NGC 6633 (Open Cluster)
A bright open cluster in Ophiuchus, about 1,270 light-years away and 470 million years old. It passes within 2° of overhead from Georgetown, so it climbs clear of the haze near the horizon.
Open cluster in Ophiuchus. Magnitude 4.6.
Can you see it from Guyana?
From Georgetown (6.8° N) it climbs to 90°, almost straight overhead at its highest; from Lethem (3.4° N), to 87° above the northern horizon. It is highest at midnight on the night of 26 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 11.1° east of Cebalrai.
- 10x50 binoculars: Resolves into dozens of stars.
- A small telescope (80 to 100 mm): A loose group of bright stars at low power; IC 4756 is 3.1° to the east.
- A camera: Tiny on camera: it needs 2,000 mm or more of focal length on a tracking mount.
How big, how bright, how far
- Size on the sky: 12′, 39% 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: 1,270 light-years. The light reaching your eye left it about 1,270 years ago, around the year 760.
- True size: 4.4 light-years across its catalogued outline; light crosses it in 4.4 years.
Apparent size from OpenNGC (Mattia Verga), CC-BY-SA-4.0.
What it is made of
- Type
- Open cluster
- Mass
- About 650 times the Sun's mass, in its stars
- Age
- About 470 million years (range 300 million years to 800 million 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: NGC 6633 390 pc; statistical 1,270-1,271 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, NGC 6633: total 645 ± 70 Suns.
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More deep-sky objects: IC 4756 (Open Cluster) · IC 4665 (Open Cluster) · Wild Duck Cluster (M11) · M14 (Globular Cluster) · M26 (Open Cluster) · Eagle Nebula (M16)