Orion Winter Wonders

How many times have you observed the Great Orion Nebula? Every winter amateurs find themselves gazing at it on cold,

clear nights almost as a matter of ritual. Many sweep through its rich surroundings in Orion's Sword and Belt until the tour becomes routine. But how few notice all that can be found here!

The Orion season has already begun. In early December the constellation stands high in the southeast by about 11 p.m., and in early January it's there by 9. When you turn your telescope its way this winter, why not try going beyond the beaten path?

Before we begin at this tour be sure to dark adapt your eyes and let your telescope have a cool off period. Work your way from Orion's lower Belt star down through his Sword using low power and find that you can indeed spend an evening discovering new wonders in this familiar ground - if armed with good guides and a highly detailed map.

1. Zeta Orionis. (Alnitak)(see chart)

The starting point for this telescopic tour will be the southeasternmost star in Orion's Belt, also known as Alnitak. Like the other Belt stars (and the dimmer of Orion's shoulders and feet), Zeta is a 2nd-magnitude blue-white giant with a spectral type close to B0. All of these are located about 1,500 light-years away.

In a 10-inch Zeta is a blaze of cold white light. Some amateur telescopes can resolve it as a triple star, but smaller scopes may only resolve one of the bright star's two companions. This was Zeta Orionis C, glimmering 58" north of the primary at magnitude 9 or 10. The A-B pair is quite dose. You'll need high power and good atmospheric seeing to resolve Zeta B, which is 2.4" south-southeast of A, the primary. An equally bright pair separated this widely should pose no problem for even a 3-inch telescope. But A and B are quite different in brightness, magnitudes 1.9 and 4.0, making the faint one hard to see in the blaze of the bright one.

2. Three faint pairs of stars.

Due west of Zeta is a faint, wide pair of stars you've probably never noticed before (see chart above). They're magnitudes 8 and 10, with the faint star 45" southwest of the bright one. Next, move ¼° north from this pair and you'll hit a wider duo, magnitudes 8.8 and 8.9, separation 181", oriented almost north-south. The northern of these two stars is slightly yellow. Go another ¼° north and you come to yet another wide pair, brighter than the previous one and with about the same separation.

3. IC 431, IC 432.

Take a very careful look at the southeastern star in that last pair. It is surrounded by a dim reflection nebula, IC 431. Compare the glow around this star with the halo around the other, brighter one of the pair. Does the first star have a slight bigger glow? Does the glow show a little irregularity? A 10 inch scope may be required to see this faint reflection nebula, so give it try. Whether or not you detect anything, you are performing a valuable warm-up exercise for the brighter reflection nebula IC 432 surrounding the star V901 Orionis just 1/5° to the east. After trying for the first nebula, you can spot the second one immediately using averted vision. The star is surrounded by very dim but unmistakable interstellar clouds. They are asymmetric, extending farthest to the star's east.

IC 431 and IC 432 are small spots of illumination in the immense, dark complex of star-forming gas and dust that sprawls all across the east-central part of Orion. V901 itself is an SX Arietis-type variable - a young, hot, strongly magnetic star that displays small changes in brightness and spectrum corresponding to changes in its magnetic field.

4. NGC 2024, the Flame Nebula.

Return south 0.4° to Zeta Orionis. Now look about ¼° east of the star, and move Zeta out of the edge of your eyepiece so its glare doesn't dazzle you. Almost right away you can see a very big, very dim gray glow, irregular and elongated somewhat northsouth.

The sight is a far cry from the famous appearance of NGC 2024 in color photographs, which show it as a brilliant turmoil of light shot across with streaks and lanes of dust. Taken on its own merits, however, the direct telescopic view holds a subtle and evocative charm of its own. You can make out the largest dust lane, which curves across the nebula from the northwest to the south. As with other dim nebulae, the visibility of NGC 2024 and its dark lanes depends critically on the darkness of your sky.

5. NGC 2023.

This small, bright reflection nebula is easy to spot under almost any sky conditions. It's easily located 0.4° southeast of Zeta, closely surrounding the 7.8-magnitude white star SAO 132464. Once here, take your time and look for detail, including a slightly brighter patch in the nebula's northeastern side. The western edge is more sharply defined than the one. Can you follow the nebular glow all the southeast to the 12th magnitude star 2.4' from the bright one? The central star is similar in brightness to a slightly yellowish star about 1/6° west-southwest. These two stars, one with a reflection nebula and one without, make a very nice side-by-side display. They are also key reference points for locating the rim of the long, dim nebula IC 434 and the dark Horsehead Nebula intruding across it. The rim runs northsouth close by the yellowish star. The Horsehead forms a flat isosceles triangle with the two stars, as shown on the map and the photograph above. But the Horsehead and IC 434 are targets for very dark sites. Good seeing and dark skies are required to see these objects.

6. IC 435.

This is another little illuminated patch in the great, dark Orion complex. It surrounds an 8.5 -magnitude B star 1/3° east of NGC 2023. Although small, this nebula too is readily visible in a 6-inch, especially when you compare it to the scattered-light halos of nearby stars that do not lie in nebulae.

7. Orion's Secret Sword Handle.

So far we have been working in the area of Orion's Belt. Now it's time to move southwest to the top of Orion's Sword. The two regions are linked by way of a faint asterism called the Secret Sword Handle. From Sigma move 1° westsouthwest. You'll come to a nearly equilateral triangle of 6th- and 7th-magnitude stars about 0.6° on a side.

The north star is white, the southwest one is strongly orange, and the east one is blue. This triangle forms the top of the Sword Handle. South of the eastern star by ¼° are two faint, wide pairs oriented at right angles to each other. The pair on the west is wider and brighter; the stars are 9th magnitude and 92" apart. The stars of the eastern pair are 11th-magnitude and 47" apart. They form a nearly equilateral triangle with another 11th-magnitude star about 220" to the south.

8. NGC 1981.

Moving another 0.7° south we come onto the first real fireworks of the night, the big, loose open cluster NGC 1981 (see chart below) at the top of Orion's Sword. Its most eye-catching feature is a north-south arc of three stars of magnitudes 6.3 to 6.5. The cluster is about a half degree wide; and fills most of the eyepiece with about a dozen very bright stars, white or bluish white, and not many faint ones. The swarm is somewhat elongated east-west.

9. NGC 1973-1975-7977.

Shifting farther south brings us into an even brighter star grouping, this one full of dim nebulosity. The most prominent feature here is an east-west arc of three stars including 5th-magnitude 42 and 45 Orionis. Between them is V359 Orionis,

another SX Arietis-type variable, which hovers around magnitude 7.3. The NGC, the New General Catalogue of Nebulae and Clusters of Stars published in 1888, lists three separate nebulae here. Modern photography, however, shows them to be patches in a single mass of cloudy blue reflection nebula. NGC 1973 is a little spot surrounding the star KX Orionis, which varies from magnitude 7 to 8; the star is a very young irregular variable that may not yet have reached the main sequence. A 6-inch will show this part of the nebula fairly easily. NGC 1977 was easier. This is a much larger glow just south and west of the arc of three bright stars. The arc seemed to form a cap along part of the nebula's north edge. The nebula appeared brightest just south of 42 Orionis. NGC 1975 is a smaller, but very dim patch north of the arc of three and northeast of KX.

10. M43.

As we scrutinize these lesser known sights, a stupendous distraction is already looming into the south edge of the field: the northern fringes of the Great Orion Nebula. The first part to come into view as we move south is M43. It's a bright circle with a northeastward swirl, all quite sharply defined, just off the north edge of the bigger M42, the Orion Nebula proper. M43 is centered on the young irregular variable star NU Orionis, which shines at magnitude 8.5 or 8.6. Although the star's spectral type is listed as BO, which ought to make it blue-white, it appears yellowish or even golden yellow. Perhaps we are seeing it through clouds of dust that redden its light.

11. M42, the Great Orion Nebuia.

This is the most famous emission nebula in the sky and a grand sight in almost any telescope. Yet it's familiar appearance is strangely deceptive. The photographs so well known to every astronomer over emphasize its faint outlying parts, which are invisible or nearly so to the eye, and they wash out the rich detail in it's bright heart. In a 6-inch the Orion Nebula looks much narrower north south than in photographs. The eye is immediately drawn to the nebula's brilliant, sharply defined "Huygenian Region" (named by John Herschel in 1826 for the 17th century astronomer Christiaan Huygens).

Right in its midst is the quadruple star Theta' (0') Orionis, which lights the area. This inner region is alive with bright, mottled detail. A landmark feature here is the intrusion of a dark nebula that extends from the east almost to the Trapezium. This dark marking is known to amateur observers as the Fish Mouth; it's easy to imagine the bright region as a fish head seen in profile facing east. A thin band of bright nebulosity cuts off the dark intrusion's rounded tip. This thin band is known as Pons Schroeteri, Schroeteri's Bridge, and is a well known amateur observing challenge. More and more subtleties come out here the longer you look. North of the big dark zone from which the Fish Mouth extends, you can glimpse a large, generalized glow every so often. See if you can perceive the dark zone not as empty sky but as a detached, ragged black cloud completely silhouetted on a luminous background. And now it took on the form of a great, dark figure with outspread arms or wings; the Fish Mouth was its head. It's body and widely spread feet were occasionally visible. One leg defines the Comma Nebula's eastern edge. It a parent dark nebula, dome shaped, poking northward almost to the Trapezium. The dome extends just past the row of three big bright stars near the nebulas heart, including Theta 2 and V361 Orionis; they're just inside the dome's head. The dome's edge near this spot coincides with the sharp, straight southeastern rim of the Huygenian Region. Even a 6-inch might see a little of the yellowish orange color of this bright rim, a contrast to the misty greenish cast of the rest. This is one of the very few places in the sky where color differences in a nebula are visually detectable. The other great bright wing of M42 extends northwest. It's much less interesting, - since it's not so sharply sculpted. Enclosed within its curve is a strong, smooth glow. The northwest wing fades out gradually, with only a slight hint of mottling to the Trapezium's west.

12. The Trapezium.
You get a strong three dimensional impression when looking down into the Trapezium region itself, unlike the flat appearance of even the best photographs. The bright multiple star seems to have blown a hollow for itself deep inside the nebula, and we are looking down into this hole as if into the mouth of a bright green cave.

The Trapeziurns brightest star is Theta C. To its northwest and northeast are A and D. These are all that may show in a very small telescope at low power, but a little more optical aid will also show B. The fifth and sixth stars of the Trapezium are famous observing tests. You can see E with difficulty in a 6-inch at 70x. Everything depends on the atmospheric seeing and the sharpness of focus. F was invisible, but has been seen plainly in a 12.5 inch at 180x on nights of good seeing.

13. Iota Orionis and family.
The Orion Nebula's southeast wing - Proboscis Major - extends at least 2/3 of the way to dazzling Iota Orionis, the next great sight looming into view. Iota is a very pretty double star - brilliant white and faint blue, magnitudes 2.8 and 6.9, with a separation of 11". It's triple if you count the fainter star Iota Orionis C, located 50" east of the main pair. Although it's not cataloged as such, the whole Iota area seems to comprise a little star cluster, distinct and isolated. In addition to Iota it includes two more wide doubles. One is bright Struve 747 to the southwest, a famous double for binocular observers. It's stars are magnitudes 4.8 and 5.7 and are separated by 36". Just west of that pair, by half the distance from Iota, is another pair that's much fainter and a little closer. Other, single stars are grouped around. If this overlooked gathering were in any other place it would be considered a showpiece star cluster.

14. NGC 1999.

To close off we'll make one more hop southward, out and away from the dazzle of Orion's Sword into the anonymous, amorphous dark. And here we discover the odd, dim little nebula NGC 1999. It surrounds the star V380 Orionis, which has ranged from magnitude 8 to 11. When all the other stars in the field focus to points, this one remains a fuzzy patch. It's quite visible in a 6 inch - a very interesting little object that would be better known were it not overshadowed by M42.