Planetary nebula luminosity function

Planetary nebula luminosity function (PNLF) is a secondary^[1] distance indicator used in astronomy. It makes use of the [O III] λ5007 forbidden line found in all planetary nebula (PNe) which are members of the old stellar populations (Population II).^[1] It can be used to determine distances to both spiral and elliptical galaxies despite their completely different stellar populations and is part of the Extragalactic Distance Scale.^[2]