who was the first man to classify stars according to their brightness
Magnitude (astronomy) The magnitude system dates back roughly 2000 years to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemyâ€”references vary) who classified stars by their apparent brightness, which they saw as size (magnitude means "bigness, size"[1]). To the unaided eye, a more prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar, which in turn appears larger than a truly faint star such as Alcor. In 1736, the mathematician John Keill described the ancient naked-eye magnitude system in this way: