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The Bortle dark-sky scale (usually referred to as simply the Bortle scale) is a nine-level numeric scale that measures the night sky's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution .
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minimum brightness [42] +1.33: star Alpha Centauri B: seen from Earth +1.86: planet Mars: seen from Earth minimum brightness [42] +1.98: star Polaris: seen from Earth mean brightness [49] +3.03: supernova SN 1987A: seen from Earth in the Large Magellanic Cloud (160,000 light-years away) +3 to +4: Faintest stars visible in an urban neighborhood ...
Stars in binary systems (or other multiples) are listed by their total or combined brightness if they appear as a single star to the naked eye, or listed separately if they do not. As with all magnitude systems in astronomy , the scale is logarithmic and inverted i.e. lower/more negative numbers are brighter.
[8] [9] Every interval of one magnitude equates to a variation in brightness of 5 √ 100 or roughly 2.512 times. Consequently, a magnitude 1 star is about 2.5 times brighter than a magnitude 2 star, about 2.5 2 times brighter than a magnitude 3 star, about 2.5 3 times brighter than a magnitude 4 star, and so on.
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(The S 10 unit is defined as the surface brightness of a star whose V-magnitude is 10 and whose light is smeared over one square degree, or 27.78 mag arcsec −2.) The total sky brightness in zenith is therefore ~220 S 10 or 21.9 mag/arcsec² in the V-band. Note that the contributions from Airglow and Zodiacal light vary with the time of year ...
By measuring these properties from a star's spectrum, the position of a main sequence star on the H–R diagram can be determined, and thereby the star's absolute magnitude estimated. A comparison of this value with the apparent magnitude allows the approximate distance to be determined, after correcting for interstellar extinction of the ...