What does magnitude mean?

Astronomers use a system of magnitudes to accurately measure the brightness of astronomical objects. An object is said to have a certain numerical magnitude–the larger the magnitude number, the fainter the object. Each object with an increased number (next larger magnitude number) is approximately 2.5 times fainter. The faintest star you can see with your unaided eye (without optical aid) is about sixth magnitude (from dark skies) or magnitude 6.0, whereas the brightest stars are negative numbers. The Sun is a whopping magnitude -26.7; Sirius, the brightest appearing star in the night sky, is magnitude -1.5; while Uranus is magnitude 5.7 (about near the naked eye limit).

Apparent magnitudes of selected objects:

Sun                             -26.7
Full Moon                   -12.6
Venus (maximum)      -4.7
Sirius                          -1.5
Saturn (maximum)     -0.5
Naked-eye limit           6.0
Uranus                        5.7
Quasar 3C-273         12.9
Pluto (maximum)       13.7
8 inch telescope        14.2 (limit)
Hubble Telescope     30.0 (limit)