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<-   The units used to measure the brightness of the stars   ->
figures/bouvier.png
The central star has a magnitude of 4,5, the second one has a magnitude of 7,2, and the stars in the sky background have magnitudes of between 15 and 18.
Copyright : CDS

The unit used to measure the brightness of the stars is magnitude: Hipparque, in 200 BC, classified visible stars between the magnitude 0, for the brightest star of the sky (Vega), and 6 for the fainter stars visible to the naked eye.

To keep this scale, we use the formula m=-2.5*log(L/L0) where L0 is the flux of a star with a magnitude of zero.