Stellar Magnitudes
Created | Updated Jan 28, 2002
Starlight, star bright
first star I see tonight...
We all remember the old nursery rhyme, but there is a bit of science behind it. The brightest stars in the sky will always be the first stars we see in the gloaming of twilight. And the measure of a star's brightness is expressed scientifically by referring to its magnitude.
Astronomical History
In the second century BC, the Greek astronomer Hipparchus described the brightest stars in the sky as being of the first magnitude, the next brightest group were referred to as being of the second magnitude and so on until he reached stars of the sixth agnitude, which were the faintest visible to the naked eye1.
Apparent Magnitudes
In the mid 1800s, astronomers determined mathematically what old Hipparchus did visually, giving his scale a definable basis. An English Astronomer, N. R. Pogson, noticed that an average first magnitude star was in fact 100 times as bright as an average 6th magnitude star. Further measurements and calculations showed that for every increase of 1 in order of magnitude there is a 2.51-fold increase in the apparent brightness of a star. Therefor, the equation for apparent magnitude m is m = -2.5 log f +constant, where f is the flux from the star.
In plain English, this means that a star of the 3rd magnitude would appear to be 6.31 times as bright as a star of the 5th magnitude, because the difference in apparent magnitude is 2.
Bright, really bright, and very, very, very bright
Difference in Magnitude | Factor in Brigtness |
---|---|
1 mag | 2.51 times |
2 mag | 6.31 times |
3 mag | 15.85 times |
4 mag | 39.81 times |
5 mag | 100 times |
6 mag | 251 times |
Incredibly, hugely, very, very bright
Some stars are so bright that they must be assigned negative magnitude values in order for the magnitude six stars to remain a the faintest visible to the naked eye. An example of this is Sirius, which is the brightest star in the sky2, shining at mag -1.4. The Moon and some planets are brighter that this. At its brightest, Venus can shine at mag -4.4. The full moon is mag -12.3 and the Sun is mag -26.8.
Absolute Magnitudes
The problem with apparent magnitudes is that there is no way to differentiate between a bright star that is a long way away, and a dimmer star which is nearer. So astronomers use absolute magnitude M to compare the intrinsic brightness of stars, as suggested by Danish astronomer E. Hertzprung.
The absolute magnitude of a star is defined as being the magnitude a star would have if it were 10 parsecs away from the Sun.
Putting it all to use
For all you backyard astronomers, here is a list of the 20 brightest stars in the sky3:
Rank | Star | Constellation | Magnitude |
---|---|---|---|
1 | Sirius | Canis Major | -1.46 |
2 | (Canopus) | Carina | -0.72 |
3 | (Rigil Kentaurus) | Centaurus | -0.27 |
4 | Arcturus | Bootes | -0.04 |
5 | Vega | Lyra | +0.03 |
6 | Capella | Auriga | +0.08 |
7 | Rigel | Orion | +0.12 |
8 | Procyon | Canis Minor | +0.38 |
9 | (Achernar) | Eridanus | +0.46 |
10 | Betelgeuse | Orion | +0.50 |
11 | (Hadar) | Centaurus | +0.61 |
12 | Altair | Aquila | +0.77 |
13 | Aldebaran | Taurus | +0.85 |
14 | (Acrux) | Crux | +0.87 |
15 | Antares | Scorpius | +0.96 |
16 | Spica | Virgo | +0.98 |
17 | Pollux | Gemini | +1.14 |
18 | (Fomalhaut) | Picis Austrinus | +1.16 |
19 | Deneb | Cygnus | +1.25 |
20 | (Mimosa) | Crux | +1.25 |