Download The magnitude scale

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Magnitudes
Understanding astronomical magnitudes
Looking at the magnitude scale
-25 Sun (-26.7)
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.
-20
-15
The diagram on the right gives you an idea
of the magnitudes of various objects.
Full Moon (-12.6)
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.
-10
-5
Venus at brightest (-4.4)
Sirius (-1.4)
The Sloan Digital Sky Survey can see
objects as faint as about magnitude 23.
0
Comparing magnitudes
5
Every increase of five magnitudes
corresponds to 100 times the quantity of
light reaching the eye or telescope.
Objects visible to the naked eye from a dark site (+6)
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.
10
Objects just visible in binoculars (+10)
15
Pluto (+15.1)
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.]
20
Objects visible to the human eye looking through a
large telescope (+21)
25
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
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