One of the most prized telescopic dark nebulae
is the Horsehead Nebula in Orion. Its widespread
popularity comes from photographic images, not
its visual impact at the eyepiece. This magnificent
celestial bust of a horse’s head — like a knight
chess piece — is one of the most challenging
objects of its kind, especially owing to its diminu-
tive size (5') and low-contrast surroundings.
Scottish astronomer Williamina Fleming
discovered the Horsehead Nebula in 1888
while scanning photographic plates at Harvard
College Observatory. Edward Emerson Barnard,
who imaged it in 1913, said, “This object has not
received the attention it deserves,” and listed it as
his 33rd object in his catalog of dark nebulae. The
nebula has ever since been known as Barnard 33
(B33), which, unfortunately, like the dark cloud that
it designates, obscures the light of Fleming’s find.
We see the dark Horsehead Nebula only
because it stands out against the diffuse glow
of emission nebula IC 434. Both objects belong to
the Orion B molecular cloud, which is one of thelargest star-forming regions near our Sun, some
1,300 light-years distant. The Horsehead itself
measures some 4 light-years tall and 3 light-years
wide, and is part of a larger cosmic landscape
hundreds of light-years across that includes the
Great Orion Nebula (see #19). If we could sweep
away the dense dust covering the Horsehead,
we’d find untold stars on the verge of being born.
To find the Horsehead, first locate IC 434.
This long reef of dim nebulosity runs for more
than 1° southeast of Alnitak (Zeta [ζ] Orionis),
the easternmost star in Orion’s Belt. Search for a
tiny notch of darkness halfway down the sharp
and straight eastern fringe of IC 434. Its appear-
ance in small telescopes is one that looks more
like a faded thumbprint than a horse’s head.
Low power helps to concentrate IC 434’s glow,
which increases contrast and aids in finding the
dark notch. The nebula has been spied through
telescopes as small as 4 inches and has even
been seen through handheld binoculars using an
H-beta filter. — S.J.O.28 The Horsehead Nebula