Shallow Sky Object of the Month: Double Cluster—NGC 884 / 869

Original article appears in GuideStar December, 2011.

Double ClusterObject: The Double Cluster Class: Open Cluster(s) Constellation: Perseus Magnitude: 5.3 R.A.: 2 h 20 m 50 s Dec: 57 deg 11 min 32 sec Distance: 7600 (NGC 884) and 6800 (NGC 869) ly Optics needed: Binoculars / naked eye / wide field telescope

These two clusters are a magnificent site in binoculars or a wide-field telescope. The picture to the right is from TheSky X software, and the light blue circle is one degree on the sky. So, you need optics that will provide a visual field of at least one degree to get both clusters in the field.

A small refractor on a dark night would be a good instrument to use on this pair of clusters. It turns out that the new moon this December is on the 24th, Christmas eve. If you’re with your friends and family on that night, this would be a great object to show them in your telescope. On that night the Double Cluster will be about 62 degrees above the horizon, and it will transit at 8:35 p.m.

Double Cluster These cluster look like two swarms of bright white to blue/white stars against a black sky. On a clear, dark night, they’re beautiful.

Here’s what you can say about open clusters...

Stars form from clouds of material, mostly hydrogen, that exist in space and which collapse into stars as the result of gravity. When the cloud gets hot enough, the stars ignite, turning hydrogen to helium. Since these stars formed from the same cloud, they’re about the same age.

Because astronomers understand the life cycle of stars, they can estimate the age of the cluster by identifying stars that are leaving the main sequence (beginning to run out of fuel). The astronomers know how long it takes a star of that size to reach this stage of life, so they know how old that star is. Since all the stars in the cluster are about the same age, they now have a good estimate of the age of the cluster.

Having done the analysis they know that NGC 884 (on the left in the image above) is about 3.2 million years old, and NGC 869 (on the right) is about 5.6 million years old.

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