The magnitude system
 
The energy arriving from any astronomical body can, in principle, be measured absolutely. The brightness of any point source can be determined in terms of the number of watts which are collected by a telescope of a given size. For extended objects similar measurements can be made of the surface brightness. These types of measurement can be applied to any part of the electromagnetic spectrum. However, in the optical part of the spectrum, absolutely brightness measurements are rarely made directly; they are usually obtained by comparison with a set of stars which are chosen to act as standards. The first brightness comparisons were, of course, made directly by eye. In the classification introduced by Hipparchus, the visible stars were divided into six groups. The brightest stars were labelled as being of first magnitude and the faintest which could just be detected by eye were labelled as being of sixth magnitude. Stars with the brightnesses between these limits were labelled as second, third, fourth or fifth magnitude, depending on how bright the star appeared.
The advent of the telescope allowed stars to be recorded with magnitudes greater than sixth; catalogues of the eighteenth century record stars of seventh, eighth and ninth magnitude. At the other end of the scale, photometers attached to telescopes revealed that some stars were brighter than the first magnitude classification and so the scale was extended to include zero and even negative magnitude stars. The range of brightnesses amongst the stars revealed that it is necessary to subdivide the unit of magnitude. The stars visible to the naked eye could have magnitudes of −0·14, +2·83 or +5·86, say, while stars which can only be detected with the use of a telescope could have magnitudes are of +6·76, +8·54 or even as faint as +23.