Solar luminosity

The solar luminosity (L_☉) is a unit of radiant flux (power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, 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 3.828×10²⁶ W.^[2] The Sun is a weakly variable star, and its actual luminosity therefore fluctuates.^[3] 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.^[4]

Solar luminosity is related to solar irradiance (the solar constant). Solar irradiance is responsible for the 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, I_☉. 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: $${\displaystyle L_{\odot }=4\pi kI_{\odot }A^{2}}$$ where A is the unit distance (the value of the astronomical unit in metres) and 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.

• Sun
• Solar mass
• Solar radius
• Nuclear fusion
• Active region
• Triple-alpha process

• 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", Astrophys. J., 583 (2): 1024–39, arXiv:astro-ph/0210128, Bibcode:2003ApJ...583.1024S, doi:10.1086/345408, S2CID 118904050
• Foukal, P.; Fröhlich, C.; Spruit, H.; Wigley, T. M. L. (2006), "Variations in solar luminosity and their effect on the Earth's climate", Nature, 443 (7108): 161–66, Bibcode:2006Natur.443..161F, doi:10.1038/nature05072, PMID 16971941, S2CID 205211006
• Pelletier, Jon D. (1996), "Variations in Solar Luminosity from Timescales of Minutes to Months", Astrophys. J., 463 (1): L41–L45, arXiv:astro-ph/9510026, Bibcode:1996ApJ...463L..41P, doi:10.1086/310049, S2CID 7372755
• 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", Proceedings of the AGU Chapman Conference: Mechanisms of Millennial Scale Global Climate Change

• LISIRD: LASP Interactive Solar Irradiance Datacenter
• Stellar Luminosity Calculator
• Solar Luminosity
• Variation of Solar Luminosity