Messier 39 – Open Cluster 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, embedded against one of the richest stellar backgrounds in the Milky Way.
At a distance of approximately 800–850 light-years from Earth, M39 is relatively close by astronomical standards and forms a loose group of bright stars spread across an area of sky roughly comparable in size to the Full Moon.
Unlike dense globular clusters such as M13, M39 has no compact central core.
Its stars are distributed much more sparsely, forming an irregular and open structure that blends naturally into the surrounding star fields of Cygnus.
Because the cluster lies in the plane of the Milky Way, the field around it is extraordinarily crowded with foreground and background stars, making it visually more difficult to separate the true members of M39 from unrelated stars projected along the same line of sight.
The cluster itself is thought to contain only a few dozen principal members, although many more stars are visible in deep photographic images of the same field.
A Young Stellar Family
The stars of M39 formed together from the same large cloud of interstellar gas and dust.
Their shared origin means that they have approximately the same age, distance and chemical composition.
M39 is estimated to be around 250–300 million years old, making it very young compared with ancient globular clusters, but significantly older than the youngest open clusters still surrounded by remnants of their natal nebulae.
At this age, the gas and dust from which the cluster formed have long since dispersed.
What remains is a loose stellar family gradually evolving and slowly drifting apart under the gravitational influence of the Milky Way.
This is the eventual fate of most open clusters.
Unlike globular clusters, which can survive for billions of years, open clusters are generally less tightly bound. Over hundreds of millions of years, gravitational interactions with other stars, molecular clouds and the Galaxy itself gradually pull their members apart.
M39 is therefore a temporary structure on Galactic timescales.
Why the Cluster Looks So Sparse
M39 is quite different from the compact star clusters that are often associated with spectacular astrophotography.
Its principal stars are widely spaced and form a broad, roughly triangular arrangement.
This sparse appearance is partly due to the relatively small number of cluster members and partly because the object is physically close to us.
Being nearby means that the cluster appears spread across a relatively large area of sky.
Its angular diameter is around 30 arcminutes, approximately the apparent diameter of the Full Moon.
In this photograph, however, the cluster lies against an exceptionally rich field of Milky Way stars.
Many of the countless faint points visible across the frame do not belong to M39 at all.
They are stars located at many different distances throughout the Galactic disc, projected by chance along almost the same line of sight.
The image therefore contains both a genuine stellar cluster and an enormous background population belonging to the surrounding Milky Way.
The Bright Blue Stars of M39
Several of the most prominent stars in M39 show a distinctive blue-white colour.
This colour is directly related to their surface temperature.
Blue and blue-white stars are hotter than yellow stars such as the Sun and considerably hotter than orange and red stars.
Their presence is consistent with the relatively young age of the cluster.
The most massive and hottest stars born when M39 formed have evolved more rapidly than lower-mass stars, but a population of luminous blue-white stars is still clearly present.
Against the enormous background of fainter yellow, orange and reddish stars in the Milky Way, these bright blue members give M39 its characteristic appearance.
Their colour is not an artificial result of image processing: the difference reflects genuine variations in stellar temperature.
A Window Through the Milky Way
One of the most striking aspects of M39 is not only the cluster itself, but the extraordinary density of stars visible around it.
Cygnus lies directly along the bright band of the Milky Way, meaning that when we look toward M39 we are looking through a substantial portion of the Galactic disc.
The countless stars visible in this image occupy very different distances.
Some are relatively nearby.
Others may lie thousands of light-years farther away.
This produces an extraordinary visual effect: the cluster itself appears almost suspended within a vast stellar ocean.
The image is therefore not simply a portrait of M39.
It is also a deep view into the structure of our own Galaxy.
Every point of light is a star within the Milky Way, but only a relatively small fraction of them belong physically to the cluster.
Open Clusters and the Evolution of the Milky Way
Open clusters are particularly valuable to astronomers because their stars provide natural laboratories for studying stellar evolution.
Because the member stars formed together, astronomers can compare stars of different masses while knowing that they have approximately the same age and chemical composition.
This makes it possible to test models describing how stars change over time.
Open clusters also help researchers trace the structure and evolution of the Galactic disc.
By measuring their distances, motions, ages and chemical compositions, astronomers can reconstruct how different regions of the Milky Way have evolved over hundreds of millions or even billions of years.
M39 is therefore more than a visually attractive collection of stars.
It is part of the fossil record of star formation within our Galaxy.
Curiosities
M39 is bright enough to be visible to the naked eye under sufficiently dark skies.
It appears as a faint misty patch in northern Cygnus and becomes easily recognisable through binoculars.
Because of its large apparent size, low magnification generally provides the best visual view.
In a telescope with too much magnification, the cluster may actually become harder to recognise because its stars spread beyond the field of view.
This is one of the reasons M39 is an excellent binocular target.
Charles Messier included the object in his catalogue in 1764, describing it as a cluster of stars with no nebulosity.
Some historical sources have suggested that the cluster may have been noticed much earlier, possibly in antiquity, but Messier's observation is the first generally accepted catalogue entry associated with the modern object M39.
Its location in Cygnus also places it in one of the most spectacular regions of the northern Milky Way, surrounded by dense star fields, dark nebulae and numerous emission nebulae.
Looking Back in Time
The light captured in this photograph has travelled through space for approximately 800–850 years.
This means that we are not seeing M39 as it exists today.
We are seeing the cluster as it appeared during the Middle Ages.
When the photons from these stars began their journey toward Earth, Europe was still living through the medieval period.
The Renaissance had not yet begun.
Christopher Columbus' voyage across the Atlantic was still centuries in the future, and the telescope itself would not be invented for several hundred more years.
While generations of people lived, kingdoms rose and fell, and human civilisation changed profoundly, the light from M39 continued travelling silently through interstellar space.
After crossing roughly eight quadrillion kilometres, a tiny fraction of those photons finally reached Earth and ended their journey on the sensor used to create this image.
When we photograph M39, therefore, we are not simply recording a cluster of stars.
We are seeing light that began its journey toward us around eight centuries ago.
Image Data
Total integration time: 12h 45m
Integration by filter:
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L-Pro: 7h (140 × 180")
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Red: 3h 3m (61 × 180")
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Green: 1h 42m (34 × 180")
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Blue: 1h (20 × 180")
Equipment:
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Telescope: Artesky ARTEC 250 Pro
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Camera: ToupTek ATR2600M
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Mount: 10Micron GM2000 HPS II
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Filters: Optolong Blue 2", Optolong Green 2", Optolong L-Pro 2", Optolong Red 2"
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Accessories: Pegasus Astro Prodigy Microfocuser, ToupTek AFW-L 7x2", WandererAstro WandererBox Pro V3
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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: