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Glenn Chaple's Observing Basics: A double take

January 2011:  Spend an autumn evening getting reacquainted with the Double Cluster.
chaple
Gaze northeastward an hour or two after sunset on a late autumn evening from a dark site and you’ll likely notice an elongated smudge of light midway between Cassiopeia and the “head” of Perseus. More than 2 millennia ago, it caught the eye of the Greek astronomer Hipparchus, who referred to it as a “cloudy spot.” Thanks to the telescope, we now know that this smudge of light is in fact a dazzling pair of starry aggregations we call the Perseus Double Cluster.

This pair, also known as h and Chi (χ) Persei, and by the NGC designations 869 and 884, respectively, is an awe-inspiring sight — especially when captured through binoculars or rich-field telescopes. A large-aperture scope (greater than 8 inches) will also work, provided it’s coupled with an eyepiece that yields a field of 3 or 4 Moon-widths.
 
By itself, either of these clusters would prove a noteworthy sight. Viewed together, they are simply breathtaking. In his 1901 classic observer’s guide, Pleasures of the Telescope, Garrett P. Serviss wrote, “As an educative object for those unaccustomed to celestial observations it may be compared among star clusters to β Cygni [Albiero, which constitutes a blue-green star and an amber one] among double stars, for the most indifferent spectator is struck with wonder in viewing it.” His words ring true more than a century later. A guaranteed “Wow!” object for the first-time viewer, the Double Cluster is a must-see at public star parties.
 
Each member of the Double Cluster spans slightly less than ½° and contains more than 100 stars between magnitudes 6.5 and 14 — most being hot, luminous supergiants. If a star equal in brightness to our Sun were placed within the bounds of either of these clusters, it would appear as a feeble 16th-magnitude speck.

Double-Cluster
The Double Cluster (NGC 869, right, and NGC 884) in Perseus is one of the heaven's most beautiful deep-sky objects. Each member spans slightly less than 1/2° and contains more than 100 stars.
Richard Jacobs
Over the years, I’ve learned one truism about deep-sky observing — never assume a “been there, done that” attitude. Al-though I had viewed the Double Cluster dozens, if not hundreds, of times, I had never noticed a feature that Iowa amateur astronomer Steve Bookout brought to my attention via e-mail a few years ago. He reported that the center of NGC 884 appears hollow, as if veiled by a tiny dark nebula. I checked out the Double Cluster with my 13-inch Dobsonian, and Bookout was right. There is, indeed, a circular star-poor region in NGC 884. I was able to make out only about a half dozen faint stars within its confines. My eyes had met the Double Cluster numerous times, but they had never really looked.

Why isn’t the Double Cluster in Charles Messier’s catalog? Despite its appearance when viewed with the naked eye, the French comet-hunter failed to add it to his roster of “comet impostors.” The omission seems particularly glaring when you consider that the catalog includes the Pleiades (M45), hardly a comet wannabe! In The Caldwell Objects (Cambridge University Press, 2003), a guidebook describing Sir Patrick Moore’s 109 finest non-Messier objects, author Stephen James O’Meara devotes four pages to the question. O’Meara suggests that the Pleiades might indeed be taken for a naked-eye comet when viewed in a twilight sky. The Double Cluster — a well-known circumpolar object visible night after night — would be less likely to trick a comet-hunter. He stresses that Messier never intended his catalog to be a list of the most spectacular deep-sky objects. Rather, it was a compilation of faux comets the observer encountered during his searches.

To appreciate the immensity of the Double Cluster, consider that we view it from a distance of between 7,400 and 7,600 light-years — some 17 times as distant as the Pleiades. What if the Double Cluster were positioned as close to us as the Seven Sisters? O’Meara writes, “One-quarter of the northern sky would be filled with the concentrated splendor of 600 suns, the brightest of which would shine with the brilliance of Vega (Alpha [α] Lyrae).”

While you can never hope to see a naked-eye sight so awe-inspiring, you can do better than Hipparchus. Simply go outside on the next clear moonless night with binoculars or a rich-field telescope and direct your attention to that “cloudy spot” in the northeast sky. If you’re a novice backyard astronomer, prepare to be dazzled. If you’re a veteran skygazer, forget that “been there, done that” attitude. Spend some time with the Double Cluster and look — really look!

Questions, comments, or suggestions? E-mail me at gchaple@hotmail.com. Next month: We go rabbit hunting. Clear skies!
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