First Light Guide #10 & #11: NGC869 & NGC884

NGC 869 & NGC 884, the Double Cluster

A detailed guide to finding / observing the two open clusters

First Light Guides 10 and 11 graphicFirst Light Guides #10 and #11 are a combined guide, as both NGC 869 and NGC 884 would be wonderful objects to observe on their own. Beyond that, seeing both requires a rather wide telescopic field of view - at least 1.5 degrees across (ideally more). Because some observers may not be able to achieve that wide a field, do go ahead and observe each cluster independently if necessary. One need not see both to still be impressed by the wonderful richness of stars located here.    

This guide is ideal for beginners, but many advanced amateur astronomers may find it useful. It will help you locate this open cluster with almost any telescope. There is a video to using a magnified finderscope, red dot finder, or setting circles along written tips on observing the Double Cluster in Perseus.

Visitors who are new to this site may wish to get started here, while more advanced observers can search the First Light Guides by constellation, or by right ascension

Find NGC 869 and NGC 884 in the sky

NGC 869 and NGC 884 are two open cluster within our own Milky Way galaxy that are very close to each other, creating a "dual" cluster for telescope observers. The two clusters are each about 7,500 light years from Earth, so the photons from it reaching your eye in a telescope left that cluster about the time copper was being used in the Middle East. The estimated age is about 12.8 million years, making it one of the younger star clusters in our galaxy.

If you have a magnified finderscope on your telescope, start here to find the Double Cluster:

If you have a red dot finder on your telescope, start here to find and observe the Double Cluster:

If you have a telescope that has an equatorial mount with fairly precise setting circles, you may find this video useful when trying to find the Double Cluster:

Video not available yet, but coming soon!

Observing tips:

After the Pleiades, the Double Cluster is perhaps the best open cluster - well, two open clusters to observe. Both of them are easily visible in most larger finderscopes even from moderately light polluted locations. For red dot finders, the real trick is aiming exactly halfway between Delta Cassiopeiae and Gamma Persei. 

Once located, these clusters are very large in the eyepiece for many telescopes. Use the lowest magnification possible (long focal length eyepiece). Shorter focal ratio telescopes (f4 to f6) will better be able to provide the wide telescopic field of view necessary to see both together, while longer focal ratios (f8 and longer) will likely require eyepieces with significant apparent fields of view. 

Lacking that expensive eyepiece glass, try viewing each cluster independently of the other. NGC 869 is the cluster with the more densely populated central region, and is also the one slightly closer to Cassiopeia. To enhance your view and see more stars, center this cluster and add a bit more magnification, to a medium-magnification eyepiece. This can darken the sky background and appear to provide a bit more contrast, pulling out a few more of the fainter stars in the cluster. 

Useful filter(s): Broadband light pollution filters may be useful in moderate to larger sized telescopes (114mm or larger) but only to darken the sky background at lower magnifications. The filter will also dim the brightness of the stars, so the benefit may be minimal or non-existent. 

What should I see?

Below is an approximate view NGC869 and NGC884 as seen with a 70mm telescope at 38x magnification, and a 1.4 degree telescopic field of view.  Demonstrates how view would look with objects on the meridian using a refractor telescope and a star diagonal.  Other telescopes or object sky positions may incur a differing view.  Various magnifications, eyepieces, telescope focal lengths and other variables may alter the view compared to this one.  This is a representation only intended to help the observer get some idea what they may see at the eyepiece.  Extreme local light pollution will significantly diminish the number of fainter stars visible.

Approximate view of Double Cluster as seen at 38x-low power

Details of NGC 869

Type: Open Cluster

Distance: 7,600 light years

Apparent dimensions: 30 arc minutes

Apparent magnitude: 3.7

Right ascension: 02h 19m 0s

Declination: +57 09' 0"

Details of NGC 884

Type: Open Cluster

Distance: 7,600 light years

Apparent dimensions: 30 arc minutes

Apparent magnitude: 3.8

Right ascension: 02h 22m 0s

Declination: +57 08' 0"

Because telescopes and observers are all different, here are some alternate sketched/drawn views of the Double Cluster:

Ferenc Lovró, using a 12" f/5 reflector

Jeremy Perez, using a 6" f/8 reflector

The constellation: Perseus

General information about Perseus the Hero, where the Double Cluster is located can be found here.  This will help you know where to find the constellation in the sky and be able to locate and identify its brightest stars.

Name of constellation: Perseus

Abbreviation: Per

Genetive form: Persei

Common names: The Hero

Associated asterisms: No major ones

Original 48 of Ptolemy: Yes

Area by size: 615 square degrees

Relative size: 24 out of 88 (Serpens is next larger, Cassiopeia is next smaller)

First Light Guide objects: Messier 34, Double Cluster, Algol

Brightest stars, in order of magnitude 

First lists the Bayer designation, then the "traditional" star name (often Arabic, but not always - see each star's notes for details). Graphic shows constellation at zenith as if looking up with body facing north.

  • α (alpha) Persei (Mirfak) magnitude 1.79, is the 37th brightest star in the sky. It is 510 light years away from us. The traditional name is derived from the Arabic Mirfaq al-Thurayya, which means, "the elbow."

  • β (beta) Persei (Algol) is an eclipsing variable star of 2.1 that dips to 3.4 every 2.86 days. At maximum, it is the 59th brightest star in the sky. Traditional name is from the Arabic رأس الغول ra's al-ghūl : head (ra's) of the ogre (al-ghūl) - or "ghoul."

  • ζ (zeta) Persei is magnitude 2.84, and about 750 light years from Earth.

  • ε (epsilon) Persei is magnitude 2.88, and approximately 640 light years distant.

  • γ (gamma) Persei is an eclipsing binary system, with an combined magnitude of 2.93, and around 243 light years distant.

  • δ (delta) Persei is magnitude 3.01 and is 520 light years away.

  • ρ (rho) Persei is magnitude 3.39 and is 308 light years distant.