Sky & Telescope - February 2022 - 57

Going Deep by Howard Banich
M42
Punching
a Hole in
the Sky
The reflection nebula NGC 1999
harbors a dark secret.
I
nconspicuously tucked away a degree
south of the Orion Nebula (M42) is
the small reflection nebula NGC 1999.
It sports a remarkable peculiarity: a
dark, keyhole-shaped feature. Astronomers
assumed that the globule was a
likely star-forming region in front of
NGC 1999, blocking light from the
reflection nebula. That was a pretty
good assumption, given that this part
of Orion is bursting with new star
formation. But then a 2010 study led
by Thomas Stanke (European Southern
Observatory) produced a startling
result: The dark nebula turned out to
be a void, or a cavity, running through
the reflection nebula.
NGC 1999 is illuminated by
V380 Orionis, a quadruple, pre-mainsequence
star system located in the
Orion A giant molecular cloud. The
dark void to the star's immediate west
is approximately 20″ × 30″ in size,
which at a distance of 1,400 light-years
translates to about 10,000 a.u. along
its longest dimension. Typical nebular
turbulent velocities should have closed
this void fairly quickly (within a few
tens of thousands of years or so).
So how did the cavity form, and why
is it staying open?
The 2010 study identified two previously
unknown stellar outflows running
through the void. The first likely originates
with the V380 Orionis system
and is responsible for carving out the
southern section of the void, while an
unrelated outflow may have excavated
the northwestern part. The convergence
of these two flows likely created and
currently maintains the keyhole-shaped
HD 37481
6.0
NGC 1999
HD 37303
6.0
HD 37209
5.7
ι
HD 36814
6.4
d
4.8
pIN ORION'S SWORD You'll find the reflection nebula NGC 1999 about 1° south-southeast of
M42. The image is approximately 1½° × 2°. North is up in all images.
void. By virtue of its close proximity,
V380 Orionis illuminates and heats the
walls of the cavity, producing the high
contrast of the dark void with the surrounding
bright nebula.
Probing the Nebula
An easy-to-remember star-hop is to
start at Iota (ι) Orionis and look for
a pair of 6th-magnitude stars a few
arcminutes due east. NGC 1999 is less
than a degree due south of the westernmost
6th-magnitude star. If you
include a third 6th-magnitude star east
of NGC 1999, we have a Corvus-like
trapezoid.
Once NGC 1999 is in the eyepiece,
it won't respond to a narrowband
nebula filter because it's a reflection
nebula. Even if a wideband light-pollution
filter might boost contrast a little,
I've always found the unfiltered view
more satisfying.
Near the center of NGC 1999 the
magnitude-10.9 star V380 Orionis
stands out quite well against the bright
nebulosity and the eastern edge of the
void. The dark cavity makes the western
side of the reflection nebula look dimmer
than the rest of NGC 1999, which
was quite evident when observing with
my 8-inch f/4 Dobsonian in a moderately
light-polluted sky:
I can see the glow of 1999 when I look
at the right spot at low power (39×)
and it's pretty evident. Pretty easy to
pass by just by sweeping, though. With
206× and 274× the glow of 1999 is
more evident and slightly oval, which is
sk yand tele scope .o r g * FEBRUARY 2022 57
POSS-II / STSCI / CALTECH / PALOMAR OBSERVATORY
Sky & Telescope - February 2022

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