Ill
THE LOCAL SYSTEM AND THE HENRY
DRAPER EXTENSION
FOR more than a century astronomers have been sys-
tematically examining the distribution of both naked-
eye and telescopic stars, recording their numbers in dif-
ferent parts of the sky, and determining their increasing
frequency with decreasing apparent brightness. Important
and laborious catalogues have been compiled of the radial
velocities, the proper motions, the magnitudes, the posi-
tions, and the spectra of thousands of stars. Although it
was realized that the stars extend in great numbers beyond
the limits of available telescopic power, it was generally
felt a generation ago that current studies were dealing with
a very considerable part of the stellar system. In recent
years, however, we have come to the realization that our
catalogues pertain to a localized bit of the sidereal universe
and that the major part of even our own Milky Way system
is comprised of stars fainter than the tenth magnitude, and
is more than ten times as remote as the average catalogue
star.
Analyses of the data of our catalogues, especially the
study of positions, numbers, and spectra, long ago showed
structural detail in the arrangement of the stars; they in-
dicated not only the galactic concentration, and the falling
off in star numbers with increasing distance (in certain direc-
tions), but also local irregularities. And the studies of star
motions revealed to Kapteyn and his successors a remarkable
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