NGC 6946 & NGC 6939 – A Wide-Field View

Key Field Data

Main objects: NGC 6946 (Fireworks Galaxy) and NGC 6939
Constellation: Cepheus, close to the Cygnus border
Field of view: approximately 4.4° × 3.4°
NGC 6946: spiral galaxy, approximately 25 million light-years away
NGC 6939: open star cluster within the Milky Way
Distance to NGC 6939: approximately 4,000 light-years
Angular separation: approximately 40 arcminutes
Additional objects: numerous faint PGC galaxies, foreground Milky Way stars and interstellar dust

Description

This wide-field image places NGC 6946, the Fireworks Galaxy, within a much larger celestial landscape than the close-up view.

Captured with a William Optics RedCat 51 III WIFD, the approximately 4.4° × 3.4° field reveals an extraordinarily rich region of the northern Milky Way. Thousands of foreground stars fill the image, while subtle variations in the background trace the presence of interstellar material along our line of sight.

NGC 6946 appears almost suspended behind this dense stellar foreground. Although it is the most immediately recognisable object in the field, the wide view transforms the photograph from a portrait of a single galaxy into a view across several very different astronomical scales.

For a closer view and detailed information about the galaxy itself — including its structure, supernova history and the remarkable N6946-BH1 event — see the dedicated NGC 6946 – The Fireworks Galaxy page.

NGC 6939 – A Neighbour Only in the Sky

Close to NGC 6946 lies NGC 6939, an open star cluster belonging to our own Milky Way.

The two objects are separated by only about 40 arcminutes on the sky, making them appear to be neighbours. In reality, their apparent proximity is entirely a perspective effect.

NGC 6939 is located roughly 4,000 light-years from Earth, while NGC 6946 lies about 25 million light-years away. The cluster is therefore thousands of times closer to us than the galaxy beside it.

This makes the pairing particularly fascinating. NGC 6939 is a relatively compact group of stars within our own Galaxy, whereas NGC 6946 is an entire galaxy containing billions of stars. Yet from Earth, both fit comfortably within the same small region of sky.

Looking Through the Milky Way

One of the defining characteristics of this image is the extraordinary density of stars across the entire frame.

The reason is geometric: this line of sight passes relatively close to the plane of the Milky Way. We are therefore looking through a substantial depth of our own Galactic disc before our view reaches NGC 6946 and the Universe beyond.

Almost every sharp individual star in the photograph belongs to the Milky Way. The differences in colour — from blue-white to yellow, orange and red — reflect different stellar temperatures, with interstellar extinction also influencing the appearance of some stars.

The background itself is not perfectly uniform. Large amounts of interstellar dust lie along this direction, absorbing and scattering light and producing subtle darker structures between the dense star fields.

The wide-field image makes this foreground environment much more apparent than the close-up photograph of NGC 6946.

A Universe Hidden Between the Stars

The annotated version of this field reveals something that is easy to miss in the unlabelled photograph: NGC 6946 is far from the only galaxy present.

Numerous tiny systems belonging to the Principal Galaxies Catalogue (PGC) are scattered across the frame. Among those identified in the field are PGC 64878, PGC 166192, PGC 166193, PGC 166194, PGC 166686 and several others.

Without annotation, many appear only as tiny elongated or diffuse patches among the thousands of foreground stars.

This produces an intriguing reversal of scale. Some of the brightest objects in the photograph are individual stars within our own Galaxy, while barely visible smudges between them may represent entire galaxies containing billions of stars.

The apparent darkness between the stars is therefore anything but empty.

Layers of Distance

The wide field creates an unusual three-dimensional perspective.

The stars scattered across the frame occupy the foreground and lie at many different distances within the Milky Way. NGC 6939 represents another layer, approximately 4,000 light-years away but still comfortably inside our Galaxy.

Far behind it lies NGC 6946 at approximately 25 million light-years.

Beyond even the Fireworks Galaxy are the numerous faint background galaxies visible throughout the image, some at distances vastly greater than that of NGC 6946.

All these objects appear projected onto the same two-dimensional photograph, despite occupying enormously different positions in space.

Why the Wide Field Matters

A close-up image is ideal for revealing the internal structure of NGC 6946. The wide-field view tells a different story.

Here, the galaxy becomes part of its apparent celestial environment. Its modest angular size can be compared directly with the surrounding Milky Way stars and with NGC 6939, while the large field reveals how densely populated this region of the sky really is.

It also demonstrates an important principle of astronomy: objects that appear close together in the sky may have almost no physical relationship at all.

NGC 6939 and NGC 6946 are a particularly striking example. They share almost exactly the same line of sight from Earth, yet one belongs to our Galaxy while the other lies millions of light-years beyond it.

Looking Back Across Different Times

A wide-field photograph such as this is also a view of several different periods of cosmic history at once.

The light from the foreground Milky Way stars may have travelled for tens, hundreds or thousands of years. The light from NGC 6939 has crossed space for roughly 4,000 years.

NGC 6946 is seen much farther into the past: its light began travelling towards us approximately 25 million years ago.

The tiny background galaxies add still deeper layers of time, with some of their light beginning its journey long before the photons we now receive from NGC 6946.

The photograph therefore has no single moment in time. Every object is seen at a different epoch determined by its distance from Earth.

What appears to be one field of sky is actually a view through the Milky Way and outward across millions of light-years of the Universe.

Image Data

Total integration time: 17h 59m 30s

Integration by filter:

• L-Pro: 5h (60 × 300")

• Red: 4h 25m

• Green: 4h 14m 30s

• Blue: 4h 20m

Equipment:

• Telescope: William Optics RedCat / Cat 51 III WIFD

• Camera: ToupTek ATR2600M

• Mount: 10Micron GM2000 HPS II

• Filters: Optolong Blue 2", Optolong Green 2", Optolong L-Pro 2", Optolong Red 2"

• Accessories: ToupTek AAF Electric Focuser, WandererAstro WandererCover V4-EC

• Software: Adobe Photoshop, Pleiades Astrophoto PixInsight, Stefan Berg Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA)

For further details and the full-resolution wide-field image, visit AstroBin:

View NGC 6946 & NGC 6939 – Wide Field on AstroBin