The Depth of NGC 7008
Scott Harrington
NGC 7008 (PN G093.4+05.4)
Central Star Magnitude: +13.88 Vmag/ +14.18 Bmag (Tylenda et al. 1991)
Distance: 2,120 l-y (Ali et al. 2022)
Expansion velocity study and knot designations from (Sabbadin et al. 1983)
I did a study of the binocular-bright planetary nebula NGC 7008 in northern Cygnus late last month. Wow, it’s an interesting planetary to view in medium to large apertures and one I’ve neglected to scrutinize for far too long! On the 25th, I observed it with my 16-inch even though the dew was heavy, the transparency was off (it had rained the last three days), and the seeing was only average. I was after the knot inside it cataloged as Kohoutek 4-44. At 300x, I caught a few glimpses of it, while at 440x it was visible 30% of the time with averted vision. Keeping at that magnification, the large, bright, northern region had dark indentations creeping in from the west and east that were easily detectable.
Sounds like an average observation, doesn’t it? In fact, it would seem like quite the accomplishment since I’ve not heard any reports of K4-44’s visibility except in Steve Gottlieb's notes using Jimi Lowrey's 48-inch. Heck, Andrew Clark didn't notice it while observing it and sketching it recently with a 30-inch! But my ability to begin splitting Condensation B (Sabbadin et al. 1983) in the outer shell from the bright clump just south of it in the inner shell was also exciting because it means I can confidently log a sighting of its outer shell.
There’s a drawing of NGC 7008 done by R. J. Mitchell using Lord Rosse’s 72-inch speculum reflector. I’ve orientated it to have north up and east left after comparing it to modern images. In Parsons (1880), he gives Mitchell’s written description for the drawing made in 1855-56. It reads “Three stars in it, faint nebulosity reaches nearly up to the star south following; Bright knots in it, the north following one seems triangular in shape and sometimes I thought it split by a dark line running from south following to north preceding.”
My understanding is that “knots” = blobs, patches. Also, “south following” = lower right, “north following” = upper right, “north preceding” = upper left. When I use those terms, I feel that I can understand the drawing. Like, the “north following [knot] seems triangular in shape and is sometimes…split by a dark line” is Condensation B and he was detecting even more of the “dark indentations” than I did. And, interestingly, he drew the knot K 4-44 as a star (though he drew it in a blob that doesn't match images)!
Regarding the planetary’s central star, the scientific literature doesn’t state that it’s a white dwarf even though a lot of places on the internet refer to it as such. In fact, Douchin et al. (2015) point out that research by David Frew indicated the central star was likely a G8 subgiant. A companion to it was discovered using the Hubble Space Telescope in Ciardullo et al. (1999). The angular separation was only 0.4” and the magnitude difference between them was about half a magnitude in visual light while in the infrared the companion was brighter. As they say, “Because the probability of a chance superposition of a 14th-magnitude star near the central star is extremely small, we consider this a good binary star candidate.”
Now to the exciting observation. I observed NGC 7008 on the 25th with my 16-inch. The next night, I observed it with my 10-inch under better transparency and excellent seeing (though the dew was just as heavy). Whereas I had some difficulty seeing K4-44 in the 16-inch the night before, tonight I was able to see it about 10% of the time at 590x with averted vision! I had noticed how little the naked-eye stars were twinkling, but this proved how great the seeing was. Truth be told, I wasn’t after K 4-44 in the 10-inch. I was actually intent on seeing if I could “split” Condensation B from the rest of the planetary at 310x when I caught a few glimpses of K 4-44. So, I tried 590x (59x per inch of aperture) and was able to get at least six strong hits in the exact spot. I’d estimate that K 4-44 is 16th-magnitude. As for Condensation B, I couldn’t quite see it as two patches, but what I saw still counts as seeing part of the outer shell.This was a wonderful observation because, to be honest, I basically don’t observe at all during the summer. So, the side-effect of that is I tend to lose some confidence in my observing skills over that period and by September and October, I worry that I’ve lost my “edge” and won’t get it back. But it comes back every autumn, so I don’t know why I worry so much!
Scott H.
