The Cocoon Nebula (Sh2-125)

If you are observing by a clear-sky night, and far from light pollution, then point your telescope to 21h 53m 28.7s right ascension and +47° 16′ 01″ declination, in the Cygnus constellation. You will be face-to-face with a prominent nebula known as the Cocoon Nebula, Sh2-125, IC 5146, or Caldwell 19. At nearly 3300 light-years away, the Cocoon Nebula is a cocoon-shaped cloud of gas and dust, hosting a young cluster of stars. Although we cannot see it without a telescope, we can localize this deep-sky object by looking near Pi-Cygni, a binary stellar system from the Cygnus constellation.

Sh2-125 is a reflection and an emission nebula. It means that it reflects light emitted by the stars in the cluster. 

Near the center of the Cocoon Nebula lies a young giant hot blue star. The whole cluster of stars constantly emits energetic radiation that heats the nebula. The most energetic radiation extracts electrons from the hydrogen atoms in the cloud. Then, these electrons and ionized hydrogen recombine together again emitting visible light glowing in red as we see in this picture.

Another part of the stellar radiation is reflected by the dust particles. It appears in blue in this picture. When we look closely at the most inner part of the nebula, we could think that this region has no stars. However, infrared images show that there is a dense nebula that hides young and energetic stars inside. These stars emit high-energy radiation that heats the nebular cloud. It is then absorbed by the dust particles and reemitted in infrared wavelengths. As the bright star that lies in the central region of the Cocoon Nebula is brightening the dense surrounding cloud, it is also constantly consuming it creating a cavity around it.

The Cocoon Nebula is about 15 light-years across. While the full Moon covers over 0.5° of the sky, the Cocoon Nebula covers around 0.2°. However, this is only the tip of the iceberg.

This image was taken with an RGB filter.

This kind of filter reproduces the object in its true colors. In this image, the long exposure permitted to highlight the trail that is part of Barnard 168: the submerged part of the iceberg. In fact, the Cocoon Nebula is thought to be physically bonded to the much larger dark cloud of dust known as Barnard 168. Having a dense starfield in the background, the dark nebula appears more clearly through a telescope than the dim Cocoon Nebula. It needs long exposures and a strong contrast to appear. The global picture looks roughly like a black snake with a pink head, represented by the Cocoon Nebula.

Besides, Barnard 168 is a good target for binoculars. It covers a large view, equivalent to nearly 3 full Moons. But with the naked eye, we will not be able to see the Cocoon Nebula. On infrared images, taken by infrared telescopes such as ESA’s Herschel Space Telescope, the long dark trail appears bright in infrared, as it is reemitting the light it receives from its surroundings. These images reveal filamentary features in Barnard 168. Astronomers think that, in the past, shockwaves caused by star explosions traveled through the nebula. These shockwaves swept up and squeezed the nebular gas and dust, producing these filaments. Even if they are dark in the visible range of wavelengths, observations indicate that inside these structures, new stars are forming. The scenery is expected to change within the next few million years.

Both of these pictures were taken using a Celestron RASA telescope, with a focal length of 620 mm. The camera that captured the image is ZWO Kamera ASI 1600 MM-Cool V3 Mono. The whole was mounted on a Celestron CGE pro mount.