|
Designations:
|
IC 5146, Caldwell C19, Cocoon Nebula, Collinder 470 (associated cluster)
|
||
|
Object Type:
|
|||
|
Constellation:
|
Cygnus
|
||
|
21 hr 53.4 min
|
+47° 16 min
|
||
|
9.3
|
Size:
|
10 X 10 arcminutes
|
|
|
Distance:
|
~3,300 light years
|
||
|
Discoverer:
|
Thomas E. Espin, 1899
|
||
|
IC 5146 (Caldwell 19) should be a paradox for visual observers, especially
for those using small-aperture telescopes. Normally one would expect a bright nebula to be more conspicuous
than a dark one, but this is not the case with the Cocoon Nebula and its neighboring field of dark nebulosity.
Under a dark sky the dark pool and its associated stream - which flows toward, then ponds around, IC 5146 -
is one of the northern sky's most visually stunning dark nebulae, whether for the naked eye, binoculars, or
low-power telescopes.(Known as Barnard 168, or B168, this dark nebula is outlined nicely in the Millennium
Star Atlas and in Sky Atlas 2000.0 second edition).
In 1899 the British clergyman and noted double-star observer Thomas E. Espin, whose observatory at Tow Law had 18- and 24-inch telescopes, first recorded the existence of the Cocoon Nebula on August 13th; he made a follow-up observation two nights later. In Astronmische Nachrichten for March 31, 1900, he refers to the nebula as a "faint glow about 8', well seen each night." That same year Max Wolf took the first photographs of the Cocoon. In a 1904 installment of Monthly Notices of the Royal Astronomical Society, Wolf wrote extensively about the "Remarkable Nebula in Cygnus connected with Starless Regions." He described the Cocoon as being somewhat round and having a complicated structure resembling that of the Trifid Nebula (M20) in Sagittarius:
It is placed centrally in a very fine lacuna, void of faint stars, which surrounds the luminous cloud like a trench. The most striking feature with regard to this object is that the star-void halo encircling the nebula forms the end of a long channel, running eastward from the western nebulous clouds and their lacunae to a length of more than two degrees. Like William Herschel before him, Wolf believed in the possibility that the location within these stellar "voids" of the Cocoon and other extended nebulae - such as the Rho Ophiuchi and Eta Carinae nebulae - was not coincidental:
. . . though partially or wholly surrounded by void zones, the nebulae are generally placed at the end of a longer extended lacuna, so that we are led to the impression that we here see the result of some cosmic movement, the end of the lacuna showing the place where this unknown event began. In regarding this nebula we are led to speculation. We might suppose the nebula were detached from the great western nebulous cloud, and as if it, or the cosmic process connected with its origin, had swept the long channel through the star-crowds of the Milky Way. Or is there a dark mass following the path of the nebula, absorbing the light of the fainter stars? We are far from knowing enough to settle these questions. Wolf also noticed the existence of some brighter suns within the confines of the Cocoon. Today we know of some 110 stars as faint as 17th magnitude inside the nebula. These constitute a very young cluster, as do the stars at the heart of the great Orion Nebula (M42), and many authorities have equated IC5146 with this young cluster. But Brent Archinal of the U.S. Naval Observatory notes that the Cocoon is "another case where the Caldwell Catalog has it right and everyone else has it wrong. IC 5146 (the Cocoon Nebula) is the nebula, while Collinder470 (not IC 5146) is the correct name of the cluster. Espin and Wolf independently discovered the nebula and are credited in the Index Catalogues. Collinder was apparently the first to notice the cluster." |
|
Telescope:
|
|
|
Focal Length:
|
1200 m
|
|
Mount:
|
|
|
Camera
|
|
|
Guider:
|
SBIG AO-7 Adaptive Optics unit
|
|
Exposures:
|
70-minutes exposure for luminance, 42-minutes for each color (total exposure 196 minutes)
|
|
Location:
|
Cicero, IN
|
|
Software:
|
CCDSoft for image acquisition, processed with CCDStack and Photoshop CS2
|
