org/diffuse.html|archive-date=1996-12-25}}</ref> Diffuse nebulae can be divided into [[emission nebula]]e, [[reflection nebula]]e and [[dark nebula]]e.

Visible light nebulae may be divided into emission nebulae, which emit [[spectral line]] radiation from excited or [[ion]]ized gas (mostly ionized [[hydrogen]]);<ref name="shu1982">{{cite book
  | author = F. H. Shu
  | title = The Physical Universe
  | publisher = University Science Books
  | date = 1982
  | location = Mill Valley, California
  | isbn = 0-935702-05-9
  | url-access = registration
  | url = https://archive.org/details/physicaluniverse00shuf
  }}</ref> they are often called [[H II region]]s, H II referring to ionized hydrogen), and reflection nebulae which are visible primarily due to the light they reflect.

Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them.<ref name="shu1982" /> Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called [[dark nebula]]e.<ref name="shu1982" />

Although these nebulae have different visibility at optical wavelengths, they are all bright sources of [[infrared]] emission, chiefly from [[cosmic dust|dust]] within the nebulae.<ref name="shu1982" />

===Planetary nebulae===
{{Main|Planetary nebula}}
[[File:A hazy nebula.jpg|thumb|The [[NGC 1501|Oyster Nebula]] is a [[planetary nebula]] located in the constellation of [[Camelopardalis]]]]
Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars (varying in size between 0.5-~8 solar masses). Evolved [[asymptotic giant branch]] stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells while leaving behind the star's core in the form of a [[white dwarf]].<ref name="shu1982" /> Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in [[star formation]] regions.<ref name="shu1982" /> They are [[H II region]]s, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions.<ref name="shu1982" />

Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, which were of more interest to them. The Sun is expected to spawn a planetary nebula about 12&nbsp;billion years after its formation.<ref name="s">{{cite book
  | author = Chaisson, E. 
  | author2 = McMillan, S. 
  | title = Astronomy: a beginner's guide to the universe
  | edition= 2nd
  | publisher = Prentice-Hall
  | date = 1995
  | location = Upper Saddle River, New Jersey
  | isbn = 0-13-733916-X}}</ref>

====Protoplanetary nebulae====
{{Main|Protoplanetary nebula}}
{{excerpt|Protoplanetary nebula}}

===Supernova remnants===

[[Image:Crab Nebula.jpg|thumb|The [[Crab Nebula]], an example of a [[supernova remnant]]]]

A [[supernova]] occurs when a high-mass star reaches the end of its life.  When [[nuclear fusion]] in the core of the star stops, the star collapses.  The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.<ref name="shu1982"/> The expanding shell of gas forms a [[supernova remnant]], a special [[diffuse nebula]].<ref name="shu1982"/> Although much of the optical and [[X-ray]] emission from supernova remnants originates from ionized gas, a great amount of the [[radio wave|radio]] emission is a form of non-thermal emission called [[synchrotron emission]].<ref name="shu1982"/> This emission originates from high-velocity [[electron]]s oscillating within [[magnetic field]]s.