Local System and Draper Extension 61
and the existence of localized star clouds. We proceed
rather to an account of a method of measuring and dis-
secting the section of the stellar universe which, though
within a few thousand light years of the Sun, nevertheless
lies beyond the range of easy trigonometric survey or meas-
urement by means of spectroscopic parallax.
Some twenty years ago the construction of the Henry
Draper Catalogue of stellar spectra was begun at the Har-
vard Observatory, after two decades of analyzing and
classifying the spectra of the brighter stars. Professor
Pickering and Miss Cannon undertook in this catalogue to
list the positions and magnitudes for stars over the whole
sky to approximately the ninth magnitude. The six thou-
sand naked-eye stars were included along with about two
hundred and twenty thousand telescopic objects. The cat-
alogue, in nine volumes, was published in fifteen years. A
study of the results showed that the catalogue was prac-
tically complete in the northern hemisphere for stars to the
magnitude 8.25, and in the southern hemisphere to 8.75;
but more than a hundred thousand stars fainter than those
limits were also included. Six major groups were estab-
lished in the classification of spectra, the sequence of classes
bearing the letters B, A, F, G, K, M, in the order of de-
creasing surface temperature. Including the aberrant types
and the subdivisions of the major classes, Miss Cannon has
differentiated approximately fifty kinds of spectra.
The average number of stars per square degree to mag-
nitude 8.75 for the whole sky is shown for the six major
classes in Figure 1 (cf. Shapley and Cannon, Harvard Re-
print 6, 1924). The conspicuously different distribution
among the spectral types for fainter stars in the Milky Way
is shown for localized regions in Cygnus, Figure 2, and in