NGC 3766, also known as the Pearl Cluster, is a superb open cluster in Centaurus. At magnitude +5.3, it's faintly visible to the naked eye but challenging to spot against the numerous stars of the background Milky Way. The cluster was born in the Carina molecular cloud, a vast star-forming region that lies at approximately between 6,500 and 10,000 light-years distant. It includes the barely-stable star eta Carinae (η Car), which one-day will explode as a spectacular supernova.

NGC 3766 is a nice binocular target, a wonderful sight through telescopes and one of the jewels of the southern sky. With a declination of 61S, it's best seen from southern latitudes during the months of March, April and May. From many southern towns and cities, the cluster is circumpolar and therefore never sets. However, from northern locations above 29N it never rises above the horizon.

Nicholas Louis de Lacaille discovered NGC 3766 on March 5, 1752.

NGC 3766 (credit:- La Silla Observatory/ESO)

Finder Chart for NGC 3766 (credit:- freestarcharts)

Finder Chart for NGC 3766 - pdf format (credit:- freestarcharts)

The Pearl Cluster is located in the southwestern corner of Centaurus, 7 degrees west and slightly north of Acrux (α Cru - mag. +0.8). Positioned 1.5 degrees south of NGC 3766 is lambda Cen (λ mag. +3.1) and surrounding open cluster and associated emission nebula, IC 2944.

Binoculars show a compact cluster, spanning 12 arc minutes, with a number of stars resolvable. An 80mm (3.1-inch) refractor at 100x magnification reveals a stunning object with dozens of stars visible. Most members are white in colour, but the look is enhanced by a couple of nice 7th magnitude red stars. One red star is positioned left of the cluster centre, the other to the right. The appearance of NGC 3766 has been compared to the Wild Duck Cluster (M11) in Scutum and M37 in Auriga.

Through a medium 200mm (8-inch) scope, NGC 3766 is a stunning sight that appears bright, colourful and fully resolvable. At the extreme northern edge of the cluster lies eclipsing binary BF Cen, which varies between magnitudes +8.5 and +9.4 every 3.7 days. In total, 137 stars have been identified, but it's not clear how many of these are actually members.

NGC 3677 is 5,500 light-years distant and has a spatial diameter of 20 light-years. It's estimated to be 20 million years old.

NGC 3766 Data Table

NamePearl Cluster
Object TypeOpen Cluster
Distance (light-years)5,500
Apparent Mag.+5.3
RA (J2000)11h 36m 14s
DEC (J2000)-61d 36m 36s
Apparent Size (arc mins)12
Radius (light-years)10
Age (years)20 Million
Number of Stars137
Other NamesCollinder 248, Melotte 107

Sky Highlights - September 2017

Neptune reaches opposition on September 5th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for September

Northern Hemisphere
West:- Jupiter (mag. -1.7)
Southwest:- Saturn (mag. +0.5)
East:- Neptune (mag. +7.8)
South:- Neptune
East:- Uranus (mag. +5.7)
West:- Neptune
South:- Uranus
East:- Venus (mag. -3.9), Mars (mag. +1.8) (from second week), Mercury (mag. +0.5 to -1.3) (from second week)

Southern Hemisphere
West:- Jupiter
Northwest:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Uranus
West:- Neptune
Northwest:- Uranus
Northeast:- Venus
East:- Mars (end of month)

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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