The magnitude system used by astronomers ranks
stars by brightness, with the brightest stars having the
lowest values of magnitude.  A star's magnitude is generally
measured after the starlight has passed through a colored
filter, which gives a measure of a star's color. Literally
dozens of filter systems are used by astronomers.  The
most common system for measuring color over the infrared,
visible, and ultraviolet wavelengths is the Johnson-Cousins
UBVRI system.