The Stellar Magnitude System

by Alan M. MacRobert, Sky and Telescope Magazine

Star Magnitudes

• Rigel, Capella, Arcturus, and Vega are magnitude 0
• Sirius shines at magnitude –1.5
• Venus reaches –4.4
• Full Moon is about –12.5 
• Sun blazes at magnitude –26.7.

Photometric Systems [UBVRI]

• Negative-Magnitudes (< 0)
  • U encompasses the near-ultraviolet
  • B is blue
  • V corresponds fairly closely to the old visual magnitude
• Positive-Magnitudes (> 0) 
  • R standard red magnitudes
  • I is  near-infrared magnitudes.

Color Index (B magnitude minus the V magnitude)  The difference between a star's photographic and visual magnitude 

• positive for yellow, orange, and red stars,
• negative for blue ones."

Absolute Magnitude Scale

• An object's absolute magnitude is simply how bright it would appear if placed at a standard distance of 10 parsecs (32.6 light-years)