The Light of the Stars 47
appear to have if it were brought to a standard distance—
which he chose as 10 parsecs. On this scale the absolute
magnitude of the Sun is +4.8, that of a star 100 times
fainter would be +9.8, that of Innes’s star is +15; while
a star 100 times brighter than the Sun would have an ab-
solute magnitude of —0.2 (which is approximately the
case for Arcturus or Vega). Still brighter stars, with
negative absolute magnitudes, exist; but to obtain any
accurate notion of their brightness we need some more
powerful method of measuring distance.
One such method has developed from the discovery of
a number of “moving clusters” of stars. All the members
of each of these clusters are moving together in space—
in parallel directions and at the same rate—though their
proper motions in the sky appear, on account of perspec-
tive, to converge towards a definite vanishing point. When
we know this point, and measure with the spectroscope the
rate at which the stars of the cluster are receding from
us (or perhaps approaching, in which case their proper
motions diverge from a common point)—it becomes a
simple matter of geometry to calculate the parallax and
distance of every star in the cluster. The first of these
clusters to be fully investigated (in a classical paper by
Lewis Boss) is the Hyades group in the constellation
Taurus, which contains about a hundred stars forming a
roughly globular group about ten parsecs in diameter, with
its centre forty parsecs from the Sun, and ranging in
luminosity from fifty times the Sun’s light to one-tenth of
its brightness.
Of more immediate interest to us is a huge group in the
southern sky studied by Kapteyn, and containing most of
the brighter stars in Scorpius, Centaurus and the Southern
Cross. The proper motions of these stars are small and