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