Absolute Magnitude

In astronomy, absolute magnitude is the apparent magnitude, an object would have if it were at a standard distance – 10 parsecs, 32.62 light years orIt allows the overall brightnesses of objects to be compared by compensating for their different distances from us.

The absolute magnitude uses a log scale. A difference of 1 magnitude corresponds to a factor of 2.512 in intrinsic brightness. A difference of 5 magnitudes corresponds to a factor of in intrinsic brightness.

You can compute the absolute magnitude M of a star given its apparent magnitude m and luminosity distance – which is a measure of the ordinary Euclidean distance and is measured in parsecs:

For nearby astronomical objects (such as stars in our galaxy) the luminosity distance D L is almost identical to the real distance to the object, because spacetime within our galaxy is almost Euclidean. For much more distant objects the Euclidean approximation is not valid, and General Relativity must be taken into account when calculating the luminosity distance of an object.

Example: Rigel has apparent visual magnitudeand is at a distance of 773 light years

The absolute magnitudes of some stars are shown in the table.


Absolute Magnitude









Barnard;s Star


Proxima Centauri


It is important to note that large positive numbers imply dimness and large negative numbers imply brightness.

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