NGC 2506 is a magnitude +7.6 rich open cluster located in the constellation of Monoceros. Although its member stars are faint the cluster itself appears quite bright and can be seen with a pair of binoculars. Through telescopes it's an impressive object and of all the Monoceros open clusters it's probably the finest. With an age of 1.1 billion years old this is an old cluster. For comparison, M45 (The Pleiades) in Taurus is a youthful 115 million years old with the Hyades cluster 625 million years old. However, NGC 2506 is not nearly as old as the 4 billion years of M67 in Cancer.

William Herschel discovered NGC 2506 on February 23, 1791. Locating the cluster can sometimes be a bit tricky as it's positioned in an area of sky devoid of bright stars. It can be found 5 degrees east-southeast of alpha Mon (α Mon - mag. +3.94) the brightest star in Monoceros. Located 19 degrees southwest of NGC 2506 is the brightest star in the sky, Sirius (α CMa - mag. -1.46). Although α Mon and Sirius are the brightest stars in their respective constellations the difference in apparent brightness between them is enormous, more than 100x.

Large open cluster M48 lies 6 degrees northeast of NGC 2506 with open cluster pair M46 and M47 positioned 6 degrees southwest of NGC 2506. They are best seen during the months of December, January and February.

NGC 2506 - Open Cluster (Jim Thommes -

Finder Chart for NGC2506

Finder Chart for NGC2506 - pdf format

NGC 2506 appears as a faint small misty unresolvable patch of light in 7x50 or 10x50 binoculars. A 100mm (4-inch) telescope under dark skies reveals about a dozen stars embedded in hazy, misty patch of light. Through 200mm (8-inch) scopes, NGC 2506 is a wonderful sight. At high magnifications more stars are visible, arranged in chains streaming across the face of the cluster. The brightest member stars are of magnitude +10.8 and the cluster contains at least 150 stars packed in an apparent diameter of 7 arc minutes. A 300mm (12-inch) scope resolves many more stars with NGC 2506 appearing more compact at the edges than in the centre. This is a wonderful object and a superb cluster for amateur astronomers, especially those with medium or larger sized scopes. In the Caldwell catalogue, NGC 2506 is listed as number 54.

NGC 2506 Data Table

Object TypeOpen Cluster
Distance (ly)11,300
Apparent Mag.+7.6
RA (J2000)08h 00m 02s
DEC (J2000)-10h 46m 11s
Apparent Size (arc mins)7.0 x 7.0
Radius (light-years)12
Age (years)1.1 Billion
Number of Stars150
Other Name (s)Collinder 170, Melotte 80

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
West:- Jupiter
North:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

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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