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Cosmic Challenge: Barnard's Loop
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One of the greatest naked-eye challenges goading amateur astronomers around the world is trying to spot the elusive arc of nebulosity known as Barnard's Loop. Cataloged officially as Sharpless 2-276, Barnard's Loop is a ghostly, 10°-wide semicircular bow of nebulosity that wraps around the eastern side of Orion, the Hunter. In long exposure photographs, it bears the unmistakable resemblance to portions of the Veil Nebula supernova remnant in Cygnus. Spotting it by eye stands as a monumental test for observers.
Above: Winter star map.
Credit: Star Watch by Phil Harrington
Above: Finder chart for this month's Cosmic Challenge.
Credit: Chart adapted from Cosmic
Challenge by Phil Harrington
Although named for the renowned American astronomer Edward Emerson Barnard (18571923), who described the scene he captured on photographs made in October 1894 as a "great nebula extending in a curved form over the entire body of Orion," Barnard was not the first person to glimpse the Loop. Records show that Barnard's Loop was discovered visually by Sir William Herschel. Herschel published observations of 52 broad regions of the sky that he thought contained traces of nebulosity. The region around Barnard's Loop is listed as Area 27 and is centered at Right Ascension 05h 48.3m, Declination +01° 09.9'. Practicing an economy of words, Herschel simply described his 27th entry as "affected with milky nebulosity."
Few confirming observations were made of Herschels 52 nebulous regions, igniting a debate over their existence that raged in certain astronomical circles for more than a century. While some of Herschel's 52 regions have subsequently been proven false, Barnard's images of Area #27 left little doubt about its existence.
Debate over Herschel's Area #27 continues to this day, but now, it revolves around seeing the Loop by eye. Many amateurs have noted sections of the Loop through surprisingly small apertures, ranging in size from 50-mm binoculars to 3- to 5-inch telescopes. But can Barnard's Loop be seen by eye alone? It certainly is large enough, spanning the height of Orion's star-studded torso. Is it too faint, or more correctly, too red for the human eye to detect? The answer is "no;" it can and has been glimpsed without optical aid. But there are a few caveats.
I have read many accounts of observers claiming to have seen Barnard's Loop by eye, but I suspect many of them are false. That's not to say the observers are falsifying what they saw. I don't doubt their honesty in the least. But from their descriptions, I suspect that they did not see the real Loop, but rather a string of faint stars that follow very nearly the same path through Orion. The False Loop is formed by 10 stars that shine between magnitude 4.5 and 5. The illusion begins north of the Belt stars at Psi (Ψ) Orionis, and then hooks counterclockwise around the Belt, connecting the stars 33, 38, Omega (ω), 56, and 60 Orionis. The False Loop then winds to the southwest, linking the faint stars SAO 132732, and 55, 49, and Upsilon (υ) Orionis as it curves between Orion's Sword and the stars Saiph and Rigel. Although these stars are widely separated, as evidenced on the chart above, the brain tends to play tricks on us when we're not careful. Rather than interpret the False Loop as a series of faint stars, our eye-brain system tends to fill in empty gaps to create a single image, especially at low light levels. This optical illusion is caused by our psychological tendency to connect indistinct features into some sort of comprehensible whole, and is exactly why Percival Lowell saw straight canals crisscrossing Mars.
In order to see the real Barnard's Loop, several factors have to come together. First, a clear, dark night free of any trace of moonlight, haze, and clouds is an absolute must. Light pollution, especially in the direction of Orion, is also a no-no. It is best to wait for Orion to be highest in the sky, to further remove any terrestrial interference. You, the observer, should be seated or lying down; standing will only cause eye strain and interference. The best solution would be to lie a chaise lounge tilted so that you are looking at Orion more or less straight on. You also need to know the point where your eyes' peripheral vision is most sensitive. Review the discussion in chapter 1 for further thoughts.
If you have them available, try narrowband and Hydrogen-Beta filters to improve image contrast. If possible, hold identical filters in front of both eyes simultaneously to take advantage of binocular vision. Some observers report good results with these, but Oxygen-III filters seem to offer little benefit.
Start with the Loop's brightest segment, which lies just south of 56 Orionis and ends just west of SAO 132732. If you spot that segment successfully, see if you can extend it toward the region in between 56 and 51 Orionis. Try staring toward the Belt stars, while focusing your peripheral vision toward SAO 132732.
Okay, take a breath. Unfortunately, the southern half of Barnard's Loop is quite a bit fainter than the northern half. To spot the segment lying between Saiph (Kappa [κ] Orionis) and Rigel (Beta [β] Orionis) try blocking both stars with your fingers using the "V for Victory" sign. Doing so just might make you victorious.
Above: Barnard's Loop. Credit: Kevin Dixon
Have a favorite challenge object of
your own? I'd love to hear about it, as well as how you did with this
month's test. Contact me through my web site or post to this
month's discussion forum.
Until next month, remember that half of the fun is the thrill of the chase. Game on!
About the Author:
Phil Harrington's Cosmic Challenge is copyright 2017 by Philip S. Harrington. All rights reserved. No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.
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