NGC869/884: The Double Cluster
| Catalogues | NGC869 and NGC884 | |
| Names | The Double Cluster | |
| Type | Open Clusters | |
| Constellation | Perseus | |
| Season Visible in Evening | Late Summer & Autumn, Winter; Sep - Mar | |
| Conversation Notes | Pair of open clusters |
Open clusters are easy to find and observe in small telescopes, pretty, and scientifically important. This is a group of stars all born from a common cloud of gas and dust. Since they came from the same gas cloud they are, astronomically speaking, all about the same age and all about the same distance from us. Knowing that they are the same age and at the same distance, the fact that they have different appearances allows us to learn a great deal about stellar evolution - the different appearances can only be a result of the different masses of the different stars.
This is a pair of clusters, separately catalogued as NGC 869 and NGC 884, but generally known as a pair called "The Double Cluster".
Finding the Double Cluster
| Although the Double Cluster is in Perseus, it's easiest to find from Cassiopeia. Cassiopeia is shaped like a W, and is found on the other side of Polaris from the Big Dipper. It rotates around the sky with the seasons, so it may be upside down or on its end. |  |
| Let's inspect Cassiopeia more closely. This is how it would be oriented on an Autumn evening. |  |
For convenience, let's name two of the stars. The star in the centre of the "W" is Gamma Casiopeiae. The next star out toward the stretched, lopsided side of the "W" is Ruchbah. |
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| Imagine a line between Gamma and Ruchbah. |  |
Now imagine extending that line in the same direction, twice its own length again. |
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The cluster is just to the side of the end of this extended line. Imagine turning a right-angle toward the centre of the "W", a short distance - about a third of the length of the original Gamma-Ruchbah line. |
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If you are observing from a dark sky site, you should be able to see the double cluster as a fuzzy patch of light at that point. |
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Even if you can't see it, point your telescope to that point. |
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In a magnifying finder you should be able to see two brighter patches of light against the background stars. Centre those. Now switch to your telescope, with your widest-field eyepiece. |
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In your main telescope, you will see two distinct clusters, each with their own bright core of stars. This image simulates the view in dark skies at about 35x magnification. In long-focal-length telescopes (such as mid-sized SCTs) you may not be able to fit both clusters in the view unless you have a very wide-field eyepiece. |
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All the above images were generated with Starry Night Pro.