IC 4665 (Open Cluster)
A large, young open cluster in Ophiuchus, 1.3° from Cebalrai, about 1,120 light-years away and 33 million years old. It passes almost overhead from Georgetown.
Open cluster in Ophiuchus. Magnitude 4.2.
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 16 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 even from Georgetown, as a hazy knot of stars. It lies 1.3° north-east of Cebalrai.
- 10x50 binoculars: Resolves into dozens of stars.
- A small telescope (80 to 100 mm): Too big for high power: a loose scatter 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: 24.6′, 79% of the Moon's width (the Moon is about 31′ wide).
- Brightness: magnitude 4.2. Brighter than the faintest stars Georgetown shows (4.3), and Lethem's (6.0).
- Distance: 1,120 light-years. The light reaching your eye left it about 1,120 years ago, around the year 910.
- True size: 8 light-years across its catalogued outline; light crosses it in 8 years.
Apparent size from OpenNGC (Mattia Verga), CC-BY-SA-4.0.
What it is made of
- Type
- Open cluster
- Mass
- About 280 times the Sun's mass, in its stars
- Age
- About 33 million years (range 18 million years to 53 million years)
What is in it, not measured:
- Stars born together: from one cloud of gas, at one time
- Hot blue stars: still shining, because the cluster is young
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 4665 343 pc; statistical 1,119-1,121 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 4665: total 279 ± 25 Suns.
More deep-sky objects: M14 (Globular Cluster) · NGC 6633 (Open Cluster) · IC 4756 (Open Cluster) · M10 (Globular Cluster) · M12 (Globular Cluster) · Wild Duck Cluster (M11)