Caldwell 91 — Open Cluster in Carina
NGC 3532
About C91
Description
NGC 3532 is one of the brightest and richest open clusters in the sky, located in Carina about 1,321 light-years away. Spanning 55 arcminutes, it contains about 150 stars brighter than magnitude 12 and over 400 confirmed members. At magnitude 3.0, it is easily visible to the naked eye.
Observing Tips
A stunning cluster in binoculars and wide-field telescopes. The large angular size means low power is essential. Many colorful stars including orange giants contrast with blue-white members. John Herschel called it the most beautiful object in the sky. Best from southern latitudes in late winter and spring.
History
Discovered by Nicolas Louis de Lacaille in 1751 from South Africa. John Herschel described it glowingly during his southern sky survey from the Cape of Good Hope in the 1830s. It was the first target observed by the Hubble Space Telescope in 1990 for calibration.
Fun Facts
NGC 3532 was the very first object pointed at by the Hubble Space Telescope after its launch. Despite the telescope's initially flawed mirror, the cluster's stars helped engineers diagnose the spherical aberration problem. The cluster is about 300 million years old.
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.
Explore
8
Classification Decoder
Discover
9
Light Travel Time Machine
10
Relativistic Travel
Community Photos (1)
Credit: ESO/G. Beccari. 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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