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
Physical 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 members: a few dozen confirmed and probable stars
Right Ascension: 21h 31m 48s
Declination: +48° 26′
Catalogued by: Charles Messier, 1764
Location: Milky Way – Galactic disc
Light-travel time: approximately 800–850 years
Description
Messier 39 is a nearby open star cluster located approximately 800–850 light-years from Earth in the constellation Cygnus, against one of the richest stellar backgrounds of the Milky Way.
Unlike compact globular clusters such as M13, M39 consists of a relatively small number of loosely distributed stars, forming a broad, roughly triangular arrangement.
Its apparent diameter of approximately 30 arcminutes makes it comparable in size to the Full Moon, while the dense surrounding star field creates a striking contrast between the cluster's bright members and the countless background stars.
A Young Stellar Family
The stars of M39 formed together from the same molecular cloud approximately 250–300 million years ago, sharing a common age, distance and chemical composition.
Although considerably younger than ancient globular clusters, M39 has already lost the gas and dust associated with its formation.
Its relatively loose structure makes it vulnerable to gravitational interactions with surrounding stars, molecular clouds and the Milky Way itself.
Over time, these forces gradually disperse open clusters, returning their stars to the general Galactic population.
The Blue-White Stars of M39
Several prominent members of M39 display a distinctive blue-white colour, indicating surface temperatures higher than that of the Sun.
These luminous stars contrast with the numerous yellow, orange and reddish stars visible throughout the surrounding field.
Their presence is consistent with the cluster's relatively young age, while the distribution of stellar colours provides information about the different temperatures and evolutionary stages of its members.
The cluster's sparse appearance is partly a consequence of its proximity: its stars are spread across a relatively large area of the sky rather than concentrated into a compact structure.
A Window into the Milky Way
M39 lies in the direction of the densely populated Galactic disc, where countless stars appear projected along the same line of sight.
Although the cluster contains only a few dozen principal members, the photograph reveals a much larger stellar population extending across different distances within the Milky Way.
Some stars are closer than M39, while others lie thousands of light-years beyond it.
This makes the image not only a portrait of an open cluster but also a remarkable view into the stellar structure of our Galaxy.
Open clusters are particularly valuable for studying stellar evolution because their members share approximately the same age and chemical composition, allowing astronomers to compare the evolution of stars with different masses.
Curiosities
M39 is bright enough to be visible to the naked eye under dark skies, appearing as a faint, misty patch in northern Cygnus.
Its large apparent size makes it particularly suitable for binocular observations and low-magnification telescopes. At high magnification, its widely separated stars may extend beyond the field of view, making the cluster more difficult to recognise.
Charles Messier catalogued M39 in 1764, describing it as a cluster of stars without nebulosity.
Its location within the rich Milky Way star fields of Cygnus makes it an excellent example of how perspective can bring together stars belonging to very different distances and populations.
Looking Back in Time
The light captured from M39 began its journey approximately 800–850 years ago, during the 12th–13th centuries, in the European Middle Ages.
At that time, medieval kingdoms dominated Europe, the Renaissance was still centuries away, and the invention of the astronomical telescope lay far in the future.
While human civilisation evolved through the following centuries, these photons travelled across interstellar space towards Earth.
The stars recorded in this photograph are therefore seen as they appeared more than eight centuries ago, providing a direct glimpse into the past of our own Galaxy.
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 03m (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 7×2", 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 details and the full-resolution image, visit AstroBin: