Luminosity distance
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Luminosity distance DL is defined in terms of the relationship between the absolute magnitude M and apparent magnitude m of an astronomical object.
which gives:
where DL is measured in parsecs. For nearby objects (say, in the Milky Way) the luminosity distance gives a good approximation to the natural notion of distance in Euclidean space.
For distant objects far beyond the Milky Way, the relation is less clear, since the apparent magnitude is affected by spacetime curvature, redshift, and time dilation. Calculating the relation between the luminosity distance and, for example, redshift of an object requires taking all of these factors into account.
