ASTRO: NGC 6939 A "dirty" open cluster

NGC 6939 is a rather well known open cluster in Cepheus near the more
famous NGC 6946 galaxy. In small scopes where only the brighter stars
are seen they seem to array in rows much like a bank of stadium lights.
That effect is totally missing from my image. The WEBDA cluster
database puts its distance at about 3850 light-years though I've seen
other uncredited estimates of 5800 light-years. Again, astronomy hasn't
yet found a good way to pin down the distance to most such clusters
leading to various estimates depending on methods used and assumptions
made about extinction from dust and gas in our galaxy which is also hard
to determine.

Sometimes you get a surprise in an image. That certainly is the case
here. The field is full of dusty nebulosity. If I'd have known it was
there I'd have put far more time into the image. But it came as a
complete surprise when I started to process the image. At the time I
processed my image of the galaxy NGC 6946 (Arp 29) I saw some of this
nebulosity but thinking it noise processed most of it out. Oops. I
need to be more trusting of my data. Still hints of it can be seen in
the image, especially just west (right) of the galaxy and toward the
west southwest of the galaxy in the image I sent on Feb, 6, 2010.
http://www.spacebanter.com/attachmen...tid=2814&stc=1

Apparently, most imaging this cluster have the same problem I did with
the galaxy and think the nebulosity is noise. It is either processed
out or not seen in most images I found on the net. Only a couple showed
the nebulosity. It apparently is mostly due to fluorescence from UV
stars illuminating the carbon compounds in it as well as reflecting
light of these very hot blue stars though one image in H alpha light
showed a general glow in the area. I found no catalog entry for this
nebulosity at Simbad.

If you look closely you will see many faint, small galaxies in the image
some well inside the cluster. Only one in the image has a red shift
value. Several others are just outside the frame. All are faint
smudges and are listed at about the same redshift as NGC 6946. They are
likely gravitationally bound to it. The one in my image with redshift
data is LEDA 166196. It is at the top at pixel 1001x130. It is hiding
behind a very orange star. It is a round reddish ball about 55 seconds
of arc (55 pixels) across. A much more obvious galaxy is below it,
2MASX J20313354+6047414. It has no red shift data same as all the
others in the image. Only 2 others are identified in the image. I
won't try to point them out.

I need to redo this field with much more exposure time to better pull
the nebulosity out of the sky glow. I've included two different size
images. One full size and one reduced to 1.5" per pixel. Due to my
short exposure time for the dust it seems to show the features of the
dust a bit better than the full size image. Sort of like looking at
summers puffy cumulus clouds at moderate magnification causes their
detail to vanish.

14" LX200R @ f/10, L=4x10' RGB=2x10'x3, STL-11000XM, Paramount ME

Rick
--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

Beautiful star colours Rick.
The dust looks good in the smaller version.

Stefan

"Rick Johnson" schrieb im Newsbeitrag
.com...
NGC 6939 is a rather well known open cluster in Cepheus near the more
famous NGC 6946 galaxy. In small scopes where only the brighter stars
are seen they seem to array in rows much like a bank of stadium lights.
That effect is totally missing from my image. The WEBDA cluster
database puts its distance at about 3850 light-years though I've seen
other uncredited estimates of 5800 light-years. Again, astronomy hasn't
yet found a good way to pin down the distance to most such clusters
leading to various estimates depending on methods used and assumptions
made about extinction from dust and gas in our galaxy which is also hard
to determine.

Sometimes you get a surprise in an image. That certainly is the case
here. The field is full of dusty nebulosity. If I'd have known it was
there I'd have put far more time into the image. But it came as a
complete surprise when I started to process the image. At the time I
processed my image of the galaxy NGC 6946 (Arp 29) I saw some of this
nebulosity but thinking it noise processed most of it out. Oops. I
need to be more trusting of my data. Still hints of it can be seen in
the image, especially just west (right) of the galaxy and toward the
west southwest of the galaxy in the image I sent on Feb, 6, 2010.
http://www.spacebanter.com/attachmen...tid=2814&stc=1

Apparently, most imaging this cluster have the same problem I did with
the galaxy and think the nebulosity is noise. It is either processed
out or not seen in most images I found on the net. Only a couple showed
the nebulosity. It apparently is mostly due to fluorescence from UV
stars illuminating the carbon compounds in it as well as reflecting
light of these very hot blue stars though one image in H alpha light
showed a general glow in the area. I found no catalog entry for this
nebulosity at Simbad.

If you look closely you will see many faint, small galaxies in the image
some well inside the cluster. Only one in the image has a red shift
value. Several others are just outside the frame. All are faint
smudges and are listed at about the same redshift as NGC 6946. They are
likely gravitationally bound to it. The one in my image with redshift
data is LEDA 166196. It is at the top at pixel 1001x130. It is hiding
behind a very orange star. It is a round reddish ball about 55 seconds
of arc (55 pixels) across. A much more obvious galaxy is below it,
2MASX J20313354+6047414. It has no red shift data same as all the
others in the image. Only 2 others are identified in the image. I
won't try to point them out.

I need to redo this field with much more exposure time to better pull
the nebulosity out of the sky glow. I've included two different size
images. One full size and one reduced to 1.5" per pixel. Due to my
short exposure time for the dust it seems to show the features of the
dust a bit better than the full size image. Sort of like looking at
summers puffy cumulus clouds at moderate magnification causes their
detail to vanish.

14" LX200R @ f/10, L=4x10' RGB=2x10'x3, STL-11000XM, Paramount ME

Rick
--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".