The Light of the Stars 59
ing about as bright as the Sun, and those of Class M a
hundred times fainter. Hertzsprung has called these by
the happily chosen names of “giant” and “dwarf” stars.
The members of either group show a dispersion in bright-
ness of a magnitude or so on either side of the mean—
and more in scattering instances—so that the two groups
are really separated only in Classes K and M. Even in
Class K, a few stragglers occupy an intermediate position,
and in Class G the brighter dwarfs and the fainter giants
overlap. In Class F the two are much intermingled, and
in Classes A and B there is no real distribution. Almost
all known stars belong to one or other of these divisions.
The few enormously luminous stars, such as Rigel and
Antares, may be regarded as scattering giants, much
brighter than the average. There are, however, a very
few white stars of very low luminosity—the faint com-
panions to Sirius and to Omicron Eridani, and one or
two others—which fall definitely outside the scheme, and
present an unsolved riddle.
The recognition of the giant and dwarf stars helps to
clear up a number of puzzling things—particularly some
connected with apparent contradictions in the properties
of the “average star” of some given kind. For example,
it is certain that a large majority of all the stars in a
given region of space (say within twenty parsecs of the
Sun) are dwarfs—most of them faint and red. Yet,
among the stars brighter than the sixth magnitude, the
large majority are giants—as is shown both by direct
measures of parallax, and by the study of their proper
motions. This is really simple enough. When we make
a list of the stars visible to the naked eye, we include only
those dwarfs which lie very near us, for, since they are
intrinsically faint, the remoter ones are invisible to us.