Surface brightness

General description

The apparent magnitude of an astronomical object is generally given as an integrated value - if a galaxy is quoted as having a magnitude of 12.5, it means we see the same total amount of light from the galaxy as we would from a star with magnitude 12.5. However, while the star is so small it is effectively a point source in most observations, the galaxy may extend over several arcseconds or arcminutes. Therefore, the galaxy will be harder to see than the star. Quoting an object's surface brightness gives an indication of how easily observable it is.

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 magnitude M extending over an area of A arcseconds, the surface brightness S is given by:

S = M + 2.5 log A

Surface brightness does not decrease with increasing object distance. For an object emitting a given amount of light, if it was twice as far away, a quarter of the intensity of light would reach us, but it would also have only a quarter the angular area, resulting in the same surface brightness.

Other resources

See also