Besides mapping the locations of stars in the sky, astronomers also determined which stars were brighter than others.
The Greek astronomer Hipparchus, a predecessor of Ptolemy, first classified stars according to their brightness. He listed six categories of brightness by magnitude.
Magnitude defines how bright the stars appear to be from Earth. A star’s magnitude is determined largely by how far away from Earth it is. Hipparchus labeled twenty stars as the first magnitude, or the brightest.
The faintest stars, those that are barely visible to the naked eye, he labeled the sixth magnitude.
The mapping of stars in the early days of astronomy was done with the naked eye as the telescope had not been invented yet.
In the 2nd century BC, Greek astronomer Hipparchus created a comprehensive catalog of 1020 stars.
Most of the common names of constellations in the northern hemisphere come from Greek astronomy.