Caldwell 76 — Open Cluster in Scorpius
NGC 6231
About C76
Description
NGC 6231 is a bright, compact open cluster in Scorpius, about 5,900 light-years away. At magnitude 2.6 it is visible to the naked eye, and it forms the core of the much larger Scorpius OB1 association. The cluster contains numerous hot O- and B-type stars.
Observing Tips
Stunning in binoculars and small telescopes. Located just north of the Scorpion's tail near Zeta Scorpii. A telescope at 40-80x reveals dozens of bright blue-white stars. Best in summer evenings from locations with a good southern horizon.
History
Known since antiquity as a bright patch near the Scorpion's stinger. Giovanni Battista Hodierna first resolved it as a cluster before 1654. Sometimes called the Northern Jewel Box for its brightness and rich star field.
Fun Facts
NGC 6231 is only about 3.2 million years old, one of the youngest clusters visible to the naked eye. Its O-type stars will burn through their fuel and explode as supernovae within the next few million years.
Observe
1Properties
Position & Identifiers
2How easy to spot?
| Telescope | Bortle 3 | Bortle 4 | Bortle 5 |
|---|---|---|---|
| 80 mm refractor 80mm refr. | Easy | Easy | Easy |
| 150 mm Newton 150mm Newt. | Easy | Easy | Easy |
| Celestron C8 (203 mm SCT) C8 203mm | Easy | Easy | Easy |
Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs
3Visibility
Set a location in User Settings to see visibility data.
4
Eyepiece View
5
Best Magnification
6Where this cluster sits in time
Open clusters span more than four orders of magnitude in age — from newborn OB associations to ancient, metal-rich survivors.
7
Colour-Magnitude Diagram
A cluster's colour-magnitude diagram reveals its age: the bluer the turn-off point where the main sequence bends into red giants, the younger the cluster.
Each point is a Gaia-DR3 member. Colour encodes spectral type; size reflects membership probability.
Discover
8
Light Travel Time Machine
9
Relativistic Travel
Community Photos (1)
Credit: Lithopsian. License: CC BY 4.0. (Wikimedia Commons)
Skybred Mar 2, 2026
Nearby in the Sky
Other targets within a few degrees — pan your scope a little and keep exploring.
Visibility scores assume a 150 mm Newton at Bortle 4.
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