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The Coma Cluster.

Want to observe a galaxy or two?

Point your scope toward the rich cluster in Coma Berenices.

Richer and denser than the Virgo Cluster and containing thousands of galaxies, the Coma Cluster (Abell 1656) is a telescopic wonderland that takes its name from its parent constellation, Coma Berenices. More than a hundred visible galaxies are packed into an area only a little greater than 3[degrees] in size; as many as 50 are visible in a 16-inch telescope. Exploring beyond its two dominant members is a challenge, though. At almost six times farther than the Virgo Cluster, its members are three or four times fainter. You can enjoy the sample tour I describe here with hardly more than a nudge of your telescope: The objects are contained in a single field of view of less than 1[degrees].

The Coma Cluster lies about 2.5[degrees] west of Beta ([beta]) Comae Berenices along an imaginary line connecting Beta and Gamma ([gamma]) Comae Berenices. The center of the cluster is about T southeast of a magnitude-7.2 star (HD 112887). Dominating the middle of the cluster are two giant elliptical galaxies just T apart, both visible in an 8-inch scope. NGC 4889 is officially the brightest galaxy in the cluster at magnitude 11.5, but neighboring NGC 4874 has a slightly higher surface brightness and appears more brilliant to the eye. Both have significantly brighter cores surrounded by large, luminous halos, with NGC 4889 being noticeably more elongated. The two galaxies each anchor their own group of smaller, fainter companions and stand out strikingly among the many neighbors that cluster around them.

William Herschel discovered both of these galaxies in April 1785. Herschel was the first to explore the cluster, but several other famous observers, including John Herschel, Heinrich d'Arrest, and Guillaume Bigourdan, are credited with discoveries here. And they are each responsible for various errors and ambiguities that John Dryer had to sort out when he compiled the New General Catalogue. Modern historians continue working to unravel some of the more puzzling discrepancies, so the pedigree of a few objects remains in contention. For instance, NGC 4889 is equivalent to NGC 4884; the dual designation is due to a second observation of the galaxy by d'Arrest that he mistakenly recorded as a new object.

NGC 4886, a smaller elliptical paired with NGC 4889, also has a dual designation (NGC 4882); d'Arrest observed the pair again on a different night, and, not recognizing them, assigned them both new designations. NGC 4886 is round with a bright core and lies at approximately the same distance as its larger neighbor, around 300 million light-years.

Averted vision will reveal a pair of similar galaxies about 2' east-southeast of NGC 4889. NGC 4898 is an elliptical galaxy that appears round, while NGC 4894 is a lenticular galaxy, a class of galaxies that are intermediate between ellipticals and spirals. NGC 4894 lies about 100 million light-years closer to us than the core of the Coma Cluster. As with most giant galaxy clusters, the center of the Coma Cluster is populated with ellipticals. The spiral galaxies that are associated with the cluster are typically found in the outskirts, and the lenticular galaxies, such as NGC 4894, are generally intermediate both in morphology and distance from the cluster core. A little more than 1.5' to the north of NGC 4889 is tiny 1C 4011, appearing as a small round spot of uniform brightness. An elliptical, it is at a distance that puts it near the center of the cluster.

Northwest of NGC 4889 at a separation of about 4.5' is NGC 4883, a small, round, barred lenticular galaxy that has a brighter core.

The western giant, NGC 4874, is surrounded by its own retinue of smaller galaxies that form an oval ring. Lying less than 1' to the southwest is NGC 4872, which has a somewhat muddled discovery history. It was most likely missed by William and Caroline Herschel and possibly first observed by d'Arrest. Errors in position and an ambiguous description by d'Arrest (his notes could apply to any of three close companions to NGC 4874) muddies the waters. Moving counterclockwise, we find NGC 4871 a little more than T to the west of NGC 4874 and NGC 4873 about 1.5' to the north. Both are similar lenticular galaxies with small bright cores. 1C 3998, some 2.5'to the east northeast of NGC 48 74, lies behind the core of the cluster at around 460 million light-years. It appears small, faint, and round. A number of small galaxies with PGC designations complete the oval ring of satellite companions, though they will require very large aperture to trace out.

Some 5' west of NGC 4874 lies the close pair comprised of NGC 4864 and NGC 4867. On the night of April 11, 1785, William Herschel described three nebulae in the core of the Coma Cluster. NGC 4889 and NGC 4874, the two brightest galaxies in the cluster, are certainly two of the three, but the third is somewhat in doubt. Harold G. Corwin speculates that the third nebula might have been the combined glow of NGC 4864 and NGC 4867 along with a superimposed star that lies just southeast of NGC 4864's core. These two galaxies are less than Y apart.

Corwin notes that the other likely possibility for William Herschel's third nebula might be NGC 4869, which lies about 4' southwest of NGC 4874. It is one of the five brightest galaxies in the heart of the cluster and appears round with a bright core.

Some 8' east-northeast of NGC 4889 is one of several concentrations of faint but detectable galaxies that surround the core of the cluster. My 30-inch Dobsonian shows eight galaxies in this quadrant easily enough. The three brightest are NGC 4908, a 13.6-magnitude elliptical that has a brighter core, 1C 4051, a more elongated 13.2-magnitude elliptical, and 1C 4040, classified as a radio galaxy. Radio galaxies are a type of active galaxy that is very bright at radio wavelengths due to the interaction of powerful jets with the interstellar medium. We can only imagine the violence of the scene; the visual galaxy is very average-looking and reveals no hint of the activity occurring there.

About 3' to the northwest of the bright field star HD 112887 is NGC 4865, a foreground lenticular that has a bright core. Some 4' farther northwest is a close pair. The bright elliptical NGC 4860 is just 38" from its fainter, smaller companion NGC 4858. The two are not proximate in space: NGC 4858 is a barred spiral that lies more than 60 million light-years behind its larger neighbor. Both galaxies have bright cores.

Our tour but scratches the surface of this cluster. Several dozen more galaxies lie within this single field, and many more galaxies will come into view with a gentle nudge of the scope in nearly any direction.

A compellingly emotive aspect of visual observing is the sense of reliving the experiences of the famous observers of the past. Exploring a distant galaxy cluster by eye, you are, at that moment, the last in a line of observers stretching back centuries to the discoverer of the object. Observing the Coma Cluster, we can feel privileged to be sharing a unique moment with William Herschel and Heinrich d'Arrest. No matter how many observers stand between them and you, the visceral impact is undiminished, the view unchanged. Contemplate that incredible fact when next you put eye to eyepiece, and it just might offer you a new perspective and add a touch of awe to your observing session.

* Contributing Editor TED FORTE explores the deep sky from his home observatory outside Sierra Vista, Arizona. His regular Backyard Astronomer column runs monthly in the Sierra Vista Herald/Review.

Caption: A SWARM OF GALAXIES Situated over 300 million light-years away in the constellation Coma Berenices, the Coma Cluster counts more than 1,000 members.

Caption: THE QUEEN IN THE CLUSTER NGC 4889, an elliptical, is the largest of the galaxies in the Coma Cluster and harbors possibly one of the most massive black holes known to date, weighing in at some 21 billion solar masses.
Coma Cluster Components

Object               Type            Surface     Mag(v)        Size

NGC 4889          Elliptical          13.3        11.5      2.9'x 1.9'
NGC 4874          Elliptical          13.1        11.7      1.9'x 1.9'
NGC 4886          Elliptical          12.8        13.9     0.6' x 0.6'
NGC 4898          Elliptical          12.0        13.5      0.6'x 0.4'
NGC 4894          Lenticular          12.6        15.2      0.5'x 0.2'
IC 4011           Elliptical           --         15.0      0.4'x 0.3'
NGC 4883      Barred lenticular       12.9        14.4     0.6' x 0.5'
NGC 4872      Barred lenticular       12.8        14.4      0.6'x 0.4'
NGC 4871          Lenticular          12.6        14.1     0.7' x 0.4'
NGC 4873          Lenticular          12.9        14.1     0.7' x 0.5'
IC 3998       Barred lenticular        --         14.6     0.6' x 0.4'
NGC 4864          Elliptical          12.4        13.6      0.7'x 0.5'
NGC 4867          Elliptical          12.8        14.5     0.5' x 0.4'
NGC 4869          Elliptical          13.1        13.8      0.8'x 0.7'
NGC 4908          Elliptical          12.9        13.6      0.8'x 0.6'
IC 4051           Elliptical          13.3        13.2      1.2'x 0.9'
IC 4040             Spiral            13.4        14.8      1.0'x 0.3'
NGC 4865          Elliptical          12.8        13.7      0.9'x 0.5'
NGC 4860          Elliptical          13.3        13.5      1.0'x 0.8'
NGC 4858        Barred spiral         13.3        15.2      0.5'x 0.4'

Object            RA               Dec.

NGC 4889       13h 00.1m     27[degrees] 59'
NGC 4874       12h 59.6m     27[degrees] 58'
NGC 4886       13h 00.1m     27[degrees] 59'
NGC 4898       13h 00.3m     27[degrees] 57'
NGC 4894       13h 00.3m     27[degrees] 58'
IC 4011        13h 00.1m     28[degrees] 00'
NGC 4883       12h 59.9m     28[degrees] 02'
NGC 4872       12h 59.6m     27[degrees] 57'
NGC 4871       12h 59.5m     27[degrees] 57'
NGC 4873       12h 59.5m     27[degrees] 59'
IC 3998        12h 59.8m     27[degrees] 58'
NGC 4864       12h 59.2m     27[degrees] 59'
NGC 4867       12h 59.3m     27[degrees] 58'
NGC 4869       12h 59.4m     27[degrees] 55'
NGC 4908       13h 00.9m     28[degrees] 02'
IC 4051        13h 00.9m     28[degrees] 00'
IC 4040        13h 00.6m     28[degrees] 03'
NGC 4865       12h 59.3m     28[degrees] 05'
NGC 4860       12h 59.1m     28[degrees] 07'
NGC 4858       12h 59.0m     28[degrees] 07'

Angular sizes and separations are from recent catalogs. Visually,
an object's size is often smaller than the cataloged value and
varies according to the aperture and magnification of the viewing
instrument. Right ascension and declination are for equinox
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Title Annotation:Going Deep; Coma Berenices
Author:Forte, Ted
Publication:Sky & Telescope
Date:May 1, 2018
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