Home > Terms > Norwegian Bokmål (NO) > Magnitude

The units used to describe brightness of astronomical objects. The smaller the numerical value, the brighter the object. The human eye can detect stars to 6th or 7th magnitude on a dark, clear night far from city lights; in suburbs or cities, stars may only be visible to mag 2 or 3 or 4, due to light pollution. The brightest star, Sirius, shines at visual magnitude -1. 5. Jupiter can get about as bright as visual magnitude -3 and Venus as bright as -4. The full moon is near magnitude -13, and the sun near mag -26. Comet C/1996 B2 (Hyakutake) reached magnitude about 0 in late March 1996. The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2. 5 times in brightness; a change of 5 magnitudes is defined as a change of exactly 100 times in brightness. It is customary in astronomy to precede a magnitude value with a left bracket (() and no space to indicate a limiting magnitude, when an object was not seen. Note that those careless people (notably dealing with variable stars) who use a less-than sign (<) instead of a left bracket to indicate limiting magnitude are, in fact, erroneously indicating that the object is brighter than the specified magnitude, because a smaller number in astronomical magnitudes means brighter than; if they wish to use a mathematical symbol to denote limiting magnitude, the only proper option is to use a greater-than sign, >, but this alone does not tell the reader whether the object was visible (and somewhat fainter than the specified value) or invisible. In the case of comets, we speak of a magnitude that is "integrated" over an observed coma diameter of several arc minutes; this is called the comet's "total (visual) magnitude", and is usually denoted by the variable m₁. Thus, a 7th-magnitude comet is much harder to see than a 7th-magnitude star -- the latter having all its light in a pinpoint, and the former having the same amount of light spread out over a large area (imagine defocussing a 7th-magnitude star to the size of a diffuse comet). Typically, however, when comets become very bright, their apparent coma sizes shrink so that the majority of visible light is in a small, intense core of the comet's head (and the comet may appear starlike with a tail emanating from the comet's head). Traditionally, in ICQ/CBAT/MPC publications, ephemerides for solar-system objects have usually given predicted/projected magnitudes of comets and minor planets in the last column, denoted m₁ and m₂ for cometary "total" and "nuclear" magnitudes, or V for minor-planet V-band ("visual") magnitudes. However, in 2003, a subcommittee of IAU Commission 20 charged with assessing cometary magnitudes for ephemerides decided that the concept of "nuclear" magnitudes should be done away with because few astrometrists ever observe a true nuclear magnitude (and in fact generally observe something between a true "total" and a true "nuclear" magnitude); CBAT publications have since then used the heading "Mag. " alone instead of the old m₁ and m₂ headings to more appropriately refer to the predicted brightness of comets.