Sky and Telescope - September 2018 - 60

SEPTEMBER 2018 OBSERVING
Going Deep

ment protruded southwestward a short
distance. So, three arms for sure and
maybe a fourth.
The pallid pinwheel showed best in a
13-mm Ethos ocular that delivered (with
the Paracorr) 180×. Numerous stars were
scattered across the galaxy, and a wavering nucleus emerged in its oval core. The
adjacent east-end appendages diverged
from the core, the inner arm heading
southward, the outer arm curving past
a 14th-magnitude star, then extending
tenuously southeastward to a fuzzy knot.
The fan-shaped third arm at the core's
opposite end stretched northward. The
stubby fourth arm south of center was
traced to a dim double of 20″ separation.
Northwest of it, a vague haze - the start

of a fifth arm - extended westward to
a broader knot.
The knots intrigued me. I figured
they were H II regions, yet they vanished
when I applied an Ultra High Contrast
nebula filter. In a Going Deep column
two years later (S&T: July 2013, p. 60),
Steve Gottlieb cleared the mystery when
he referred to the larger knot as "a huge
stellar and gas complex spanning 2,000
light-years and containing more than
a dozen tightly packed clusters." Those
nebulous-looking knots are incredibly
rich in luminous blue giant stars!

Pushing Deeper
I'm always poking around my chosen
telescopic subjects for obscure "back-

q CONSTELLATION GERRYMANDERING? The cluster NGC 6939 and galaxy NGC 6946 are
celestial neighbors located a short, 2° star-hop southwest of 3rd-magnitude Eta (η) Cephei. The
cluster resides in southwestern Cepheus, while the galaxy, around 38′ southeastward, straddles
the Cepheus-Cygnus border. That NGC 6946 is a borderline resident of Cygnus results from a
seemingly arbitrary sliver of Cygnus real estate extending into Cepheus at that location.
Star magnitudes

+64°
3
4
5
6
7
8

CEPHEUS
_

e
+62°

DRACO
d
"Anonymous"

+60°

6939

ground" galaxies. During this project, I
struck pay dirt twice.
PGC 64824, a 13.7-magnitude
galaxy of very low surface brightness,
lies almost three-quarters of the way
from the 7.2-magnitude star in the
tail of my "kite" to a 7.7-magnitude
star (HD 195607) 20′ east. Measuring
2.3′ × 0.6′, PGC 64824 is a thin vapor
elongated east-west. A 14.6-magnitude star flickers near its eastern tip. I
glimpsed the feeble fog in my friend's
20-inch Dob at 290×, then again in my
18-inch at 228×, on the perfect night
mentioned above. The high magnification was helpful, as it permitted me
to exclude an unwanted "streetlamp"
- the 7.7-magnitude field star. I also
noticed an anonymous galaxy 90″
south of a 10.5-magnitude star about
20′ east-northeast of NGC 6939. In my
scope at 228×, the mystery smudge was
tiny and faint.
Let me end with some encouragement for owners of smaller telescopes.
As I indicated earlier, a 10-inch will
provide at least partial access to these
delicate wonders. I once observed
essentially everything described here
(except the "fog" galaxy) in a 15-inch
f/5 Dobsonian. Cruising at 146× in
crystal-clear conditions, the 15-inch
gave me three good arms and both knots
on NGC 6946. The neighboring cluster
was fabulous. Never forget: The key to
appreciating this seriously attractive
combo is a seriously dark sky.

6946
PGC 64824
21h 20m

21h 00m

20h 40m

CYGNUS

20h 20m

20h 00m

+58°

¢ Contributing Editor KEN HEWITTWHITE probes the deep sky from the
6,000-foot summit of Mount Kobau in
southern British Columbia.

On the Border of Cygnus and Cepheus
Object
NGC 6939
NGC 6946
PGC 64824
"Anonymous"

Type

Surface
Brightness

Mag(v)

Size/Sep

RA

Dec.

Open cluster

-

7.8

10′

20h 31.5m

60° 40′

8.8

11.5′ × 9.8′

20h

34.9m

60° 09′

20h

30.2m

60° 26′

20h 34.3m

60° 46′

Spiral galaxy

13.8

Irregular galaxy

13.9

13.7

2.3′ × 0.6′

Undefined

-

-

-

Angular sizes and separations are from recent catalogs. Visually, an object's size is often smaller than the cataloged value and varies according to the aperture and
magnification of the viewing instrument. Right ascension and declination are for equinox 2000.0. Coordinates for the last object listed are estimated.

60

S E P TE M B E R 2 018 * SK Y & TELESCOPE



Table of Contents for the Digital Edition of Sky and Telescope - September 2018

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