52 ASTRONOMY • FEBRUARY 2018
offers a different perspective.
Black skies, low powers, and
averted vision work best to see
faint, outlying, cloudy wisps
that go unnoticed by casual
observers. Medium powers
reveal the complexity of the
nebula’s twisted structure,
with its varying contrasts. For
instance, look for the dark
cloud that protrudes into M42
from the northeast corner.
That finger-shaped cloud was
dubbed the “fish’s mouth” by
mid-19th-century observer
Adm. William Smyth, a British
naval officer and astronomer.
High magnification lets you
explore the heart of the nebula
around Theta itself, with all its
intricate subtleties.
It doesn’t take high magnifi-
cation, however, to see there is
more to Theta than just a single
star. Even small binoculars will
reveal that it is actually two
stars. These are appropriately
labeled Theta^1 (θ^1 ) and Theta^2
(θ^2 ) Orionis. High magnifica-
tions through 4-inch and larger
telescopes further show that
Theta^1 is actually a multiple-
star system known as the
Trapeziu m.
The Trapezium has
four main components.
Traditionally, these stars are
referred to as A, B, C, and D,
in order of right ascension.
Their magnitudes are 7, 8, 5,
and 7, respectively. A and B
are also eclipsing binaries,
raising the star count to six,
though you never will see
those companions.Depending on your tele-
scope’s aperture and seeing
conditions, you might be able
to spot as many as three other
stars in the Trapezium. In
1826, German astronomer
F.G.W. Struve discovered star E,
a 10th-magnitude point about
5" north of A. Star F is also
10th magnitude about 4" south-
east of C. Lastly, G shines
weakly at only 15th magnitude.
It lies inside the Trapezium,
about 6" due west of D.
The Orion Nebula shines by
fluorescence, so it is called an
emission nebula, or HII region.
Its hydrogen is excited intoluminescence by the strong
ultraviolet radiation from the
Trapezium’s stars as well as
others buried within. Ionized
hydrogen is principally respon-
sible for the cloud’s magnificent
red color seen so vividly in
photographs. But our eyes are
all but colorblind to red under
dim light. Instead, many see
the nebula as slightly greenish,
chief ly due to light given off by
doubly ionized oxygen.
M42 is about 1,300 light-
years away. It measures 24
light-years across, making it
more than 15,000 times larger
than the distance between the
Sun and Pluto. It is probably no
more than 3 million years old.
M43 appears as a detached
portion of the Orion Nebula,
about 7' north of its more
impressive neighbor. Although
cataloged as a separate entity,
M43 is part of the same com-
plex, only appearing to be cut
off by a dark dust cloud.
Indeed, photographs show that
not only the immediate area of
Orion’s Sword, but nearly the
entire constellation, is engulfed
in nebulosity.
One such patch, a ref lection
nebula this time, lies just half a
degree north of M42. This
cloud of cosmic dust is visible
only by ref lecting light from
nearby stars. The cloud carries
three entries in the New
General Catalog: NGC 1973,
1975, and 1977. The divisions
are caused by intertwined,
darker regions. Collectively
they create what observers call
the Running Man Nebula. I
don’t see anyone running, but
through my 10-inch ref lector, I
can make out NGC 1973 and
NGC 1975 as small, faint
patches. Both require averted
vision to be seen. NGC 1977
is more difficult still, since it
overlaps 4th-magnitude
42 Orionis.The Running Man Nebula,
collectively NGC 1973/5/7, is
a region of faint nebulosity
half a degree north of the
Orion Nebula. ADAM BLOCK/MOUNT
LEMMON SKYCENTER/UNIVERSITY OF ARIZONAThe Horsehead Nebula is one of
the most famous dark nebulae
in the sky, but it’s difficult to
spot visually. R. JAY GABANY