Download Figure 10-6 The same star field shown in Figure

.'
'*'..,~
Figure 10-6
The same star field shown in Figure 2-10
indicated.
(Courtesy of M. Walker, Lick Observatory)
with apparent
.'
•.
><'•••••
magnitudes
Hipparchus misjudged the magnitudes of some of the brighter
stars, however. When the magnitude scale was extended and expressed by a mathematical formula, it developed that the brighter
stars are brighter than those of the first magnitude; indeed they
are even brighter than those of zero magnitude.
The only way to
express these hitherto unsuspected magnitudes and yet retain the
old scale (thereby avoiding a wholesale revision of stellar catalogs)
was to adopt a few negative magnitudes.
As a result, Sirius, the
brightest star in the sky aside from the sun, has an apparent magnitude of -1.4 and the star 61 Cygni A has an apparent magnitude
of +5.4 and is just visible to the naked eye on a clear moonless
night when viewed from a place far from the lights, smoke, and
smog of our modern cities.
The upshot is that a magnitude scale set up some 2000 years
ago is still in use, although stretched somewhat on a mathematical
rack. After the revisions, a zero-magnitude
star is exactly 100
times brighter than a fifth-magnitude
star, which is fairly close
to Hipparchus' scale. Thus for a magnitude difference of 5 there
is a change in brightness by a factor of 100.