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Thread: Object of the Week January 12 2014 - Abell 21, a large evolved PN in Gemini

  1. #1
    Member reiner's Avatar
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    Feb 2012
    Freiburg, Germany

    Object of the Week January 12 2014 - Abell 21, a large evolved PN in Gemini

    Abell 21 (Medusa Nebula)

    RA 07 29 03
    DEC +13 14 30

    Type: large and ancient Planetary Nebula

    Size: 12'x 9'

    Abell 21 is one of those large and ancient PN that have expanded (and diluted) far into interstellar space such that their density is approaching that of the interstellar medium (ISM). The shape of the PN is therefore no longer governed by properties of its progenitor star (as in young PNe), but by its interaction with the ISM.

    According to data by David Frew, Abell 21 is about 1800 LYs away, it has expanded to a diameter of roughly 3 LYs and is about 27 000 years old. Flux in OIII and H-alpha are comparable, so this is an object best viewed with OIII filter.

    Image of Abell 21 from the DSS

    Extremely deep image by Heidi Schweikert and Travis Rector (NOAO)

    Similarly deep image by Ken Crawford

    Most of the large and ancient PN are extremely faint and require excellent conditions. Compared to these, Abell 21 is "reasonably" bright and under fair conditions not overly difficult. With my 22" Dob, it is visible also without filter (and it should also be with smaller aperture). Abell 21 is a bipolar PN. Due to its interaction with the ISM, Abell 21 appears lopsided at the telescope. The brightest section of the PN is its SE part that forms a crescent shaped shell with internal filamentary structure that is quite obvious at the eyepiece already in medium sized telescopes.

    One step more difficult: Much fainter, but still detectable, is the interior of the PN. This interior of the PN is filled with a very faint glow visible with OIII filter. It enlarges the brighter crescent shaped part to a largely round shape.

    And another step more difficult: Only during one night, I tried to trace the NW counterpart to the brighter SE crescent. This part is visible as a limb-brighened shell in the narrow band images. Under excellent conditions I suspected an extremely faint crescent with OIII filter that runs roughly parallel to a distinct chain of faint stars and beyond. Such chains of stars are deceptive, in particular in combination with emission line filters. Can anybody confirm sighting of this NW crescent?


    And finally the Holy Grail of Abell 21: Beyond this extremely faint NW crescent, there is further glowing gas visible in the very deep narrow band images. It is not clear whether this is material of the PN that had been stripped off while moving through the ISM or rather ISM itself being ionized by the White Dwarf central star of the PN. Assuming that the brighter SE side of Abell 71 is a bowshock due to movement of the PN through the ISM, this material would be on the "right side" to be a trail. Has anybody tried to observe this feature ? I did not even think of tackling this visually :-)
    Last edited by reiner; January 12th, 2014 at 05:47 PM.

    22" and 14" Dobs on EQ platforms and Deep Sky Observing

  2. #2
    Member Howard B's Avatar
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    Feb 2012
    Portland, Oregon USA
    Hi Reiner,

    I have only one sketch of Abell 21 from the late 90's with my 20" f/5:

    AbellPN21_crop.jpg AbellPN21_crop_invert.jpg

    "Big and surprisingly bright at 92x without a filter, and adding the OIII brought it to life. Big crescent shape with both ends as the brightest areas with the rest of the crescent having an undulating brightness. Several hand fulls of stars are strewn across this lovely scene and these are only the stars visible through the OIII filter - there's lots more without the filter. 167x."

    Next time I get a good night I'll give this a shot and see if I can detect the fainter details you've mentioned.
    28 inch f/4 alt-az Newtonian

  3. #3
    One note, don't let this Abell PNe destination make you think that you need a huge scope to see it. I've seen it pretty easily with my 4" refractor. I've read reports that it was seen in scopes as small is 80mm.
    Clear skies,
    Alvin #26

  4. #4
    Join Date
    Feb 2012
    Wow Reiner,

    as always - wonderful compilation of information.

    I didn't know the outer shell structure in the NW before. But when I remember right I still didn't look at A 21 with my 27", I have to revisit it.

    With my old 16" I tried to see the main (double) shell structure as a challenge which is mainly shining in Ha. But with [OIII] it was visible, also like the faint interior of the PN. Howard has sketched the double shell also like Rainer Mannoff with 16" under Moroccan skies. My sketch with 16" is down below.

    And Alvin is right, the PN is visible with very small aperture. Together with Martin Schoenball we both observed this PN with a very simple 70mm Reflector which has a spherical mirror. With [OIII] it was visible without any problems and I think it could be visible even with 50mm-60mm or a 50mm binocular with filters.

    16", 100x, [OIII], NELM 6m5+

    Clear Skies, uwe

    27" f/4,2

  5. #5
    Member Marko's Avatar
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    Feb 2012
    Bay Area, CA
    Nice reminder Reiner. I revisit this rather bright Abell each year as many of us probably do on this list. Your tossing the gauntlett is motivating as I like to look at this object but have only fairly scant records of what I have observed of Sh2-274. My notes only discuss the brighter crescent with mention of the darker strands along that bright part. Time to revisit since my last 2010 record of this curious and semi-bright Abell. Thanks
    Let me roam the deep skies and I'll be content.
    Mark Johnston
    18" StarMaster f/3.7
    12" Meade LightBridge f/5

  6. #6
    Member Atlas's Avatar
    Join Date
    Nov 2012
    Rottenburg / Germany
    Hi Reiner,

    I had a chance to observe Abell 21 a few nights ago. Conditions were rather poor. Seeing was mediocre and transparency was low due to very high humidity and patches of fog in my vicinity.

    Abell 21 was visibile even without filter, but the OIII filter enhanced the image a lot. I could see the crescent plus the interior filling. Also the large bar shaped exterior structure to the NW, that appears reddish in the NOAO image, was visible, as well as the reddish patch to the W. Both features did not seem very difficult to me. Furthermore I suspected some of the filamental structures NW of the nebula itself. Only the holy grail did not show up. I assume it is very difficult to observe since it runs parallel the a star chain.

    25" f4 home built Dobsonian, Argo Navis, ServoCAT
    My astronomy website: Blick ins All

  7. #7
    Member FaithJ's Avatar
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    Feb 2012
    50.65 N, 1.15 W
    13thm March 2010: Eastern side is the brighter and is crescent shaped, in fact almost triangular. There is also some nebulosity on the western side. 20" Obsession (Magnification unknown) + OIII
    18" f/4.3 David Lukehurst Dob
    8" f/6 Dark Star Dob
    8x42 binoculars
    100% visual observing

    Visual Observing - FJ Astronomy

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