AstroProjects Magnitudes
Astronomers measure the brightness of an object in the sky using the magnitude scale .
The scale is somewhat strange because brighter objects have smaller magnitudes , while fainter objects have larger magnitudes - the opposite of what you might expect.
The diagram on the right gives you an idea of the magnitudes of various objects.
The faintest object visible to the naked eye from a dark site has magnitude six . Sirius, the brightest star, has magnitude -1.4. The planets, Mercury, Venus, Mars, Jupiter and
Saturn vary in brightness, but are generally quite bright - the brightest being Venus which can reach a magnitude of -4.4.
The Sloan Digital Sky Survey can see objects as faint as about magnitude 23.
Every increase of five magnitudes corresponds to 100 times the quantity of light reaching the eye or telescope.
For example, a star of magnitude 11 is 100 times as faint as one at the naked eye limit, magnitude 6. A star of magnitude 16 is 100 times fainter again, i.e. 10,000 times fainter than one at the naked eye limit, and so on.
An increase of one magnitude corresponds to approximately 2.5 less light reaching the eye or telescope.
[A scale like this where a quantity is multiplied by a fixed amount for a fixed increase in its scale value, is called a logarithmic scale . Another example that you may be familiar with is the decibel scale for sound intensity.]
-25 Sun (-26.7)
-20
-15
-10
-5
Venus at brightest (-4.4)
0
5
10 Objects just visible in binoculars (+10)
15 Pluto (+15.1)
20
Objects visible to the human eye looking through a large telescope (+21)
25
Full Moon (-12.6)
Sirius (-1.4)
Objects visible to the naked eye from a dark site (+6)
30
Objects visible to cameras on the Hubble Space
Telescope and large Earth based telecopes (+30)
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Richard Beare, 18th December, 2007 Version 1.00 Page 1 of 1