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Orion
Considered by many to be the heaven's single most beautiful constellation, Orion
the Hunter is a veritable playground for the deep-sky observer. Within its
borders lies the magnificent Orion Nebula, the elusive yet captivating Horsehead
Nebula, and several other deep-sky treats.
Orion culminates at midnight in mid-December and stands high in the southern sky
on January and February evenings.
NGC 1788, a bright nebula, resides in the constellation's southwest
corner, about 2 degrees north of Beta Eridani. Look for a cigar-shaped object
extending southward from a faint double star. A 6-inch is probably the smallest
telescope that will show this tiny reflection nebula well, although it has been
seen with averted vision through instruments as small as 3 inches. Larger
instruments will also show several other dim suns superimposed on the cloud.
Regardless of telescope used, however, its small 8'x5' size will necessitate
high power.
M42, the Great Nebula in Orion, is unquestionably the grandest deep-sky
object visible from the northern hemisphere. Seen as a hazy patch of light
surrounding Theta Orionis, the middle star in Orion's sword, this region quickly
explodes into a glowing cloud of great intricacy when viewed with any optical
aid.
On
especially transparent winter nights, a medium-to-large aperture telescope
displays a tremendous greenish cloud engulfing many stars. As the low-power view
in Eyepiece Impression 41 demonstrates, the nebula fills the field. To me, it
looks like a cupped hand as tenuous fingers of glowing gas extend from the main
body of the nebula toward the myriad of field stars.
The famed 19th-century observer Admiral Smyth was the first to call attention to
the fish's mouth, a dark protrusion on the cloud's northeast corner. Near its
tip lie the stars of the Trapezium, also known as Theta-1 Orionis, one of the
season's finest examples of a multiple star. Small telescopes show the system's
four brightest suns, while larger instruments reveal half a dozen surrounded by
an array of dim points of light. Their combined energy excites the Orion Nebula
into luminescence, resulting in the reddish color seen in photographs. Though
difficult to discern visually, hints of red can be seen along the nebula's misty
fringes through large telescopes on exceptional nights.
At first glance, the Trapezium appears to be a
quadruple-star system. Traditionally, the four brightest Trapezium members have
been labeled A, B, C, and D according to their increasing Right Ascension rather
than in order of brightness like other binaries. These stars, rated magnitudes
6.8, 7.9, 5.1, and 6.7, respectively, shine with a fiery blue-white intensity
against the soft, gossamer clouds of the nebula itself.
In 1826, Struve discovered an
11th-magnitude star only 4" north of A, while a second 11th-magnitude orb
may be glimpsed about 4" southeast of C. Now known as Theta1 Orionis E and
F, respectively, both may be spotted in high-quality 6-inch instruments when
seeing is steady. Another Trapezium member, christened G, shines at 16th
magnitude and therefore can only be seen in the largest amateur telescopes. It
is found about 6" due west of D.
In 1975, Theta Orionis A, the Trapezium's westernmmost star, was discovered to
be an eclipsing binary, fading a full magnitude for 20 hours every 65.4 days. B
is also known as BM Orionis. It too is an eclipsing binary, with a 6.5-day
period.
M43 (NGC 1982) appears about 7' to the north of M42. Frequently,
observers fail to distinguish this detached portion of the Great Nebula as a
separate object. In fact, while M42 was telescopically discovered in 1610, M43
was not recognized as an individual entity until 1731.
Mere words cannot possibly do justice to this magnificent pair. Even the finest
drawings and photographs cannot capture the thrill of viewing M42-43 visually.
All eyepiece powers work well, with each offering a different perspective. Low
powers are best for spotting faint, outlying wisps usually overlooked by the
casual observer. Medium magnification reveals the nebula's complex structure and
its varying colors and contrasts, while the area in and around the Trapezium is
best served with high magnification. What a view!
NGCs 1973, 1975, and 1977 are the three brightest regions of a large
emission/reflection nebula network just north of M42 and surrounding
5th-magnitude 42 Orionis.
NGC 1977 stands out the best. Amateur telescopes show it as a large, bright,
bluish arc of nebulosity stretching between 42 Orionis and two dimmer suns. NGC
1973 is a comparatively small, faint object surrounding the variable star KX
Orionis, while NGC 1975, the dimmest of the three, has been recorded as greenish
in larger telescopes. All require dark skies for best visibility.
NGC 2022. Although Orion overflows with vast regions of bright and dark
nebulosity, it can only claim a single planetary nebula that bright enough to be
seen in most backyard telescopes. That nebula is NGC 2022, found just east of
Orion's triangular head. From Phi-2 Orionis, move 1 degree east to a pair of
8th-magnitude suns. Center the eastern star in the field of a medium-power
eyepiece and shift the view half a degree southeast to spot the star-like disk
of NGC 2022.
At low power, a high-quality 3-inch telescope is all that is needed to show NGC
2022 as a tiny 11.5-magnitude point of light nestled in a starry field. While
some amateurs comment on the nebula's greenish tint, most (including me) have a
difficult time detecting any tint to its 18" disk. Any detail in the
planetary requires a medium- or large-aperture telescopes and high
magnifications. Only then, and by using averted vision, will the cloud's
ring-like structure be seen. If you are observing from light-polluted
surroundings, a narrow-band nebula filter might help, although from dark-sky
sites, it seems to make little difference. And you can just about forget trying
to spot the cloud's central star. It shines at a bleak magnitude 15.2,
restricting visibility to only the largest amateur telescopes, if at all.
NGC 2023/NGC 2024/IC 434. Photographs reveal that much of Orion is
blanketed with faint wisps of nebulosity. The region surrounding Alnitak (Zeta
Orionis) is especially noteworthy, with no fewer than six clouds having been
recorded within 2 degrees of the star. NGC 2024 is the brightest nebula in the
area. Visually, it appears as a complex, mottled gray patch of light with a
couple of faint stars sprinkled in the foreground. The actual source of
excitation for this 0.5-degree-diameter object is hidden behind a dark, roughly
rectangular absorption region, making direct observation impossible.
Though not as conspicuous as its larger neighbor, NGC 2023 is still relatively
easy to find. Look for a fairly bright, though small mist diffusing outward from
a centralized 8th-magnitude star. High magnification enhances its appearance.
IC 434, stretching 1 degree due south of Zeta, is famous as being the backdrop
nebula for Barnard 33, the Horsehead Nebula. Both are well known as notoriously
difficult objects. Yet, would you believe that once, under the dark skies of the
Florida Keys, observer extraordinaire Tom Lorenzin (author of the book 1000+)
actually showed it to me through his nebula-filtered 10x70 binoculars? Hard to
believe, but true. From my 5th-magnitude suburban backyard, the Horsehead is a
little more difficult to spy, although I have seen it faintly through my
13.1-inch Newtonian equipped with a 12mm Nagler eyepiece and a Hydrogen-Beta
nebula filter. In both instances, IC 434 could be seen as a faint foggy
"wall" with a perfectly straight eastern edge, while the surprisingly
large Horsehead requires ultimate concentration to be seen.
M78/NGC 2064/NGC 2067/NGC 2071. A little less than 4 degrees east of
Mintaka (Delta Orionis) is a clump of nebulosity highlighted by M78. You might
find it easiest to find M78 by centering on Mintaka, turning your telescope's
clock drive off, and taking a 15-minute warm-up break. When you return, the
earth will have aimed your telescope toward M78 for you!
M78, a reflection nebula discovered by Pierre Méchain in 1780, stands out well
in small telescopes. Eyepiece Impression 42 shows its famous comet-like form,
with a broad 6'x4' fan-like "tail" extending to the south. Two nearly
identical 10th-magnitude stars pose as "nuclei" to complete the comet
illusion.
Several other patches surround M78. The brightest is NGC 2071, a fairly obvious
glow that appears a little smaller than M78. Look for a circular mist
encompassing a 10.5-magnitude star north-northeast of M78. While clearly visible
in 8-inch telescopes under dark, rural skies, averted vision and/or a nebula
filter will probably be required to make it out in smaller instruments or from
suburbia. NGC 2067 is a wispy target which will prove difficult in telescopes
smaller than 12-inch aperture. It is illuminated by the same stars as M78, and
is positioned about 4' to its northwest. Sharp-eyed observers might also be able
to make out NGC 2064, a difficult cloud just southwest of M78. Although it has
always eluded my probing eye, you might have better luck.
Text and chart copyright 1997 Sky Publishing Corp.
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