Exploring Orion in depth: with a good chart and guide, your telescope will start to show a lot more than you might have imagined.
So astronomy, like politics, is the art of the possible. Your scope is never big enough, your targets are never near enough. But if you put aside that thinking and instead plan observing sessions based on the known limits of your equipment, using the abundant amateur observing resources now available, you can have endless successes and much richer experiences than those who observe in a less focused manner.
Every winter, for example, amateurs find themselves gazing at the Great Orion Nebula (Messier 42) 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!
So I created a detailed guided tour of this area's sights. Coming with what I hoped were fresh eyes, I worked my way from Orion's lower Belt star down through his Sword using reference books and a 6-inch reflector at 70 power in my light-polluted suburban backyard. Even under these rather humble circumstances, I found that you can indeed spend an evening discovering new wonders in this familiar territory--with good advance planning indoors and a highly detailed map.
Zeta ([zeta]) Orionis, the lower-leftmost star of Orion's Belt, is the starting point for our telescopic tour. It shines to us from about 750 light-years away. Like the other Belt stars, it's a blue-white dazzler much more luminous than our Sun with a spectral type on the border between classes O and B (see page 60).
In my 6-inch, Zeta is a blaze of cold white light. A 6-inch scope can resolve it as a triple star on a night when the air is perfectly steady, but this night I noticed only one of the bright star's two companions. This was faint Zeta Orionis C, glimmering one arcminute (1') north of the primary (Zeta A).
Let's see what that means. The Moon appears 30' (one-half-degree) wide--about the same as a pencil eraser at arm's length. So in the eyepiece at a magnification of 70x, Zeta A and C appear more than twice as far from each other as opposite sides of the Moon do to the unaided eye. Easy enough!
Unlike A and C, the A-B pair is quite close. You'll need high power and good atmospheric seeing to resolve Zeta B, which is 2.4" south-southeast of A. (There are 60 arcseconds, 60", in one arcminute.) Two stars of equal brightness separated by that much should pose no problem for even a 3-inch telescope on a night of steady seeing. But Zeta B is just 1/7 as bright as Zeta A, making it hard to see in the brighter star's glare.
Three faint pairs. West of Zeta by just 0.2[degrees] is a faint pair of stars that even lifelong Orion scanners may never have noticed. It's the lowest of the "Three pairs" labeled on the map on the previous page. Its components are magnitudes 8 and 10, just 1/250 as bright as Zeta A and B. (The higher the magnitude the fainter the star.) They're an ample 45" (3/4') apart, making them an easy split. They're a nice landmark for getting oriented and checking your navigating skills before pressing on.
To make sure you have a good grasp on directions in the eyepiece, nudge the telescope slightly westward. New sky will enter the view from the west edge of your eyepiece field. Now nudge the telescope in the direction of Polaris, the North Star; new sky enters from the north edge. Turn the map around to match this orientation. (North is up on the map.)
The other thing you need to know how to do is judge apparent distances in the eyepiece. Remember that a typical inexpensive 50x eyepiece shows a circle of sky about 1[degrees] wide. This is the about size of the black circles on the map on the previous page. As the magnification goes up, the field shrinks. For instance, my 70x Plossl eyepiece gives a true field 0.7[degrees] wide. So I expect my view to be 0.7 as wide as the black circles.
Once you've located the little star pair west of Zeta, you can use its 0.2[degrees] distance from Zeta to check your eyepiece's field size directly.
Next, move 1/4[degrees] north from this pair and you'll hit a wider duo, magnitudes 8.8 and 8.9, separation 3', oriented almost north-south. The northern of these two stars is slightly yellow.
Go another 1/4[degrees] north and you come to yet another wide pair of stars, brighter than the previous one and with about the same separation.
NGC 2024, the Flame Nebula. Return to Zeta Orionis. Now look about 1/4[degrees] east of the star, and move Zeta out of the edge of your eyepiece so its glare doesn't dazzle you. Almost right away I see a very big, very dim gray glow, irregular and elongated somewhat north-south. The sight is a far cry from the appearance of NGC 2024 in color photographs like the one here, 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. I 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.
NGC 2023. This small, bright reflection nebula is easy to spot under almost any sky conditions. It's easily located 0.4[degrees] southeast of Zeta, closely surrounding a 7.8-magnitude white star named SAO 132464. It's more obvious to me than the larger but dimmer Flame. 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 eastern one. (Remember, west on star charts is to the right.) Can you follow the nebular glow all the way 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 10' to the west-southwest, which is also plotted on the map. These two stars, one with a reflection nebula and one without, make a very nice side-by-side display.
They're 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 north-south close by the yellowish star. The Horsehead forms a flat isosceles triangle with that star and NGC 2023, as shown on the map and in the photograph on the facing page. But the Horsehead and IC 434 are targets for very dark sites. I have never detected anything here with the 6-inch through my light pollution.
Sigma ([sigma]) Orionis and Struve 761 (2761). Sigma Orionis is a dazzling quintuple star with a fainter triple in the same view. The diagram on the bottom of the facing page shows the layout of the scene.
The brightest component is Sigma AB, a pair separated by only 0.2", much too close together to be resolved in amateur telescopes. Except for B, all the other stars in the diagram are easily visible in my 6-inch scope at 70x. Only Sigma C, the faintest, takes a couple of moments to come into view, after which it remains pretty obvious.
Some 5' southeast of Sigma, in the opposite direction from Struve 761, is an 8th-magnitude star that seems to balance the Struve triplet across the fulcrum of Sigma.
Orion's Secret Sword Handle. So far we've been working near Orion's Belt. Now it's time to move southwest to the top of Orion's Sword. We can link the two regions by way of a faint star pattern that no one seems to have noted before. I call it the Secret Sword Handle.
From Sigma, move 1[degrees] west-southwest. You'll come to a nearly equilateral triangle of 6th- and 7th-magnitude stars about 0.6[degrees] on a side. The northern star is white, the southwest one is strongly orange, and the east one is bluish. This triangle forms the top of the Sword Handle.
South of the eastern star by 1/4[degrees] 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 (not plotted) are 11th magnitude and 47" apart. They two pairs form a nearly equilateral triangle with another 11th-magnitude star about 220" to the south. The whole pattern makes a nice little landmark in the 6-inch.
NGC 1981. Moving another 0.7[degrees] south we stumble onto the first real fireworks of the night: the big, loose open cluster NGC 1981 at the top of Orion's Sword. Its most eye-catching feature is a north-south arc of three stars shining from magnitudes 6.3 to 6.5. They're near the top of the photo below. The whole grouping is about a half degree wide; it fills most of my 70x eyepiece with about a dozen very bright stars, white or bluish white, and not many faint ones.
NGC 1973-1975-1977. Shifting 1/2[degrees] farther south brings us into an even brighter star grouping. The most prominent feature here is an east-west arc of three stars including 5th-magnitude 42 and 45 Orionis. Between them is fainter V359 Orionis, a young variable star, which hovers around magnitude 7.3. Deep images show this star group embedded in faint nebulosity. Can you see any of this hazy light from your backyard?
M43. As we scrutinize these lesser-known sights, a stupendous distraction is looming in 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, which I call the Comma Nebula. 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 usually shines at magnitude 8.5 or 8.6.
M42, the Great Orion Nebula. This is the most famous star-forming nebula in the sky and a grand sight in any telescope. Yet its familiar appearance is strangely deceptive. The photographs so well known to every astronomer usually overemphasize its faint outlying parts, which are invisible or nearly so to the eye, while overexposing and washing out the rich detail in the nebula's bright heart.
In my 6-inch telescope the Orion Nebula looks much narrower north-south than it does in photographs, showing two bright, outstretched arms rather than the nearly 1[degrees]-wide, tulip-like enclosure pictured on the previous page. My eye immediately goes to the nebula's brilliant, sharply defined "Huygenian Region" (named by John Herschel in 1826 for the 17th-century Dutch astronomer Christiaan Huygens). Right in its midst is the quadruple star Theta1 (01) Orionis, also called the Trapezium, which illuminates the area. This inner region is alive with bright, mottled detail, as shown above.
A landmark feature here is the intrusion of a dark nebula that extends from the east almost to Theta1. This dark marking is known to amateur observers as the Fish Mouth; it's easy to imagine the entire bright region as a fish head seen in profile facing east. More and more subtleties come out here the longer you look. Pay particular attention to the two great arcing wings that form the base of the tulip bell on photographs.
The Trapezium. I get a strongly three-dimensional impression when looking down into the Trapezium region itself, unlike the flat appearance of even the best photographs. The bright multiple star has blown a hollow for itself deep inside the nebula, and we're looking down into this hole as if into the mouth of a bright, slightly greenish cave.
The Trapezium's brightest star is Theta1 C. To its northwest and northeast are A and D. These stars are all that may show in a very small telescope at low power, but a little more optical aid will also reveal B.
The fifth and sixth stars of the Trapezium are famous observing tests. I can see E with difficulty in the 6-inch at 70x. Everything depends on the atmospheric seeing and the sharpness of focus. F is invisible on many nights, but I've seen it plainly at 180x on nights of excellent sharp seeing.
Iota ([iota]) Orionis and family. The Orion Nebula's southeast wing extends at least 2/3 of the way south 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 a 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 ([summation] 747) to the southwest, a famous double for binocular observers. Its 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 bunch were in any other place it would be considered a showpiece cluster.
NGC 1999. That might have been a nice stopping place. But to close, 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, a variable that 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 the 6-inch scope--a very interesting little object that would be better known were it not overshadowed by grander stuff. How many skywatchers who have gazed at Orion's Sword a hundred times even know it exists?
Alan MacRobert, a Sky & Telescope senior editor, has published 28 "star hop" guided tours like this one. Fourteen are collected in his book Star Hopping for Backyard Astronomers.
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|Title Annotation:||Visual Observing: Deep Space from Your Backyard|
|Date:||Jan 1, 2015|
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