The Stars In The Sky

In the 2nd century BC, Hipparchus classified the visible stars in the 
night sky according to their brightness, and by the time of Ptolemy
(around 140 AD) it was customary to categorize the stars into six 
"magnitudes", with the "first magnitude" being the brightest stars, 
and the sixth being the faintest stars visible to the naked eye.

In the mid 1800's this traditional classification scheme was quantified
based on precise luminosity measurements (with the aid of telescopes),
and it was found that 5 "magnitudes" represents a factor of 100 in
actual brightness.  A log scale was established, with the "first
magnitude" ranging from 0 to +1, the second magnitude ranging from
+1 to +2, and so on.  An increase of 1 in magnitude corresponds to
a decrease in brightness by a factor of 100^(1/5) = 2.51188...

Using this scale, it was found that the stars traditionally placed
in the same category sometimes had brightnesses that fall outside
the numerical bounds for that category.  In fact, a few stars are
brighter than the first magnitude, so to accommodate these extreme
cases it's necessary to assign them negative magnitudes.  The 
brightest star in the sky is Sirius, whose magnitude is -1.45.
(Incidentally, Sirius is called the Dog Star, since it's the most
prominent star in the constellation Canis Major, and it becomes
visible in the Northern hemisphere only during a certain span of
time in the summer, which is why we call that period the "dog days
of summer".)  The next brightest star visible from Earth is Canopus, 
which has a magnitude of -0.7.  However, this star is in the Southern 
sky and is not visible from most places in the Northern hemisphere.

Sirius and Canopus are the only two stars with significantly negative
"magnitudes".  Alpha Centauri and Arcturus (the 3rd and 4th brightest
stars in the sky) have magnitudes very close to 0.0 (actually just
slightly negative), and all the remaining stars have positive
magnitude.  Only about a dozen stars have magnitude less than (i.e.,
brighter than) 1.0.

Here's a little chart showing the maximum apparent magnitudes of 
various astronomical objects:

       Sun                          -26.74
       100 Watt bulb at 100 ft      -13.70
       Moon (full)                  -12.73
       Venus                         -4.22
       Jupiter                       -2.60
       Mars                          -2.02
       Sirius                        -1.45
       Mercury                       -0.20
       Alpha Centuri                 -0.10
       Vega                           0.00
       Saturn                         0.70
       Polaris (North Star)           2.00
       Stars in Big Dipper      1.8 to 3.3
       Andromeda galaxy               3.50
       Uranus                         5.50
       Naked eye visibility limit     6.00=======
       Neptune                        7.90
       Crab Nebula                    8.60
       3C273 (brightest quasar)      12.80
       Pluto                         14.90
       Ground telescope limit        25.00
       Hubble Telescope limit        28.00

To see an animated Java illustration showing the positions of all 
the stars with magnitudes less than 3.5, click on

        The Stars In The Sky

This applet shows the 297 brightest stars projected onto a sphere,
like a planetarium.

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