Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona
The Cocoon Nebula (designated IC 5146 or Sh2-125) is a reflection/emission nebula of about 15 light-years across that surrounds the young, open star cluster Collinder 470. It lies some 4,000 light-years away in the northern constellation of Cygnus (the Swan), while it is approaching us at approximately 17.8 kilometers per second.
This stellar nursery is actually a “blister” on the front edge of a large molecular complex. It combines an emission nebula of red, glowing, hydrogen gas and a reflection nebula, seen as blue, dust-reflected starlight at the edge of an otherwise invisible molecular cloud, cut by long, dark, dusty filaments where stars are forming (which can only be seen at infrared wavelengths).
The open cluster it surrounds is made up of mostly young, hot stars which clear out a cavity in the molecular cloud’s star forming dust and gas. One of them, the bright star near the center of this nebula has a surface temperature of 30,000 to 35,000 degrees and is primarily responsible for lighting up the nebula. This star is likely only a few hundred thousand years old, while the two or three hundred other stars in the area have a range of ages averaging a million or so years, suggesting that several episodes of star formation took place in the region, continuing to the present day.
This color view of the Cocoon Nebula traces remarkably subtle features within and surrounding the dusty stellar nursery. Surrounding the bright nebula is the end of a dark (absorption) nebula, Barnard 168, which separates the emission nebula from the surrounding starry background.
A dark sky is probably the most important factor in observing this nebula. It appears as a diffuse round glow in small telescopes. In larger instruments hints of dark lanes can be glimpsed against the otherwise smooth haze.