The magnitude of star is the measure of its brightness. When measured from the earth, two stars that differ by one magnitude have a brightness ratio of 2.512

\dfrac{\text{The brightness of star of nth magnitude}}{\text{The brightness of the star of (n+m)th magnitude}} = (2.512)^m

If two stars of magnitude m_1 and m_2 possess brightness l_1 and l_2 respectively, then

m_2 - m_1 = -2.5 log\dfrac{l_2}{l_1}