The original magnitude system is a flux-based system: The more light that reaches an observer on Earth, the lower its magnitude number. This is called apparent magnitude, because it is the apparent brightness of the star as seen from Earth.

The absolute magnitude system is a luminosity-based system: The more light that is emitted by a star regardless of where it is, the lower its magnitude number. It is defined as follows: The absolute magnitude of a star is what the apparent magni tude of the star would be if it were at a distance of 10 parsecs (about 32.6 light-years). Since flux and luminosity are related to one another by the distance between a light source and the observer, the difference between the apparent magnitude (m) and absolute magnitude (M) of a star is called its distance modulus (m–M).