Observing the Great Orion Nebula

Observing the great Orion Nebula with this amazing guide from Sky and Telescope.
With its gossamer tendrils and sparkling stars, the Great Orion Nebula (Messier 42) is the grandest of the many attractions adorning the Hunter’s Sword. This luminous gas cloud can be admired on January and February evenings by stargazers worldwide. A glowing bubble along its upper-left edge, looking like a fat apostrophe, is known as Messier 43. The bluish, star-studded cloud above center is often called the Running Man Nebula; can you pick out the runner's left-facing silhouette?
Jeff Ball

No praise is too high for the Great Orion Nebula. Over the years it has garnered superlatives like “spectacular,” “magnificent,” “gorgeous,” and “celestial wonder of wonders.” That last one is mine.

My first telescopic observation of this glowing gas cloud came soon after my 15th birthday. It was a bitterly cold night in January 1966, but the sky over my Ottawa, Ontario, backyard was incredibly clear. The stars of Orion, the Hunter, blazed brilliantly in the south. A stargazing newbie, all I had in the way of observing equipment was my 2.4-inch (60-mm) refractor and two poor eyepieces. Even so, the entry in my observing log from that long-ago night reminds me how thrilled I was. “Saw Orion Neb. at 64´ — wow!” My optics may have been modest and my observing skills not yet honed, but I could tell that the Great Orion Nebula was a sky object different from all the rest.

The 42nd entry in the popular Messier catalog of “faint fuzzies,” the Orion Nebula is admired by stargazers worldwide. Not only is it plainly visible through binoculars; M42 (Messier 42) is one of only a handful of deep-sky objects visible to the unaided eye. It’s easy to locate, too.

Right now Orion is climbing high in the southeastern sky after sunset. First locate Orion’s Belt, which contains three bright stars. Next, look to the lower right for a vertical row of fainter stars marking the hunter’s Sword. See the fuzzy “star” in the middle of the Sword? There you’ll find the Orion Nebula.

A Stellar Nursery

About 1,500 light-years of empty space separate the city of Ottawa from the Orion Nebula. Actually, the space isn’t completely empty. Our home galaxy is permeated with an incredibly tenuous mixture of atoms and molecules. Here and there, hydrogen and other chemical elements have gravitated together to form giant molecular clouds. One of the nearest of these is the Orion Molecular Cloud, whose dark cloak of dust and gas sprawls across 100 light-years of celestial real estate in southern Orion. The nebula we call M42 is a luminous “blister” about 25 light-years wide on the side of the Orion Molecular Cloud facing us.

In technical terms, M42 is an emission nebula. It’s being energized by a quartet of hot young stars, called the Trapezium, located near the center of the blister. Ultraviolet radiation produced by the Trapezium stars is heating the surrounding gas, making it fluoresce like the glowing gas in a neon bulb.

These massive stars are also sculpting the nebula. Powerful winds of energetic particles stream from their hot “surfaces,” and, along with the stars’ searing light, these winds have been blowing a hole in the dark fog of the Orion Molecular Cloud for tens of thousands of years. It is only because the growing cavity has broken through the edge of the cloud that we can observe the wonders within.

In the 1970s, astronomers peering into this brightly lit cavern discovered that the Orion Nebula is populated by hundreds of young stars, each less than a million years old. They also uncovered protostars — unborn suns still incubating inside dense clumps of gas. Since then, scores of studies have confirmed that the Orion Nebula is a prolific cradle of starbirth.

Paradoxically, the nebula is also hazardous to its embryonic stars. The Trapezium’s harsh ultraviolet light is dispersing the raw material the protostars need to become full-fledged suns, like a tyrannical toddler stealing food from her baby brother’s plate.

Scoping the Great Orion Nebula

You can see these stellar newborns yourself. Just aim your telescope at that fuzzy patch in Orion’s Sword. The nebula should appear in your finderscope as a faint mist enveloping a pair of stars. (These stars don’t have common names, so astronomers use their official-sounding designations, Theta-1 and Theta-2 Orionis.) If you’re using a telescope with at least 3 inches of aperture, an eyepiece yielding a magnification of 40´ or 50´ will resolve the brighter star, Theta-1 Orionis, into the four tightly spaced members of the Trapezium.

This chart shows most if not all of the stars that typically are visible through handheld binoculars with front lenses that are 35 to 50 mm wide. Only the brightest stars (the largest dots) on this chart are readily visible to the unaided eye.
This chart shows most if not all of the stars that typically are visible through handheld binoculars with front lenses that are 35 to 50 mm wide. Only the brightest stars (the largest dots) on this chart are readily visible to the unaided eye.
Intern B (Shannon Hall)

Once your eyes have adapted to darkness, you’ll discover that the tiny trapezoid is embedded in an intricate weave of nebulosity spanning a patch of sky larger than that covered by the full Moon. Unless your telescope has a short focal length (less than 500 mm or thereabouts), you’ll probably need your lowest-power eyepiece — the one with the largest number on its barrel — to capture the entire nebula in a single field of view.

Binoculars can frame the Orion Nebula in its entirety, with room to spare. Viewed under dark country skies with 7´50 or 10´50 binoculars, M42 blossoms into a misty tulip hanging downward from Theta-1 and Theta-2 Orionis.

Because M42 is a large, bright target that can stand up to a fair amount of light pollution, it is a deeply satisfying study even for citybound stargazers. I observed the nebula from my suburban backyard recently, using a stubby 4-inch reflector at magnifications between 50´ and 75´. I glimpsed a rectangular glow surrounding the Trapezium and noticed that one corner of this cloudy box was formed by two remarkably straight sides. The cloud itself was distinctly mottled. A dark bay dubbed the Fish’s Mouth bit deeply into the core, nearly reaching the Trapezium. Long threads of gauzy nebulosity extended widely to either side. Nearly 40 years after my first peek at M42, my reaction was still “Wow!”

Under a rural sky with my much larger 10-inch Dobsonian reflector, the view takes a quantum leap. With its wispy extensions, the Great Nebula becomes a gossamer gull soaring in space. Its elegant wings sweep back into a broad tail of feathery material entangling many dim stars. But you don’t have to have a big scope to see stunning subtleties in M42. My observing friend Lee Labuschagne of Cape Town, South Africa, uses a reflecting telescope with a light-gathering mirror just 4½ inches wide. Yet to her, these delicate features still evoke “sheets of the finest, almost transparent silk studded with celestial diamanté.”

Sue French traced the nebula's winglike appendages and other faint features while viewing through a high-quality 4-inch refractor.
Sue French traced the nebula's winglike appendages and other faint features while viewing through a high-quality 4-inch refractor.
Sue French

One of the trickiest attributes of the Orion Nebula to observe is color; our eyes do a poor job of discerning color when light levels are low. The vibrant crimson so obvious in pictures of M42 is the spectral hue emitted by the nebula’s huge stores of energized hydrogen. Yet Night Sky columnist Sue French notes that in many backyard telescopes “the bright mist holding the Trapezium is mottled with sea-green luminescence.” Green is the color emitted by oxygen ions — superheated oxygen atoms that have lost some of their electrons. Oxygen exists in M42, and our eyes are much more sensitive to green light than to red light.

Even so, the Trapezium region appears edged with rust when I view it with my 10-inch scope, and some observers report seeing hints of red through apertures as small as 6 inches. Others, however, see the nebula only in shades of gray. The next clear night, gaze at the Orion Nebula intently. What colors do you see?

Orion’s Myriad Wonders

If all this isn’t satisfying enough, the fuzzy midpoint of Orion’s Sword offers two deep-sky objects for the price of one. A rift of dusty material at the base of the Fish’s Mouth separates M42 from a dimmer nebula to the north. This more modest object, visually speaking, is a second blister within the Orion Molecular Cloud, and it has a separate designation — M43 (Messier 43). In small telescopes it’s a faint round glow enveloping a bright star. Larger scopes used at moderately high magnifications will show that M43 is shaped like a fat apostrophe or comma.

You’ll find yet more goodies if you poke around Orion’s Sword. Below (south of) M42 is blue-white Iota Orionis, the Sword’s brightest star. Iota has a faint companion visible with modest magnification. The half dozen or so stars around Iota Orionis form a ragged cluster.

Above (north of) M42 is a line of three stars embedded in a small, ghostly nebula that I can just make out with my 4-inch under suburban skies (see the first photo for its pale blue glow, starlight scattered by dust grains). And just above that is a loose but obvious star cluster called NGC 1981. A wide-field instrument will show all these extras in a single breathtaking view.

Take my word for it: M42 and its attendant wonders in Orion’s Sword are celestial landmarks you’ll want to visit again and again.

Canadian astronomy enthusiast Ken Hewitt-White admires the Orion Nebula from a variety of observing sites in southern British Columbia.

This article originally appeared in the January/February 2005 issue of Night Sky.

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