The Cocoon Nebula (IC 5146, S125)

Distance 3900 light years

Right Ascension: 21 : 53.4 (hours : minutes)
Declination: +47 : 16 (degrees : minutes)

IC 5146 is a beautiful emission and reflection cloud that surrounds the illuminating central star BD +46°3474. The circular HII region is catalogued separately as Sharpless 125. The powerful B-type star, BD +46°3474 is the dominant illumination source of IC 5146 and lies in the foreground of a large molecular cloud complex. The colorful nebula IC 5146 is located at the eastern end of a series of dark clouds known as LDN 1035 and LDN 1045 (Lynds 1962) (also catalogued as Barnard 168). The central cluster of IC 5146 is a very young open cluster with a median age of about one million years. The cluster is so young that many of its members are still pre-main sequence stars. Among the young stars several young Herbig-Haro objects and many T-tauri objects are found. T-tauri stars are the pre-main sequence counterpart to low mass stars similar to our sun. They are detectable in h-alpha light as their dusty envelopes emit strongly in the shorter wavelengths. The bright star BD +46°3471, just west of the main IC 5146 illuminates a small reflection cloud lying near the western edge of the cloud complex. It is classified as a Herbig Be star. These stars are pre-main sequence intermediate mass stars (2 to 8 solar masses) which are often associated with dust or reflection nebulosity.

The cluster of stars which lie at the center of IC 5146 are mostly low mass stars like our sun but much younger at only 1 million years old. The nebula is powered by the hot and luminous B0 type star BD +46°3474 whose surface temperature of 30,000 to 35,000 degrees allows it to generate the ultraviolet flux needed to ionize the surrounding gases. The central star began to shine only one hundred thousand years ago along the near side of the current molecular cloud it illuminates.

The formation and evolutionary history of IC 5146 is suggested by its three dimensional structure. The mass tied up in the cluster members (64 solar masses) is much greater than the mass of the surrounding cloud (18 solar masses) suggesting that the young cluster likely formed in a foreground cloud that has since dissipated. Once formed, BD+46°3474 quickly evaporated the foreground cloud and went on to ionize a thin "blister" on the large background molecular cloud. Although BD +46°3474 is central within the bright nebulosity, the young T-tauri stars are asymmetrically distributed around the star suggesting they outline the boundary of the original cloud. Since that time the energetic winds of BD+46°3474 have carved out a cavity from the near side of the molecular cloud complex. Heated gas and dust flow like champagne from the molecular cloud, exiting the cavity in the direction of our sun where it is expelled. Fortuitously for astronomers the cavity has cleared a visual portal allowing direct observation of the innermost regions of the nebula and its cluster.