IC 5146, Cocoon Nebula


IC 5146 was discovered on July 28, 1894 by the German astronomer Max Wolf. It is an open star cluster, involved in a large galactic nebula consisting of emission nebulae, reflection nebulae and dark nebulae. The designation IC 5146 applies to the star cluster including the nebula. The nebula is also known as Sharpless 125 (Sh2-125) and the star cluster as Collinder 470. It was nicknamed Cocoon Nebula because of its appearance, which resembles a glowing insect cocoon in a dark cloud.

IC 5146, B 168
IC 5146, B 168: Cluster with nebula IC 5146 and dark cloud Barnard 168; Celestron RASA 11" f/2.22; ZWO ASI6200 Pro; Tentlingen; © 2020 Peter Kocher
IC 5146
IC 5146: Cluster with nebula IC 5146 and dark cloud Barnard 168; Meade Schmidt-Newton 254mm f/4; Canon EOS 20Da; ISO 1600; 16 x 362s; Obwalden; © 14. 7. 2007 Eduard von Bergen
IC 5146
IC 5146: Cluster with nebula IC 5146 and dark cloud Barnard 168; 500 mm Cassegrain 5800 mm f/11.4; SBIG STL11K; 30+10+10+10 min LRGB; Bernese Highlands; © 2005 Radek Chromik
IC 5146
IC 5146: Cluster with nebula IC 5146 and dark cloud Barnard 168; Takahashi Mewlon 250 CR (f=2500 mm / f10), SBIG ST-8300; 20L x 1200 sec 1×1, 13R, 12G, 14B 2×2 x 1200 sec; Bernese Highlands; © 2016 Bernhard Blank, Dragan Mihajlovic

Physical Properties

The star cluster with the bright part of the nebula has an angular diameter of about twelve arc minutes. Distance measurements are quite inaccurate. They range from around 2500 to 6400 light years. The star cluster consists of two compact groups that orbit massive, bright stars. The brightest of these is a main sequence star of the spectral type B0 and is primarily responsible for the fact that the nebula shines. It is believed that the bright part of the nebula originated around 100'000 years ago. The two to three hundred hot, bright, young stars in the area, however, have an average age of around one million years, which suggests that star formation took place here in several episodes, which continues to this day. The blue areas of the nebula are caused by the reflection of the starlight on dust grains, while the reddish regions are caused by ionized hydrogen (mainly Hα), caused by the star's non-violet radiation.

IC 5146
IC 5146: Image taken with the ESA Herschel space telescope in infrared light at 500 µm, 250 µm and 70 µm [300]

Around the light nebula lies the end of a dark nebula that extends far to the west Barnard 168 which makes it stand out clearly against the starry background of the Milky Way. The dark nebula appears on the infrared image of the ESA Herschel telescope in Fig. 4 brightly. Over 350 compact, starless knots have been discovered in the filaments of the dark nebula. About 45 of these are candidates for future star formation. [145, 196, 300]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationIC 5146
Right Ascension21h 53m 24.0s
Declination+47° 16' 00"
Diameter10.00 × 10.0 arcmin
Dreyer DescriptionpB, vL, iF, * 9.5 in M
IdentificationOCL 213, LBN 424, Cocoon nebula

Finder Chart

IC 5146 is located in the constellation Cygnus (Swan) towards Lacerta (Lizard). The best observation time is April to November. If you connect the imaginary line from α Lacertae (3.8 mag) - 4 Lacertae (4.6 mag) - ρ Cygni (3.9 mag), then IC 5146 lies on it approximately at the level of π Cygni (4.2 mag).

Chart IC 5146
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]


145SIMBAD astronomical database; simbad.u-strasbg.fr/simbad
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
196Celestial Atlas by Curtney Seligman; cseligman.com/text/atlas.htm (2020-12-28)
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)
300HERSCHEL IMAGE OF IC 5146; sci.esa.int/web/herschel/-/48624-herschel-image-of-ic-5146 (2021-03-31)