Loterre: Astronomy: stellar magnitude

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astronomical technique > astrometry > stellar magnitude

stellar magnitude

The magnitude is the brightness of a celestial object, measured on a scale in which lower numbers mean greater brightness. The magnitude system stems from the ancient Greeks and, in particular, Hipparchus who, in about 120 BC, ranked stars from first to sixth magnitude: those of first magnitude being the first to appear after sunset, those of sixth magnitude being at the limit of naked-eye visibility in a dark sky. In the nineteenth century, when the advent of photometers made it possible to measure accurately the relative brightness of stars, the system was put on a strict quantitative footing by the English astronomer Norman Pogson (1829–1891). In 1856, the magnitude scale was fixed so that a difference of 5 magnitudes corresponded to a ratio of apparent brightness of 100. On this new scale, known as the Pogson scale, defined so that most of the traditional magnitudes of stars stayed roughly the same, a difference of one magnitude corresponds to a change in brightness by a factor of 2.512, while a jump of 5 magnitudes equals a brightness change of exactly 100-fold. (Encyclopedia of Science, by David Darling, https://www.daviddarling.info/encyclopedia/M/magnitude.html)

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