Star Magnitude System For Classifying The Brightness Of Stars
by Kevin Brown : last updated: July 17, 2008A system of magnitude numbers (mag for short) is used to classify the brightness of the stars we see in the sky.
This magnitude system was actually first devised by Hipparchus, the Greek astronomer, a long time ago in the second century BC.
It is based on the principle that the stars we can see with the unaided eye, should be classified between magnitude 1 for the brightest and magnitude 6, for the faintest.
Please note that the magnitude classification refers to the apparent brightness of the star as it appears to us, observing from the surface of the Earth. Magnitude figures usually have nothing to do with the actual brightness of the star.
A very bright star a great distance away, could easily appear less bright than a fainter star, much closer to Earth. Magnitude is (usually) not used as a measure of actual brightness.
However, sometimes books will quote actual magnitudes, although this should be clearly stated. You need to watch out for this so you do not get confused.
Back in the time of Hipparchus, magnitudes would have just been judged by eye.
In more recent times, sensitive scientific measurement has been applied to the classification of apparent brighness, but the fundamentals of the system have not really changed.
Magnitude 1 is now defined as exactly one hundred times brighter than magnitude 6. This means that each magnitude number is approximately 2.5 times brighter than the next. So a mag 1 star is 2.5 times brighter than a mag 2 star, and so on.
The faintest stars that people with good, but unaided eyesight can see, are still classified as magnitude 6.
As a consequence, some objects in our sky are so bright that negative magnitudes are needed, if the scale is to remain consistent.
So the brightest star we can see, Sirius, is magnitude -1.4. And the planet Venus is magnitude -4.
