<P> The apparent magnitude (m) of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth . The brighter an object appears, the lower its magnitude value (i.e. inverse relation). The Sun, at apparent magnitude of − 27, is the brightest object in the sky . It is adjusted to the value it would have in the absence of the atmosphere . Furthermore, the magnitude scale is logarithmic; a difference of one in magnitude corresponds to a change in brightness by a factor of √ 100, or about 2.512 . </P> <P> The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry . Apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths . An apparent magnitude is usually measured in a specific passband corresponding to some photometric system such as the UBV system . In standard astronomical notation, an apparent magnitude in the V ("visual") filter band would be denoted either as m or often simply as V, as in "m = 15" or "V = 15" to describe a 15th - magnitude object . </P>

The apparent visual magnitude of a star is a measure of the star's