Object of the Week November 29th, 2015 - NGC7662 The Blue Snowball

NGC7662

Andromeda

RA 23 25 53.9
DEC 42 32 06

Planetary Nebula

Mag: 8.3

Size: 37"

CS Mag: 13.2

(Please excuse the brevity of this weeks OOTW. I am still recovering from surgery to my left arm and am unable to type properly. Needless to say, one handed typing is a long winded process.)

This week's OOTW is the beautiful planetary NGC 7662 in Andromeda. Discovered by William Herschel in 1784, NGC7662 very much resembles its namesake. Dubbed the "Blue Snowball" by Leland Copeland in the February 1960 issue of Sky and Telescope, Copeland described this objects appearance as "a light blue snowball". The name has since stuck.

One curiosity of this object has to do with its central star. Many sources cite the CS as being in the neighborhood of 12th or 13th magnitude. According to the Strasborgh ESO Catalog of Galactic PN, magnitudes range from 11th to 18th magnitude. 18th! In 1908, EE Barnard using the 40" Yerkes refractor, which happens to be just north of me on Lake Geneva WI, deemed the CS as a variable, varying by as much as 3 magnitudes! So is it? Noones really sure. One possible explanation is that density variations in the shell of the PN is what makes the star appear to vary in brightness.

In the eyepiece, NGC7662 is bright enough to be seen in scopes down to 4". In larger scopes the blue turquoise colors tend to be most apparent and you'll probably need a scope north of 15" to tease out both the inner and outer shells. Elongated SE-NW, the inner shell appears annular in shape surrounding a darker center and both shells appear to be brightest on their NE limbs. Lastly, OIII filters appear to work best with NGC7662.

One final note, has anyone ever seen the extended halo of NGC7662? It extends out quite about from the PN with a diameter of approximately 130". Its visible in many photographs and I assume its within the abilities of many observers here. I do not have any notes annotating observation. Does anyone?

As always,

"Give it a go and let us know!
Good luck and great viewing!"

Attachment 1889
Credit:Mike Landherr & the ESA/ESO/NASA Photoshop FITS Liberator

Hi Dragan,

Nice object choice and very interesting information. I wasn't aware the central star was reported as variable. I have two observing reports. The first is with my 6" telescope logged on Sept 11, 2006:

Observing conditions: dusk still ongoing, the Moon is just over the horizon and the sky is hazy. At 29x I can clearly see a fuzzy ball. When I defocus nearby HD 220822 until it has the same size as NGC 7662, then the nebula is slightly brighter than the star. At 170x the nebula is surrounded by very faint stars. The nebula itself is a bright sphere. It has a brighter part in the center. The rest of the nebula resembles an unresolved globular cluster.

Then with my 20" a few weeks ago:
Stunning object! At 83x I already see a bright ring in the center of the nebula. At 692x it resembles a spiral galaxy or two non-concentric rings that interlock. The central star is clearly visible.


Clear skies, Wouter

Barnard chronicles his observations of the central star (as well as others such as Lord Rosse) for over 10 years in exquisite detail in this MNRAS paper. He mentions magnitudes from 12th to 16th with a possible period of about 28 days, and this range is still quoted today as if it is established fact, though as we know sky conditions are often the determining factor on the visibility of a faint CS. At the end of the paper Barnard also includes a sketch of the planetary.

Here's an observation from 2001 with my old 20 inch f5 Obsession:

"...similar in some respects to NGC 6543 but with its own distinctive personality. A beautiful blue-ish green colored oval at low power, this color is completely washed out at high power. The general oval shape has seemingly flattened edges which are also very sharply defined and distinct. Within the outer boundary are two "C" shaped bright arcs facing each other with the center of the planetary at their common center. Although a slight brightening is seen at the center no star is visible. The two "C's" almost touch on one side but are definitely apart at the other, forming what at first seemed to be a large crescent shape. This is the brightest part of the nebula and is quite distinct at 719x."

Attachment 1890 Attachment 1891

Thanks for the link to Barnard's paper on 7662 Steve, he was a great observer and there's little reason to doubt what he saw. Regardless of whether the central star actually varies in brightness it certainly seems to vary in visibility. I haven't seen it at all yet, but it seems just as reasonable to speculate that observing conditions play a role in seeing a faint star surrounded by bright nebulosity.

Very nice object Dragan. And also thanks to Steve for the fabulous sketch of Barnard, the master itself.

I have an older sketch of NGC 7662 with 20". To the CS I noted under good to average Seeing "very faint and only popping in and out of view"
20", 620x, NELM 6m5+, Seeing II-III
Attachment 1892

For the CS I have two other observations:
14", High Power, NELM 7m0+, Seeing I-II - under very good seeing and transparency I could hold the CS steadily with averted vision; confirmation by another observer
27", 419x, NELM 6m5+, Seeing III-IV - difficult under soft seeing, could hold CS for seconds but not steadily
27", 586x, NELM 7m0+, Seeing II - easy object, direct vision

Another often overlooked detail is the halo of NGC 7662. I could spot it even with 14" but under very good transparency. With the 27" the Halo was an easy detail with [OIII] and 4mm EP. The NW part was visible even with direct vision.