Even a casual glance at the night sky will show that some stars are very bright while others hover on the limit of naked eye visibility. As long ago as 150BC, the Greek astronomer Hipparchus devised a system whereby the stars were classed according to their brightness, with the brightest stars being given a magnitude value of 1 and the faintest a value of 6. The system is still used today, although with certain refinements. Magnitudes can be estimated to within 0.01 with modern instruments. The brightest stars have had to be designated zero or even negative values to fit in with the current scale. However, it isn't just the stars that have had there magnitudes assessed. For example, the sun has a magnitude of -26.7, other magnitudes can be seen in the table below.

Pluto, on the other hand, hovers at around 15th magnitude, although this can vary according to it's distance from the sun. Uranus can just become visible to the naked eye at magnitude 5.6. A pair of 7 X 50 binoculars will show stars down to 8th or 9th magnitude, although in comparison the world's largest teelscopes will reveal objects such as distant galaxies down to 25th magnitude.
A difference of 1 magnitude between two stars is equivalent to a difference in brightness of 2.512. For example, a star of magnitude 3 is 2.512 times as bright as one of magnitude 5 and so on. In 1856, Pogson standardised the magnitude system and he estimated that a star of 1st magnitude was 100 times brighter than one of 6th magnitude.

The above system classifies celestial objects according to their apparent magnitude, which is a measurement of their brightness in visible light only. However, stars emit radiation at many different wavelengths and these are taken into account when the bolometric magnitude is assessed. It is sometimes difficult to calculate the bolometric magnitude of a star due to the fact that the Earth's atmosphere absorbs certain types of radiation.
There is also absolute magnitude, which is the magnitude a star would have at a distance of 10 parsecs, or 32.6 light years. A parsec (3.26 light years) is the distance at which a star would have an angle of parallax of 1 second of arc. At this distance the sun would shine with a magnitude 4.83, while Sirius would be much brighter at magnitude 1.3. Meanwhile, Polaris, the Pole Star, is situated at a distance of several hundred light years and has an apparent magnitude of 1.99, although it's absolute magnitude is minus 4.6. It can be seen that the absolute magnitude of a star is far better indication of its actual luminosity.