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In astronomy it is common to use the concept of Apparent Magnitude, which is the measure that expresses brightness as we see it.

The magnitude scale has a historical origin but has been redefined in the modern era. The magnitude Scale is explained in detail in our EBook Magnitude & Distance.



In Summary apparent magnitude (symbol m):

  • indicates the brightness of a star as we see it
  • is a scale that runs opposite to brightness (the smaller the magnitude, the brighter the star)
  • is centred at the apparent magnitude of Vega (m = 0)
  • allows negative values for bright stars
  • is a logarithmic scale in which five units in magnitude correspond to a factor of 100 in brightness

Absolute magnitude uses the same scale as apparent magnitude. It has a value which is defined as the apparent magnitude a star would have if it were located at a standard distance of 10 pc from Earth.

As an example the Sun has an apparent magnitude m = -26.73 but it has an absolute magnitude of +4.75. That is the magnitude we should see if the Sun was at a distance of 10 pc. Therefore absolute magnitude is directly related to the luminosity of a star and is a way to express its luminosity in the magnitude scale.


Now if:

  • we have a measure of the absolute magnitude (M) of a star and
  • we compare that with the apparent magnitude (m),

then we can use the Inverse Square Law to find the distance.


The method used for this in astronomy is related to (m - M) which is called the Distance Modulus.






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