Editing Convection zone

{{Short description|Region of a star}}
[[File:Structure of Stars (artist’s impression).jpg|thumb|upright=1.5|An illustration of the structure of the [[Sun]] and a [[red giant]] star, showing their convective zones. These are the granular zones in the outer layers of the stars.]]

A '''convection zone''', '''convective zone''' or '''convective region''' of a [[star]] is a layer which is unstable due to convection. Energy is primarily or partially transported by [[convection]] in such a region. In a [[radiation zone]], energy is transported by [[radiation]] and [[conduction (heat)|conduction]].

Stellar convection consists of mass movement of plasma within the star which usually forms a circular convection current with the heated plasma ascending and the cooled plasma descending.

The [[Schwarzschild criterion]] expresses the conditions under which a region of a star is unstable to convection. A parcel of gas that rises slightly will find itself in an environment of lower pressure than the one it came from. As a result, the parcel will expand and cool. If the rising parcel cools to a lower temperature than its new surroundings, so that it has a higher density than the surrounding gas, then its lack of buoyancy will cause it to sink back to where it came from. However, if the [[temperature]] [[gradient]] is steep enough (i.e. the temperature changes rapidly with distance from the center of the star), or if the gas has a very high [[heat capacity]] (i.e. its temperature changes relatively slowly as it expands) then the rising parcel of gas will remain warmer and less dense than its new surroundings even after expanding and cooling. Its buoyancy will then cause it to continue to rise. The region of the star in which this happens is the convection zone.

==Main sequence stars==
{{See also|Radiation zone#Stability against convection}}
In [[main sequence stars]] more than 1.3 times the mass of the Sun, the high core temperature causes [[nuclear fusion]] of [[hydrogen]] into [[helium]] to occur predominantly via the [[CNO cycle|carbon-nitrogen-oxygen (CNO) cycle]] instead of the less temperature-sensitive [[proton–proton chain]].  The high temperature gradient in the core region forms a convection zone that slowly mixes the hydrogen fuel with the helium product. The core convection zone of these stars is overlaid by a [[radiation zone]] that is in thermal equilibrium and undergoes little or no mixing.<ref>{{cite journal|doi=10.1051/0004-6361:20010585|arxiv = astro-ph/0105054 |bibcode = 2001A&A...373..190B |title = Formation of massive stars by growing accretion rate |journal = Astronomy and Astrophysics |volume = 373 |pages = 190–198 |year = 2001 |last1 = Behrend |first1 = R. |last2 = Maeder |first2 = A. |s2cid = 18153904 }}</ref>  In the most massive stars, the convection zone may reach all the way from the core to the surface.<ref>{{cite journal|doi=10.1051/0004-6361/201321282|arxiv = 1304.3337 |bibcode = 2013A&A...554A..23M |title = Evidence of quasi-chemically homogeneous evolution of massive stars up to solar metallicity |journal = Astronomy & Astrophysics |volume = 554 |pages = A23 |year = 2013 |last1 = Martins |first1 = F. |last2 = Depagne |first2 = E. |last3 = Russeil |first3 = D. |last4 = Mahy |first4 = L. |s2cid = 54707309 }}</ref>

In main sequence stars of less than about 1.3 solar masses, the outer envelope of the star contains a region where partial [[ionization]] of [[hydrogen]] and [[helium]] raises the heat capacity.  The relatively low temperature in this region simultaneously causes the [[Opacity (optics)|opacity]] due to heavier elements to be high enough to produce a steep temperature gradient. This combination of circumstances produces an outer convection zone, the top of which is visible in the Sun as [[Granule (solar physics)|solar granulation.]] Low-mass main-sequence stars, such as [[red dwarf]]s below 0.35 [[solar mass]]es,<ref name=aaa496_3_787>{{cite journal
 | last1=Reiners | first1=Ansgar | last2=Basri | first2=Gibor
 | title=On the magnetic topology of partially and fully convective stars
 | journal=Astronomy and Astrophysics | volume=496 | issue=3 | pages=787–790
 |date=March 2009 | doi=10.1051/0004-6361:200811450
 | bibcode=2009A&A...496..787R |arxiv = 0901.1659 | s2cid=15159121 }}</ref> as well as pre-main sequence stars on the [[Hayashi track]], are convective throughout and do not contain a radiation zone.<ref name=antona>{{cite journal|doi=10.1051/0004-6361:20031410 |arxiv = astro-ph/0309348 |bibcode = 2003A&A...412..213D |title = Efficiency of convection and Pre-Main Sequence lithium depletion |journal = Astronomy and Astrophysics |volume = 212 |pages = 213–218 |year = 2003 |last1 = d'Antona |first1 = F. |last2 = Montalbán |first2 = J. |s2cid = 2590382 }}</ref>

In main sequence stars similar to the Sun, which have a radiative core and convective envelope,  the transition region between the convection zone and the [[radiation zone]] is called the [[tachocline]].

==Red giants==
In [[red giant star]]s, and particularly during the [[asymptotic giant branch]] phase, the surface convection zone varies in depth during the phases of shell burning.  This causes [[dredge-up]] events, short-lived very deep convection zones that transport fusion products to the surface of the star.<ref>{{cite journal|doi=10.1051/0004-6361:200809363|arxiv = 0805.3242 |bibcode = 2008A&A...486..511L |title = AGB stars of the intermediate-age LMC cluster NGC 1846 |journal = Astronomy and Astrophysics |volume = 486 |issue = 2 |pages = 511 |year = 2008 |last1 = Lebzelter |first1 = T. |last2 = Lederer |first2 = M. T. |last3 = Cristallo |first3 = S. |last4 = Hinkle |first4 = K. H. |last5 = Straniero |first5 = O. |last6 = Aringer |first6 = B. |s2cid = 18811290 }}</ref>

==References==
{{reflist}}

==Further reading==
*{{Cite book |last=Hansen |first=Carl J. |url=https://archive.org/details/springer_10.1007-978-1-4419-9110-2 |title=Stellar Interiors: physical principles, structure and evolution |last2=Kawaler |first2=Steven D. |last3=Trimble |first3=Virginia |date=1994 |publisher=Springer |isbn=978-0-387-20089-7 |edition=2nd |series=Astronomy and Astrophysics Library |location=Berlin |name-list-style=amp}}
*{{Cite book |last1=Zeilik |first1=Michael |title=Introductory astronomy and astrophysics |last2=Gregory |first2=Stephen A. |date=1998 |publisher=Brooks/Cole |isbn=978-0-03-006228-5 |edition=4th |location=Australia}}

==External links==
* [http://alienworlds.southwales.ac.uk/sunStructure.html#/convectionzone Animated explanation of the Convection zone] {{Webarchive|url=https://web.archive.org/web/20151116133527/http://alienworlds.southwales.ac.uk/sunStructure.html#/convectionzone |date=2015-11-16 }} (University of South Wales).
* [http://alienworlds.southwales.ac.uk/sunStructure.html#/convectiontempden Animated explanation of the temperature and density of the Convection zone] {{Webarchive|url=https://web.archive.org/web/20151116133527/http://alienworlds.southwales.ac.uk/sunStructure.html#/convectiontempden |date=2015-11-16 }} (University of South Wales).

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[[Category:Convection|Zone]]
[[Category:Stellar phenomena]]