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 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. 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. Relationship to physical units The surface brightness in magnitude units is related to the surface brightness in physical units of solar luminosity per square parsec by where and are the absolute magnitude and the luminosity of the Sun in chosen color-band respectively. Surface brightness can also be expressed in candela per square metre using the formula [value in cd/m2] = 10.8×104 × 10(-0.4*[value in mag/arcsec2]). There is an online calculator available here http://unihedron.com/projects/darksky/magconv.php?ACTION=SOLVE&txtMAGSQA=21.83 Examples A truly dark sky has a surface brightness of cd m−2 or 21.8 mag arcsec−2. The peak surface brightness of the central region of the Orion Nebula is about 17 Mag/arcsec2 (about 14 millinits) and the outer bluish glow has a peak surface brightness of 21.3 Mag/arcsec2 (about 0.27 millinits). See also Araucaria Project Low-surface-brightness galaxy Limiting magnitude Sigma-D relation References General references Observational astronomy