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Designations:  NGC869 & NGC 884, Double Cluster
Object Type:  Open Cluster
Constellation:  Perseus

NGC 869

RA: 02h 19.0m
Dec :  +57 deg 08m
Visual Magnitude:  5.3
Size:  18 min
Distance: 7300 light years

NGC 884

RA: 02h 22.0m
Dec :  +57 deg 08 m
Visual Magnitude: 6.1 
Size:  18 min
Distance: 7300 light years

Discoverer:  Known since antiquity

Although Herschel is credited with dis­covering these objects' star-cluster nature, the combined glow of NGC 869 and NGC 884 has been known since antiquity. Hipparchus recorded them as a "cloudy spot." In his book Star Names: Their Lore and Meaning, Richard Hinckley Allen says that Ptolemy noted them as a "dense mass," and subsequent pre-telescopic astronomers saw them as nebulous entities. Allen also says that the twin clusters "seem strangely to have escaped the notice of astrologers, who, as a rule, devoted much attention to clusters as harmful objects which portended accidents to sight and blindness." Without the aid of a telescope, however, it's doubtful astrologers would have associated this "cloud" with an obvious cluster, such as the Pleiades. Today NGC 869 and 884 are simply known as the Double Cluster in the Sword Hand of Perseus.

One long-lived mystery concerning the Double Cluster is why the 18th-Century French comet hunter Charles Messier did not include it in his famous catalog. An equally compelling mystery is that, while Hipparchus and other early skywatchers listed NGC 869 and NGC 884 as two objects, few of today's amateurs seem to believe the components can be resolved with the unaided eye. Most popular astronomy books and articles state that the Double Cluster can be seen with the unaided eye only as a hazy patch of light, which becomes two beautiful objects with the aid of binoculars or a rich-field telescope. With a sweep of the hand that statement is true. But I wonder how many observers have missed seeing two distinct components with the unaided eye (not to mention the brightest stars superposed on them) because no one told them to look.

Together the two clusters occupy a full degree of sky, and their bright cores are separated by 25' — nearly a full Moon diameter. Why, then, have these distinct cores escaped the notice of the unaided eye? The problem may be one of contrast. With a casual glance the faintly luminous background of the Milky Way tends to fill the gap between the cores, making the split less distinct. Atmospheric clarity also may be a critical factor. When the Double Cluster is seen from nearly 14,000 feet atop Hawaii's Mauna Kea, a distinct lane of darkness separates the two components, even with direct vision. At an altitude of 4,200 feet, however, resolving the components requires a slight effort. The odds seem to worsen at sea level. You can avoid the issue by looking for the Double Cluster at the start of astronomical twilight, about 45 minutes after sunset; it emerges at about the same time as the brightest stars in Cassiopeia. Before the sky darkens completely the two cores pop clearly into view for several minutes, without any interference from the Milky Way. 

It is also at this time that one can make an accurate estimate of each component's brightness. Published magnitudes for the individual clusters are wide-ranging. The Observing Hand­book and Catalogue of Deep-Sky Objects lists the components' visual magnitudes as 3.5 (NGC 869) and 3.6 (NGC 884). Visual Astronomy of the Deep Sky pegs them at 4.4 and 4.7, respectively. And both the Deep Sky Field Guide to Uranometria 2000.0 and Star Clusters list values of 5.3 for NGC 869 and 6.1 for NGC 884. Most sources determine a cluster's total apparent brightness by summing the contributions of the stars in each cluster as recorded on photo­graphic plates. The differences among published values may reflect (1) differences in the numbers of stars used, and (2) whether the total magnitude includes any bright stars superimposed on the clusters. (On photographic plates the overexposed images of bright stars will lead to spuriously bright total magnitude estimates.) Motivated by these discrepancies, in February 1998 I estimated the brightness of the individual clusters with the naked eye, both in deep twilight and under a dark sky, and got an average visual magnitude of 4.5 for NGC 869 and 5.7 for NGC 884. Brent Archinal suspects that these visual estimates are close to what one would get by including the stars that are too bright to be measured on photographs.

The Double Cluster resides in a Milky Way spiral arm (the Perseus Arm) that lies farther from the galactic center than the one containing our solar system (the Orion Arm). A recent study places the clusters 7,300 light-years from Earth. These clusters probably formed from a single cloud of dust and gas, perhaps 13 million years ago. We see the Double Cluster through thick clouds of interstellar dust that line the plane of the Milky Way. Were it not for this dust, we would see the Double Cluster shining 1.6 magnitudes (4.4 times) brighter. The obscuration is unfortunate because, of the 300 or so suns that populate each of the two clusters, many are hot, blue-white supergiants; yet interstellar dimming lets even the brightest of these appear no brighter than magnitude 6.5. Imagine the grandeur of the Double Cluster if it were at the distance of the Pleiades (407 light-years). It would recall the starscape described in Isaac Asimov's story, "Nightfall." The twin mounds of blue supergiants would each span 5 Moon diameters and shine 300 times brighter than they actually do in our skies. 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. The earliest skywatchers would have succumbed to the Double Cluster's presence, looked to it for meaning, and planned their affairs in step with its eternal rhythms. Poets and artists would have looked upon these twin celestial cities to fuel their passions, or perhaps simply to dream.

But we have to settle for the more subtle view. To the naked eye, each cluster has a central condensation and a mottled outer halo.

In 10x50 binoculars the core of NGC 884 looks like a translucent diamond surrounded by a cloud of white smoke, as if it has just fallen into a heap of lime. To its west the core of NGC 869 is punctuated by a pair of stars that form the center of a cross-shaped asterism. A stream of 8th- to 10th-magnitude stars divides the two cores; flowing in from the north, the stream curves eastward around the southern periphery of NGC 884. A second river of stars starts at the wide double star 7 Persei and follows the western fringe of NGC 869 before it meanders toward 9 Persei, more than 1° south of NGC 884. When viewed with east "up," the "weight" of NGC 869 seems to press down on this western river of stars, causing it to sag. Sweeping the binoculars to the south, then east toward 10 Persei should bring into view two rings of stars, each roughly 1° in diameter. The Double Cluster lies on the northern edge of the overlapping rings, each of which appears to have been set with a cluster of semiprecious stone. One could imagine the stars of the Double Cluster having been swept out of these rings into two piles. The view brings to mind two frozen ponds cleared of snow for skating. Skaters' tracks can be seen faintly etched in starlight within each circle.

At 23 x in the Genesis the Double Cluster is transformed into an abundance of scintillating jewels.  Here are sapphires, rubies, topaz, and diamonds bursting out of treasure chests mired in the black sands of a volcanic beach. Few of : heavens' sights are better. A closer look with higher magnification reveals a dark Y-shaped t at the heart of NGC 869, just east of the cluster's elongated core. In comparison the core of NGC884 resembles a hollow rib cage of stars rounding a ruby heart — the pulsating semiregular variable star RS Persei. (The star's pulse is slow, its magnitude varying between 7.8 and 10.0 about every 224 days.) The Millennium Star Atlas shows a half dozen other variables in and around the Double Cluster. Finally, near the Double Cluster lies the radiant of the Perseid meteor shower, which peaks in the predawn morning hours of August 12th. Allen notes that Dante may have made re­erence to the Perseid meteors in the Purgatorio:

Vapors enkindled saw I ne'er so swiftly
At early nightfall cleave the air serene,
Nor, at the set of sun, the clouds of August;

"In the Middle Ages," Allen continues, the Perseids "were known as the Larmes de Saint Laurent, Saint Laurence's Tears, his martyrdom upon the red-hot gridiron having taken place on the 10th of August, 258." Thus together with the upwellings of starlight in the Double Cluster we have tears of fire and ice.

Telescope: Stellarvue SV80s
Focal Length: 384 mm (480 mm  * 0.8 with Televue field flattener)
Mount: Takahashi NJP 160
Camera: SBIG ST-10XME
Exposure:  10 30-second exposures and 6 30-second exposures for each color.
Other:  

Image Processing:

 Processed using CCDStack and Photoshop CS2

Astro Images

doug@dougsastro.net

Copyright(c) 2007 Doug Sanqunetti. All rights reserved