Convection zone

Short description: Region of a star

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.

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.

## Contents

* 1 Main sequence stars
* 2 Red giants
* 3 References
* 4 Further reading
* 5 External links

## Main sequence stars

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 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.[1] In the most massive stars, the convection zone may reach all the way from the core to the surface.[2]

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 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 solar granulation. Low-mass main-sequence stars, such as red dwarfs below 0.35 solar masses,[3] as well as pre-main sequence stars on the Hayashi track, are convective throughout and do not contain a radiation zone.[4]

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 stars, 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.[5]

## References

1. ↑ Behrend, R.; Maeder, A. (2001). "Formation of massive stars by growing accretion rate". Astronomy and Astrophysics 373: 190–198. doi:10.1051/0004-6361:20010585. Bibcode: 2001A&A...373..190B.
2. ↑ Martins, F.; Depagne, E.; Russeil, D.; Mahy, L. (2013). "Evidence of quasi-chemically homogeneous evolution of massive stars up to solar metallicity". Astronomy & Astrophysics 554: A23. doi:10.1051/0004-6361/201321282. Bibcode: 2013A&A...554A..23M.
3. ↑ Reiners, Ansgar; Basri, Gibor (March 2009). "On the magnetic topology of partially and fully convective stars". Astronomy and Astrophysics 496 (3): 787–790. doi:10.1051/0004-6361:200811450. Bibcode: 2009A&A...496..787R.
4. ↑ d'Antona, F.; Montalbán, J. (2003). "Efficiency of convection and Pre-Main Sequence lithium depletion". Astronomy and Astrophysics 212: 213–218. doi:10.1051/0004-6361:20031410. Bibcode: 2003A&A...412..213D.
5. ↑ Lebzelter, T.; Lederer, M. T.; Cristallo, S.; Hinkle, K. H.; Straniero, O.; Aringer, B. (2008). "AGB stars of the intermediate-age LMC cluster NGC 1846". Astronomy and Astrophysics 486 (2): 511. doi:10.1051/0004-6361:200809363. Bibcode: 2008A&A...486..511L.

## Further reading

* Hansen, Carl J.; Kawaler, Steven D.; Trimble, Virginia (1994). Stellar Interiors: physical principles, structure and evolution. Astronomy and Astrophysics Library (2nd ed.). Berlin: Springer. ISBN 978-0-387-20089-7. https://archive.org/details/springer_10.1007-978-1-4419-9110-2.
* Zeilik, Michael; Gregory, Stephen A. (1998). Introductory astronomy and astrophysics (4th ed.). Australia: Brooks/Cole. ISBN 978-0-03-006228-5.

## External links

* Animated explanation of the Convection zone (University of South Wales).
* Animated explanation of the temperature and density of the Convection zone (University of South Wales).

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The Sun

Internal structure|

* Core
* Radiation zone
* Tachocline
* Convection zone

Atmosphere|

| Photosphere|

* Supergranulation
* Granule
* Faculae
* Sunspot

|

Chromosphere|

* Plage
* Spicule
* Moreton wave

Corona|

* Transition region
* Coronal hole
* Coronal loop
* Coronal mass ejection
* Prominence
* Helmet streamer
* Supra-arcade downflows

Variation|

* Solar cycle
* List of solar cycles
* Solar maximum
* Solar minimum
* Wolf number
* Solar wind
* Flare
* Helioseismology

Heliosphere|

* Current sheet
* Termination shock
* Heliosheath
* Heliopause
* Bow shock

Related|

* Heliophysics
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* Solar dynamo
* Eclipse
* Solar energy
* Solar neutrino
* Solar observation
* Solar phenomena
* Solar physics
* Solar System
* Solar telescope
* Solar time
* Space climate
* Space weather
* Standard solar model
* Star
* Sunlight radiation

* Spectral class: G-type main-sequence star

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Stars

Formation|

* Accretion
* Molecular cloud
* Bok globule
* Young stellar object
* Protostar
* Pre-main-sequence
* Herbig Ae/Be
* T Tauri
* FU Orionis
* Herbig–Haro object
* Hayashi track
* Henyey track

Evolution|

* Main sequence
* Red-giant branch
* Horizontal branch
* Red clump
* Asymptotic giant branch
* Blue loop
* Protoplanetary nebula
* Planetary nebula
* PG1159
* Dredge-up
* Instability strip
* Luminous blue variable
* Blue straggler
* Stellar population
* Supernova
* Superluminous supernova / Hypernova

Spectral
classification|

* Early
* Late
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* O
* B
* A
* F
* G
* K
* M
* Brown dwarf
* WR
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* Lead
* Technetium
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* Shell
* B[e]

Remnants|

* White dwarf
* Helium planet
* Black dwarf
* Neutron
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* Binary
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* Stellar black hole
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Hypothetical|

* Blue dwarf
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* Black dwarf
* Exotic
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* Quark
* Q
* Black
* Gravastar
* Frozen
* Quasi-star
* Thorne–Żytkow object
* Iron
* Blitzar

Stellar
nucleosynthesis|

* Deuterium burning
* Lithium burning
* Proton–proton chain
* CNO cycle
* Helium flash
* Triple-alpha process
* Alpha process
* Carbon burning
* Neon burning
* Oxygen burning
* Silicon burning
* S-process
* R-process
* Fusor
* Nova
* Symbiotic
* Remnant
* Luminous red nova

Structure|

* Core
* Convection zone
* Microturbulence
* Oscillations
* Radiation zone
* Atmosphere
* Photosphere
* Starspot
* Chromosphere
* Corona
* Stellar wind
* Bubble
* Bipolar outflow
* Accretion disk
* Asteroseismology
* Helioseismology
* Eddington luminosity
* Kelvin–Helmholtz mechanism

Properties|

* Designation
* Dynamics
* Effective temperature
* Luminosity
* Kinematics
* Magnetic field
* Absolute magnitude
* Mass
* Metallicity
* Rotation
* Starlight
* Variable
* Photometric system
* Color index
* Hertzsprung–Russell diagram
* Color–color diagram

Star systems|

* Binary
* Contact
* Common envelope
* Eclipsing
* Symbiotic
* Multiple
* Cluster
* Open
* Globular
* Super
* Planetary system

Earth-centric
observations|

* Sun
* Solar System
* Sunlight
* Pole star
* Circumpolar
* Constellation
* Asterism
* Magnitude
* Apparent
* Extinction
* Photographic
* Radial velocity
* Proper motion
* Parallax
* Photometric-standard

Lists|

* Proper names
* Arabic
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* Extremes
* Most massive
* Highest temperature
* Largest volume
* Smallest volume
* Brightest
* Historical
* Most luminous
* Nearest
* Nearest bright
* With exoplanets
* Brown dwarfs
* White dwarfs
* Milky Way novae
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* Timeline of stellar astronomy

Related articles|

* Substellar object
* Brown dwarf
* Sub-brown dwarf
* Planet
* Galactic year
* Galaxy
* Guest
* Gravity
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* Starfield
* Tidal disruption event

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