In astronomy, the magnitude is a measure of the brightness of a celestial object expressed on a logarithmic scale.
The magnitude scale, which comes from the visual astronomy, is only defined in the optical to mid-infrared wavelength range (300 nm to 20 μm); in other parts of the electromagnetic spectrum (X, radio, etc.) physical units are used, such as W m−2, Jy, etc. The measure is generally defined for a specific filter, that is, for a given wavelength and a given bandpass. A set of filters corresponds to a photometric system, one of the most widely used being the Johnson system. With no other indication, the term magnitude refers to the magnitude in the V (“visible”) band of the Johnson system (λ = 550 nm; bandpass = 89 nm). The magnitude scale is a relative scale, the reference of brightness being the star Vega. It is thus a quantity with no units. The magnitude of a given object whose measured flux in a given filter X is FX is given by the expression: mX = −2.5...