The Double Cluster in Perseus
The Double Cluster in Perseus (NGC 869 and NGC 884) are two open clusters in the constellation Perseus. Also named Caldwell 14 or simply “The Double Cluster”, both star clusters are very similar to each other in size, magnitude, and age.
This pair of open clusters lie about 7,000 light-years away from Earth. They are an easy binocular target, and an impressive sight through a small telescope.
The magnitude 3.7-3.8 star clusters are bright enough to be spotted with the naked eye under the right conditions. Each of the clusters contains more than 300 blue/white super-giant stars. The photo shown above was captured from a dark sky site (Bortle Scale class 4) using a Canon EOS 60Da camera, and a Rokinon 135mm F/2 lens.
The image was created by capturing several 90-second image exposures and stacking the images together. A star tracker was used to compensate for the apparent rotation of the night sky, to capture sharp stars across the field.
The long-exposure image below displays the beautiful mix of warm and cool stars in the area, with the prominent blue/white stars of the double cluster. This image was captured through a refractor telescope (Astro Tech 72ED) and a Canon EOS T3i DSLR by Brent Newton.
The Double Cluster in Perseus. Brent Newton.
NGC 869 and NGC 884 (The Double Cluster)
- Common Name: The Double Cluster
- Cataloged: NGC 869, NGC 884, Caldwell 14
- Constellation: Perseus
- Distance: 7,500 ly (2,300 Pc)
- Apparent dimensions (V): 60′
- Apparent magnitude (V): 3.7 and 3.8
- Age: 12.8 Million Years
The Double Cluster occupies a total area of about 60 arc minutes. For comparison, the full Moon is approximately 30 arc minutes in size. Each cluster is surrounded by an extended halo of stars, and are only separated by a few hundred light-years.
The open star clusters were first cataloged around 130 BC when the Greek astronomer Hipparchos mentioned it as a “nebula” or “cloudy spot” in his writings. It was not until the invention of the telescope in the early 17th century, that the true nature of the Double Cluster in Perseus was realized.
The Messier Catalog does not include the Double Cluster, but it is listed in the Caldwell, and NGC catalogs. Astronomer William Herschel was the first one to recognize that this “cloudy spot” in the night sky consisted of two separate clusters of stars.
Views of NGC 869 and NGC 884 at varying magnifications.
Each cluster contains roughly 300-400 stars that are approximately 300 million years old. From a dark sky location during the new moon phase, you may be able to see the hazy pathway of the winter Milky Way crossing this part of the sky.
The Double Cluster lies directly between the constellations Perseus, and Cassiopeia. At magnitude 3.7 and 3.8, NGC 869 and NGC 884 are brighter than many of the other open clusters in the night sky, and this makes them easier to find.
The easiest way to find the Double Cluster in Perseus is to travel downwards from the constellation Cassiopeia. Under moderately dark skies, you should see a faint glow of stars below the 2 bright stars at the bottom of the “W” shape of Cassiopeia (Ruchbah and Epsilon Cassiopeiae).
Related Video: Two Ways to find the Double Cluster in Perseus
Some find it helpful to draw an imaginary line from the stars Navi, and Ruchbah in Cassiopeia. Following this path, you will eventually run into the Double Cluster in a pair of binoculars.
Through a pair of binoculars, the two clusters of stars turn from a faint smudge, into two distinct groupings of stars. From mid-northern latitudes, the best time to look for the Double Cluster in Perseus is in the fall.
The location of the Double Cluster between Cassiopeia and Perseus.
I remember the first time I saw the Double Cluster through the eyepiece of my telescope, and it was one of the most exciting observations I had made at the time. The number of intensely bright stars in a single field of view is remarkable, and it is one of the most satisfying views in the entire night sky.
Try scanning past the area of Perseus where the clusters are located using a wide-field eyepiece. When the Double Cluster appears in view, you’ll want to enjoy the sight for an extended period of time. A wide-field telescope eyepiece (25mm or more) is recommended.
The Double Cluster, the Heart Nebula, and the Soul Nebula. Robin Onderka.
The Constellation Perseus.
What is an Open Cluster?
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. Open clusters are loosely bound together by gravitational attraction, yet become disrupted by close encounters with other clusters and gas clouds as they orbit the galactic center.
The Pleiades is perhaps the most popular open cluster in the night sky, easily observed with the naked eye in the constellation Taurus. Over 1000 open clusters have been discovered within the Milky Way Galaxy, and many more are presumed to exist. Open clusters can survive for over a hundred million years. Globular clusters exert a stronger gravitational attraction on their members and can survive even longer.
Open clusters are found in the disk of the Galaxy and therefore lie largely in the plane of the Milky Way. Many of the closest open clusters are easily visible to the naked eye. To help you find the location of the brightest Open Clusters in the night sky, it is helpful to use a stargazing app on your smartphone.
The Double Cluster with surrounding H-alpha nebulosity. (View Full-Size version on Flickr)
The image above was captured by Nico Carver (Nebula Photos) to highlight the hydrogen gas present in the area. Nico combined a DSLR shot of the double cluster with a deep shot of the surrounding h-alpha emission from a mono camera.
Well-Known Open Clusters
- Pleiades Star Cluster
- Hyades Star Cluster
- The Alpha Persei Cluster
- The Wild Duck Cluster
- The Butterfly Cluster
- The Beehive Cluster
- The Coma Star Cluster
This article was originally published on February 16, 2016, and updated on October 22, 2020.