The Caldwell Objects

sion of a "thin flat ring of enormous dimensions,
seen very obliquely." Since there was no reliable
way at the time to determine the distances to
these nebulae, their true sizes and natures
remained a mystery. It wasn't until 1924 that
astronomers finally learned that spiral nebulae
weren't baby solar systems within our own
galaxy but external Milky Ways of their own,
scattered throughout a universe whose dimensions
humankind was just beginning to grasp.
We now know that NGC 4565, like several
other Caldwell objects, is a member of the
cosmologically local Coma-Sculptor Cloud of
galaxies. It is also the largest and most famous
edge-on spiral galaxy in the night sky. If we
accept its estimated distance of 32 million lightyears,
this moderately late-type spiral has a true
linear diameter of 150,000 light-years and a total
mass of 200 billion Suns. Interestingly, the
Hubble Space Telescope has identified roughly
200 globular clusters around NGC 4565, a total
virtually indistinguishable from that attending
our own Milky Way. Recent radio observations
have revealed that the galaxy's "rotation curve"
— the orbital speed of its disk's constituents,
plotted as a function of distance from the nucleus
— mim-
The Caldwell Objects
38
ics that of our galaxy; as with our Milky Way,
NGC 4565's rotation curve indirectly argues for
an extended halo of invisible "dark matter"
enveloping the disk.
In high-resolution photographs the flattened
disk of NGC 4565 displays bright and dark
patches with much dumpiness and branching. As
with NGC 891, photographs also show dark
fingers of dust projecting above and below the
equatorial plane of NGC 4565; this material may
have been spewed out of the disk by supernova
explosions, collisions of interstellar clouds, or
magnetic forces. It would be quite a challenge, I
imagine, to see these dusty projections at the
eyepiece of even a large telescope.
Shining at a respectable magnitude 9.6, NGC
4565 is a ¼°-long needle of light about 2° due east
of 17 Comae Berenices, a fine binocular double
star in the diffuse, naked-eye Coma Berenices star
cluster (also known as Melotte 111). It is a
fabulous sight in a telescope of any size. The
most dramatic view of NGC 45651 can recall was
through the 16-inch Boiler & Chivens reflector at
Harvard's Oak Ridge Station in Massachusetts.
On one warm spring night in the mid-1970s I
found myself galaxy-hopping with this telescope
through the rich Coma-Virgo galaxy cloud. I did
not know what to expect as I slowly slewed the
telescope toward NGC 4565, for I had only seen
photographs of the great spiral in books. Looking
intently in the main eyepiece for a possible dim
glow, I watched countless field stars sail across
the eyepiece. Suddenly the sharp tip of a blade of
light entered the field from the upper left. Deeper
and deeper it cut into the field of view, until the
galaxy's robust hub and girdle of darkness all but
shattered the visual serenity that had preceded
its appearance. I continued to slew the telescope,
but the galaxy did not end — not until its leading
edge began to exit the opposite edge of the field
of view This
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