Magnitude of a star

(Astron.)

the rank of a star with respect to brightness. About twenty very bright stars are said to be of first magnitude, the stars of the sixth magnitude being just visible to the naked eye; called also visual magnitude, apparent magnitude, and simply magnitude. Stars observable only in the telescope are classified down to below the twelfth magnitude. The difference in actual brightness between magnitudes is now specified as a factor of 2.512, i.e. the difference in brightness is 100 for stars differing by five magnitudes.

See also: Magnitude