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 brightness's 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