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'''Planetary Nebula Luminosity Function''' ('''PNLF''') is a secondary<ref name="Ferrareseetal2000" /> [[standard candle|distance indicator]] used in [[astronomy]]. It makes use of the [[O III|[O III]]] λ5007 [[forbidden line]] found in all [[planetary nebula]] which are members of the old stellar populations ([[Population II]]).<ref name="Ferrareseetal2000">{{harvnb|Ferrarese|Ford|Huchra|Kennicutt|2000}}</ref>
It works well for both [[spiral galaxy|spiral]] and [[elliptical galaxy|elliptical]] galaxies despite their completely different [[stellar populations]] and is part of the [[Extragalactic Distance Scale]].<ref name="=Schoenberneretal2007">{{harvnb|Schoenberner|Jacob|Steffen|Sandin|2007}}</ref>
== History and background ==
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