Editing Planetary nebula (section)

===Morphology===
[[File:Artist’s view of how a planetary nebula’s wobbling jets are sculpted.ogv|thumb|300px|This animation shows how the two stars at the heart of a planetary nebula like [[Fleming 1]] can control the creation of the spectacular jets of material ejected from the object.]]

Only about 20% of planetary nebulae are spherically symmetric (for example, see [[Abell 39]]).<ref name=Jacoby2001>{{harvnb|Jacoby|Ferland|Korista|2001|pp=272–86}}</ref> A wide variety of shapes exist with some very complex forms seen. Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, [[bipolar nebula|bipolar]], quadrupolar,<ref name=KwoketalDec2005>{{harvnb|Kwok|Su|2005|pp=L49–52}}</ref> and other types,<ref name=Kwok8>{{harvnb|Kwok|2000|pp=89–96}}</ref> although the majority of them belong to just three types: spherical, elliptical and bipolar. Bipolar nebulae are concentrated in the [[galactic plane]], probably produced by relatively young massive progenitor stars; and bipolars in the [[galactic bulge]] appear to prefer orienting their orbital axes parallel to the galactic plane.<ref>{{harvnb|Rees|Zijlstra|2013}}</ref> On the other hand, spherical nebulae are probably produced by old stars similar to the Sun.<ref name=Osterbrock/>

The huge variety of the shapes is partially the projection effect—the same nebula when viewed under different angles will appear different.<ref>{{cite journal | first = Z | last = Chen |author2=A. Frank|author3=E. G. Blackman|author4=J. Nordhaus|author5=J. Carroll-Nellenback| s2cid = 119073723 | title = Mass Transfer and Disc Formation in AGB Binary Systems| journal = Monthly Notices of the Royal Astronomical Society | volume = 468 | issue = 4 | pages = 4465 | date = 2017 | doi = 10.1093/mnras/stx680 | doi-access = free |arxiv = 1702.06160 |bibcode = 2017MNRAS.468.4465C }}</ref> Nevertheless, the reason for the huge variety of physical shapes is not fully understood.<ref name=Kwok8/> Gravitational interactions with companion stars if the central stars are [[binary star]]s may be one cause. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae.<ref name=Morris>{{harvnb|Morris|1990|pp=526–30}}</ref> In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae,<ref name="Jordanetal2005" /> predicted already in 1960s,<ref>{{cite book | last1 = Gurzadyan | first1 = Grigor | year = 1969 | title = Planetary Nebulae| publisher= Gordon & Breach |location=New York |isbn= 90-277-0117-2}}</ref> and hypothesized that the fields might be partly or wholly responsible for their remarkable shapes.<ref>{{harvnb|SpaceDaily Express|2005}}</ref>