The Double Cluster

Under the dry crisp skies of autumn, you should have no trouble spying a unique spray of stars lying in the winter Milky Way that is actually two star clusters in one. Facing northeast around 9 o'clock and looking halfway up into the sky you'll see Cassiopeia highest in the sky, with Perseus situated just below her. Between these two star groups - 7° southeast of Epsilon (e) Cassiopeiae and 4° northwest of Eta (n) Persei -is a gauzy patch of light that is a slightly brighter portion of the Milky Way. Binoculars (7X50) reveal two clumps of stars lying in the part of the

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Look for the Double Cluster

in the northeastern sky

around 9 p.m., October 9.

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Perseus figure known as the 'sword handle.' Each clump is a fine open cluster in its own right, but seen side by side in the sky, they make a stunning pair. The westernmost cluster is cataloged NGC 869 or 'h' Persei; the other is NGC 884 or Chi (%) Persei. Collectively, this object is known as the Double Cluster.

It is well worth taking a look at these remarkable star clusters through a small telescope at low magnification so that both fit into the same field of view. Suddenly, the clumps resolve into dozens of individual strands of stars radiating from two concentrated groups. Astronomers have counted nearly 400 stars in the clusters, though there are undoubtedly many more than this shrouded in interstellar dust. Most of the visible stars are very hot and massive. Sprinkled along the outskirts of NGC 884, however, are a few red giant stars that can easily be picked out among the white and blue-white stars.

Astronomers think the Double Cluster first condensed out of interstellar dust and gas only a few million years ago, although the red giant stars associated with NGC 884 suggest that it is the older of the two. And though they appear to be next to each other in space, they are not. NGC 869 is about 7,000 light-years distant, and NGC 884 is approximately 1,200 light-years beyond that.

The Double Cluster is truly one of the night sky's most enthralling sights. Once you've seen it in binoculars or a telescope, you will eagerly return to it again and again.

Also this week:

• The Small Magellanic Cloud crosses the meridian at midnight in the southern sky. The Large Magellanic Cloud is easily visible 20° to the southeast. (See February 8 - 14.)

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