Distance modulus

Distance modulus is the difference between an astronomical object's apparent magnitude and absolute magnitude. It encodes distance on a logarithmic scale, with corrections needed when extinction dims the object.

Apparent magnitude: A logarithmic measure of an object's observed brightness as seen from Earth. It is the observed-brightness term in the difference defining distance modulus.
Standard candle: An astronomical object whose intrinsic luminosity is known or calibrated, enabling distance estimates from observed brightness. Known absolute magnitude makes its distance modulus directly useful for measuring distance.
Astronomical magnitude: A logarithmic scale for expressing the brightness of celestial objects, with smaller values indicating greater brightness. Distance modulus is a difference on this scale.
Parallax: The apparent angular shift of a nearby object against distant background objects as the observer changes position. Parallax measures nearby distances geometrically rather than through brightness differences.
Absolute magnitude: A logarithmic measure of an object's intrinsic brightness, defined as its apparent magnitude at ten parsecs. It supplies the standardized intrinsic-brightness term compared with apparent magnitude.
Cepheid variable: A pulsating star whose period-luminosity relation allows its intrinsic luminosity to be estimated. Cepheid absolute magnitudes provide distance moduli beyond the reach of direct parallax.
Luminosity: The total energy emitted by an object per unit time across all wavelengths. Absolute magnitude represents intrinsic brightness, which is tied to an object's luminosity.
Luminosity distance: The distance defined by relating an object's luminosity to its observed flux, including cosmological effects. It is a physical distance quantity inferred from brightness, while distance modulus is a logarithmic magnitude difference.
Parsec: A unit of distance equal to about 3.26 light-years, defined by a parallax angle of one arcsecond. The distance-modulus formula expresses distance in parsecs.
Type Ia supernova: A thermonuclear supernova whose peak luminosity can be standardized for measuring extragalactic distances. Its standardized peak brightness yields distance moduli across large cosmic distances.

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