M21 (Open Cluster)
A compact, very young open cluster in Sagittarius, about 5 million years old, 0.7° from the Trifid Nebula. The two share one binocular field and one photograph.
Open cluster in Sagittarius. Magnitude 5.9.
Can you see it from Guyana?
From Georgetown (6.8° N) it climbs to 61° above the southern horizon at its highest; from Lethem (3.4° N), to 64° above the southern horizon. It is highest at midnight on the night of 20 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 6.2° north-west of Kaus Borealis.
- 10x50 binoculars: A small hazy knot with a few stars resolved. The Trifid Nebula (M20) is 0.7° to the south-west.
- A small telescope (80 to 100 mm): A tight group of brighter stars at low power, with the Trifid Nebula 0.7° to the south-west.
- A camera: Tiny on camera: it needs 2,000 mm or more of focal length on a tracking mount. The Trifid Nebula fits in a wider frame with it.
How big, how bright, how far
- Size on the sky: 6′, 19% of the Moon's width (the Moon is about 31′ wide).
- Brightness: magnitude 5.9. Fainter than Georgetown's faintest stars (4.3) but brighter than Lethem's (6.0).
- Distance: 3,920 light-years. The light reaching your eye left it about 3,920 years ago.
- True size: 6.8 light-years across its catalogued outline; light crosses it in 6.8 years.
Apparent size from OpenNGC (Mattia Verga), CC-BY-SA-4.0.
What it is made of
- Type
- Open cluster
- Mass
- About 910 times the Sun's mass, in its stars
- Age
- About 5.4 million years (range 3.9 million years to 7.2 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: NGC 6531 1,202 pc; statistical 3,910-3,929 ly (16th-84th percentile); Gaia DR3 parallax zero-point systematics (about 0.02 mas) add about 2%. 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 6531: total 913 ± 85 Suns.
More deep-sky objects: M23 (Open Cluster) · M28 (Globular Cluster) · M18 (Open Cluster) · M25 (Open Cluster IC 4725) · Globular Cluster M22 · Eagle Nebula (M16)