Editing Nebula (section)

===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}}