IC 5146, Cocoon- Nebula
The Cocoon Nebula (IC 5146) is an HII region with an embedded star cluster at a distance of 3,100 light-years - located in the northern part of the
constellation Swan. It was discovered photographically by E. Barnard in 1893 and independently by Max Wolf a few months later.
The apparent dimension of IC 5146 is 10 arcminutes. Using the known distance, the true diameter can be determined from this to be 9 light years.
The gas masses consist mainly of ionized atomic hydrogen. There are emitting parts as well as reflecting (vdB 147) and absorbing (B168) ones.
This is because the Cocoon Nebula is part of a much larger molecular cloud.
In the center of the nebula is an open star cluster called "Collinder 470", which is best seen in the infrared. The cluster is very young with an age
of 100,000 years and contains several hundred stars and protostars. The brightest star is "BD +46°3474", which belongs to spectral class B0 and
is nearly 30,000 Kelvin hot at the surface. With a mass of 14 solar masses the star is a white supergiant. Together with some other young, hot stars
it emits a lot of UV radiation, which causes the surrounding nebular masses to glow and make the Cocoon Nebula visible to us.
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Even with a 20" telescope the Cocoon Nebula does not reveal much of its secrets. You can see a faint glow around a brighter 10th magnitude star
at low magnification in combination with a nebula filter. Only with time a few structures and brighter areas appear - hard to see because of the
low brightness differences.
The very young star cluster in the center is not visible even with 50cm aperture, also a magnification of V=270x does not help much.
Worth mentioning is in any case a long tube of dust, starting from the Cocoon Nebula and running through the dense star field of the Milky Way,
leaving the impression as if someone had sucked away all the stars in a narrow strip with a cosmic vacuum cleaner.
