The Caldwell Objects

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stars." Ernst Hartung calls it a "fine scattered
cluster . . . containing orange, yellow, white and
bluish stars." And Philip Harrington saw "some
appearing golden, others bluish." The most
obvious color can be seen in a pair of stars at the
cluster's northwestern edge. This wide pair
marks the foot of the T. The brighter of this pair's
two stars is V910 Centauri, which shines at
magnitude 7.2 and has a ruddy spectral class of
M0Ib.
Now relax your gaze, and see if the southeastern
side of the Τ doesn't widen into a triangle
or wedge of starlight. A pair of red stars appears
at the triangle's northern tip. The arms of the Τ
are marked by a 6'-long stream of stars flowing to
the north and a fainter one flowing an equal
distance to the south. The wedge of starlight then
can be seen as a body — using my imagination it
looks like a great frigate bird with outstretched
wings and a long forked tail.
With 72x and averted vision, look for a
fantastic 1°-wide pool of darkness abutting the
390
southeastern corner of the triangle. This dark pool
has a slightly elliptical shape that manifests itself
more clearly the more you concentrate on it. If
you suspect that you may have seen this dark
lagoon, but aren't sure, try defocusing the image
ever so slightly. This will blend the bright stars
surrounding the pool, helping the latter to stand
out. Focus the image again and look closely at the
multitude of stars in the triangle. How many
doubles do you see? There should be at least a
half dozen bright pairs and an equal number of
fainter ones. Immediately east of the triangle, a
roughly 9th-magnitude star forms part of a
minicluster of dim 12th- to 13th-magnitude suns.
Sweep your eyes east and west, then north and
south, across NGC 3766, and see if you don't pick
up all manner of loops and star chains. At one
moment I saw a ring of 11th-magnitude stars
within the triangle; it had a string of equally
bright stars dropping to the south. At high power
the cluster begins to lose its luster, though the
extra magnification does help separate the many
fine doubles. My drawing shows only the
cluster's brightest stars, and serves mainly to
show the location and extent of the dark pool
mentioned earlier. Use the photograph on page
388 as your guide to the many star patterns
visible in this wonderful cluster.
The eclipsing binary star BF Centauri lies at
NGC 3766's extreme northern edge. Its light
oscillates between magnitude 8.5 and 9.4 every
3.7 days; the star spends 20 percent of this period
in eclipse. Following this star's light curve would
make a good project for an eager observer with a
keen eye, a photometer, or a CCD.
Deep-Sky Companions: The Caldwell Objects