A Star's Magnitude

Magnitude is the degree of brightness of a star. In 1856, British astronomer Norman Pogson proposed a quantitative scale of stellar magnitudes, which was adopted by the astronomical community. He noted that we receive 100 times more light from a first magnitude star as from a sixth; thus with a difference of five magnitudes, there is a 100:1 ratio of incoming light energy, which is called luminous flux.

Because of the nature of human perception, equal intervals of brightness are actually equal ratios of luminous flux. Pogson's proposal was that one increment in magnitude be the fifth root of 100. This means that each increment in magnitude corresponds to an increase in the amount of energy by 2.512, approximately. A fifth magnitude star is 2.512 times as bright as a sixth, and a fourth magnitude star is 6.310 times as bright as a sixth, and so on. The naked eye, upon optimum conditions, can see down to around the sixth magnitude, that is, +6. Under Pogson's system, a few of the brighter stars now have negative magnitudes. For example, Sirius is –1.5. The lower the magnitude number, the brighter the object. The full Moon has a magnitude of about –12.5, and the Sun is a bright –26.51!

The Brightest Stars

Star Constellation Mag. Dist
Sirius Canis Major -1.58
Canopus Carina -0.9 650
Alpha Centauri Centaurus +0.1 4
Vega Lyra 0.1 23
Capella Auriga 0.2 42
Arcturus Boötes 0.2 32
Rigel Orion 0.3 545
Procyon Canis Minor 0.5 10
Achernar Eridanus 0.6 70
Beta Centauri Centaurus 0.9 130
Altair Aquila 0.9 18
Betelgeuse Orion 0.9 600
Aldebaran Taurus 1.1 54
Spica Virgo 1.2 190
Pollux Gemini 1.2 31
Antares Scorpius 1.2 170
Fomalhaut Piscis Austrinus 1.3 27
Deneb Cygnus 1.3 465
Regulus Leo 1.3 70
Beta Crucis Crux 1.5 465
Eta Carinae Carina 1–7
Alpha-one Crucis Crux 1.6 150
Castor Gemini 1.6 44
Gamma Crucis Crux 1.6
Epsilon Canis Majoris Canis Major 1.6 325
Epsilon Ursae Majoris Ursa Major 1.7 50
Bellatrix Orion 1.7 215
Lambda Scorpii Scorpius 1.7 205
Epsilon Carinae Carina 1.7 325
Mira Cetus 2–10 250

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