Of principle importance to general astronomical observation is the observable brightness of the stars. Magnitude is the unit used in astronomy to describe a star's brightness. Although stellar magnitude in the visible spectrum dictates which stars can be observed under particular visible light conditions— variable due to time of observation, moon phase, atmospheric conditions, and the amount of light pollution present—magnitude also describes the relative amount of electromagnetic radiation observable in other regions of the electromagnetic spectrum (e.g., the x ray region of the spectrum).

Stars emit different amounts of radiation in different regions of the spectrum, so a star's "brightness" or magnitude will differ from one part of the spectrum to the next. An important field of research in modern astronomy is the accurate measurement of stellar brightness in magnitudes in different parts of the spectrum.

The Greek astronomer Hipparchus devised the first...