The Double Cluster In Perseus

Two Open Clusters: NGC 869 and NGC 884

Best time to observe: November - March

To locate:

In the winter, find Cassiopeia high overhead and chasing the Dipper around the North Star. It's shaped like a large W (or M). To the west of Cassiopeia lies the somewhat faint constellation Perseus, shaped a bit like a stick figure man with curly arms and legs. Look at your Telrad chart and notice that the double cluster lies between these two constellations. There aren't any bright stars nearby, so you might not be able to locate the fuzzy patch in your Telrad. Try pointing the telescope in roughly the right direction, then scanning the sky, looking through your eyepiece for a sudden,bright collection of stars.

What to notice:

Both clusters will fit in your telescope's field of view, using a low power eyepiece. One of them, NGC 869, has two stars distinctly brighter than the rest. They look a bit like owl eyes. You'll also notice a haze of light behind its bright stars, light from stars too faint and close together to be resolved individually. The other cluster is a bit bigger and more open in the middle. Sometimes it looks like there's a little stick figure in it. Several stars in these clusters are distinctly reddish. Don't use averted vision to search for colored objects, since your eye can see color best straight on. There's a small red star almost in the middle of your field of view, between the 2 clusters. Can you find any more? These red stars are Red Giants, old stars once like our Sun but which have swollen with age. Some of them may be as large as Jupiter's orbit.

What you are looking at:

The stars in each of these clusters formed together in a nebula. Each contains about 300-350 stars, in a compact grouping 70 light years in diameter. Both are about 7500 light years from us, although astronomers are not sure whether the clusters are really associated with each other or simply happen to lie along the same line of sight from us. One cluster, NGC 869 (old Owl Eyes) is young, barely 5 million years. The other is twice that old. You can tell which is older by looking -- the Red Giant stars will be in the older cluster, since they represent stars nearing the end of their lifetimes. The brightest stars of this older cluster are supergiants some 50,000 times as bright as our Sun.

One one of your observing pages, sketch the Double Cluster in Perseus

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