Star brightness

The brightness of stars appears to be different, because they give out different amounts of light and they lie at vastly different distances, so a closer star may appear brighter than a distant one even if the latter is much more powerful. A star's exact brightness is described by astronomers as its apparent magnitude, and the brightness a star appears to have if all the stars were the same distance away, which we have decided as 10 parsecs (32.6 light years), is known as its absolute magnitude.

The Greek astronomer Hipparchus in AD 129 divided the naked eye stars into 6 classes of brightness, from the brightest 1st magnitude, to the faintest 6th magnitude. In 1856, the English astronomer Norman Pogson (1829-91) mathematically defined a magnitude 1 as being exactly 100 times brighter than a star of magnitude 6.

On this scale, a difference of 5 magnitudes corresponds to a brightness difference of 100 times, so a step of one magnitude is equal to a brightness difference of just over 2.5 times.

Objects more than 250 times brighter than 6th magnitude are given negative magnitudes, so Sirius, the brightest star in the sky, is of magnitude -1.46. Stars fainter than magnitude 6 are given progressively larger positive magnitudes. Therefore, the brighter the star, the lower its magnitude.

The 30 brightest stars in our sky

Star	Constellation	Magnitude
Sirius	αCma	-1.46
Canopus	αCan	-0.72
Right Centurus	​αCen	-0.27
Arcturus	αBoo	-0.04
Vega	​αlyr	​0.03
​Capella	αAur	0.08
​Rigel	​βOri	0.12
Procyon	​αCMI	0.38
​Achernar	αEri	​0.46
Betelgeuse	​αOri	​0.50
​Hadar	​βCen	​0.61
​Altair	αAql	0.77
Acrux	αCru	0.79
Aldebaran	αTau	0.85
Spica	​αVir	​0.98
Pollox	βGem	​1.14
Formalhaut	​αPsA	1.16
​Antares	αSco	1.22
​Mimosa	​βCru	1.25
Deneb	αCyg	​1.25
​Regulus	​αLeo	​1.35
​Adhara	​εCMa	​1.50
​Castor	​αGem	1.58
​Gacrux	γCru	1.63
Shaula	​λSco	​1.63
Bellatrix	​ γOri	1.64
​Elnath	​βTau	​1.65
​Miaplacidus	​βCar	1.68
Alnilam	​εOri	​1.70
​Alnair	αGru	1.74 ​