What is Apparent Magnitude (m)?

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      -26.73mVega     0m
The full moon      -12.6mBrightest stars you can see in Middlesbrough      3m
maximum brightness of Venus      -4.4mfaintest stars seen with the Hubble Space Telescope      30m

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