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Object of the Week August 18, 2019 - NGC 6826 The Blinking Planetary
NGC 6826, PK 083+12.1, PN G083.5+12.7
Type: Planetary Nebula
Constellation: Cygnus
RA: 19h 44m 48.2s
DEC: +50° 31’ 30”
Mag: 8.8
Size: 27” x 24”
Mag (central star): 10.4
High in the Northern skies during August and September, the planetary nebula NGC 6826 in Cygnus is the prototypical example of a planetary nebula that blinks, hence it bears the nickname: The Blinking Planetary. More about what causes the blinking in a moment.
NGC 6826 was first observed by William Herschel on September 6, 1793 and cataloged as H IV-73. He noted of his observation: “A bright point, a little extended, like two points close to one another; as bright as a star of the 8.9 magnitude, surrounded by a very bright milky nebulosity suddenly terminated, having the appearance of a planetary nebula with a lucid center; the border however is not very well defined. It is perfectly round, and I suppose about a half minute in diameter. It is of a middle species, between the planetary nebulae and nebulous stars, and is a beautiful phenomenon.” Quite an amazingly accurate observation in my humble opinion!
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Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918 xxxxxxxx Sketch by DSF member and OOTW Contributer Uwe Glahn, 16" f/4.5 600x
At a magnitude of 8.8, NGC 6826 is hardly a difficulty object. The PN can be observed with an aperture as small as 4” where at lower magnification it is virtually stellar. However be patient and give it a few minutes and you should be able to discern its non-stellar form. Double or triple the magnification and you should be able to discern a small disk with soft aqua hue. Add some more magnification again in the 4” and the PN’s 10.4 magnitude central star should begin to stand out. So it shouldn’t be surprising that NGC 6826 takes to increased aperture and magnification quite well, with every increase presenting a uniquely different view of the planetary nebula’s curious nature.
Here are characteristics you should look for and make note of as you observe NGC 6826, The Blinking Planetary.
- Color: Soft aqua hue to a blue-green hue using larger aperture
- Central star: Visible in a 4” telescope with moderate magnification
- Morphological constructs:
> Circular or elliptical
> Central shell: Diffuse; uniform or hint of brightening toward the central star; complex structure at extremely high magnification
> Fast Low-Ionization Emission Regions (FLIERs): Dense elliptical inner disk and knots at either end of the ellipse embed in an outer halo (try an O-III filter);
> Outer shell: FLIERs extend beyond bright outer shell at extremely high magnification
> Western limb: Possible dark region; possible uneven edge
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Foreground image credit Fred Calvert, Cold Spring Observatory; superimposed on DSS background
I promised to explain what causes this planetary nebula to blink. Quite simply - it is an optical illusion caused by its very bright central star. When focusing on it with your eye (direct vision), the surrounding - much fainter - nebula seems to disappear somewhat. When you then turn your gaze away from the star (indirect or averted vision), the nebula reappears. No magic, just our eyes having difficulties adapting to different brightness levels when already observing under dark circumstances.
Be patient and The Blinking Nebula will reward you. The keys to digging out the details are using extremely high (stupid crazy) magnifications in above average or excellent seeing conditions.
Now it is your turn.
Give it a go and let us know!
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HST image of the central portion of the nebula
