The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation.[1] Effective temperature is often used as an estimate of a body's surface temperature when the body's emissivity curve (as a function of wavelength) is not known.
When the star's or planet's net emissivity in the relevant wavelength band is less than unity (less than that of a black body), the actual temperature of the body will be higher than the effective temperature. The net emissivity may be low due to surface or atmospheric properties, including greenhouse effect.
Star
The effective temperature of the Sun (5777 kelvins) is the temperature a black body of the same size must have to yield the same total emissive power.
The effective temperature of a star is the temperature of a black body with the same luminosity per surface area (FBol) as the star and is defined according to the Stefan–Boltzmann law FBol = σTeff4. Notice that the total (bolometric) luminosity of a star is then L = 4πR2σTeff4, where R is the stellar radius.[2] The definition of the stellar radius is obviously not straightforward. More rigorously the effective temperature corresponds to the temperature at the radius that is defined by a certain value of the Rosseland optical depth (usually 1) within the stellar atmosphere.[3][4] The effective temperature and the bolometric luminosity are the two fundamental physical parameters needed to place a star on the Hertzsprung–Russell diagram. Both effective temperature and bolometric luminosity depend on the chemical composition of a star.
The effective temperature of our Sun is around 5780 kelvins (K).[5][6] Stars have a decreasing temperature gradient, going from their central core up to the atmosphere. The "core temperature" of the Sun—the temperature at the centre of the Sun where nuclear reactions take place—is estimated to be 15,000,000 K.
The color index of a star indicates its temperature from the very cool—by stellar standards—red M stars that radiate heavily in the infrared to the very hot blue O stars that radiate largely in the ultraviolet. The effective temperature of a star indicates the amount of heat that the star radiates per unit of surface area. From the warmest surfaces to the coolest is the sequence of stellar classifications known as O, B, A, F, G, K, M.
A red star could be a tiny red dwarf, a star of feeble energy production and a small surface or a bloated giant or even supergiant star such as Antares or Betelgeuse, either of which generates far greater energy but passes it through a surface so large that the star radiates little per unit of surface area. A star near the middle of the spectrum, such as the modest Sun or the giant Capella radiates more energy per unit of surface area than the feeble red dwarf stars or the bloated supergiants, but much less than such a white or blue star as Vega or Rigel.
Planet
Main article: Planetary equilibrium temperature
Blackbody temperature
To find the effective (blackbody) temperature of a planet, it can be calculated by equating the power received by the planet to the known power emitted by a blackbody of temperature T.
Take the case of a planet at a distance D from the star, of luminosity L.
Assuming the star radiates isotropically and that the planet is a long way from the star, the power absorbed by the planet is given by treating the planet as a disc of radius r, which intercepts some of the power which is spread over the surface of a sphere of radius D (the distance of the planet from the star). The calculation assumes the planet reflects some of the incoming radiation by incorporating a parameter called the albedo (a). An albedo of 1 means that all the radiation is reflected, an albedo of 0 means all of it is absorbed. The expression for absorbed power is then:
\( {\displaystyle P_{\rm {abs}}={\frac {Lr^{2}(1-a)}{4D^{2}}}} \)
The next assumption we can make is that the entire planet is at the same temperature T, and that the planet radiates as a blackbody. The Stefan–Boltzmann law gives an expression for the power radiated by the planet:
\( P_{{{\rm {rad}}}}=4\pi r^{2}\sigma T^{4} \)
Equating these two expressions and rearranging gives an expression for the effective temperature:
\( {\displaystyle T={\sqrt[{4}]{\frac {L(1-a)}{16\pi \sigma D^{2}}}}} \)
Where \( \sigma \) is the Stefan–Boltzmann constant. Note that the planet's radius has cancelled out of the final expression.
The effective temperature for Jupiter from this calculation is 88 K and 51 Pegasi b (Bellerophon) is 1,258 K. A better estimate of effective temperature for some planets, such as Jupiter, would need to include the internal heating as a power input. The actual temperature depends on albedo and atmosphere effects. The actual temperature from spectroscopic analysis for HD 209458 b (Osiris) is 1,130 K, but the effective temperature is 1,359 K.[ The internal heating within Jupiter raises the effective temperature to about 152 K.
Surface temperature of a planet
The surface temperature of a planet can be estimated by modifying the effective-temperature calculation to account for emissivity and temperature variation.
The area of the planet that absorbs the power from the star is Aabs which is some fraction of the total surface area \( A_{total }= 4\pi r^{2} \) , where r is the radius of the planet. This area intercepts some of the power which is spread over the surface of a sphere of radius D. We also allow the planet to reflect some of the incoming radiation by incorporating a parameter a called the albedo. An albedo of 1 means that all the radiation is reflected, an albedo of 0 means all of it is absorbed. The expression for absorbed power is then:
\( P_{{{\rm {abs}}}}={\frac {LA_{{{\rm {abs}}}}(1-a)}{4\pi D^{2}}} \)
The next assumption we can make is that although the entire planet is not at the same temperature, it will radiate as if it had a temperature T over an area Arad which is again some fraction of the total area of the planet. There is also a factor ε, which is the emissivity and represents atmospheric effects. ε ranges from 1 to 0 with 1 meaning the planet is a perfect blackbody and emits all the incident power. The Stefan–Boltzmann law gives an expression for the power radiated by the planet:
\( P_{{{\rm {rad}}}}=A_{{{\rm {rad}}}}\varepsilon \sigma T^{4} \)
Equating these two expressions and rearranging gives an expression for the surface temperature:
\( {\displaystyle T={\sqrt[{4}]{{\frac {A_{\rm {abs}}}{A_{\rm {rad}}}}{\frac {L(1-a)}{4\pi \sigma \varepsilon D^{2}}}}}} \)
Note the ratio of the two areas. Common assumptions for this ratio are 1/4 for a rapidly rotating body and 1/2 for a slowly rotating body, or a tidally locked body on the sunlit side. This ratio would be 1 for the subsolar point, the point on the planet directly below the sun and gives the maximum temperature of the planet — a factor of √2 (1.414) greater than the effective temperature of a rapidly rotating planet.[7]
Also note here that this equation does not take into account any effects from internal heating of the planet, which can arise directly from sources such as radioactive decay and also be produced from frictions resulting from tidal forces.
Earth effective temperature
Main article: Earth's energy budget
The Earth has an albedo of about 0.306.[8] The emissivity is dependent on the type of surface and many climate models set the value of the Earth's emissivity to 1. However, a more realistic value is 0.96.[9] The Earth is a fairly fast rotator so the area ratio can be estimated as 1/4. The other variables are constant. This calculation gives us an effective temperature of the Earth of 252 K (−21 °C). The average temperature of the Earth is 288 K (15 °C). One reason for the difference between the two values is due to the greenhouse effect, which increases the average temperature of the Earth's surface.
See also
iconStar portal
Brightness temperature
Color temperature
List of hottest stars
Learning materials related to Atmospheric retention at Wikiversity
References
Archie E. Roy, David Clarke (2003). Astronomy. CRC Press. ISBN 978-0-7503-0917-2.
Tayler, Roger John (1994). The Stars: Their Structure and Evolution. Cambridge University Press. p. 16. ISBN 0-521-45885-4.
Böhm-Vitense, Erika (1992). Introduction to Stellar Astrophysics, Volume 3, Stellar structure and evolution. Cambridge University Press. p. 14. Bibcode:1992isa..book.....B.
Baschek (June 1991). "The parameters R and Teff in stellar models and observations". Astronomy and Astrophysics. 246 (2): 374–382. Bibcode:1991A&A...246..374B.
Lide, David R., ed. (2004). "Properties of the Solar System". CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. p. 14-2. ISBN 9780849304859.
Jones, Barrie William (2004). Life in the Solar System and Beyond. Springer. p. 7. ISBN 1-85233-101-1.
Swihart, Thomas. "Quantitative Astronomy". Prentice Hall, 1992, Chapter 5, Section 1.
"Earth Fact Sheet". nssdc.gsfc.nasa.gov. Archived from the original on 30 October 2010. Retrieved 8 May 2018.
Jin, Menglin and Shunlin Liang, (2006) “An Improved Land Surface Emissivity Parameter for Land Surface Models Using Global Remote Sensing Observations” Journal of Climate, 19 2867-81. (www.glue.umd.edu/~sliang/papers/Jin2006.emissivity.pdf)
External links
Effective temperature scale for solar type stars
Surface Temperature of Planets
Planet temperature calculator
vte
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
Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova
Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]
White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster
Hypothetical
Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar
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 Stellar 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 Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy
Related articles
Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event
Hellenica World - Scientific Library
Retrieved from "http://en.wikipedia.org/"
All text is available under the terms of the GNU Free Documentation License
