The Visual Magnitude Scale
Visual brightness of stars, planets and other astronomical objects is based on the visual magnitude scale (Vo). Every integer increase in magnitude represents a 2.5 increase in brightness. So the Sun is 6 trillion times brighter than a 6th magnitude star, which in turn is 4 billion times brighter than a 30th magnitude star - the limit of the Hubble Space Telescope.
Magnitude   Object or Limit  
-27 : The Sun
-20 : Brightest Meteors
-13 : Full Moon
-4 : Venus (brightest planet)
-1 : Sirius (brightest star)
+6 : Naked-eye limit
+9 : Binoculars limit
+13 : 8" backyard telescope limit
+18 : Large telescope limit(visual)
+23 : Large telescope limit(photograpic)
+27 : Large telescope limit(CCD imaging)
+30 : Hubble Space Telescope limit