Apparent magnitude

Apparent magnitude (m) is a unit of measure of the relative brightness of a star or other astronomical object as seen by an observer on Earth or theoretically from other celestial objects. Apparent magnitude is often understood to mean apparent visual magnitude (v), defined as the brightness of a star across the visible part of the electromagnetic spectrum as viewed by the human eye. This also relates to an object's intrinsic luminosity, its distance, and the atmospheric or interstellar extinction, reducing its brightness. Comparison of apparent magnitudes are arbitrarily fixed at the distance of 10.0 parsecs (32.6 light-years) and are termed absolute magnitudes. The magnitude scale is an inverse logarithmic relation, where a difference of 1.0 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude, with the brightest astronomical objects having negative apparent magnitudes: for example, Venus at −4.2 or Sirius at −1.46. The faintest naked-eye stars visible on the darkest night is around +6.5 magnitude. Apparent magnitudes typically range between −26.7 (Sun) to below +30 (faintest star). Instrumental measurement of the apparent magnitudes of celestial objects can be obtained by photometry. Apparent magnitudes of astronomical sources are often quantified from ultraviolet, visible, and infrared wavelengths, measured through standard passband filters corresponding to various adopted photometric systems such as the UBV system (or the Strömgren uvbyβ system.) In accepted astronomical notation, an apparent magnitude in the V ("visual") filter band could be denoted either as mV or often simply as V, as in "mV = 15" or "V = 15" or simply as 15.0V magnitude to describe a 15th-magnitude object. A stricter definition used since 1938 is known as the apparent bolometric magnitude (mbol). This is based on the measure of the corresponding irradiance received from the standard 'zero-point' 0.00 magnitude star, and equivalent to received radiant flux (power) of 2.518×10-8 W·m-2. (Watts per square metre.)


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