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 2 times.
This was hardly very scientific.
Apparent Magnitude
demonstrate an understanding of the apparent magnitude scale and
how it relates to observed brightness of stars
When we look at the stars in the sky some seem very bright whilst others are just bright enough to be visible.
How bright a star appears to be is known as its apparent magnitude (m).
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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 2 times. This was hardly very scientific. |
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In 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.
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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 |
Remember
- The apparent magnitude of a star depends on two things: How luminous it actually is (its absolute magnitude) and how far away it is.
- a star with m = 3 is 2.5 times brighter than a star with m = 4