Friday, May 15, 2015

Apparent Brightness and Magnitude

A basic observable quantity for a star is its brightness. Because stars can have a very broad range of brightness, astronomers commonly introduce a logarithmic scale called a magnitude scale to classify the brightness.

The Magnitude Scale

The magnitudes m1 and m2 for two stars are related to the corresponding brightnesses b1 and b2 through the equation

where "log" means the (base-10) logarithm of the corresponding number; that is, the power to which 10 must be raised to give the number. Because this relation is logarithmic, a very large range in brightnesses corresponds to a much smaller range of magnitudes; this is a major utility of the magnitude scale. 

  

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