What is Absolute Magnitude?

Absolute magnitude (M) is a measure of a celestial object’s luminosity, on an astronomical magnitude scale of the inverse logarithm. The absolute magnitude of an object is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs (32.6 light-years), without extinguishing (or dimming) its light due to interstellar matter absorption and cosmic dust. By positioning all objects hypothetically within a standard reference distance from the observer, their luminosities can be measured directly on a magnitude scale.

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As for all astronomical magnitudes, the absolute magnitude can be defined for various wavelength ranges corresponding to specific filter bands or passbands; the absolute visual magnitude used by the visual (V) band of the spectrum (in the UBV photometric system) is the widely cited absolute magnitude for stars. A capital M denotes absolute magnitudes, with a subscript describing the filter band used for the calculation, such as MV for absolute magnitude in the V band.

The luminous an object, the smaller the absolute magnitude of its numerical value. A difference of 5 magnitudes between the absolute magnitudes of two objects corresponds to a ratio of 100 in their luminosities, and a difference in absolute magnitude of n magnitudes corresponds to a ratio of 100(n/5). For example, as calculated in the V filter band, a star of absolute magnitude MV=3.0 will be 100 times as luminous as a star of absolute magnitude MV=8.0. The absolute magnitude for the Sun is MV=+4.83. Highly luminous objects can have absolute negative magnitudes: the Milky Way galaxy, for example, has an absolute B magnitude of around −20.8.

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