Editing Solar luminosity

{{Short description|Unit of light in stars and galaxies}}
[[File:Solar evolution (English).svg|right|thumb|upright=1.5|Evolution of the solar luminosity, [[Solar radius|radius]] and [[effective temperature]] compared to the present-day Sun. After Ribas (2010)<ref name=ribas2010>{{citation | last=Ribas | first=Ignasi | contribution=The Sun and stars as the primary energy input in planetary atmospheres | title=Solar and Stellar Variability: Impact on Earth and Planets, Proceedings of the International Astronomical Union, IAU Symposium  | volume=264 | pages=3–18 |date=February 2010 | doi=10.1017/S1743921309992298 | bibcode=2010IAUS..264....3R |arxiv = 0911.4872 | s2cid=119107400 | contribution-url=https://www.cambridge.org/core/services/aop-cambridge-core/content/view/293C0314C44A1A4AAF8175A3C288B50B/S1743921309992298a.pdf/sun_and_stars_as_the_primary_energy_input_in_planetary_atmospheres.pdf }}</ref>]]
The '''solar luminosity''' ('''{{Solar luminosity}}''') is a unit of [[radiant flux]] ([[Power (physics)|power]] emitted in the form of [[photon]]s) conventionally used by [[astronomer]]s to measure the [[luminosity]] of [[star]]s, [[galaxy|galaxies]] and other celestial objects in terms of the output of the [[Sun]].

One nominal solar [[luminosity]] is defined by the [[International Astronomical Union]] to be {{val|3.828|e=26|ul=W}}.<ref>{{cite web |url=https://www.iau.org/static/resolutions/IAU2015_English.pdf|title=Resolution B3 on recommended nominal conversion constants for selected solar and planetary properties|publisher=International Astronomical Union|date=2015|access-date= 5 June 2018}}</ref>  This corresponds almost exactly to a [[bolometric magnitude|bolometric absolute magnitude]] of +4.74.

The Sun is a weakly [[variable star]], and its actual luminosity therefore [[Solar variation|fluctuates]].<ref>{{Cite journal | last1 = Vieira | first1 = L. E. A. | last2 = Norton | first2 = A. | last3 = Dudok De Wit | first3 = T. | last4 = Kretzschmar | first4 = M. | last5 = Schmidt | first5 = G. A. | last6 = Cheung | first6 = M. C. M. | doi = 10.1029/2012GL052950 | title = How the inclination of Earth's orbit affects incoming solar irradiance | journal = Geophysical Research Letters | volume = 39 | issue = 16 | page = L16104 (8 pp.) | year = 2012 | bibcode=2012GeoRL..3916104V | url = https://pubs.giss.nasa.gov/docs/2012/2012_Vieira_vi06000k.pdf | id = [https://hal-insu.archives-ouvertes.fr/insu-01179873 insu-01179873] | doi-access = free }}</ref> The major fluctuation is the eleven-year [[solar cycle]] (sunspot cycle) that causes a quasi-periodic variation of about ±0.1%. Other variations over the last 200–300 years are thought to be much smaller than this.<ref name="Noed">{{cite journal |last=Noerdlinger |first=Peter D. |arxiv=0801.3807 |title = Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System |journal = Celestial Mechanics and Dynamical Astronomy |bibcode=2008arXiv0801.3807N |volume=801 |date=2008 |page=3807}}</ref>

== Determination ==
Solar luminosity is related to [[Irradiance|solar irradiance]] (the [[solar constant]]). Slow changes in the axial tilt of the planet and the shape of its orbit cause cyclical changes to the solar irradiance. The result is [[orbital forcing]] that causes the [[Milankovitch cycles]], which determine Earthly glacial cycles. The mean irradiance at the top of the Earth's atmosphere is sometimes known as the solar constant, {{math|''I''<sub>☉</sub>}}. Irradiance is defined as power per unit area, so the solar luminosity (total power emitted by the Sun) is the irradiance received at the Earth (solar constant) multiplied by the area of the sphere whose radius is the mean distance between the Earth and the Sun:
<math display="block">L_\odot = 4\pi k I_\odot A^2 </math>
where {{math|''A''}} is the [[Astronomical unit|unit distance]] (the value of the [[astronomical unit]] in [[metre]]s) and {{math|''k''}} is a constant (whose value is very close to one) that reflects the fact that the mean distance from the Earth to the Sun is not exactly one astronomical unit.

==See also==
* [[Solar mass]]
* [[Solar radius]]
* [[Nuclear fusion]]
* [[Active region]]
* [[Triple-alpha process]]

==References==
{{Reflist|30em}}

== Further reading ==
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* {{citation | author = Sackmann, I.-J.| author2 = Boothroyd, A. I. | title = Our Sun. V. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars | journal = [[Astrophysical Journal|Astrophys. J.]] | date = 2003 | volume = 583 | issue = 2 | pages = 1024–39 | bibcode = 2003ApJ...583.1024S | doi = 10.1086/345408 |arxiv = astro-ph/0210128 | s2cid = 118904050 }}
*{{citation | author = Foukal, P.| author2 = Fröhlich, C.| author3 = Spruit, H.| author4 = Wigley, T. M. L. | date = 2006 | title = Variations in solar luminosity and their effect on the Earth's climate | journal = [[Nature (journal)|Nature]] | volume = 443 | pages = 161–66 | doi = 10.1038/nature05072 | pmid = 16971941 | issue = 7108|bibcode = 2006Natur.443..161F | s2cid = 205211006}}
*{{citation | author = Pelletier, Jon D. | date = 1996 | title = Variations in Solar Luminosity from Timescales of Minutes to Months | journal = [[Astrophysical Journal|Astrophys. J.]] | volume = 463 | issue = 1 | pages = L41–L45 | doi = 10.1086/310049|arxiv = astro-ph/9510026 |bibcode = 1996ApJ...463L..41P | s2cid = 7372755 }}
*{{citation | display-authors = 4| author = Stoykova, D. A.| author2 = Shopov, Y. Y.| author3 = Ford, D.| author4 = Georgiev, L. N.| author5 = Tsankov, L. | date = 1999 | contribution = Powerful Millennial-Scale Solar Luminosity Cycles and Their Influence Over Past Climates and Geomagnetic Field | url = http://www.ncdc.noaa.gov/paleo/chapconf/stoykova_abs.html | title = Proceedings of the AGU Chapman Conference: Mechanisms of Millennial Scale Global Climate Change}}
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== External links ==
*[https://web.archive.org/web/20120511173325/http://lasp.colorado.edu/lisird/ LISIRD: LASP Interactive Solar Irradiance Datacenter]
*[http://astro.unl.edu/classaction/animations/stellarprops/stellarlum.html Stellar Luminosity Calculator]
*[https://archive.today/20091116175134/http://www.universetoday.com/guide-to-space/the-sun/solar-luminosity/ Solar Luminosity] 
*[https://web.archive.org/web/20031113174453/http://web.sfc.keio.ac.jp/~masudako/edu/text/quatbook/variat/node4.html Variation of Solar Luminosity]
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[[Category:Sun|Luminosity]]
[[Category:Stellar astronomy]]
[[Category:Units of power]]
[[Category:Units of measurement in astronomy]]