What is distance modulus?

Apparent magnitudeThe apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth. The brighter the object appears, the lower the value of its magnitude., absolute magnitudeThe apparent magnitude an object would have if it were located at a distance of 10 parsecs. This is a measure of the intrinsic brightness of a celestial object. and distance are related by an equation:

m - M = 5 log d - 5

m is the apparent magnitude of the object

M is the absolute magnitude of the object

d is the distance to the object in parsecs

The expression m - M is called the distance modulus and is a measure of distance to the object. An object with a distance modulus of 0 is exactly 10 paresecs away. If the distance modulus is negative, the object is closer that 10 parsecs, and its apparent magnitude is brighter than its absolute magnitude. If the distance modulus is positive, the object is farther than 10 parsecs and its apparent magnitude is less bright than its absolute magnitude.  

The following table gives values of d corresponding to different values of m - M.