apparent magnitude
/ ap-PAIR-ent /
Long before telescopes, the Greek astronomer Hipparchus sorted the stars by how bright they looked, calling the brightest 'first magnitude' and the faintest the eye could see 'sixth magnitude.' That ancient ranking survives, slightly cleaned up, as the apparent magnitude system. Its first odd feature: brighter stars have smaller numbers, so a magnitude-1 star is bright and a magnitude-6 star is barely visible.
Apparent magnitude measures how bright an object looks from Earth — that is, its flux, the light we actually receive. The scale is logarithmic, because the eye responds to ratios, not differences. The modern definition fixes the steps precisely: a difference of 5 magnitudes corresponds to exactly a factor of 100 in flux, so each single magnitude step is a brightness ratio of about 2.512 (the fifth root of 100). The scale runs past zero into negative numbers for the very brightest things: Sirius is about minus 1.5, the full Moon about minus 13, and the Sun about minus 27.
Apparent magnitude is the workhorse number of practical astronomy because it is exactly what you can measure directly with a camera. But it confuses two things — true power and distance — into one number, so it cannot tell you how luminous a star really is. A magnitude-2 star might be a nearby modest star or a distant brilliant one. To untangle power from distance, astronomers pair apparent magnitude with absolute magnitude and the distance modulus.
Vega is defined to be near magnitude 0; the faintest stars the unaided eye can see from a dark site are about magnitude 6; the Hubble Space Telescope can reach objects fainter than magnitude 30 — that is, more than a billion times fainter than the dimmest stars you can see.
Smaller (even negative) magnitude means brighter; the scale is logarithmic, 5 steps = 100x in flux.
Counterintuitively, brighter means a smaller number, and the scale is logarithmic, not linear. Apparent magnitude tells you how bright something looks, never how luminous it truly is — distance is baked in.