Editing Planetary nebula (section)

===Numbers and distribution===
About 3000 planetary nebulae are now known to exist in our galaxy,<ref name="Parkeretal2006">{{harvnb|Parker|Acker|Frew|Hartley|2006|pp=79–94}}</ref> out of 200 billion stars. Their very short lifetime compared to total stellar lifetime accounts for their rarity. They are found mostly near the plane of the [[Milky Way]], with the greatest concentration near the [[Galactic Center]].<ref name=majaess2007>{{harvnb|Majaess|Turner|Lane|2007|pp=1349–60}}</ref>