Absolute and apparent magnitude

 St Mary's in Wigton is famous for having gold stars painted on the ceiling of the chancel.

Some of the stars are bigger than others. It's like that in the night sky - some stars look brighter than others. The Greek astronomer Hipparchus decided that he could discern 6 levels of brightness and this has turned out to be reliable to this day. The brightest stars he called 1 and the dimmest 6. This is called the apparent brightness because it is what it looked like to his eye. But the differences in brightness could be because one identical star is further away than the other - or that two stars of different intrinsic brightness are the same difference away. This intrinsic brightness is called the absolute magnitude. To put this on a scientific setting, it was decided to calculate how bright each star would seem if they could all be moved to the same difference away - 10 parsecs. This distance appears in the equation linking apparent magnitude m and absolute magnitude M. It is m - M = 5log(d/10) where d is the distance to the star in parsecs.