Cocoon (IC5146) and Associated Dark (B270) Nebulae - LHaRGB (Cygnus), June 2023
Televue 127is; AP Mach2
ASI6200MM, FIRST LIGHT - Baader CMOS-Opt Filters (L,R,G,B and Ha(6.5nm))
L: (40 x 160s exposures, Bin 1x1, Gain 100)
Ha: (20 x 720s exposures, Bin 1x1, Gain 100)
R,G,B: (20,20,20 x 180s, Bin 1x1, Gain 100)
Total Integration Time = 8.8 hours

This view of Cygnus is prototypical in a couple of ways.
Compositionally, it contains a rich Milky Way starfield and faint hydrogen emission signal to provide the perfect backdrop to illustrate the three primary types of nebulosity. The Cocoon "Stellar Nursery" is bright with red hydrogen emissions, fueled by the UV light primarily from the bright, infant star at its centre.  The red colour indicates hydrogen and this is termed an emission nebula - emitting visible light with the energy converted from the UV.   Surrounding the red emission nebulosity,  the same gas can be seen this time reflecting light, mainly in blue, from other nearby stars.   This is termed refection nebulosity because it is not being emitted, rather visible starlight is being reflected to us.  Throughout the image the gas is generally semi-transparent (ultra-low density), giving off, at best, a greyish glow of reflected light.  The red hue in the background that comes and goes, indicates areas where low-level hydrogen emissions are taking place.   In certain parts of the image, the gas is more dense and is visible only due to the virtue that it nearly or completely blots out the background glow and stars behind it.  This is the aptly named "dark nebulosity" - so all three "kinds" of nebulosity; emission, reflection, and dark,  are visible in this image. In addition, all three forms of nebulosity are composed of essentially the same stuff - a hydrogen rich molecular cloud that is the sole ingredient to create new stars.

The image also demonstrates two phases of star formation.  The molecular cloud, which began in a more homogeneous state, can be seen fragmenting and coalescing into dark nodules losing its transparency completely in places.  This is the caused by gravity that pulls the cloud into more dense discrete high density points where stars can potentially form,  but it requires the gases to be very cold (and not jiggling about) to coalesce to the extent required to form stars.  Light, whether it be visible or UV, when shined on the cloud will heat it up, disrupting the initial star creation process (conception).    Parts of the molecular cloud is hidden from this light by the cloud itself - within areas where it completely blots out background too.     It is within these darkest nodules that the nucleus of gaseous material that will form a star will occur.

If sufficient material comes together in a cloud "nodule", a proto-star will be conceived and will continue to grow as the gravitational effects will snowball and continue to pull material in.   At this point, even the warming effects caused by gas compression will now be overcome by gravity.   However the cloud will continue to be opaque to the outsider, so we don't know where exactly where these protostars are in this image.  (IR can better see through the cloud, so protostars can be seen with an appropriate camera and filter.  Sometimes  the presence of protostars is given away by Herbig Haro streams)

Once large and hot enough, fusion will be ignited and a proper star will be born. This new born star will be rich in its own UV radiation which will heat, light up, and begin to blow away the gases and dust that used to surround, feed, and protect  it - creating an emission nebula such as the Cocoon.   It can be seen that even within the Cocoon Nebula, there remains dark nodules, where gases are likely continuing to coalesce to additionally forming future stars - possibly a cluster of them.   If the cloud is large enough,  more stellar nurseries will be created and the area will be termed a "star forming region"