58 AUSTRALIAN SKY & TELESCOPE February | March 2019
GOING DEEPwhich produces its characteristic red
glow seen in colour photos. We see the
Horsehead as a dark nebula, but on its
opposite side it’s also photoionised by
Sigma. We see a tiny bit of that glow
around the top edge of the Horsehead’s
silhouette, which enhances its
contrast. Seeing this thin bright edge
is a challenge, but it is possible. On the
best nights you might be able to see the
faintly glowing equine outline of the
horse’s mane, head and brow, as well as
the top of the muzzle. It’s an exciting
observation and demonstrates that the
Horsehead is an extension of IC 434
that’s silhouetted against itself.
More difficult to see is the bottom
(eastern) edge of the Horsehead’s
muzzle. It’s much less distinct than
the top, and whether you can see
any contrast between IC 434 and the
bottom of the muzzle will determine
if you see a dark nebula shaped like a
horse’s head in profile or something
more like Fleming’s “semicircular
indentation.”
Even more challenging is a very
slightly lighter area in the neck that
extends east-west and highlights the
Horsehead’s mane. Detecting this area
requires a pristine sky and superior
observing skills. I have yet to identify
it unambiguously, but it’s been seen by
other observers.
Tracing the full extent of IC 434 isa challenge because it blends ever so
gently into the sky background in all
directions. It takes careful examination
of the field of view to determine where
it disappears, and I’ve been surprised
how far I’ve been able to trace it. Subtle
east-west striations in the area west of
the Horsehead can also be detected.Spectacular surroundings
Just northeast of the Horsehead Nebula
is NGC 2023, a compact reflection
nebula surrounding the 7.8-magnitude
star HD 37903. Discovered by William
Herschel in 1785, on the best nights you
may be able to see a fairly sharp edge on
NGC 2023’s northwestern edge, which
highlights how far off HD 37903 liesfromthetruecentreofthenebula.
IfNGC2023isanobviousglow
withoutanebulafilter,youhaveagood
chanceofseeingtheHorsehead.Ifyou
can’tseeNGC2023,thenyou’llneed
towaitforadarkerand/oramore
transparent night.
By far the most obvious bright nebula
in the area isNGC 2024, the Flame
Nebula. Also discovered by Herschel, it’s
just east of brilliant Alnitak, or Zeta (ζ)
Orionis, the easternmost star of Orion’s
Belt.TheFlamecanbewashedoutby
theglareofAlnitaksoplacethestar
just outside the field of view. The Flame
is an H II star-forming region, energised
bynearbyAlnitak.Streaksofdark
nebulosity divide it into three distinct
partsthatcreateitsflamelikeshape.
Although it shows well without a
nebula filter, I prefer the view using
aDGMOpticsNarrowPassBandThe Horsehead and its nebulae
Object Type of Nebula Size RA Dec.
B33 Dark 6 ′× 4 ′ 05 h40.9m –02° 28′
IC 434 Emission 60 ′× 10 ′ 05 h41.0m –02° 24′
NGC 2023 Emission + Reflection 10 ′× 10 ′ 05 h41.6m –02° 16′
NGC 2024 Emission 30 ′× 30 ′ 05 h41.9m –01° 51′
IC 431 Reflection 5 ′× 3 ′ 05 h40.3m –01° 27′
IC 432 Reflection 8 ′× 4 ′ 05 h40.9m –01° 29′
IC 435 Reflection 4 ′× 3 ′ 05 h43.0m –02° 19′
Angular sizes and separations are from recent catalogues. Visually, an object’s size is often smaller
than the catalogued value and varies according to the aperture and magnification of the viewing
instrument. Right ascension and declination are for equinox 2000.0.WTHE HORSE’S HEAD SKETCHED The
author’s drawing of the Horsehead Nebula and
its surroundings combined many fields of view
to produce this composite sketch. A little more
than 4.5 hours of sketching time over several
nights using a 70-cm f/4 Newtonian went
into this drawing, which is based on views
using magnifications of 131× to 253× and a
1990s-era Lumicon hydrogen-beta filter. Note
how the top edges of the Horsehead’s profile
have the greatest contrast. One of the more
spectacular objects in this 3°-wide field of
view, and certainly the brightest, is the double
star Alnitak, Zeta Orionis, the easternmost star
of Orion’s Belt — note that its double nature is
not at all evident in the previous photos. The A
component is the brightest O star in the sky.HOWARD BANICHNE