ASTRO: NGC 6826 The Blinking Planetary

NGC 6826 is apparently the only known triple shell, giant halo
planetary. Considering there are only 3 in the first category and 6 in
the second that isn't all that surprising. It is in northwestern
Cygnus. Distances to planetary nebulae are very hard to determine. One
paper cited two other papers giving a range from 2450 to 7385 light-years.

It is also known as the Blinking (Eye) Planetary Nebula. When you look
right at the central star the nebula fades greatly only to reappear when
you look off to the side. I have seen lots of observers blame this on
the lack of sensitivity of the eye's fovea. It's been long known to
visual observers that the center of our vision has fewer low level rod
cells so doesn't see faint objects all that well. This is involved but
other equally faint planetary nebulae don't exhibit this trait nearly so
strongly. Those though don't have 10.4 magnitude central stars. Theirs
are several magnitudes fainter. I used a cross hair eyepiece to block
the light of the central star. When that was done even looking at where
it was the nebula only faded slightly. I think most of the "blink" is
due to how our eye/brain handles the fainter nebula around the bright
star. Something to test at your next dark sky star party.

You can read a bit more on it at:
http://adsabs.harvard.edu/full/1981PASP...93..719F

The other unusual characteristic of this one (not mentioned in the above
link) is the the FLIERS (Fast Low-Ionization Emission Regions) seen in
it. They are the two red regions seen in the HST image (link below).
In my RGB image they come out as bright blue regions on either side of
the center shell (mostly hidden by the bright central star). Why they
came out blue I don't understand but they did. These FLIERS may not be
so "fast" however. There's some indication they are relatively
stationary and somehow the gas of the second shell is speeding past them
rather than pulling them along with the shell's expansion. Either way
FLIERs are hard to explain with our current understanding of Planetary
Nebulae.

The only galaxy with a redshift measurement in the image is the
brightest in the field. It is west (right) of NGC 6826 and a bit north.
It is CGCG 257-010 with a redshift that puts it some 330 million
light-years away. All others in the field that NED includes are from
the 2MASS survey, as is CGCG 257-010.

Due to the huge brightness difference between the central part of the
nebula and the large halo I used two sets of exposures to record this
one. The core was exposed using 14 2 minute exposures for the luminance
and 2 5 minute exposures for the color data. The rest of the image used
my "standard" 4 ten minute luminance and 2 10 minute color frames. The
two were then blended together. As usual all were binned 2x2. I really
need a better night for the core. If it happens I'll redo the core
region using 1x1 binning.

Hubble's image of the inner two shells using narrow band filters:
http://hubblesite.org/newscenter/arc...97/38/image/d/

14" LX200R @ f/10, L=4x10' and 14x2' RGB=2x10' and 2x5', STL-11000XM,
Paramount ME

Rick
--
Prefix is correct. Domain is arvig dot net

Rick,

amazing how clearly you could record the halo...

Stefan

"Rick Johnson" schrieb im Newsbeitrag
.com...

NGC 6826 is apparently the only known triple shell, giant halo
planetary. Considering there are only 3 in the first category and 6 in
the second that isn't all that surprising. It is in northwestern
Cygnus. Distances to planetary nebulae are very hard to determine. One
paper cited two other papers giving a range from 2450 to 7385 light-years.

It is also known as the Blinking (Eye) Planetary Nebula. When you look
right at the central star the nebula fades greatly only to reappear when
you look off to the side. I have seen lots of observers blame this on
the lack of sensitivity of the eye's fovea. It's been long known to
visual observers that the center of our vision has fewer low level rod
cells so doesn't see faint objects all that well. This is involved but
other equally faint planetary nebulae don't exhibit this trait nearly so
strongly. Those though don't have 10.4 magnitude central stars. Theirs
are several magnitudes fainter. I used a cross hair eyepiece to block
the light of the central star. When that was done even looking at where
it was the nebula only faded slightly. I think most of the "blink" is
due to how our eye/brain handles the fainter nebula around the bright
star. Something to test at your next dark sky star party.

You can read a bit more on it at:
http://adsabs.harvard.edu/full/1981PASP...93..719F

The other unusual characteristic of this one (not mentioned in the above
link) is the the FLIERS (Fast Low-Ionization Emission Regions) seen in
it. They are the two red regions seen in the HST image (link below).
In my RGB image they come out as bright blue regions on either side of
the center shell (mostly hidden by the bright central star). Why they
came out blue I don't understand but they did. These FLIERS may not be
so "fast" however. There's some indication they are relatively
stationary and somehow the gas of the second shell is speeding past them
rather than pulling them along with the shell's expansion. Either way
FLIERs are hard to explain with our current understanding of Planetary
Nebulae.

The only galaxy with a redshift measurement in the image is the
brightest in the field. It is west (right) of NGC 6826 and a bit north.
It is CGCG 257-010 with a redshift that puts it some 330 million
light-years away. All others in the field that NED includes are from
the 2MASS survey, as is CGCG 257-010.

Due to the huge brightness difference between the central part of the
nebula and the large halo I used two sets of exposures to record this
one. The core was exposed using 14 2 minute exposures for the luminance
and 2 5 minute exposures for the color data. The rest of the image used
my "standard" 4 ten minute luminance and 2 10 minute color frames. The
two were then blended together. As usual all were binned 2x2. I really
need a better night for the core. If it happens I'll redo the core
region using 1x1 binning.

Hubble's image of the inner two shells using narrow band filters:
http://hubblesite.org/newscenter/arc...97/38/image/d/

14" LX200R @ f/10, L=4x10' and 14x2' RGB=2x10' and 2x5', STL-11000XM,
Paramount ME

Rick
--
Prefix is correct. Domain is arvig dot net