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3.2 The system of magnitudes

Theoretically you could measure the brightness of stars in candela like the brightness of a light bulb. But this is rather awkward. Instead, astronomers use the system of magnitudes (m).

The brighter a star, the smaller is its magnitude. Sirius is the brightest star in the sky, and its brightness is -1.6m. Vega has a brightness of 0.0m, and Polaris, the Pole Star, of 2.0m. The faintest stars that you can see with the naked eye in a very clear sky have 6m.

The faintest stars in the standard catalogue of PP3 are of approx. 7m, however extremely good terrestrial telescopes can see up to 22m.