magnitude - (n.)
A measure of the brightness of a star. It is based on a system established by Hipparchus, in which stars were ranked according to how bright they appeared to the unaided eye. In the modern system, a difference of five magnitudes corresponds exactly to a brightness ratio of 100, so that a star of a given magnitude has a brightness that is 1001/5 = 2.51 times that of a star one magnitude fainter.