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In physics, luminosity is the density of luminous intensity in a given direction.

In astronomy, luminosity is the amount of energy a body radiates in unit time. Stellar luminosities are usually expressed as solar luminosities, L; that is, how many times more energy the object radiates in comparison to the Sun. In X-ray and radio wavelengths, luminosity is sometimes expressed in watts. Stars with bolometric magnitudess of -10 have luminosities of 106L and stars with bolometric magnitude of +17 have luminosities of 10-5L. The Hertzsprung-Russell diagram relates luminosity with absolute magnitude, stellar classification, and surface temperature.