Gravity darkening

Not to be confused with gravitational redshift.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "Gravity darkening" – news · newspapers · books · scholar · JSTOR (November 2024) (Learn how and when to remove this message)

|

Achernar is a rapidly spinning star flattened into an oblate spheroid hotter and brighter at the poles.

Gravity darkening, also referred to as gravity brightening, is an astronomical phenomenon where the poles of a star are brighter than the equator, due to rapid rotation and oblate shape. When a star is oblate, it has a larger radius at its equator than it does at its poles. As a result, the poles have a higher surface gravity, and thus higher temperature and pressure is needed to maintain hydrostatic equilibrium. Thus, the poles are "gravity brightened", and the equator "gravity darkened".[1]

The star becomes oblate (and hence gravity darkening occurs) because the centrifugal force resulting from rotation creates additional outward pressure on the star. The centrifugal force is expressed mathematically as

F centrifugal = m Ω 2 ρ , {\displaystyle F_{\text{centrifugal}}=m\Omega ^{2}\rho ,}

where  m {\displaystyle m} is mass (in this case of a small volume element of the star),  Ω {\displaystyle \Omega } is the angular velocity, and  ρ {\displaystyle \rho } is the radial distance from the axis of rotation. In the case of a star, the value of  ρ {\displaystyle \rho } is largest at the equator and smallest at the poles. This means that equatorial regions of a star have a greater centrifugal force than the pole. The centrifugal force pushes mass away from the axis of rotation, resulting in less overall pressure on the gas in the equatorial regions of the star. This causes the gas in this region to become less dense, and cooler.

Von Zeipel's theorem states that the radiation from a star is proportional to the local effective gravity, that is to the gravity reduced by any centrifugal force at that location on the star's surface. Then the effective temperature is proportional to the fourth root of the effective gravity.[2]

## References

[edit]

1. ^ Inglis-Arkell, Esther. "The Mysterious Phenomenon of "Gravity Darkening"". io9. Archived from the original on 1 July 2015. Retrieved 28 June 2015.
2. ^ Lucy, L. B. (1967). "Gravity-Darkening for Stars with Convective Envelopes". Zeitschrift für Astrophysik. 65: 89. Bibcode:1967ZA.....65...89L.

* v
* t
* e

Stars

* List

Formation|

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

Evolution|

* Main sequence
* Red-giant branch
* Horizontal branch
* Red clump
* Asymptotic giant branch
* post-AGB
* super-AGB
* Blue loop
* Planetary nebula
* Protoplanetary
* Wolf–Rayet nebula
* PG1159
* Dredge-up
* OH/IR
* Instability strip
* Luminous blue variable
* Stellar population
* Supernova
* Superluminous
* Hypernova

Classification|

* Early
* Late
* Main sequence
* O
* B
* A
* F
* G
* K
* M
* Subdwarf
* O
* B
* WR
* OB
* Subgiant
* Giant
* Blue
* Red
* Yellow
* Bright giant
* Supergiant
* Blue
* Red
* Yellow
* Hypergiant
* Yellow
* Carbon
* S
* CN
* CH
* White dwarf
* Chemically peculiar
* Am
* Ap/Bp
* CEMP
* HgMn
* He-weak
* Barium
* Lambda Boötis
* Lead
* Technetium
* Be
* Shell
* B[e]
* Helium
* Extreme
* Blue straggler

Remnants|

* Compact star
* Parker's star
* White dwarf
* Helium planet
* Neutron
* Radio-quiet
* Pulsar
* Binary
* X-ray
* Magnetar
* Stellar black hole
* X-ray binary
* Burster
* SGR

Hypothetical|

* Blue dwarf
* Black dwarf
* Exotic
* Boson
* Electroweak
* Strange
* Preon
* Planck
* Dark
* Dark-energy
* Quark
* Q
* Black hole star
* Black
* Hawking
* Quasi-star
* Gravastar
* Thorne–Żytkow object
* Iron
* Blitzar
* White hole

Nucleosynthesis|

* Deuterium burning
* Lithium burning
* Proton–proton chain
* CNO cycle
* Helium flash
* Triple-alpha process
* Alpha process
* C burning
* Ne burning
* O burning
* Si burning
* s-process
* r-process
* p-process
* Nova
* Symbiotic
* Remnant
* Luminous red nova
* Recurrent
* Micronova
* Supernova

Structure|

* Core
* Convection zone
* Microturbulence
* Oscillations
* Radiation zone
* Atmosphere
* Photosphere
* Starspot
* Chromosphere
* Stellar corona
* Alfvén surface
* Stellar wind
* Bubble
* Bipolar outflow
* Accretion disk
* Protoplanetary disk
* Proplyd
* Asteroseismology
* Helioseismology
* Circumstellar dust
* Cosmic dust
* Circumstellar envelope
* Eddington luminosity
* Kelvin–Helmholtz mechanism

Properties|

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

Star systems|

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

Earth-centric
observations|

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

Lists|

* Proper names
* Arabic
* Chinese
* Extremes
* Most massive
* Highest temperature
* Lowest temperature
* Largest volume
* Smallest volume
* Brightest
* Historical brightest
* Most luminous
* Nearest
* bright
* Most distant
* With resolved images
* With multiple exoplanets
* Brown dwarfs
* Red dwarfs
* White dwarfs
* Milky Way novae
* Supernovae
* Candidates
* Remnants
* Planetary nebulae
* Timeline of stellar astronomy

Related|

* Substellar object
* Brown dwarf
* Desert
* Sub
* Planet
* Galactic year
* Galaxy
* Guest
* Gravity
* Intergalactic
* Neutron star merger
* Planet-hosting stars
* Stellar collision
* Stellar engulfment
* Tidal disruption event

* Category
* Stars portal

*[v]: View this template
*[t]: Discuss this template
*[e]: Edit this template