Messier 39 – Wide Field in Cygnus
Key Scientific Data
Object type: Open star cluster
Constellation: Cygnus
Other designations: M39, NGC 7092
Distance from Earth: approximately 800–850 light-years
Diameter: approximately 7–10 light-years
Apparent size: approximately 30–32 arcminutes
Apparent magnitude: approximately 4.6
Estimated age: approximately 250–300 million years
Estimated number of members: a few dozen confirmed and probable member stars
Right Ascension: 21h 31m 48s
Declination: +48° 26′
Discovery: generally attributed to Charles Messier, 1764
Location in the Milky Way: Galactic disc
Light-travel time: approximately 800–850 years
Description
Messier 39 is a nearby open star cluster located in the constellation Cygnus, one of the richest regions of the northern Milky Way.
At a distance of approximately 800–850 light-years, M39 is relatively close to Earth by Galactic standards and appears as a loose group of bright stars spread across an area of sky roughly comparable in size to the Full Moon.
In this wide-field image, however, M39 is only one element within a much larger and extraordinarily complex region.
The cluster lies against an immense background of Milky Way stars, dark interstellar clouds and faint emission structures extending across the field.
This broader perspective reveals the true environment in which M39 is located: not an isolated group of stars, but a small stellar family embedded within the densely populated disc of our Galaxy.
A Cluster Inside the Milky Way
M39 belongs to the Galactic disc, the broad flattened structure containing most of the stars, gas and dust of the Milky Way.
Because Cygnus lies directly along one of the richest portions of the Milky Way, the line of sight toward M39 intersects enormous numbers of stars at very different distances.
This is why the background appears so densely populated.
Only a relatively small number of the stars visible here are physically associated with M39.
Most belong to unrelated stellar populations lying either in front of or behind the cluster.
The result is a remarkable three-dimensional scene compressed into a two-dimensional photograph.
Nearby stars, M39 itself, distant stellar fields, dark clouds and faint nebular structures all overlap along the same line of sight.
A Wide-Field View of Cygnus
The strength of a wide-field image lies in context.
At higher magnification, M39 is easily recognised as a sparse and irregular open cluster.
At wider scale, its visual importance changes.
The cluster becomes part of the surrounding Galactic landscape.
This field contains numerous dark nebulae, faint emission regions and dense concentrations of stars.
Several of the dark structures correspond to clouds of interstellar dust that absorb the light of more distant stars behind them.
These clouds are not empty regions.
They are composed of cold gas and dust and form part of the complex interstellar medium of the Milky Way.
Their silhouette becomes visible only because they lie in front of an exceptionally bright stellar background.
Dark Nebulae and Interstellar Dust
One of the most interesting features of this field is the presence of numerous dark nebulae.
These structures are dense clouds of dust and molecular gas capable of blocking visible light from stars located behind them.
They therefore appear as dark patches, lanes and irregular voids against the glowing star fields of Cygnus.
In reality, these clouds may contain enormous quantities of material.
Some are associated with active or potential regions of star formation, where gravity can slowly compress dense pockets of gas until new stars begin to form.
Wide-field astrophotography is particularly effective at revealing these structures because it shows how individual dark clouds connect into much larger complexes across the sky.
Faint Emission Nebulae
The image also reveals faint areas of reddish emission.
These structures are produced primarily by ionised hydrogen, commonly referred to as H-alpha emission.
Ultraviolet radiation from hot stars can ionise surrounding hydrogen gas.
When the electrons later recombine with hydrogen nuclei, the atoms emit light at characteristic wavelengths, including the deep red H-alpha line at 656.3 nanometres.
These emission regions can be extremely faint compared with the stars surrounding them.
Long integration times are therefore essential to reveal their delicate structure without overwhelming the field with stellar light.
In this image, the faint reddish nebulosity helps reveal that the region around M39 is not simply full of stars but also contains large quantities of diffuse interstellar gas.
The Rich Star Fields of Cygnus
Cygnus is one of the most spectacular constellations for wide-field astrophotography because we are looking through a particularly dense section of the Galactic plane.
The countless stars visible across this frame belong to many different populations and distances.
Some are blue-white and relatively hot.
Others are yellow, orange or red, reflecting cooler surface temperatures and different stages of stellar evolution.
The apparent colour differences are therefore physically meaningful.
They reveal variations in stellar temperature and, indirectly, differences in mass, age and evolutionary state.
Against this extraordinary stellar background, the stars of M39 form only a small local grouping.
M39 as a Stellar Family
The stars of M39 formed from the same molecular cloud approximately 250–300 million years ago.
They therefore share a common origin, similar chemical composition and approximately the same age.
Unlike the stars of a globular cluster, however, the members of M39 are only loosely gravitationally bound.
Over time, gravitational interactions with the Milky Way, passing stars and interstellar clouds gradually alter their motions.
Eventually, the members of M39 will disperse into the Galactic disc.
At that point, they will no longer be recognisable as a cluster.
Open clusters such as M39 therefore represent temporary stages in the evolution of stellar populations.
They are born together, travel together for hundreds of millions of years, and eventually dissolve into the wider Milky Way.
A Galactic Landscape
This image illustrates an important characteristic of wide-field astrophotography.
The objective is not always to isolate a single astronomical object.
Sometimes the wider environment is just as important as the object itself.
Here, M39 provides a recognisable reference point within a much larger landscape composed of stars, dust and gas.
The image therefore reveals several different components of the Milky Way simultaneously:
the stars of an open cluster,
foreground and background stellar populations,
dark molecular clouds,
diffuse interstellar dust,
and faint ionised hydrogen emission.
Together they provide a small cross-section of the Galactic disc.
Curiosities
M39 is bright enough to be visible to the naked eye under sufficiently dark skies.
Because of its large apparent size, it is generally better observed with binoculars or a small telescope at low magnification than with a large telescope at high power.
Charles Messier catalogued the cluster in 1764.
Its position in Cygnus places it close to numerous dark nebulae and faint emission regions that become especially noticeable in modern wide-field photographs.
The region is also particularly rich in very faint background galaxies.
Although the Milky Way dominates the image, sensitive astronomical imaging can detect distant galaxies shining through less obscured regions of the Galactic foreground.
This produces an extraordinary range of distances within a single frame, from nearby Galactic stars to objects located millions of light-years beyond the Milky Way.
Looking Back in Time
The principal stars of M39 are approximately 800–850 light-years away.
The light we see from them today therefore began its journey toward Earth roughly eight centuries ago.
When those photons left M39, Europe was still in the Middle Ages.
The Renaissance, the great oceanic voyages and the invention of the astronomical telescope had not yet occurred.
But this wide-field image contains objects at many different distances.
Some foreground stars are much closer and therefore seen as they were only decades or centuries ago.
Other stars lie thousands of light-years away.
The faint nebulae and background stellar fields may also lie considerably farther than M39 itself.
And some tiny background galaxies visible in the field may be millions of light-years distant.
This means that the photograph does not represent a single moment in cosmic history.
It is a mosaic of different times, superimposed along the same line of sight.
Every star, cloud and galaxy is seen at a different point in its past.
In this sense, a wide-field image of Cygnus is not only a view across space.
It is a view through many different layers of time.
Image Data
Total integration time: 8h 9m
Integration by filter:
-
L-Pro: 8h 9m (163 × 180")
Equipment:
-
Telescope: William Optics RedCat 51 III WIFD
-
Camera: ToupTek ATR2600M
-
Mount: 10Micron GM2000 HPS II
-
Filters: Optolong L-QEF 2", Optolong L-Ultimate 2"
-
Accessories: ToupTek AAF Electric Focuser, WandererAstro WandererBox Pro V3, ZWO EFW 7 × 2″
-
Software: Adobe Photoshop, Pleiades Astrophoto PixInsight, Stefan Berg Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA)
For further information and the full-resolution image, visit AstroBin: