IC 4756 (Open Cluster)
A large, old open cluster in Serpens, about 840 million years old and 1,520 light-years away. From Georgetown it passes almost exactly overhead.
Open cluster in Serpens. Magnitude 4.6.
Can you see it from Guyana?
From Georgetown (6.8° N) it climbs to 89°, almost straight overhead at its highest; from Lethem (3.4° N), to 88°, almost straight overhead. It is highest at midnight on the night of 29 Jun, 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 13.8° east of Cebalrai.
- 10x50 binoculars: Resolves into dozens of stars.
- A small telescope (80 to 100 mm): Too big for high power: a wide spray of stars nearly a Moon-width across; NGC 6633 is 3.1° to the west.
- 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: 24′, 77% 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: 1,520 light-years. The light reaching your eye left it about 1,520 years ago, around the year 510.
- True size: 11 light-years across its catalogued outline; light crosses it in 11 years.
Apparent size from OpenNGC (Mattia Verga), CC-BY-SA-4.0.
What it is made of
- Type
- Open cluster
- Mass
- About 1,100 times the Sun's mass, in its stars
- Age
- About 840 million years (range 550 million years to 1.3 billion years)
What is in it, not measured:
- Stars born together: from one cloud of gas, at one time
- Red giants: stars that have already aged and swollen
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: IC 4756 467 pc; statistical 1,522-1,523 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, IC 4756: total 1,074 ± 118 Suns.
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More deep-sky objects: NGC 6633 (Open Cluster) · Wild Duck Cluster (M11) · IC 4665 (Open Cluster) · M26 (Open Cluster) · M14 (Globular Cluster) · Eagle Nebula (M16)