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Surface brightness
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==Calculating surface brightness== Surface brightnesses are usually quoted in magnitudes per square arcsecond. Because the magnitude is logarithmic, calculating surface brightness cannot be done by simple division of magnitude by area. Instead, for a source with a total or integrated magnitude ''m'' extending over a visual area of ''A'' square arcseconds, the surface brightness ''S'' is given by <math display="block">S = m + 2.5 \cdot \log_{10} A.</math> For astronomical objects, surface brightness is analogous to photometric [[luminance]] and is therefore constant with distance: as an object becomes fainter with distance, it also becomes correspondingly smaller in visual area. In geometrical terms, for a nearby object emitting a given amount of light, radiative [[flux]] decreases with the square of the distance to the object, but the physical area corresponding to a given [[solid angle]] or visual area (e.g. 1 square arcsecond) decreases by the same proportion, resulting in the same surface brightness.<ref>{{harvtxt|Sparke|Gallagher|2000|loc=Β§ 5.1.2}}</ref> For extended objects such as nebulae or galaxies, this allows the estimation of spatial distance from surface brightness by means of the distance modulus or [[luminosity distance]].{{Clarify|date=February 2018|reason=How can we get a distance from the surface brightness if it doesn't depend on distance?}}
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