The difference between the apparent magnitude of an object, m, corrected for interstellar absorption, and its absolute magnitude, M. This difference is directly linked to the distance in parsecs, r, by the formula
m − M = 5 log (r/10).
Put another way, if the distance modulus is known, the distance in parsecs can be found from
r = 10 × 1.585(m − M).
Distance moduli are used for finding the distances to objects too far away to show a measurable parallax (i.e. beyond a few hundred light years). The distances of extragalactic objects are often referred to in terms of their distance modulus, rather than in parsecs or light years.
Subjects: Astronomy and Astrophysics.