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Magnitude scale theory
How bright a star looks is given by its apparent magnitude. The absolute magnitude of a star is defined as the apparent magnitude that it would have if placed at a distance of 10 parsecs from the Earth.

Consider two stars where star A appears to be brighter than star B.
Let the apparent magnitude of star A = mA and the apparent magnitude of star B be mB.
IA/IB = 100(mB mA)/5

Therefore taking logs of both sides : mB mA = 5/2[lg(IA/IB)]

Now let the magnitude of A (mA) be that at 10 parsecs, in other words the absolute magnitude of the star (M) and let mB be the magnitude (m) at some other distance d (also measured in parsecs).

Therefore: lots m M = 5/2[lg(IA/IB)] lots of space

M m = 5 - 5lg(d)

© Keith Gibbs 2009