When we look at the stars in the sky, some seem very bright while others are just bright enough to be visible.
How bright a star appears to be is known as its apparent magnitude (m).
Originally (Hipparchus 190 – 120 BC) the scale was from 1 to 6.
The brightest stars were m = 1 and the faintest, just barely visible with the naked eye, were m = 6. Going from 1 magnitude to another meant an increase or decrease in brightness of about two times.
This was hardly very scientific.
n 1856 Pogson made things more formal by saying that a magnitude 1 star was 100 times brighter than a magnitude 6 star.
The difference between magnitudes is, therefore, the fifth root of 100 = 2.51, known as Pogson’s ratio.
So if star A has an apparent magnitude of 5 and star B has an apparent magnitude of 4 then B appears 2.51 x brighter than A.
We also need a reference star, a star of known brightness that we can compare all the others by. Various stars have been used for this including Polaris and Vega.
Here are some apparent magnitudes.
|The Sun -26.73m||Vega 0m|
|The full moon -12.6m||Brightest stars you can see in Middlesbrough 3m|
|maximum brightness of Venus -4.4m||faintest stars seen with the Hubble Space Telescope 30m|