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NGC 6891 (16,121 of 18,816)

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NGC 6891

NGC 6891, HD 192563, PK 054-12 1, PN VV' 529, PN VV 253, PN G054.1-12.1

RA: 20h 15m 8.84s
Dec: +12° 42′ 15.6″

Con: Delphinus
Ch: MSA:1242, U2:208, SA:16

Ref: SIMBAD

(reference key)

Type: planetary nebula

Mag: B=10, V=12.4

Size: ?
PA: ?

Historical observations

Burnham, S.W. (1894)

Burnham, S. W. () "Measures of planetary nebulae with the 36-inch equatorial of the Lick Observatory", Pub. Lick Obs., vol 2, p159-167.

Published comments

Terzian, Y. (1980)

Q.J. R.astr.Soc vol 21, p82-92 [09.16.1] notes that the nebula has a faint outer giant halo.

Sulentic & Tifft (1973)

(Sulentic and Tifft 1973) notes that this is a 11.5 mag planetary nebula.

Photo index

Photo Index by Jim Lucyk: Sky&Tel. 9/83 p277.

Modern observations

Walter Scott Houston

Houston writes: "Though only 11th mag and 0.1' x 0.2' in size, it is easier to identify because the central star is also 11th mag. In my 4-inch refractor the star is evident at 60x, but the 20x apogee scope has trouble."

Sanford (1989) Observing the Constellations

Sanford notes that this planetary nebula in Delphinus measures 7x15 arcseconds, is round and has a bluish colour.

Hartung, E.J. (1968) Astron.Obj.South.Tel

Hartung notes: "This is a bright bluish nebula, round and fairly well defined, about 12 arcsec across and easily picked out in a rather barren field."

Tom Lorenzin

Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "10M; 15" x 7" extent; with 11M center star; disk surrounded by fainter ring."

Steve Coe

Steve Coe, observing with a 13" f/5.6, notes: "Bright, pretty small and somewhat elongated at 100X. It is a little difficult to find in a rich Milky Way field. The central star of this planetary comes and goes with the seeing at 270X. It is very nice aqua or blue-green in both the 13" and my old 18" f/6 Dobsonian."

Birkmann, Mark [IAAC]

Observer: Mark Birkmann Your skills: Intermediate (some years) Date/time of observation: 7/9/99, 6:50 UT Location of site: New Haven, Missouri (Lat N 38, Elev ~700') Site classification: Rural Sky darkness: 5 1-10 Scale (10 best) Seeing: 5 1-10 Seeing Scale (10 best) Moon presence: None - moon not in sky Instrument: 30" f/4, dob Magnification: 150x-540x Filter(s): Object(s): NGC 6891 Category: Planetary nebula. Class: 2a+2b Constellation: delphinus Data: mag 11.7p size 15" Position: RA 20h:15m 8.9s DEC +12:42' 15"

Description: This small pn was green/gray at low power, gray at high power, the mag 12.4 central star was suspected, the outer halo was about twice the diameter of the slightly brighter central area.

[amastro] Size of Planetary NGC 6891

This is a case where you can use a field star to estimate the size that you see in the eyepiece. The mag. 14 star on the east side of the nebula is 13".5 from the center. Scale from there. L&S do indeed list a large size for the object, presumably selected from the Perek & Kohoutek catalogue; there was probably nothing else available to us at the time. Referring to the earlier discussion about using a CCD to get isophotal dimensions of planetaries, here's another case where we need a better number.

Brian

------------------------------------------------------------------------

It seems that all sources are right. The problem (as it's with so many catalog data - you must always consult original papers) is that they refer to different parts. A recent image of NGC 6891 (which was, by the way, one of the few PNs which appeared in the BD atlas and catalog unrecognized, that is as faint stars - this one as BD+12o4266) appeared together with others PNs (I much like IC 1295) in ApJ 507:889-908, 1998. The paper is not freely available at ADS, but it's available (gzipped postscript, 2.2 Mb!) at the IAC preprint website:

http://www.iac.es/publicaciones/preprints.html

The overexposed central nebula is slightly oval with p.a. of the major axis roughly in the N-S direction, and ends just at the 14th mag star mentioned by Brian. A faint halo around is not limb-brightened (as the halo of NGC 6826, NGC 6720...) nor it has obvious higher-surface-brightness condensations (as the halo of NGC 6543, NGC 7662, etc.). In fact, according to the text and tables of the paper, NGC 6891 is a triple-shell PN. The bright core has dimensions 12x10 arc seconds, so-called 'attached shell' (nearly linear surface brightness profile) has 23x21 arc seconds and finally the 'detached halo' has size 68 arc seconds.

Best,

Leos

------------------------------------------------------------------------

David,

Some of my personal notes from previous research on NGC 6891 mention that this is a multiple-shell PN with a Carbon-type Wolf-Rayet nucleus and a 74" outer halo structure. I have the main disk of the nebula listed at 15" and 11.7p mag. However, just to give you a little reference as to exactly how faint most of these post-AGB envelopes are around many of these PNe, one of the more definitive articles concerning the subject is a 1992 ApJ vol.392 article entitled "Stellar Paleontology" by some of the top PN researchers themselves (Balick, Frank, Jacoby, and Gonzalez). On page 588 of this piece, the authors mention that the outer structure of NGC 6891 is some 2500 times fainter than the main disk of the PN! And that's determined via deep H-alpha and [NII] images. I've seen a number of these halo structures around other PNe myself (M 57, NGC 3242, NGC 6543, M 76 to name a few); however, after playing with the DSS image contrast for a while, it's quite obvious that this one would certainly be challenging (if even possible) for us backyard-types...

Jay McNeil

------------------------------------------------------------------------

After doing a little lunar and double star work in my ten inch Newtonian, I went after (and easily found) NGC 6891, a small planetary in Delphinus. It appeared nearly stellar at 59x with a nice bluish color, and with 0.5 arc second seeing (somewhat variable though), I kicked the power up to 310x and 440x. The planetary appears as a small somewhat diffuse disk with a hint of a slight north-south elongation, which has a brighter slightly oval diffuse core, also which shows a possible elongation. The brightness rise is somewhat gradual, and the edge of the planetary made it very difficult to get a decent size estimate with only a ten inch aperture and the moon a day past first quarter. My guess was perhaps 10 to 15 arc seconds. On consulting various authors, I am getting mixed signals. Sky Catalog 2000.0 give 12"/72", Uranometria lists >14", the Web Society Handbook gives it a 12" arc size, and my old fallback (Luginbuhl & Skiff) gives 74" x 62" arc. Looking at the POSS images, I measure the overexposed image at about 20" x 18" at most, with no clear signs of an outer shell. By doing some enlargement and putting my nose on the screen, I might be seeing a suggestion of an outer shell, but it may just be glare. Anybody have some answers to this?

--

David Knisely KA0CZC dk84538@navix.net

------------------------------------------------------------------------

Brian Skiff

POSS: *s 15" in pa85 and (brtr) at 55" in pa290.

Marling: mv=10.5

Kaler: cen * V=12.4

15cm - brtr than expected, pn shape seen @ 122x. no features, well defined edges. HHM, Roof, June 1977.

- res as non*ar @ 50x. 295x: circ, 10" diam, or half sep of m13,14.2 wide pair WNW. faint halo, steps up to much brtr core and poss tiny annularity in center w/cen *. BS, 28Jun1989, Anderson Mesa.

25cm - sharply def. cen brtning but no cen *. * on NE edge of neb.

- 190x: sm, 10"-12" across. fairly consp cen * even though neb has hisfcbr. no other details. discernable from a * @ 50x. BS, 6Jun1981, Slate Mtn.

30cm - 476x: indef elong (?). circ, 10" diam w/broad br core. SE of m9.5 *. m13.5 * 25" NE. smooth texture. CBL, Roof.

[L&S text]: a m13.5 * vis on E edge, lying 15" from center. 425x: neb 10" across w/o any discernable elongation. broadly concen w/smooth texture. CBL, Roof.

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