Hipparchus, an ancient Greek astronomer, pioneered this classification by dividing stars into six categories called magnitudes. The brightest stars were assigned first magnitude, followed by less bright stars at second magnitude, and so on, until reaching the faintest stars at sixth magnitude. Although this initial system was imprecise and excluded the Moon and planets, it laid the groundwork for our modern understanding of apparent magnitude.
Today's magnitude scale has been refined, with the star Vega serving as the benchmark with a magnitude of zero. The brightness of all other celestial objects is compared to Vega's brightness, with a specific criterion determining the differences between successive magnitudes. Currently, the Pogson criterion (established in 1856) is utilized, stating that a first magnitude star must be 100 times brighter than a sixth magnitude star. Consequently, a first magnitude star is 2.512 times brighter than a second magnitude star.
The contemporary magnitude system encompasses not only stars, but all observable celestial objects. Magnitudes are no longer confined between one and six and can now assume negative values for brighter objects or larger values for extremely faint objects.
To illustrate this concept, consider the following examples: From Earth, the Sun has an apparent magnitude of -26.74, while Venus reaches a minimum magnitude of -4.89. Sirius, the brightest star in the sky, has an apparent magnitude of -1.47. The North Star, which exhibits slight variability, possesses an apparent magnitude fluctuating between +1.89 and +2.13. The threshold of visibility to the naked eye ranges between the sixth and seventh apparent magnitudes, depending the observer's location. Neptune's apparent magnitude varies from +7.78 to +8.02, based on its distance from Earth, making it observable only through instruments, not with the naked eye.
Credit: Tragoolchitr Jittasaiyapan.
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