Absolute Magnitude (M)

Apparent magnitude (m) is how bright a star appears to be but this obviously depends on how far away it is. The closer the star is to us the brighter it will appear to be.

The Absolute Magnitude (M) of a star is a measure of how luminous it actually is.

It is defined as how bright a star would appear to be if it were at a distance of 10 parsecs from Earth. (so if a star were actually 10 parsecs from Earth then M and m would be equal)

If we know the apparent magnitude (m) and distance in parsecs (d) a star is away from us we can calculate its absolute magnitude (M) using this equation;

M = m + 5 - 5 log d

I am not going to explain logarithms here. All you need to know is that there is a "log" button on your calculator which you can use to find log d.

Examples:

e.g. for Sirius  M = -1.47 + 5 - (5 x log 2.64)      =      3.53 - 2.11    =   1.42   (near enough!)

Show that the absolute magnitude of Vega is the value shown in the table and calculate the absolute magnitudes of Betelgeuse and Polaris yourself.