The Caldwell Objects

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"cluster variables" were of fundamental importance
because they all had roughly the same
intrinsic brightnesses. If an astronomer compared
how bright (on average) a cluster variable
appeared through a telescope to how bright it
really was, he or she could determine the distance
to the variable, and hence to the
galaxy or cluster it is in.
Shapley's observations
of 11 RR Lyrae stars
in NGC 7006 showed the
cluster to be about five
times as remote as M3 or
M5, the most distant
globulars known at the
time. After discovering
and tracking both RR
Lyraes and Cepheid variable
stars in other globulars
across the heavens, he
placed his cosmic
yardstick between the
Earth and the clusters and
mapped the threedimensional
structure of
our host galaxy. With
globulars lying tens or
hundreds of thousands of
light-years from our
Earth, our galaxy swelled
to magnificent proportions in Shapley's eyes.
These were pioneering years, however, and
Shapley did not know then, as we do now, just
how dramatically our galaxy's interstellar medium
dims the light of globular clusters at low
galactic latitudes. Shapley's underestimation of
this effect led him to overestimate the distances
of globulars, and with them the size of our
galaxy. For instance, Shapley had positioned
NGC 7006 about 180,000 light-years from Earth,
roughly 33 percent farther than we know it is
today. But his data and analyses gave the first
170
reliable indication that the Milky Way was a
much larger system than previously had been
believed. His results also offered astronomers
their first real glimpse of the Milky Way's fantastic
three-dimensional structure. Armed with the
distances to globulars, Shapley had the power to
dethrone the Sun from its central position in our
galaxy, just as Copernicus had done to the Earth
in our solar system four centuries earlier.
Small-telescope users will have to keep in mind
the role this dim and distant globular cluster
played in helping Shapley model our galaxy,
since its visual appearance is understandably
underwhelming. The cluster resides at a
relatively low galactic latitude (-19°), so its
brightness is dimmed by intervening dust. We
see its 3.6'-wide disk shining at a modest apparent
visual magnitude of 10.6. NGC 7006's surface
brightness is also modest; on average,
Deep-Sky Companions: The Caldwell Objects