Hipparchus

An ancient Greek astronomer named Hipparchus (190 B.C.E.- 120 B.C.E.) invented the first Magnitude scale to measure the brightness of stars. He gave the brightest stars he could see a value of 1 and the dimmest a value of 6.

For example, Sirius was the brightest star in sky with a value of -1.6.

Pogson

In ancient times, stars were ranked in six magnitude classes, but presently it is used the system proposed by Norman Robert Pogson in 1850. One magnitude is defined as a ratio of brightness of 2.512 times (⁵√100).

For example, a star of magnitude 5.0 is 2.512 times as bright as one of magnitude 6.0.