Editing Planetary nebula (section)

==Role in galactic enrichment==
[[File:An_Interstellar_Distributor.jpg|thumb| ESO 455-10 is a [[planetary nebulae|planetary nebula]] located in the constellation of [[Scorpius]] (The Scorpion).<ref>{{cite web|title=An Interstellar Distributor|url=https://esahubble.org/images/potw2104a/|work=Picture of the Week|publisher=ESA/Hubble|access-date=29 January 2020}}</ref>]]

Planetary nebulae may play a very important role in galactic evolution. Newly born stars consist almost entirely of [[hydrogen]] and [[helium]],<ref>{{cite web| author=W. Sutherland| url=http://www.maths.qmul.ac.uk/~wjs/MTH726U/chap4.pdf| title=The Galaxy. Chapter 4. Galactic Chemical Evolution| date=26 March 2013| access-date=13 January 2015}}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> but as stars evolve through the [[asymptotic giant branch]] phase,<ref>{{Cite journal | last1 = Sackmann | first1 = I. -J. | last2 = Boothroyd | first2 = A. I. | last3 = Kraemer | first3 = K. E. | title = Our Sun. III. Present and Future | doi = 10.1086/173407 | journal = The Astrophysical Journal | volume = 418 | pages = 457 | year = 1993 |bibcode = 1993ApJ...418..457S | doi-access = free }}</ref> they create heavier elements via [[nuclear fusion]] which are eventually expelled by strong [[stellar wind]]s.<ref>{{cite journal| bibcode=1975ApJ...200L.107C |last1= Castor|first1=J. |last2=McCray|first2=R. |last3=Weaver|first3=R. | title=Interstellar Bubbles| date=1975| journal=Astrophysical Journal Letters| volume=200| pages=L107–L110| doi=10.1086/181908| doi-access=free}}</ref> Planetary nebulae usually contain larger proportions of elements such as [[carbon]], [[nitrogen]] and [[oxygen]], and these are recycled into the interstellar medium via these powerful winds. In this way, planetary nebulae greatly enrich the [[Milky Way]] and their [[nebula]]e with these heavier elements – collectively known by astronomers as ''metals'' and specifically referred to by the [[metallicity|metallicity parameter]] ''Z''.<ref name=Kwok19>{{harvnb|Kwok|2000|pp=199–207}}</ref>

Subsequent generations of stars formed from such nebulae also tend to have higher metallicities. Although these metals are present in stars in relatively tiny amounts, they have marked effects on [[stellar evolution]] and fusion reactions. When stars formed earlier in the [[universe]] they theoretically contained smaller quantities of heavier elements.<ref name=PopIII>{{cite journal|last=Pan|first=Liubin|author2=Scannapieco, Evan|author3= Scalo, Jon|s2cid=119233184|title=Modeling the Pollution of Pristine Gas in the Early Universe|journal=The Astrophysical Journal|date=1 October 2013|volume=775|issue=2|page=111|doi=10.1088/0004-637X/775/2/111|arxiv = 1306.4663 |bibcode = 2013ApJ...775..111P }}</ref> Known examples are the metal poor [[Population II]] stars. (See [[Stellar population]].)<ref name=Marochnik>{{harvnb|Marochnik|Shukurov|Yastrzhembsky|1996|pp=6–10}}</ref><ref name=Gregory>{{cite book|last2=Gregory|first2=Stephen A. |first1=Michael |last1=Zeilik|title=Introductory astronomy & astrophysics|date=1998|publisher=Saunders College Publishing|location=Fort Worth [u.a.]|isbn=0-03-006228-4|page=322|edition=4.}}</ref> Identification of stellar metallicity content is found by [[Astronomical spectroscopy|spectroscopy]].