A Star's Magnitude

Updated August 27, 2019 | Factmonster Staff

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

StarConstellationMag.Dist
(l.-y.)
SiriusCanis Major-1.58
CanopusCarina-0.9650
Alpha CentauriCentaurus+0.14
VegaLyra0.123
CapellaAuriga0.242
ArcturusBoötes0.232
RigelOrion0.3545
ProcyonCanis Minor0.510
AchernarEridanus0.670
Beta CentauriCentaurus0.9130
AltairAquila0.918
BetelgeuseOrion0.9600
AldebaranTaurus1.154
SpicaVirgo1.2190
PolluxGemini1.231
AntaresScorpius1.2170
FomalhautPiscis Austrinus1.327
DenebCygnus1.3465
RegulusLeo1.370
Beta CrucisCrux1.5465
Eta CarinaeCarina1–7
Alpha-one CrucisCrux1.6150
CastorGemini1.644
Gamma CrucisCrux1.6
Epsilon Canis MajorisCanis Major1.6325
Epsilon Ursae MajorisUrsa Major1.750
BellatrixOrion1.7215
Lambda ScorpiiScorpius1.7205
Epsilon CarinaeCarina1.7325
MiraCetus2–10250

 

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