'''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 ==
Starting with the time of [[Edwin Hubble]], the brightest stars have been employed as [[Extragalactic astronomy|extragalactic]] distance indicators. However, it was not until the early 1960s that planetary nebula (PNe) were recognized as being some of the “brightest stars” and consequently useful as extragalactic distance indicators. During the early stages of their evolution, a planetary nebula's luminosity is on par with their [[asymptotic giant branch]] (AGB) ancestors. Even though most of their continuum [[Emission (electromagnetic radiation)|emission]] emerges in the [[far-ultraviolet]], rather than the [[visible light|optical]] or [[near infrared]], their detectability is not hampered. In fact, since most of the [[central star|central star's]] [[flux]] is emitted at energies below 13.6 [[electron volt|eV]], the [[photoionization]] physics ensures that their energy is transformed into a series of optical, [[infrared]], and [[near-ultraviolet|near-UV]] [[emission line]]s. Fortuitously, approximately 10% of the flux emitted by a young PNe is in the one emission line of [[doubly-ionized oxygen]] at 5007 [[Ångström|Å]]. Therefore, for the purposes of [[cosmology]], a PNe may be thought of a cosmic machine that turns continuum emission into monochromatic [[flux]].
It was not until late 1970s that the initial PNe derived distance estimates were computed. The first study of the PNLF was {{harvnb|Jacoby|1989}}. Ironically, the technique was first applied to galaxies outside our [[Local Group]] before being applied to it. The reason for this odd order of adoption is because any one PNe is not a standard candle and that distance estimates to individual PNe within our own galaxy are very inaccurate, an error factor of 2<ref name="Ciardullo2003" /><ref name="Jacoby1989">{{harvnb|Jacoby|1989}}</ref> being normal. However, by way of sampling a large number of PNe, one may apply the PNLF to produce accurate distance estimates to galaxies. Because PNe are found in all galaxies, the PNLF is unique in that it may be utilized to estimate distances to all large galaxies within the [[Local Supercluster]] independent of their environment and [[Hubble type]].<ref name="Ciardullo2004">{{harvnb|Ciardullo|2004}}</ref>
