Local System and Draper Extension 77
when of apparent magnitude 11.5, which is the working
limit of the Henry Draper Extension. This means that the
completion of the Extension all around the Milky Way
will give us most of the giant stars throughout the local
system and, out to distances of two or three thousand light
years, some hundreds of thousands of less brilliant bodies.
Although distances of the individual stars will not be ob-
tained, the material should be of considerable value in
statistical investigations of the structure of the local star
cloud.
Useful though it may be for the analysis of the local
system, the Henry Draper Extension will deal very little
with most of the star clouds that make up the Milky Way.
Except for their supergiant stars, the clouds are composed
of objects fainter than magnitude 11.5. We may be able
to get some rather uncertain information concerning their
structure from undifferentiated star counts; some rather
better knowledge from surface photometry of the Milky
Way. But a more direct way of investigating the distances
and structure of the Milky Way star clouds is described in
the next section. Leaving the local system, we enter the
fourth territory of our sidereal explorations and tackle the
difficult problem of measuring directly the dimensions of
the galactic system.