http://www.skyandtelescope.com.au 57CENTER FOR ASTROPHYSICS / HARVARD & SMITHSONIAN
Discovery
Photographed in 1888 with a 20-cm
astrograph at the Harvard College
Observatory (HCO) in Cambridge,
Massachusetts, the discovery plate,
B2312, was examined by Williamina
Fleming of the HCO, who published
her description of the photo in 1890. In
Volume 18 of the Annals of the Harvard
College Observatory, she mentioned a
“semicircular indentation” that we
now call the Horsehead Nebula:A large nebulosity extending nearly
south from ζ Orionis for about 60′.
More intense and well marked on the
following side, with a semicircular
indentation 5′ in diameter 30′ of ζ.Although the discovery was initially
credited to the HCO’s Director Edward
Pickering in the 1895 edition of the
Index Catalog (IC), in Volume 60 of its
Annals in 1908 the HCO correctly listed
Fleming as discoverer of the Horsehead.
The nebula was catalogued as B33
by E. E. Barnard in 1919. The “large
nebulosity,” now known as IC 434,
was originally discovered by William
Herschel in 1786.Visual detection
IC 434 provides the slightly brighter
background against which the
Horsehead is silhouetted. Without this
background it would be invisible to
visual observers, so it’s IC 434 that’s
crucial to detect — if you can’t see IC
434 you won’t see the Horsehead.
An important tool for visually
observing IC 434 is a hydrogen-beta
nebula filter. Sometimes referred to
as the ‘Horsehead Filter,’ it increases
the contrast between IC 434, the
background sky and the Horsehead. If
you have one, use it. If you don’t, get
one. It’s useful on more than just the
Horsehead, of course, but its nickname
is deserved because it can make this
famously difficult object a great deal
easier to see.
Although I was able to detect
the Horsehead using an Ultra-High
Contrast filter with my 50-cm,borrowing Chuck’s hydrogen-beta
filter increased contrast immensely
and produced a much more satisfying
image. On the other hand, if you’re
ever under a pristine dark sky with
Orion near the meridian, try to see
the Horsehead without a filter and you
may be pleasantly surprised. On these
rare nights I’ve thought it looked like a
knight, the chess piece.
If you’ve never seen the Horsehead
visually, there are two confounding
factors to overcome. The first is to
get yourself under a truly dark and
transparent sky. Without a quality dark
sky you have almost no chance to see it
visually no matter how large or fine a
telescope you’re looking through. Even
the hydrogen-beta filter won’t help much.
The second factor is expectation,
and being aware of this is just as
important as the observing conditions.
We’ve all seen wide-field photographs
of the Horsehead and its environs (like
those in this article) showing it as a
rather small object in relation to its
spectacular surroundings. Because of
photos like these, there’s a good chance
you’ll expect the Horsehead to appear
smaller through your eyepiece than
it actually appears, and to have more
contrast with its background. If so,
your eye might skip over its much larger
apparent size and subtle contrast.
To calibrate your expectations, the
Horsehead is approximately the same
apparent size as M27, the Dumbbell
Nebula, and has even less contrast
than the Pillars of Creation in M16.A big telescope isn’t required to
see the Horsehead. I’ve been able to
see it quite well through scopes down
to a 114-mm Newtonian from the
unspoiled skies of Steens Mountain in
southeast Oregon, illustrating that a
top-notch sky is more important than
the size of the telescope.Details
In her plate notes from 1890,
Williamina Fleming mentioned that
IC 434 is “more intense and well
marked on the following (east) side,”
but visually it doesn’t have a sharp
eastern edge, even though it does
fade more gradually westward. The
sharpest boundary is created by the
Horsehead Nebula itself, which helps
tremendously with its detectability.
Even so, on most nights the Horsehead
will appear as a dark, fuzzy-edged
scoop into IC 434 that’s only slightly
darker than its surroundings. IC 434
is quite faint even with the help of a
hydrogen-beta filter.
We see IC 434 because of the O-type
star Sigma (σ) Orionis. Its intense
ultraviolet light causes IC 434 to glow
by a process called photoionisation,M43 M42RigelIC 431IC 434
IC 43520232024B33βτηδυιε
σζORION–8° 5 h 40 m 5 h 30 m 5 h 20 m–6°–4°–2°0 °
IC 432M78Star magnitudes21 3 4 5 6 7S THE HORSE’S HEAD APPEARS A cropped
scan of plate B2312, the discovery image of the
Horsehead Nebula (at centre) from the Harvard
College Observatory, has also had the colours
reversed for better contrast. Taken with the 20-
cm Bache doublet refractor in 1888, this plate
was examined by Williamina Fleming in 1890
when she discovered the Horsehead Nebula.
The Horsehead is included in E. E. Barnard’s
1919 catalogue of dark nebulae as B33. We
now know the Horsehead is the closest nebula
pillar sculpted by photoionisation, and that
protostars are forming within it.NE