Seeing double with star cluster


If you're outside just after dark this week, aim binoculars along the Milky Way low in the northeastern sky. About midway between the constellations Cassiopeia and Perseus, you may think you're seeing double.

That's because you are.

You're looking in the direction of the famous Double Star Cluster of Perseus, one of — or should I say two of — the most beautiful showpieces in the heavens. Under a clear dark sky you might even be able to see the pair with the unaided eye as a single fuzzy wisp of light.

I remember the Double Cluster with fondness because, in 1971, it was the final view I had through the 16-inch telescope at the Weitkamp Observatory at Ohio's Otterbein College. You see, I was a student there at the time, and I was showing this sight to a small public group when someone smelled smoke. A fire in the lab beneath the observatory spread quickly and it was only minutes before the entire facility was destroyed. The memory of seeing the Double Cluster on that night, however, will always remain.

Prehistoric stargazers almost certainly spotted this object but didn't know what it was. Not until around 150 B.C. did stargazers officially catalogue it. The ancient astronomers Hipparchus and Ptolemy wrote about the pair, but they referred to it only as a "nebula" or "cloudy spot," one of the half dozen or so they knew at the time.

Modern astronomers identify this pair as "h and chi Persei", as well as by the catalog numbers NGC 869 and NGC 884; most of us, however, know it simply as the Double Cluster of Perseus. The easternmost of the pair (NGC 884) is slightly larger but contains fewer stars — about 150 or so — while NGC 869 is physically smaller and contains about 200 stars.

At a distance of between 7,000 and 7,500 light years, these two star clusters lie within only a few hundred light years of each other — pretty close by cosmic standards. They also seem to be quite young — most likely less than 13 million years old — and travel through the galaxy tethered by gravitation.

If you're unable to find the Double Cluster right away, try this "star hopping" technique to help you out.

First, find the constellation Cassiopeia. It appears during autumn evenings as a number "3" standing high in the northeastern sky. This star grouping and the area around it represented to the ancients a lovely queen who was so smitten by her own beauty that, when the gods placed her among the stars, they positioned her so she'd spend half of her time upside down.

If you watch her over an entire night — or through an entire year — you'll easily understand the origin of this idea. As she revolves around Polaris, the North Star, she'll go from appearing like a "3" to an "M" to an "E" to a "W" and back to a number "3."

Simply trace its two nearly vertical stars, and extend that line downward about the same distance as their separation. Aim binoculars or a small telescope in this direction and you'll soon be rewarded with a double cosmic treat!

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