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Cosmic Challenge: IC 5146 and B168

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Cosmic Challenge:
IC 5146 and B168

November 2016

Phil Harrington


This month's suggested aperture range

Giant (≥70mm) binoculars

3- to 5-inch (75cm to 127cm) telescopes


As a group, emission nebulae, or Hydrogen-II regions, are the most difficult deep-sky objects to see visually. The problem is that they radiate light in very narrow segments of the visible spectrum, with their brightest emissions in the red wavelengths. As luck would have it, the human eye is all but color blind to red light under dim light conditions.

Arguably, the only objects more difficult to spot than emission nebulae are the opaque profiles of dark nebulae. These cosmic dust clouds are themselves invisible; we only see their silhouettes against the starry backdrop. No starry backdrop, no dark nebula; it's that simple. 

And that brings us to this month's double challenge in Cygnus. IC 5146, known to many by its nickname, the Cocoon Nebula, is a taxing patch of glowing gas, while Barnard 168 is a thin, sinuous lane of darkness that seems to start at the nebula and extend far to its northwest.

Above: Autumn star map from Star Watch by Phil Harrington.


Above: Finder chart for this month's Cosmic Challenge.  Chart adapted from Cosmic Challenge by Phil Harrington.
Click on the chart to open a printable PDF version in a new window.

To spot this celestial odd couple for yourself, begin at the bright open cluster M39 to the northeast of Deneb [Alpha (
α) Cygni]. Famous as a bright, loose congregation of stars and covering an area of sky as large as the Full Moon, M39 is best appreciated at very low powers. Be sure to take a moment to enjoy the view. 

From M39, steer your telescope 2½° east-northeast to 4th-magnitude Pi2 (π2) Cygni, and then slowly scan southward, watching for the starry background to drop off abruptly. That will be Barnard 168. Because of its length -- more than a degree tip-to-tip -- Barnard 168 is best appreciated with binoculars. My 16x70s reveal a winding stream of black ink flowing through a valley in the stars, as I've tried to recreate in the sketch below.

Above: Sketch of IC 5148 and B168 as viewed through the author's 16x70 binoculars.

By following the dark cloud to its eastern end, you will come to a pair of 9.5-magnitude stars. Both are engulfed in the subtle clouds of the Cocoon. Armed with a 22-mm Tele Vue Panoptic eyepiece (46x), my 4-inch refractor can only muster the slightest hint of the nebula itself, looking like an oval glow surrounding those stars. 

So-called nebula filters prove only moderately successful with the Cocoon. The biggest boost, modest as it is, through my 4-inch refractor is with a narrowband filter. A hydrogen-beta line filter also has a positive effect on the Cocoon, but only in larger apertures. A hydrogen-beta filter on my 4-inch scope renders the nebula invisible. Surprisingly, an oxygen-III filter, considered by experienced observers to be the most useful filter of all on emission nebulae, proves worthless with IC 5146 regardless of telescope aperture. 

The question of who discovered IC 5146 is the subject of some debate. Most references state Thomas E. Espin was first to spot it on August 13, 1899. Espin was a British clergyman and astronomer who specialized in the study of binary stars using his observatory in Tow Law, a small town in the Wear Valley district of County Durham, England. Some dispute Espin's role as discoverer, however. While his is likely the first visual observation of the Cocoon, it was actually discovered photographically by Edward Emerson Barnard on October 11, 1893, using the 6-inch Willard lens at Lick Observatory.

Some sources claim that IC 5146 is a star cluster, not a nebula. In fact, a cluster of more than 100 young stars is embedded within the Cocoon Nebula. Espin's original notes, however, refer to his discovery as a "faint glow about 8 arc-minutes [across], well seen each night." The entry in Dreyer's Index Catalogue echoes Espin's words, referring to IC 5146 as "pretty bright, very large, irregularly faint, magnitude 9.5 star in the middle."

Credit for the first separate mention of the Cocoon's cluster actually belongs to the Swedish astronomer Per Collinder. Collinder's 1931 listing of open star clusters includes it as Collinder 470. According to Brent Archinal and Steven Hynes in their book Star Clusters (Willmann-Bell, 2003), Collinder himself apparently caused the confusion when he incorrectly cross-labeled the cluster as IC 5146. This error has since been carried over to many other references, including the first edition of the popular Uranometria 2000.0 star atlas. Modern studies show that 110 stars belong to Collinder 470, including the two 9.5-magnitude stars embedded within the Cocoon's clouds. Unfortunately, most of the other cluster stars are far too faint to be visible through amateur telescopes.

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 writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2016 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.


  • R.bak and John O'Hara like this


John O'Hara
Oct 30 2016 04:02 PM



With the dark moon period upon us, so is the lake effect engine here in NW Pennsylvania.  I'd love to go after this object with my new 100 mm Pro ED.  I once tried the Cocoon many years ago with my old 6" f/8 AP, but I think the only filter I had in those days was the OIII and I did not succeed.  As popular as this object is, I've not revisited it despite the fact that I have better eyepieces and a better filter assortment and more experience.  Hopefully I'll get a chance before Cygnus recedes low into the northwest sky. 


BTW, are you still offering certificates for Star Watch?  My wife completed all of the objects and our attempts to e-mail you through your web site have all bounced.


Clear skies!

John O'Hara

    • PhilH likes this

Dark nebula B168 is one of the best I know for small telescopes. I have observed it several times from small 10x35 bino, and from 63mm up to 120mm refractors. Under dark sky, it is striking object, one of my favorite. Not so much under the light polluted sky from my backyard.


I have been trying to spot IC5146 numerous times during last couple of years, mostly using 63mm and 80mm refractors. Even under dark side I was not able to convince myself that I saw the nebulosity and not just atmospheric halos around the two stars.


Finally, this year late summer I got the first positive observation. It came from 150mm Cassegrain at 42x. Without filter there were two rounded halos around the two stars. With Hbeta filter, I noticed some irregularities, namely V shaped brighter line with vertex on the south star and the V was opened towards the north star. When I looked later at images, it looked like I have registered the southern edge of the nebula superimposed with the atmospheric halos. Definitely a challenging target.

    • PhilH and JerryOrr like this

Here is a very rough sketch from my logbook showing this V-shape line:



    • PhilH, John O'Hara and JerryOrr like this

Nice job all around!


And John, I haven't in years, but sure.  She needs to submit through my Star Watch web site.

John O'Hara
Nov 12 2016 06:13 PM

Thanks, Phil.  She'll give it another try.



John O'Hara
Nov 19 2016 08:26 AM



I decided to try for this object pair with my 100 SW Ed (my first try with this scope) from my local dark sky site (about Bortle 4).  Unfortunately, the skies had high haze which hampered the observation.  At 22x with my trusty 40 Pentax XL, I could easily see B-168 and traced it to the position of the nebula.  However, with my Orion Ultrablock filter I only suspected the nebula.  I hope to try on a better night.  I later noted in that you used 46x with your 4" scope, but I'm not sure it would have made the difference with the hazy skies.  Actually, I was surprised at how well I could see B-168 in the haze. 

I've had this 4" refractor for a few months now, and wish I'd obtained such a convenient scope years ago!  I've been missing a lot of astronomy not having a good grab and go.



    • JerryOrr likes this

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