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<title>Solar luminosity</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Solar luminosity</span></span>

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<p>The <b>solar luminosity</b> (<b><span class="nowrap"><var>L</var><sub>☉</sub></span></b>) is a unit of <a href="Radiant_flux" title="Radiant flux">radiant flux</a> (<a href="Power_(physics)" title="Power (physics)">power</a> emitted in the form of <a href="Photon" title="Photon">photons</a>) conventionally used by <a href="Astronomer" title="Astronomer">astronomers</a> to measure the <a href="Luminosity" title="Luminosity">luminosity</a> of <a href="Star" title="Star">stars</a>, <a href="Galaxy" title="Galaxy">galaxies</a> and other celestial objects in terms of the output of the <a href="Sun" title="Sun">Sun</a>.

</p><p>One nominal solar <a href="Luminosity" title="Luminosity">luminosity</a> is defined by the <a href="International_Astronomical_Union" title="International Astronomical Union">International Astronomical Union</a> to be <span class="nowrap">3.828<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>26</sup> <a href="Watt" title="Watt">W</a></span>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> This corresponds almost exactly to a <a href="Bolometric_magnitude" class="mw-redirect" title="Bolometric magnitude">bolometric absolute magnitude</a> of +4.74.

</p><p>The Sun is a weakly <a href="Variable_star" title="Variable star">variable star</a>, and its actual luminosity therefore <a href="Solar_variation" class="mw-redirect" title="Solar variation">fluctuates</a>.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> The major fluctuation is the eleven-year <a href="Solar_cycle" title="Solar cycle">solar cycle</a> (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.<sup id="cite_ref-Noed_4-0" class="reference"><a href="#cite_note-Noed-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Determination">Determination</h2></div>

<p>Solar luminosity is related to <a href="Irradiance" title="Irradiance">solar irradiance</a> (the <a href="Solar_constant" title="Solar constant">solar constant</a>). 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 <a href="Orbital_forcing" title="Orbital forcing">orbital forcing</a> that causes the <a href="Milankovitch_cycles" title="Milankovitch cycles">Milankovitch cycles</a>, which determine Earthly glacial cycles. The mean irradiance at the top of the Earth's atmosphere is sometimes known as the solar constant, <span class="texhtml"><i>I</i><sub>☉</sub></span>. 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:

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<annotation encoding="application/x-tex">{\displaystyle L_{\odot }=4\pi kI_{\odot }A^{2}}</annotation>

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where <span class="texhtml"><i>A</i></span> is the <a href="Astronomical_unit" title="Astronomical unit">unit distance</a> (the value of the <a href="Astronomical_unit" title="Astronomical unit">astronomical unit</a> in <a href="Metre" title="Metre">metres</a>) and <span class="texhtml"><i>k</i></span> 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.

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<ul><li><a href="Solar_mass" title="Solar mass">Solar mass</a></li>

<li><a href="Solar_radius" title="Solar radius">Solar radius</a></li>

<li><a href="Nuclear_fusion" title="Nuclear fusion">Nuclear fusion</a></li>

<li><a href="Active_region" title="Active region">Active region</a></li>

<li><a href="Triple-alpha_process" title="Triple-alpha process">Triple-alpha process</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFRibas2010" class="citation cs2">Ribas, Ignasi (February 2010), <a rel="nofollow" class="external text" href="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">"The Sun and stars as the primary energy input in planetary atmospheres"</a> <span class="cs1-format">(PDF)</span>, <i>Solar and Stellar Variability: Impact on Earth and Planets, Proceedings of the International Astronomical Union, IAU Symposium</i>, vol. 264, pp. <span class="nowrap">3–</span>18, <a href="ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/0911.4872">0911.4872</a></span>, <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2010IAUS..264....3R">2010IAUS..264....3R</a>, <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1017%2FS1743921309992298">10.1017/S1743921309992298</a>, <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:119107400">119107400</a></cite></span>

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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.iau.org/static/resolutions/IAU2015_English.pdf">"Resolution B3 on recommended nominal conversion constants for selected solar and planetary properties"</a> <span class="cs1-format">(PDF)</span>. International Astronomical Union. 2015<span class="reference-accessdate">. Retrieved <span class="nowrap">5 June</span> 2018</span>.</cite></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFVieiraNortonDudok_De_WitKretzschmar2012" class="citation journal cs1">Vieira, L. E. A.; Norton, A.; Dudok De Wit, T.; Kretzschmar, M.; Schmidt, G. A.; Cheung, M. C. M. (2012). <a rel="nofollow" class="external text" href="https://pubs.giss.nasa.gov/docs/2012/2012_Vieira_vi06000k.pdf">"How the inclination of Earth's orbit affects incoming solar irradiance"</a> <span class="cs1-format">(PDF)</span>. <i>Geophysical Research Letters</i>. <b>39</b> (16): L16104 (8 pp.). <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2012GeoRL..3916104V">2012GeoRL..3916104V</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1029%2F2012GL052950">10.1029/2012GL052950</a></span>. <a rel="nofollow" class="external text" href="https://hal-insu.archives-ouvertes.fr/insu-01179873">insu-01179873</a>.</cite></span>

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<li id="cite_note-Noed-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-Noed_4-0">^</a></b></span> <span class="reference-text"><cite id="CITEREFNoerdlinger2008" class="citation journal cs1">Noerdlinger, Peter D. (2008). "Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System". <i>Celestial Mechanics and Dynamical Astronomy</i>. <b>801</b>: 3807. <a href="ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/0801.3807">0801.3807</a></span>. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2008arXiv0801.3807N">2008arXiv0801.3807N</a>.</cite></span>

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<div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2></div>

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<ul><li><cite id="CITEREFSackmann,_I.-J.Boothroyd,_A._I.2003" class="citation cs2">Sackmann, I.-J.; Boothroyd, A. I. (2003), "Our Sun. V. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars", <i><a href="Astrophysical_Journal" class="mw-redirect" title="Astrophysical Journal">Astrophys. J.</a></i>, <b>583</b> (2): <span class="nowrap">1024–</span>39, <a href="ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/astro-ph/0210128">astro-ph/0210128</a></span>, <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2003ApJ...583.1024S">2003ApJ...583.1024S</a>, <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1086%2F345408">10.1086/345408</a>, <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:118904050">118904050</a></cite></li>

<li><cite id="CITEREFFoukal,_P.Fröhlich,_C.Spruit,_H.Wigley,_T._M._L.2006" class="citation cs2">Foukal, P.; Fröhlich, C.; Spruit, H.; Wigley, T. M. L. (2006), "Variations in solar luminosity and their effect on the Earth's climate", <i><a href="Nature_(journal)" title="Nature (journal)">Nature</a></i>, <b>443</b> (7108): <span class="nowrap">161–</span>66, <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2006Natur.443..161F">2006Natur.443..161F</a>, <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1038%2Fnature05072">10.1038/nature05072</a>, <a href="PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/16971941">16971941</a>, <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:205211006">205211006</a></cite></li>

<li><cite id="CITEREFPelletier,_Jon_D.1996" class="citation cs2">Pelletier, Jon D. (1996), "Variations in Solar Luminosity from Timescales of Minutes to Months", <i><a href="Astrophysical_Journal" class="mw-redirect" title="Astrophysical Journal">Astrophys. J.</a></i>, <b>463</b> (1): <span class="nowrap">L41 –</span> <span class="nowrap">L45</span>, <a href="ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/astro-ph/9510026">astro-ph/9510026</a></span>, <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1996ApJ...463L..41P">1996ApJ...463L..41P</a>, <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1086%2F310049">10.1086/310049</a>, <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:7372755">7372755</a></cite></li>

<li><cite id="CITEREFStoykova,_D._A.Shopov,_Y._Y.Ford,_D.Georgiev,_L._N.1999" class="citation cs2">Stoykova, D. A.; Shopov, Y. Y.; Ford, D.; Georgiev, L. N.; et al. (1999), "Powerful Millennial-Scale Solar Luminosity Cycles and Their Influence Over Past Climates and Geomagnetic Field", <a rel="nofollow" class="external text" href="http://www.ncdc.noaa.gov/paleo/chapconf/stoykova_abs.html"><i>Proceedings of the AGU Chapman Conference: Mechanisms of Millennial Scale Global Climate Change</i></a></cite></li></ul>

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<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20120511173325/http://lasp.colorado.edu/lisird/">LISIRD: LASP Interactive Solar Irradiance Datacenter</a></li>

<li><a rel="nofollow" class="external text" href="http://astro.unl.edu/classaction/animations/stellarprops/stellarlum.html">Stellar Luminosity Calculator</a></li>

<li><a rel="nofollow" class="external text" href="https://archive.today/20091116175134/http://www.universetoday.com/guide-to-space/the-sun/solar-luminosity/">Solar Luminosity</a></li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20031113174453/http://web.sfc.keio.ac.jp/~masudako/edu/text/quatbook/variat/node4.html">Variation of Solar Luminosity</a></li></ul>

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