Object of the Week October 7th, 2018 – NGC 246

[Paul Alsing]

Object of the Week – NGC 246 = PN G118.8-74.7 = PK 118-74.1 = ARO 43 – Planetary Nebula in CETUS
R.A.: 00h47m03.3s Dec.: -11°52'19" (2000)
Apparent Size: 4.0'
Magnitude: 10.40
Magnitude of Central Star: 12.0

Like Howard last week, I couldn’t find any previous mention of this terrific object here on Deep Sky Forum, and can’t believe that it hasn’t been mentioned yet! Howard chose NGC 247, which immediately reminded me that NGC 246 was nearby and needed to be my OOTW this week.

NGC 246, also known as the Skull Nebula because of 2 dark spots that show up nicely in astrophotos, is a favorite of mine, and is always a great go-to object whenever we have visitors at the telescope wanting to see “something cool”, and under dark and clear skies it always pleases when viewed employing my favorite filter, the DGM NBP Filter. Here is a wonderful APOD of this object…

https://apod.nasa.gov/apod/image/0604/ngc246_gemini_f.jpg

Discovered by William Herschel on November 27th, 1785, NGC 246 is a large, dim planetary nebula in an area of Cetus that has very few bright stars. Using his typical short hand he wrote “vF, L, 4 st in dif neb,” by which he meant “very faint, large, 4 stars in a diffuse nebula”. NGC 246 is located just a little east of Diphda (beta Cetus), about 6° away.

The central star of NGC 246 (HIP 3678A) is easy enough to see, being about 12th magnitude, which is pretty bright for a central star in a planetary nebula… and what do you know, it is also a double star, the secondary being a mag 14.4 K-class star (HIP 3578B), discovered in 1965 by Rudolph Minkowski! The separation is about 4 arc-seconds to the SE of the primary. In volume 5 of Annals of the Deep Sky I learned that this actually a triple system, with the 3rd member being an M-class dwarf (HIP 3678C) about 1 arc-second NE of HIP 3678A. Interestingly, only 2 other planetary nebulae may host triple-system central stars, those being Abell 63 and NGC 7008 (another favorite of mine), although NGC 246 is the only one that has been confirmed, to date. I have never split the central star, but the truth is that before today I didn’t realize that it was a visual double, so I never actually tried to split it. The above referenced volume 5 tells me that it is possible, but that excellent seeing is absolutely required. I’ll put this on my to-do list for the immediate future, should I be observing when the seeing is super-duper.

This planetary nebula has always been very obvious to me, starting back in the early 70’s when I had my serial #14 orange 8” Celestron, with the sand-cast forks, and it is quite a sight in my 25” f5 Obsession, too. The central star is not quite in the center, and the nebulosity is not quite round, the stars within shine brightly, but the contrast is good enough to make the edge of the nebulosity obvious, rather than just melting away, especially on the SW side. There are a couple of dim cavities within the nebulosity, which are easier to see when the view is filtered, which explains why is has the nickname “Skull Nebula” since they kinda look like the empty eye sockets of a skull. Check out an astrophoto to see what I mean, like this one from Greg Crinklaw’s terrific web page…

https://observing.skyhound.com/archives/oct/NGC_246_01.gif

Did I mention that it is a big object? It is a big object, filling about half of the field of my 13mm Ethos, and is pretty much a wondrous thing.

As always, give it a go and let us know.

[Ivan Maly]

Big favorite - also literally, as you noted. I have seen it in 4", and the best I have so far is this from five years ago from our near club site (16", SQM-L 21.30 mag/arcsec2 overhead, 21.20 in Cetus):

"PN NGC 246 ("Pacman Nebula") at 45x (Pentax XW40) looks like Pleiades might to the naked eye: a few stars with nebulosity that has a very slight bluish (cool) tint. At 225x (Ethos 8), the S two stars arc free from nebulosity, which forms a broad arc from NE to NW to S, involving 4 other stars, including the acute triangle of the brightest stars. There are 5 enhancements in the ring: NE T-shaped (leg of the “T” directed inwards), two brightest off the NE [clearly I meant NW in this instance] side of the triangle, and two to the S of its narrow base. The last one (going CW) is round and diffuse S of the star that the “Pacman” is swallowing in photos. The interior of the nebula in the region of the triangle, however, is visually almost as dark as around the last star. The view with OIII is poorer: only enhancements 1, 2, and 3 (CW in the arc visible without the filter) are visible."

[Raul Leon]

Hi here's my observation from 10/19/2017 :large oval with several stars superimposed, mottling observed within disc. NPB filter used with a 17mm Ethos at 93x magnification with my 14.5 Starstructure Dob f/4.33220

[Bertrand Laville]

Hi All,

Very Intresting object, and very intresting central star,

Here is my observation of september 2006, under namibian sky, for this intriguing CS**:


3221


T508 x 195/OIII-12nm*:
I did not notice anything about the duplicity of the CS *, and unfortunately I have not thought about observe high G and without filter, but I find that I designed this one with two tangent points, which is more accurate in the AP. Coincidence? I have no memory. However, the companion is m14. 3v, which seems low to have been seen with the OIII,

In September 2013, I reobserved it, from southern french Alps, under also a good sky*: SQML 21056, NELM 6,5v, S 1,3*", T635 x 520 without filter*:


3222

CS *, for which I want to check duplicity (an old discussion Webastro suggests that CS * is not dual, while my Tivoli drawing 2006 shows otherwise). The duplicity of the CS * is difficult, but absolutely safe; the companion is low, the imbalance torque is large, the separation is complete, the black stripe has a width equivalent to the companion disk.

You could have much more details of these two observations, particulary about bright and dark areas, at*:
http://www.deepsky-drawings.com/ngc-246/dsdlang/fr
and
http://www.deepsky-drawings.com/ngc-0246-t635/dsdlang/fr

Clear skies
Bertrand
http://www.deepsky-drawings.com/

[Uwe Glahn]

I did't try the double CS yet but a good reason to revisit the PN.

I found a very old sketch, one of the first sketches I made when I visited the Alps for the first time.
16", 129x, UHC, NELM 7m0+
3229

Beside quick and dirty observations with 18" and 27" from the Alps and with 24" from Namibia I also found an entry with a 4" telescope. With 27x I described "three stars within the PN, faint glow around".

[MikeWiles]

I was unaware that the central star was a double. I guess it goes back onto the observing list for next time out.

Here's my observation from Portal, Arizona on November 8, 2018

Ethos 21mm, 83x - Large, round nebula with two 11th magnitude stars involved in the nebulosity. Nebula appears somewhat elongated N/S. With the OIII, there is a void in the southeastern quadrant.

Ethos 6mm, 292x - Nebula appears annular at this magnification, with the two 11th magnitude stars peering out. Obviously why it earns the name the Skull nebula. The nebulosity is shown to be more uneven around the edges at this magnification. A 16th magnitude star touches southeastern edge of the nebula.