In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background.
The total magnitude is a measure of the brightness of an extended object such as a nebula, cluster, galaxy or comet.
Alternatively, a photometer can be used by applying apertures or slits of different sizes of diameter.
[1] The background light is then subtracted from the measurement to obtain the total brightness.
However, a star is so small it is effectively a point source in most observations (the largest angular diameter, that of R Doradus, is 0.057 ± 0.005 arcsec), whereas a galaxy may extend over several arcseconds or arcminutes.
What counts as small or large depends on the specific viewing conditions and follows from Ricco's law.
[4] In general, in order to adequately assess an object's visibility one needs to know both parameters.
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
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.
[clarification needed] The surface brightness in magnitude units is related to the surface brightness in physical units of solar luminosity per square parsec by[citation needed]
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]).
[9][clarification needed] 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).