Different Kinds of Distance Moduli
Distance is not the only quantity relevant in determining the difference between absolute and apparent magnitude. Absorption is another important factor and it may even be a dominant one in particular cases (e. g. in the direction of the galactic center).
Thus a distinction is made between distance moduli uncorrected for interstellar absorption (whose values would overestimate the distance if used naively) and absorption-corrected moduli.
The first ones are termed visual distance moduli and are denoted by while the second ones are called true distance moduli and denoted by .
Visual distance moduli are computed by calculating the difference between the observed apparent magnitude and some theoretical estimate of the absolute magnitude. True distance moduli require a further theoretical step, that is the estimation of the interstellar absorption coefficient.
Read more about this topic: Distance Modulus
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