Editing Planetary nebula (section)

==Membership in clusters==
[[File:Abell 78.jpg|thumb|[[Abell 78]], 24 inch telescope on Mt. Lemmon, Arizona.]]

Planetary nebulae have been detected as members in four Galactic [[globular clusters]]: [[Messier 15]], [[Messier 22]], [[NGC 6441]] and [[Palomar 6]]. Evidence also points to the potential discovery of planetary nebulae in globular clusters in the galaxy [[Andromeda Galaxy|M31]].<ref name=ja2013>Jacoby, George H.; Ciardullo, Robin; [[Orsola De Marco|De Marco, Orsola]]; Lee, Myung Gyoon; Herrmann, Kimberly A.; Hwang, Ho Seong; Kaplan, Evan; Davies, James E., (2013). [http://esoads.eso.org/abs/2013ApJ...769...10J ''A Survey for Planetary Nebulae in M31 Globular Clusters''], ApJ, 769, 1</ref> However, there is currently only one case of a planetary nebula discovered in an [[open cluster]] that is agreed upon by independent researchers.<ref name=fr2008>Frew, David J. (2008). [http://esoads.eso.org/abs/2008PhDT.......109F ''Planetary Nebulae in the Solar Neighbourhood: Statistics, Distance Scale and Luminosity Function''], PhD Thesis, Department of Physics, Macquarie University, Sydney, Australia</ref><ref name=parker2011>{{harvnb|Parker|2011|pp=1835–1844}}</ref><ref name=ma2014>Majaess, D.; Carraro, G.; Moni Bidin, C.; Bonatto, C.; Turner, D.; Moyano, M.; Berdnikov, L.; Giorgi, E., (2014). [http://esoads.eso.org/abs/2014A%26A...567A...1M ''On the crucial cluster Andrews-Lindsay 1 and a 4% distance solution for its planetary nebula''], A&A, 567</ref> That case pertains to the planetary nebula PHR 1315-6555 and the open cluster Andrews-Lindsay 1. Indeed, through cluster membership, PHR 1315-6555 possesses among the most precise distances established for a planetary nebula (i.e., a 4% distance solution). The cases of [[NGC 2818]] and NGC 2348 in [[Messier 46]], exhibit mismatched velocities between the planetary nebulae and the clusters, which indicates they are line-of-sight coincidences.<ref name=majaess2007/><ref name=kiss2008>{{harvnb|Kiss|Szabó|Balog|Parker|2008|pp=399–404}}</ref><ref name=mermilliod2001>{{harvnb|Mermilliod|Clariá|Andersen|Piatti|2001|pp=30–9}}</ref> A subsample of ''tentative'' cases that may potentially be cluster/PN pairs includes Abell 8 and Bica 6,<ref name=bo2008>Bonatto, C.; Bica, E.; Santos, J. F. C., (2008). [http://esoads.eso.org/abs/2008MNRAS.386..324B ''Discovery of an open cluster with a possible physical association with a planetary nebula''], MNRAS, 386, 1</ref><ref name=tu2011>Turner, D. G.; Rosvick, J. M.; Balam, D. D.; Henden, A. A.; Majaess, D. J.; Lane, D. J. (2011). [http://esoads.eso.org/abs/2011PASP..123.1249T ''New Results for the Open Cluster Bica 6 and Its Associated Planetary Nebula Abell 8''], PASP, 123, 909</ref> and He 2-86 and NGC 4463.<ref name=mo2014>Moni Bidin, C.; Majaess, D.; Bonatto, C.; Mauro, F.; Turner, D.; Geisler, D.; Chené, A.-N.; Gormaz-Matamala, A. C.; Borissova, J.; Kurtev, R. G.; Minniti, D.; Carraro, G.; Gieren, W. (2014). [http://esoads.eso.org/abs/2014A%26A...561A.119M ''Investigating potential planetary nebula/cluster pairs''], A&A, 561</ref>

Theoretical models predict that planetary nebulae can form from [[main-sequence]] stars of between one and eight solar masses, which puts the progenitor star's age at greater than 40 million years. Although there are a few hundred known open clusters within that age range, a variety of reasons limit the chances of finding a planetary nebula within.<ref name=majaess2007/> For one reason, the planetary nebula phase for more massive stars is on the order of millennia, which is a blink of the eye in astronomic terms. Also, partly because of their small total mass, open clusters have relatively poor gravitational cohesion and tend to disperse after a relatively short time, typically from 100 to 600 million years.<ref name=Allison>{{harvnb|Allison|2006|pp=56–8}}</ref>