The Caldwell Objects

magnitude field star 2' to the north-northeast.)
The best way to detect Aldebaran's true
companion star is during a lunar occultation of
Aldebaran itself. Aldebaran is the brightest star
that the Moon can occult (other than the Sun!).
These occultations occur in bunches, and
magazines like Sky & Telescope announce the
events in their pages. The last series to be seen
from the United States ended in 1999. Observers
in the contiguous United States will have to wait
for the year 2015 to see the next series begin.
Despite Aldebaran's prominent position in
the Bull's face, it is not a true member of the
Hyades cluster. That much has been confirmed
by the Hipparcos satellite. Launched in August
1989, the High Precision Parallax Collecting
Satellite was one of modern astronomy's true
technological wonders. Equipped with two
telescope systems aimed 58° apart, the rotating
spacecraft was designed to measure star
positions, brightnesses, and motions. Most
fundamentally, by utilizing the Earth's orbit as a
baseline for triangulation, Hipparcos pro-vided
astronomers with parallaxes, and thereby
distances, for tens of thousands of stars. Of the
118,218 stars in the Hipparcos catalog, the
distances of 22,396 are now known to an accura-
cy of better than 10
percent. (Before Hipparcos's
data were
released in 1997 the
distances to fewer than
1,000 stars were known
so precisely.) Among
those 22,396 precisely
triangulated stars are
many in the Hyades.
The satellite has ended
a century-long debate
The Caldwell Objects
41
over the cluster's distance — a critical quantity
for calibrating the distance scale of the entire
visible universe. Hipparcos has pinpointed the
Hyades' center of mass at 151 light-years from
Earth, plus or minus a mere 0.9 light-year.
Taking advantage of the Hipparcos data, Sky
& Telescope editor and Millennium Star Atlas
coauthor Roger W Sinnott . created the 3-D
stereogram of the Hyades shown below. T see o
the cluster in three dimensions, hold the page
against your face so each box is in front of one
eye. Relax and gaze into the distance "through"
the paper while slowly moving it away to reading
distance or a little beyond. The two views
should merge in your mind's eye. Note that
Aldebaran is omitted; at its distance of 65 lightyears
(also measured by Hipparcos), Aldebaran
is less than half as far away as the Hyades.
Of course, few will argue that Aldebaran's
rosy hue does not attract our gaze toward the
Bull's illustrious V. If you disagree, go outside,
close one eye, and block the light of Aldebaran
with your thumb while gazing outward with
your open eye. (You can also do this with a star
chart by simply covering Aldebaran with a finger.)
Without Aldebaran, the Bull's face is lost,
and the V suddenly transforms into the letter
NorZ.
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