In this blog and my next, I'm going to try to demystify the magnitude system — the scale astronomers use to measure the brightness of a celestial object. Let me start with a little history. The first known observer to describe and catalog differences in star brightnesses was Greek astronomer Hipparchus (ca. 190–120 B.C.). He divided his catalog of roughly 850 visible stars into six brightness ranges, or magnitudes. He called the brightest 1st...