What is the magnitude of a star? | Behind the Lore
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Rafal is proud: “Yesterday, I saw my first magnitude-six star. It was incredible!” Later, Hubert checks his readings and says: “You saw a magnitude-six star. You have good eyes.” Magnitude serves here as a yardstick for an observer’s sharpness.
The subject in depth
Apparent magnitude records a body’s brightness as perceived from Earth [Wikipedia] . Its first oddity is that it runs backwards: a magnitude-1 star is brighter than a magnitude-6 one. That is a legacy of Antiquity, where stars were ranked by “magnitudes”, from the first (the finest) to the sixth (the faintest).
Its second oddity is that it is logarithmic. The human eye does not perceive brightness proportionally but by ratios. A difference of 5 magnitudes corresponds exactly to a brightness ratio of 100. Each whole magnitude is therefore a factor of about 2.5. This rule, already nearly true in the ancient ranking, was formalized mathematically in the 19th century.
Magnitude 6 has a concrete meaning: it is, roughly, the faintest star a healthy human eye can make out in a perfectly dark sky, with no Moon and no artificial light. Seeing a magnitude 6 is therefore not a feat of instruments, but of acuity and conditions. A crucial point often forgotten: magnitude says nothing about the star’s distance or true size. A star can look bright because it is close, or because it is intrinsically very luminous but far away.
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