This is a measure of the brightness of a celestial object. The lower the value, the brighter the object, so magnitude 4 is brighter than magnitude 0, which is in turn brighter than magnitude +4. The scale is logarithmic, and a difference of 5 magnitudes means a brightness difference of exactly 100 times. A difference of one magnitude corresponds to a brightness difference of around 2.51 (the fifth root of 100).
The system was started by the ancient Greeks, who divided the stars into one of six magnitude groups with stars of the first magnitude being the first ones to be visible after sunset. In modern times, the scale has been extended in both directions and more strictly defined.
Examples of magnitude values for wellknown objects are;
Sun  26.7 (about 400 000 times brighter than full Moon!) 
Full Moon  12.7 
Brightest Iridium flares  8 
Venus (at brightest)  4.4 
International Space Station  2 
Sirius (brightest star)  1.44 
Limit of human eye  +6 to +7 
Limit of 10x50 binoculars  +9 
Pluto  +14 
Limit of Hubble Space Telescope  +30 
