Sky and Telescope - September 2018 - 58

A Celestial Set Piece
If your sky is dark, you'll enjoy scrutinizing this
cluster-galaxy combo.

icture in your mind two deep-sky
objects: They're totally different
types, completely unrelated, similar
in angular size and apparent brightness, and visible in a single telescopic
ﬁeld of view. That describes the open
cluster NGC 6939 and spiral galaxy
NGC 6946 (also known as the Fireworks Galaxy, famed for its numerous
supernovae), located just 39′ apart on
the Cepheus-Cygnus border.
Much of what's been written about
this unusual duo concentrates on the
alluring face-on galaxy. My desire here
is to treat both items equally as a celestial set piece that can be appreciated
at a wide range of magniﬁcations in a
variety of apertures.

Taking Their Measure
Frankly, NGC 6939 is no prize. Almost
6,000 light-years from Earth and
1.0-1.3 billion years old, the cluster
has become a slowly dispersing family
of several hundred evolved suns, some
having reached the red giant stage. For
experienced telescope users, the "vital
signs" of NGC 6939 are hardly noteworthy: a total visual magnitude of 7.8,
no members better than magnitude
11.4, and a ragged diameter of perhaps
10′. By any measure, it's only a modest
catch among the many glittering glories
along the Milky Way.
NGC 6946 is a challenging study.
Its relatively close distance - 18 million light-years - seems telescopefriendly, but NGC 6946 lines up less
than a dozen degrees from the crowded
galactic equator. The 8.8-magnitude
object would be nearly two magnitudes
brighter if it resided farther from the
58

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

obscuring band of the Milky Way. Moreover, the galaxy's sprawling 11.5′ × 9.8′
dimensions yield an overall surface
brightness of a measly 13.8 magnitudes
per square arcminute. Unless your sky
is velvety dark, you'll declare NGC 6946
an armless ghost.
But, hey, if we assessed deep-sky
wonders by statistics alone, we'd never
get outside to observe. In truth, our
coarse cluster and gauzy galaxy together
display a unique, if subtle, beauty. Let's
have a look.

A Rare Pair
For two specimens so different,
NGC 6939 and NGC 6946 in certain
circumstances appear amazingly alike.
My 4¼-inch f/6 reﬂector at 22× detects
them simply as twin pale clouds. A
7.2-magnitude foreground star (HD
196085) beams blue-white two-thirds
of the way from the southeastern cloud

(the galaxy) to the northwestern one
(the cluster).
Even in my 10-inch f/5.5 Newtonian
at 34×, the tandem targets remain comparable in size since barely more than
the central portion of the bigger cloud
- the galaxy - materializes. An undramatic scene, yes, but the low-power view
includes an attractive bonus. NGC 6939
is enclosed by a diamond-shaped asterism, 12′ by 16′ in extent, delineated by
four 10th-magnitude stars. If we add
the 7.2-magnitude foreground star and
a few others, the asterism outlines a
kite whose tail of ribbons extends to a
7.8-magnitude star (HD 196053) a little
more than 30′ south of the cluster and
some 20′ west of the galaxy. Nice!
A longer inspection in the 10-inch
at 47× is tantalizing. The galaxy is still
extremely diffuse; however, it's heftier
in the middle and clearly elongated
north-northeast by south-southwest. At
the northeast end the misty mass bends
eastward, while at the southwest end it
veers westward - a hazy-lazy backward
S. By contrast, the cluster is a crisp,
grainy patch. Patient staring at 58×
morphs the grain into a salting of suns
detached from the surrounding stars of
the Milky Way.

The Cluster in Detail
NGC 6939 sparkles in my 18-inch f/4.5
Dobsonian. Fully resolved at 69×, the
cluster exhibits two remarkably straight

PINWHEEL A ND K ITE: GIA NLUCA M ASI / VIRT UA L TELESCOPE PROJECT / MICH A EL SCHWA RT Z /
TEN AG R A OBSERVATORIES, LTD.; NGC 6946: GIA NLUCA M ASI / VIRT UA L TELESCOPE PROJECT;
NGC 6939: GIA NLUCA M ASI / VIRT UA L TELESCOPE PROJECT

SEPTEMBER 2018 OBSERVING
Going Deep by Ken Hewitt-White

Sky and Telescope - September 2018

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