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"GO TO"....ORION - - The Marvelous Stars of the Winter's Hunter
....the most unforgettable constellation of the cosmos....
Observing and Understanding the Incredible Messier 42 - the Orion Nebula / The Belt and Sword Stars of Orion
As the 32nd, 33rd, and 34th Constellation Guides, "GO TO ORION" of the series "GO TO GUIDES for all GO TO Telescope Users, the magnificent splendor of Orion justifies that it be presented in THREE PARTS:
PART I - ORION: "Making a Mark in the Sky - Legends for Mankind" (last installment)
PART II - ORION: "Observing and Understanding Messier 42 - the Orion Nebula / The Belt and Sword Stars of Orion" (this current Guide)
PART III - "Revealing the Treasures Within" (the next and final installment)
Anyone who has ever studied this wonderful constellation realizes that there is simply too much that must be discussed for one presentation. Thus, PART I featured an overview of the constellation, its stars, mythology, nomenclature and many, many fascinating facts that will make your time with "the Hunter" not only more enjoyable, but more meaningful and educational as well. If that overview did not get you excited enough to pursue further the wonderful world of the Hunter, then your DC power source needs a fresh set of batteries. Perhaps this current installment will charge you up:
This Guide - PART II - is comprised of a very in-depth version of our typical "GO TO" TOUR Constellation Guides, but with added focus on the true splendor of the Orion Nebula (Messier 42), the immediate surrounding area and the fascinating variety of nebulae and variable stars through the "sword" of Orion. There is not a more eye-pleasing, research-generating, and thought-provoking object anywhere in the sky that can take the place of this magnificent emission and reflection nebula, the birth place of hundreds of stars and the bearer of yet stars to emerge into the beautiful heavens.
The remaining installment, PART III, will be our typical "Concise GO TO TOUR" and discusses the remaining (but certainly not less interesting nor exciting) objects, including the "Horsehead Nebula" (IC 434), many other reflection nebulae, clusters and several of the hundreds of spectacular double and multiple stars held within the Hunter's haven.
So......join in with me as we continue to explore this constellation of mythology, cosmology, astronomy and anthropology. Here we go deep into the stellar nursery of the famous and beautiful ORION NEBULA, an object that continues to amaze even seasoned observers in telescopes, binoculars and to the naked eye!
TOOLS OF HIS TRADE....THE "BELT" AND "SWORD" OF ORION (with some wanderings into "Dark Adapted Eyes")
The closeup star chart featured below shows details within the remarkably symmetrical star field known as Orion's "belt" and his "sword." This "asterism," or group of familiar stars, is often mistaken by novice skywatchers as "the big dipper" because....well, because that's what it LOOKS like! It is a striking view with the naked eye, and even more dramatic in good binoculars on a very dark moonless night when very elusive nebulae such as the "Horsehead" (IC 434 and discussed in detail in Part III) can be sometimes detected by the keen eyed with eyes "dark adapted" over a long period of time.
A Brief Word Concerning "Dark Adapted" Eyes -
"Dark adapted?" Perhaps more in Orion's borders than anywhere else because of the wealth of very extended and faint gaseous nebulae, but important for ALL deep sky observing, is the aspect of allowing your eyes to become dark adapted. All this means is allowing your eyes an opportunity to adjust themselves as the very slow-to-respond low light receptors become increasingly active under dark conditions. We are all familiar with two types of "night blindness." This can happen: 1) when going from a bright room or moving away from a bright flashlight into an inky black environment...for many minutes the eyes cannot see the subtle detail that is actually faintly illuminated; and, 2) going from a very dark condition into an extremely bright one, such as driving down a dark highway and suddenly encountering bright headlights that "temporarily blind" the eyes that gaze directly at it.
In astronomical observing, it is very important - particularly for the deep sky observer - to allow much time after coming from a lighted room for the eyes to become dark adapted. The minimum time is 15 minutes with NO white light....optimum time before the very faintest objects and stars can be seen is a full 45 minutes. It is, indeed, a long time to wait....but the results in what your eyes can see by comparison is phenomenal. Full dark adaptation can result in TWO FULL MAGNITUDES gain on your limiting magnitude, both naked eye and telescopic for stars, and up to THREE magnitudes on very faint and small nebulae, galaxies, comets and other subtle deep sky objects.
Once your eyes are fully capable of seeing the faintest objects possible it is important to keep them that way. Many of us are forced to routinely observe with interruptions, particularly when observing at home. If you are forced to go the lighted indoors for a phone call or "family crisis" and attempt to return to observing once done, your eyes are no longer dark adapted and the entire process (for the entire length of time) must start anew.
Also, the use of a white-light flashlight - no matter how large or small - will destroy the night vision that prolonged darkness has provided. I highly recommend NOT buying an expensive "astronomer's flashlight", but take one you already own and remove the front lens; at your local camera store you can by or special order WRATTEN 25 or 25-A red mylar-like filters in sheet form. One sheet is enough to do about 4-6 common flashlights by cutting out a slightly oversized circle to insert under the glass flashlight lens. This is the same filtration that is used in darkroom lighting and provides a "color" of light that your eyes are not sensitive to, thereby preserving night vision.
A complete discussion of "The Eye in Observational Astronomy" is found in my posting at: http://www.weasner.com/etx/buyer-newuser-tips/eye.html.
Orion's Belt Stars -
Nearly everyone who has looked more than casually at the winter night skies has stopped momentarily and marveled at the perfect little "diagonal line" that is outlined by the three nearly-equally-bright stars that comprise Orion's Belt (see description in Part I). A complete description of Orion's "anatomy" as outlined by the brilliant stars of the constellation is give in Part I. This area truly invites exploration with a good pair of binoculars, and this discussion of the Orion Nebula would not be complete without exploring the area around it in brief. A complete look at many of the objects and double/multiple stars within the "belt" and "sword" of Orion is found in Part III of this three-part series.
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The belt of Orion is one of the most identifiable and most unforgettable of all asterisms of the night sky. The belt is marked by the bright stars (in order from East to West):
- ALNITAK (zeta Orion, pronounced "ALL-knee-tack") - RA 05 41 / DEC -01 57 - from the Arabic "al Nitak" or the "girdle star of the Giant," this is a 1.8 magnitude very "young O-type" white star. It is a triple star, with the closest two being 1.9 and 5.5 magnitude, with the brighter star being very white or yellow-white and the fainter component clearly a lavender or green-blue star. Although separated by 2.6", this is not a particularly easy double star because of the brilliant glare of the primary star. For the 4" and larger telescopes, there is a 10th magnitude third star, although not actually linked gravitationally, nearly due north of the close pair. Like all three of the belt stars, Alnitak is about 1500 light years distant, the three of them actually comprising a VERY loose triple star of sorts! It is nearly due south of Alnitak that we find the famous "Horsehead Nebula," so photogenic, but nearly impossible to glimpse visually.
- ALNILAM (epsilon Orionis, pronounced "ALL-knee-lamb") - RA 05 36 / DEC -01 12 - from the Arabic "al Ni-lam", or "belt of pearls" which is a moniker that is thought to have represented originally the entire span of all three stars. This is the central of the three belt stars and the brightest, magnitude 1.7
- MINTAKA (delta Orionis, pronounced "men-TAH-kuh") - RA 05 32 / DEC -00 18 - from the Arabic "al Mintakah" or "the Giant's belt." This is the westernmost and the faintest of the three belt stars, yet varies almost 0.2 magnitude in the course of nearly six days. Look north of the bright star for a very lavender-colored 6th magnitude companion to Mintaka. An interesting aspect of this companion is that it is spaced almost exactly a half light year from the primary star, about the same distance that the famous "Oort Comet Cloud" is located from our own sun.
Orion's Sword Stars -
Referring to the chart above, the "sword of Orion" is not only an interesting naked eye and binocular object, but it is also the home of perhaps the most famous of all deep sky objects, the Great Nebula in Orion, Messier 42 and much additional diffuse gases that actually glow for us to see them! As can be seen in the chart, the "sword" dangles just as it should from Orion's "belt" stars. There are three brighter stars that comprise the sword and are described following, from north to south: (try hitting these targets using your R.A. and DEC "entered coordinates" by pressing the "mode" key and holding for about 2-3 seconds and entering the correct coordinates given below via the keypad).
- 42 Orionis - RA 05 33 / DEC -04 50 - A fairly faint naked eye star of only magnitude 4.5, but a fine double star in 4" and larger telescopes, although the 3" scope may show both components. Look for the secondary star, about 8th magnitude nearly due south and a bit west of the brighter star. This star clearly is distinguishable as a member of the "sword trio" in binoculars, and represents the "top" or northernmost extent of Orion's sword. As with EVERY star discussed here as members of either the "belt" or "sword", 42 Orionis is surround by nebulosity - NGC 1977, a very faint (actually magnitude 4.7, but spread over a 40' x 25' area!) moon-sized cloud centered on the 4th magnitude star. ALSO....look immediately NORTH of 42 Orionis for the very loose star cluster NGC 1981 which contains 10 stars magnitude 8 to 10. This cluster is clearly visible in nearly all telescopes, but very low power wide field instruments provide an incredibly striking view and actually are MORE likely to reveal NGC 1977 than "higher power" instruments!
- Theta Orionis - RA 05 35 / DEC - 05 23 - The famous "Trapezium," or the heart of the fabulous Orion Nebula (NGC 1976) which will be discussed in much length following. It is located about 1/2 degree, or about one lunar diameter, SOUTH of 42 Orionis. Theta - which curiously has remained unnamed throughout antiquity - is a fine multiple star, with four brighter components (about magnitude 6.5 each) easily seen in a small telescope at moderately high magnification. However, the 8" telescope will reveal a total of SIX (6) stars in this group, and a 16" scope can show EIGHT on a very dark and steady night. The positions of these incredibly challenging stars relative to the four brightest members of the Theta multiple group are clearly shown a chart I have prepared in the following detailed description. Indeed, modern estimates suggest that the eight stars are merely the brightest members of newly-forming galactic star cluster! Most of the 300-plus stars that have been identified within only 5 arc minutes of Theta and thought to be a part of this most unusual cluster are well beyond amateur observation. Thought to be ONLY 300,000 years old, this could likely be the youngest of all known star clusters in the Milky Way galaxy! The image of the hundreds of stars centered on the Trapezium shown below was drawn by the author using astrometric grids on the 24" f/4 reflector some two decades ago.
- HATYSA - (iota Orionis, pronounced "hot-E-suh") - RA 05 35 / DEC -05 55 - This southernmost star of Orion's sword is also the brightest at magnitude 2.8. It is also the only named star of the grouping. This star also carries another Arabic name, "Nair al-Saiph," or the "jewel of the sword." Hatysa is located about 1/2 degree south of Theta and the Great Orion Nebula. This is a great triple star for most telescopes. The primary, 2nd magnitude, star has a secondary star about 10 arc seconds southeast; it is an easy object at 6th magnitude and distinctly blue in color. Nearly due east of the bright Hatysa is a faint 11th magnitude star some four times more distant in your telescope than the 6th magnitude companion. Another double very nice "pure white" double star - Struve 747 - is found only 8' to the southwest of Iota, and with is a very easy double for all telescopes with magnitudes of 5.4 for the primary and 6.5 for the secondary star about one "Jupiter diameter" southwest. The 14' round and very faint nebula NGC 1980 surrounds Hatysa, but is an extremely difficult object visually.
As mentioned, every star of the BELT and every star of the SWORD is surrounded by glowing diffuse nebulosity! Indeed, it is likely that every one of these stars - all nearly at the same distance, from 1,400 to 2,000 light years from us - and many, many other very faint ones that are not so obvious, were actually formed out of the primordial gas cloud that is now nearly all "accreted" or gravitationally compacted into masses that we call "stars." The great Orion Nebula is one such example of a very rich cloud of this gas that is still forming "protostars" today; even the smallest telescope can probe deep into the Orion Nebula and see very faint stars which appear embedded in the nebulosity.....stars which were effectively "born" from the very gas cloud through which you view.
THE GREAT ORION NEBULA - PHYSICAL ASPECTS
Before discussing the thrill of "what you can see" within and around the Great Nebula in Orion, let us explore some of the facts, details and speculations about this magnificent deep sky object. With the incredible view of the Orion Nebula that is immediately overwhelming when seen in even a pair of common binoculars, it is sometimes easy to overlook the physical attributes that this "classroom of stellar life" affords us.
Distance and Dimensions -
For the record books.....the Orion Nebula was the FIRST deep sky object photographed, this in a 50-minute exposure by pioneer astrophotographer Henry Draper in 1880. Years before, William Bond at Harvard College Observatory had recorded the first photograph of ANY celestial object - the bright star Sirius - utilizing the great 15-inch refractor in Massachusetts, the largest of its kind in the world at that time.
The nebula and most of its associated stars which have formed from it (see "variable star" discussion below), is located some 1,300 light years distant which places it somewhat closer than the "belt stars" (1,500 to 2,000 light years). The star association and gas cloud is HUGE....the overall width being some 25 THOUSAND TIMES larger than the entire span of our own solar system, or a total of 32 light years. The very dense central portion (which the majority of what is seen visually in amateur instruments) comprises a portion of space the equivalent of the distance of nearly 5 light years: the space between our sun and its closest neighbor....Alpha Centauri!
The Nature of the Nebula -
In celestial terms, this nebula is one of the most recent creation in all of the heavens. Indeed, the nebula and its associated stars, are thought to have begun its formation only 23,000 years ago.....in the recent midst of the Great Ice Age here on Earth. It is in that immediate past that the Trapezium stars actually began to glow while early hunter-gatherers on our planet searched frozen plains for traces of the life-sustaining Mastodon....perhaps there were some inquisitive people among them who glanced skyward and wondered whether that tiny string of star-like objects we now know as the "sword" was indeed becoming brighter night to night in the frozen winter sky.
As discussed in our AURIGA "GO TO" TOUR Constellation Guide (see the Constellation Guide on ASO GUIDES), the expansion of the nebula within this 23,000 year period has resulted in the "expulsion" of three brighter stars (the "runaway stars"): AE Auriga, 54 Arietis and Mu Columbae which for whatever reason were ejected at high speeds (the central part of the Orion Nebula is expanding from its center at an incredible rate of 6 miles per second!) at very near the very earliest years of this remarkable stellar nursery.
Unlike the Merope Nebula and other "reflection" nebula associate with the nearby Pleiades star cluster (Messier 45), we see the Orion Nebula and much of the other nebula filling this constellation because it ACTUALLY GLOWS through fluorescence, much as you see the desk in your office from the overhead lighting provided. The outstanding wide field photograph above from the Lick Observatory in California reveals the tremendous extension of this energetic gases of the nebula. The energy of the hot and energetic young stars (particularly the Theta Orionis group) produces strong ultraviolet radiation, just like electrical current is producing energy within the glowing gases of your fluorescent light bulb. Although - like so much of our universe - the nebula is nearly completely comprise of hydrogen and helium, its "greenish color" that we see visually in telescopes is due to the much smaller percentage of "energized oxygen" in the nebula. On the other hand, nearly all familiar photographs of Messier 42, including spectacular amateur photographs rivaling those of the professional astrophotographer, display the distinctive color of the nebula - DEEP RED - the color associated with the more common and film-sensitive Hydrogen atoms so dominant in the Orion Nebula.
The Variable Stars Within -
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The chart above from the American Association of Variable Star Observers (AAVSO at www.aavso.org ) provides the locations of the dozens of variable stars that are within "walking distance" of Theta Orionis and all embedded in the clouds of the Orion Nebula. Without going into considerable detail about these "nebular variables", it will serve to say that each of these stars, like the "runaway stars" and the bright Theta Orionis grouping, was "born" from the nebula. For a review on methods and techniques of observation of such stars, and a discussion on nebular variables, see my in-depth guide at ASO GUIDES/Variable Star Observing. The brightest of these stars - V372 - varies from magnitudes 7.4 to 8.6 while the faintest shown here - AI Ori - starts at a very faint maximum of 12.1 and dims to 14.2 with no regular period known. All of the "Theta Orionis Variables" are irregular "T-Tauri" type variables with no distinct period nor clearly defined range of maximum and minimum light. For charts of specific stars, use the AAVSO chart creator at:
https://www.aavso.org/apps/vsp/ . Note for these charts, simply type in the NAME of the variable at top to generate your choice of chart.
THE GREAT ORION NEBULA - VISUAL ASPECTS - The Thrill of the Hunt.....the Thrill of "The Hunter"
There is truly "something for everybody" in this wonderful winter object. The naked eye can clearly detect at first glance that this is NOT a stellar object....it is diffuse and the eye struggles as if to focus it into a point of light like all other bright stars surrounding it. One look with binoculars and you clearly detect why: in addition to the fabulous cloud of gas, the field of view in a wide field glass is remarkable, with stars filling the field of view like sugar spilled on black satin. On a dark night, the gaseous clouds can be traced in all directions away from the central portion of this nebula, and the nearby fainter one, Messier 43. Averted vision (not looking or staring directly at an object but attempting to see fleeting glimpses of subtle detail as the eye moves and "sees" the object with the eye's perimeter) will reveal countless streaks and globs of nebulae not first seen when merely looking casually at this area of sky.
Perhaps what surprises me so much about this remarkable object is that Charles Messier - from whom the famous "Catalog" originated with no particular order season-to-season - decided to observe and record 41 objects BEFORE including the Orion Nebula!
The Many Stars of the Orion Nebula -
We will start our "GO TO" TOUR of the Great Orion Nebula at its very heart.....the multiple star known as Theta Orionis, or "the Trapezium," and subsequently the many hundreds of stars that are known members of the "family" propagated by the Orion Nebula.
My diagram above shows a "reversed" image of the Trapezium stars as might be seen at very high magnification with a large Maksutov or Schmidt-Cassegrain telescope with the diagonal fitted (north at top and east at right). The stars "A", ''B", "C" and "D" are of course the brighter members and form the "trapezoid" shape from whence the name originates; all four of these stars are visible in small telescopes using fairly high magnifications of about 40 to 50x per inch aperture. Note that star "C" at magnitude 5.4 is the brightest of the quadrangle and is actually visible to the naked eye, as is the combined brightness of ALL four stars. Magnitudes of the remaining stars are: "A" = 6.8; "B" = (variable, 8 to 8.7); and "D" = 6.3. On a very dark and steady night with Orion very high in the sky, a good 4" scope at about 200x might reveal the 11.3 magnitude star "E", immediately southeast of bright "C". Another faint target star for a 5" and larger scope is "F" at magnitude 11.6 an fainter than the "E". These fainter members are NOT as easy as one might expect....they appear fainter than the magnitudes provided, but have been confirmed at that brightness. The stars "G" and "H" are visible in a 16" telescope under incredibly steady conditions and can be held steady in a 24" telescope. None but the primary four stars can be photographed by amateur means since the bright clouds of nebulosity accumulate their light on film or CCD, thus masking the fainter stars in exposures long enough to reach a limiting magnitude of less than 8.0.
But these eight stars of Theta Orionis - as spectacular a multiple star as they might be - are NOT the only stars of note within the cloud we know as the Orion Nebula. From 1976 through 1982 I developed and maintained an extensive study which both mapped and monitored (visually and photoelectrically) the seemingly countless stars associated with the Orion Nebula.
An interesting aspect of observing this nebula is "tunnel vision" for nearly all observers, myself included. Even seasoned observers are so mesmerized by the beauty of the gas cloud and the striking contrast of the Trapezium, that little attention is addressed toward the hundreds of very tiny stars that are visible in virtually every telescope....the larger your scope, the more stars that are revealed, as shown in the detail drawing made from astrometric measurements of these Orion Nebula stars as seen under dark mountaintop skies with the 24" telescope operating at about 170x. Nearly ALL of these stars, using standard good quality eyepieces can be seen in a quality 8" or 10" telescope and more than half in telescopes of 4", 5" and 6" diameter!
If one zooms in even closer to the Trapezium area, even fainter stars might be seen as seen in the next star field positional chart from 1982 using the 24" at about 400x. This is one of the interesting aspect of the Orion Nebula AND its field stars: both taken higher magnifications very well. The nebula, as discussed following, delivers considerable fine "mottling", like "lambs' wool" clouds of a March evening with higher magnifications in very dark skies in any telescope; higher magnification reveals additional stars as well, since the added contrast will allow the very faint points of light to be more easily differentiated from the very bright surrounding nebula.
I encourage every observer to take the time to explore the wonderfully rich star field associated with the Orion Nebula. Start, using the two charts above, in the "center," at Theta Orionis and use the charts as a guide to determine how many of these stars you might visually detect....I can guarantee you that, on a very dark night, you will see the majority of them in most good telescopes of any size! One note, however, is that the drawings made from my 1982 measurements are "astronomically correct"; Maksutov and Schmidt-Cassegrain modern telescopes using the diagonal prism have NORTH at top and EAST to the right, due to a "mirror-imaging" by the prism or mirror diagonal.
Exploring Messier 42 - Touring the Orion Nebula -
As mentioned, the French comet hunter and "faint-fuzzy-cataloger" Charles Messier took his time getting around to including the Great Orion Nebula as his 42nd object. It certainly was known to him far earlier, and is a conspicuously none-stellar object even to the naked eye.....no doubt this object has been looked upon with wonder by all humankind during the last 23,000 years. Curiously, however, we have mentioned that this region of Theta Orionis was never named by the Arabian stargazers who named everything around it. Surely they would have noted the unusual naked eye appearance of this area.....a "star", but not quite. Galileo likewise missed the Orion Nebula in his first telescope around 1610, not surprising as the semi-permanent stars of the sky apparently were of little interest to his earliest explorations. It was not until the following year that a fellow Italian observer "discovered" the unusual nature of Theta Orionis. The first detailed record was by mathematician/optician/astronomer Christian Huygens of Saturn notoriety when he actually sketched the stars of Theta while describing them immersed in the nebulous cloud.
Today - even in ordinary binoculars - the Orion Nebula is simply something that cannot be overlooked so easily. Indeed, I have always encouraged each and every astronomy/science teacher who is attempting to encourage an interest in astronomy to beginners, merely show the student the Orion Nebula on a dark and moonless night. That will do the trick, for Messier 42 is an overwhelming, almost hypnotizing, sight even to the uninspired and the uniformed.
The famed double star observer at Yerkes Observatory, E.E. Barnard, noted that the nebula always appear to him as a "ghostly bat" in the field of view of the remarkable 40" Clark refractor. Indeed, this shape - likened more in modern times to friendlier creatures - reminds me of a sea gull gliding through the sky, wings outstretched and arched gracefully from one edge of a wide angle eyepiece to the other.
Every telescope of every size will show remarkable detail of the Orion Nebula. As mentioned, the view in each telescope will likewise change when inserting medium and high magnification eyepieces. Thus, one rather different approach of this "GO TO" TOUR guide to the Orion Nebula will be a discussion of relative images based on MAGNIFICATION more so than telescope aperture. Where appropriate I will, of course, interject limitations and expectations of various telescope apertures.
Messier 42 - Wide Angle and Very Low Magnification (telescopes and binoculars) - Stretching the Limits:
In binocular and very wide angle richest field telescopic views, the Orion Nebula is spectacular when viewed on the proper night, including:
1) no moonlight to interfere;
2) very clear and crisp night;
3) Orion on the celestial meridian, as high in the sky as it will get from your observing location;
4) eyes totally dark adapter with no extraneous light interference.
Perhaps the most awe-inspiring view of this region is with richest field instruments with about a three degree or more field of view; for with that area covered, all three of the "sword stars" can be included in one very dramatic field of view. In addition the combination of good optics and wide field will ALSO allow the observer the begin to detect the very faint nebulosity that surrounds nearby Iota and 42 Orionis in addition the grandeur of M-42 and M-43 to its immediate north.
In wide field low power instruments, look for the "gull wings" that stretch far outward from the central region of the nebula. These are very conspicuous filaments of gases that stream outward from the dark "gap" in the central dense region of the nebula known as "the fish's mouth" (see my drawings following....the fish's mouth is the dark gap that appears to intrude into the brightest area of the core of M-42). One such filament streaks to the west from the fish's mouth for nearly 2 degrees. Larger telescopes will reveal further extensions of this streamer, but with the sacrifice of field of view. A second and even more interesting filament that contains much lumps and dense clouds and streaking nebulosity arcs gracefully in an opposite direction, southeastward toward Iota Orionis, also extending well over one degree.
In low power instruments, the bright central region near Theta Orionis will be clearly seen in stark contrast to the rest of the faint nebula; also, increasing power just a bit will reveal the fish's mouth in much better detail. The brighter stars of the Orion Nebula (refer to the chart above) will be seen sparkling within the gas cloud, with occasional hints of very faint stars that are just at the limit of sight at this very low magnification.
Messier 42 - Medium Magnification (telescopic only views) - Unlocking the Fine Details:
The drawing immediately above was made by the author about 40 years ago with a 6" Unitron refractor at f/16. Note the intense center of nebulosity and the gull wings that are present all the way to the edge of this field of view.
In medium magnifications (about 15x per inch aperture) the observer will begin to see more and more faint stars; within the nebula, expect this limiting magnitude for stars:
3" - 10.2
4" - 10.9
5" - 11.4
8" - 12.5
10" - 13.4
12" - 14.1
16" - 14.9
Note that these limiting magnitudes are NOT the faintest star your telescope is capable of revealing under ideal circumstances; but observing faint stars that are embedded in bright nebulosity will not allow you to reach that faint limit, and thereby these limitations are empirical limits based on actual observing sessions within the Orion Nebula.
At this medium magnification, examine the full extent....a bit at a time.....of each of the "gull wings" for some very fine filamentary detail. You will be absolutely amazed at the wealth of fine detail once your eye becomes fully dark adapted. Of course, larger telescopes will reveal more difficult detail and fainter extended filaments, but there is more to see in all telescopes by increasing magnification.
Also, center the "fish's mouth" and examine the remarkable mottled detail that becomes clearly visible within the bright central area that is just south of the dark feature. You will note in 5" and larger instruments that this bright concentration of nebulosity describes a bit of a "box" shape with considerable texture within it. Once again, let you eyes wander throughout this area in search of the VERY faint stars that suddenly pop into view with this increased magnification. You will find that if you scan, the peripheral view of your eyes - "averted vision" - will reveal far more faint stars than if you peered directly at the area in which they are located. At this magnification, you might begin to locate some of the very few RED stars that are found in the nebula...search and discover!
Messier 42 - Higher Magnifications - Exploring the Heart of the Great Nebula:
At magnifications of about 20x to 25x per inch, you will begin to reveal stars near the limit of light grasp of your telescope...thus the faintest stars will likely be seen at this power. I suggest that you will see TWICE the number of stars at this magnification as with medium magnification. Below is the latest drawing made through a 12" Meade f/10 with the diagonal mirror removed to match orientation with that done with the 6" refractor; you will note that the very fine and delicate detail suddenly becomes richer, when at first glance there might appear to be more detail in the smaller aperture instrument.
Also holding up very well under high powers is the central region of the nebula; there will be countless knots and tiny filaments and dark globules that suddenly become visible. At higher magnifications it might require some time for your eye to adapt to the lowered contrast between dark sky and faint nebula, but this wait is well worth it. Also showing up remarkably well under very good conditions are the places "where stuff ain't" as I like to say. These are areas that are totally absent of nebulosity, scattered stars or other objects of note....these are places that stand out in inky stark contrast to nearby areas illuminated by the fascinating nebula. These dark areas are fascinating and add yet another dimension to the incredible and seemingly never-ending discoveries that you can make with the Great Orion Nebula.
And a Word About Messier 43 -
Messier 43 (ngc1982 at coordinates [2000 epoch] R.A. 05h 36m / DEC -05d 16m) is immediately north of the Orion Nebula and actually appears much as an "appendage" of the great nebula....but it is not. Messier 43 is also an emission nebula with many embedded stars that have formed from the dust and gas of this cloud, just as has happened in the Orion Nebula. It surrounds an 8th magnitude star and is a vague but clearly-definable cloud, misty-appearing due north of M-42 and within the same low power field of view as the "fish's mouth" (above). A beautiful image of this nebula (north up) is seen below as captured in the wide field camera of the Anglo-Australian Observatory.
NEXT INSTALLMENT OF THE ORION "TOUR" - PART III - "Revealing the Treasures Within"
With our next "GO TO" TOUR, part three of this three part series, we resume the "normal" Concise Directory of interesting and unusual deep sky objects to locate in your computerized telescope. Not ignoring Messier 42, but many observers overlook countless of other fantastically rich and exciting objects in Orion....from planetary nebulae and wonderful galactic star clusters....to beautifully colored and challenging double and multiple stars.
Like the Great Galaxy in Andromeda, it is easy for one magnificent object to overshadow and shortchange other objects that are not only worthy of observation, but further away from such showcase deep sky splendors they would be showcase objects in their own rights. So we will explore such wonderful objects in the grand constellation of Orion in Part III.
Yes, there is life in Orion after the Great Nebula....