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ASTRO: NGC 2420



 
 
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Old August 1st 14, 09:00 AM posted to alt.binaries.pictures.astro
Rick Johnson[_2_]
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Default ASTRO: NGC 2420

NGC 2420 is an open cluster in Gemini not far from the far more famous
Eskimo Nebula. It is another Herschel 400 object I picked up. It was
discovered by William Herschel on November 19, 1783 using his 18.7"
reflector. In looking this one up I found this is about all the various
sources I found agree on. Even the visual description varies greatly.
One using a 12" Dob said it was mostly a smudge that became more obvious
if he moved the scope -- a common way of enhancing faint visual objects.
His drawing shows only the bright orange stars and none of the fainter
blue ones. Yet my log says: "Tight, rich cluster... It is a good object
well worth the side trip when looking at NGC 2392". And I was using a
10" scope. Things get worse when looking at other factors. Various
sources put it at 7 to 10 thousand light-years away. One says it is in
the Perseus Arm of the galaxy, another that it lies 3000 light-years
above the plane of the galaxy so wouldn't be in any arm of the galaxy.
It's age is said to be 1.1 billion years in WEBDA but a press release by
the NOAO ( http://www.noao.edu/outreach/press/pr02/pr0204.html ) puts
its age as 1.7 billion years. Either way it is unusually old for an
open cluster. These are normally torn apart after much less time by
tidal forces in the galaxy's disk. So the age, either of them, support
the idea it is well above the plane of the galaxy. Of course it will
have to dive through the plane and spend some time below the plane as
well but if its orbit carries it well above or below the plane then such
trips would be few aiding its longevity. The NOAO release says its
stars have the same metal content as the sun but they are younger. Also
its location is very odd for having the necessary ingredients to even
form a cluster let alone one with this metal content which may mean it
formed in the disk and somehow got its orbit changed. Could this mean
it formed in some galaxy ours has cannibalized? That might explain its
odd orbit but then other pieces of that galaxy should be evident and I
see no mention of that. Lots more work is in order here!

The field has several "bright" and interesting galaxies but only two
have redshift data. They appear to be related as they are less than a
minute of arc apart and have almost the same redshift values. The rest
have no redshift data. I annotated the brighter ones anyway. I'd not
have annotated the image at all except it contains 4 rather obvious
asteroids all a bit fainter than 19th magnitude.

This image was taken over several nights. Blue data was highly focused
by good seeing that was lost due to clouds. This made the blue stars
much smaller than those taken other nights by the luminance, red and
green filters under much poorer seeing. Even blurring the blue to try
and make it match I ended up with some color fringing I couldn't seem to
solve. Seems combining color data taken under near one arc second
seeing with that taken under 3.5 arc second seeing doesn't work all that
well. Note seeing got so bad for the luminance data I used only 3 of 8
frames taken so this doesn't go as deep as usual. After lots of work
this will have to do.

Please note I don't trust either my color balance nor that of the NOAO
image. Mine is iffy due to the treatment I gave the blue data to try
and match the much poorer red and green nights that may have put too
much blue into the blue stars. The NOAO image seems too orange. The
bright "red" star below the NOAO image is shown as being G0 which
usually means a rather white star not the red star their image shows.
It is "bright" (magnitude 9.22) as it is only about 200 light-years away.

14" LX200R @ f/10, L=3x10' RGB=2x10', STL-11000XM, Paramount ME

Rick

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