M39 (Open Cluster)
A large, loose open cluster in Cygnus, one of the nearest Messier clusters at about 960 light-years. Of the Messier clusters, only the Pleiades, the Beehive and M7 are closer.
Open cluster in Cygnus. Magnitude 4.6.
Can you see it from Guyana?
From Georgetown (6.8° N) it climbs to 48° above the northern horizon at its highest; from Lethem (3.4° N), to 45° above the northern horizon. It is highest at midnight on the night of 12 Aug, in the middle of its season. The page works out tonight's rise, highest point and set for both towns in Guyana time (UTC-4), and where the Moon is.
What you'll see
- Your eyes: Visible to the eye only from a dark site, as a hazy knot of stars; lost in city skies. It lies 9.2° north-east of Deneb.
- 10x50 binoculars: Resolves into dozens of stars.
- A small telescope (80 to 100 mm): Too big for high power: a sprinkle of bright stars, best at the lowest power.
- A camera: Spans about 40% of an APS-C frame at 1,500 mm (field 0.9° × 0.6°); needs a tracking mount.
How big, how bright, how far
- Size on the sky: 19.5′, 63% of the Moon's width (the Moon is about 31′ wide).
- Brightness: magnitude 4.6. Fainter than Georgetown's faintest stars (4.3) but brighter than Lethem's (6.0).
- Distance: 970 light-years. The light reaching your eye left it about 970 years ago, around the year 1060.
- True size: 5.5 light-years across its catalogued outline; light crosses it in 5.5 years.
Apparent size from OpenNGC (Mattia Verga), CC-BY-SA-4.0.
What it is made of
- Type
- Open cluster
- Mass
- About 500 times the Sun's mass, in its stars
- Age
- About 130 million years (range 67 million years to 220 million years)
What is in it, not measured:
- Stars born together: from one cloud of gas, at one time
How we know: Distance from Gaia parallaxes of its stars: Hunt & Reffert 2024, A&A 686, A42 (Gaia DR3 cluster catalogue, VizieR J/A+A/686/A42), retrieved 2026-10-21: Theia 517 296 pc; statistical 965-965 ly (16th-84th percentile); Gaia DR3 parallax zero-point systematics (about 0.02 mas) add under 1%. True size: the apparent size times the distance (apparent size: OpenNGC (Mattia Verga, CC BY-SA 4.0), commit 4e2398a2). Mass: Hunt & Reffert 2024, A&A 686, A42 (Gaia DR3 cluster catalogue, VizieR J/A+A/686/A42), retrieved 2026-10-21, Theia 517: total 500 ± 26 Suns.
More deep-sky objects: IC 1396 (Elephant's Trunk Nebula) · Wizard Nebula (NGC 7380) · M29 (Open Cluster) · M52 (Open Cluster) · NGC 7789 (Caroline's Rose / White Rose Cluster) · M56 (Globular Cluster)