Editing Planetary nebula (section)

==Current issues in planetary nebula studies==

The distances to planetary nebulae are generally poorly determined,<ref>{{cite news|title=Distances to Planetary Nebulae|author=R. Gathier|url= https://www.eso.org/sci/publications/messenger/archive/no.32-jun83/messenger-no32-20-22.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.eso.org/sci/publications/messenger/archive/no.32-jun83/messenger-no32-20-22.pdf |archive-date=2022-10-09 |url-status=live |access-date=31 May 2014|newspaper=ESO Messenger}}</ref> but the ''[[Gaia (spacecraft)|Gaia]]'' mission is now measuring direct [[Stellar parallax|parallactic distances]] between their central stars and neighboring stars.<ref>{{Cite web|url=http://simbad.u-strasbg.fr/simbad/sim-ref?bibcode=2018yCat.1345....0G|title=SIMBAD references}}</ref> It is also possible to determine distances to nearby planetary nebula by measuring their expansion rates. High resolution observations taken several years apart will show the expansion of the nebula perpendicular to the line of sight, while spectroscopic observations of the [[Doppler shift]] will reveal the velocity of expansion in the line of sight. Comparing the angular expansion with the derived velocity of expansion will reveal the distance to the nebula.<ref name=Reed1999/>

The issue of how such a diverse range of nebular shapes can be produced is a debatable topic. It is theorised that interactions between material moving away from the star at different speeds gives rise to most observed shapes.<ref name=Kwok8/> However, some astronomers postulate that close binary central stars might be responsible for the more complex and extreme planetary nebulae.<ref name="Soker2002">{{harvnb|Soker|2002|pp=481–6}}</ref> Several have been shown to exhibit strong magnetic fields,<ref name=Gurzadyan4>{{harvnb|Gurzadyan|1997|p=424}}</ref> and their interactions with ionized gas could explain some planetary nebulae shapes.<ref name="Jordanetal2005">{{harvnb|Jordan|Werner|O'Toole|2005|pp=273–9}}</ref>

There are two main methods of determining [[metallicity|metal abundances]] in nebulae. These rely on recombination lines and collisionally excited lines. Large discrepancies are sometimes seen between the results derived from the two methods. This may be explained by the presence of small temperature fluctuations within planetary nebulae. The discrepancies may be too large to be caused by temperature effects, and some hypothesize the existence of cold knots containing very little hydrogen to explain the observations. However, such knots have yet to be observed.<ref name="Liuetal2000">{{harvnb|Liu|Storey|Barlow|Danziger|2000|pp=585–587}}</ref>