NGC 869 and NGC 884 are two bright open clusters in the constellation of Perseus, that are separated by only half a degree of apparent sky. Together they are commonly known as the "Double Cluster" and form a famous showpiece object, that's easily visible to the naked eye and a wonderful sight in binoculars and telescopes. Both clusters have been known since antiquity and probably pre-historically. Greek astronomer Hipparchus first catalogued them around 130 B.C, with early celestial cartographers naming them as "h Persei" (NGC 869) and "χ Persei" (NGC 884).

The Double Cluster is located in the far northwestern part of Perseus, close to the border with Cassiopeia. With a declination of 57N, it's circumpolar from many northern locations and therefore never sets. To locate the object, draw an imaginary line from Mirfak (α Per - mag +1.8) in a northwesterly direction towards the centre of the "W" of Cassiopeia. The Double Cluster lies just over halfway along this line.

It's listed as number 14 in the Caldwell catalogue.

NGC 884 (left) and NGC 869 (right) The Double Cluster (credit:- Michael Fulbright - MSFAstro.net - http://msfastro.net)

Finder Chart for NGC 869 and NGC 884 The Double Cluster (credit:- freestarcharts)

Finder Chart for NGC 869 and NGC 884 The Double Cluster - pdf format (credit:- freestarcharts)

To the naked eye, the Double Cluster appears like a large unresolved detached portion of the Milky Way. The view through binoculars is addictive, a breath-taking sight with both clusters covering more than half a degree of sky and almost touching. They are embedded in a hazy mist and surrounded by many unrelated Milky Way stars. NGC 869, at mag. +4.3, is marginally the brighter and richer cluster, while NGC 884 glows at mag. +4.4. The brightest individual members shine at 7th magnitude.

The extra aperture and magnification provided by small scopes reveals two large bright compact clusters with dozens of stars visible, concentrated towards the cluster centres. Many other fainter stars are scattered throughout. A 200mm (8-inch) telescope reveals several hundred stars that completely fill the field of view. There are strands of bright A and B type blue and white supergiants streaming outwards in curving patterns, with a few fainter M type red supergiants in and around cluster NGC 884.

Recent data measurements put the distances of NGC 884 and NGC 869 at 7,600 and 6,800 light-years respectively. Both clusters are extremely young and spatially close to each other. It's estimated that NGC 869 is 5.6 million years old and NGC 884 is 3.2 million years old. For comparison, the Pleiades (M45) open cluster is 115 million years old.

In total, NGC 869 and NGC 884 contain at least 400 hot blue-white supermassive and super luminous giant stars.

NGC 869 Data Table

NGC869
Caldwell14
Nameh Per
Object TypeOpen Cluster
ConstellationPerseus
Distance (light-years)6,800
Apparent Mag.+4.3
RA (J2000)02h 19m 03s
DEC (J2000)+57d 08m 06s
Apparent Size (arc mins)35 x 35
Radius (light-years)35
Age (years)5.6 Million
Number of Stars250
Other NamesCollinder 24, Melotte 13

NGC 884 Data Table

NGC884
Caldwell14
Nameχ Per
Object TypeOpen Cluster
ConstellationPerseus
Distance (light-years)7,600
Apparent Mag.+4.4
RA (J2000)02h 22m 32s
DEC (J2000)+57d 08m 39s
Apparent Size (arc mins)35 x 35
Radius (light-years)38
Age (years)3.2 Million
Number of Stars200
Other NamesCollinder 25, Melotte 14

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

Southern Hemisphere
Evening
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
Midnight
West:- Jupiter
North:- Saturn
East:- Neptune
Morning
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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