SUMMER SKIES contain countless telescopic treasures for observers. As our nighttime window opens upon the Summers Milky Way's soft glow, we find ourselves face to face with the center of our own galaxy. Surrounding the galactic nucleus, much as moths gather around a flame, are huge, spherical conglomerations of stars - the magnificent globular clusters.
The story of globular clusters begins in 1665, when the German astronomer Abraham IhIe stumbled across a smudge of light near Lambda Sagittarii while observing Saturn. Today we know this to be the globular M22. Little is known about Ihle, however, and it has even been suggested that M22 had been seen earlier by another German astronomer, Johannes Hevelius.
By 1781 an additional 31 globulars had been discovered, but none, barring M4 in Scorpius, were recognized as more than dim nebulous glows. The English observer William Herschel was the first person to recognize them as massive star gatherings. He used the term "globular cluster" when identifying several of these objects in his catalogue of 1786.
Presently there are more than 130 known globulars associated with the Milky Way. Amateur telescopes are capable of revealing over half of them. These clusters are all similar in appearance except for
their shape, brightness, and apparent size. The primary difference among globulars is their density or richness, which is generally measured on a 12-step scale. Class I globulars are very rich and compressed, while those of class XII are quite loosely structured.
Not all the objects in the following list can be seen easily from mid-northern latitudes. Each object is special in some way, and all can be seen with a 6-inch telescope. Many are visible in binoculars.
NGC 6093 (M80)
Constellation:
Scorpius
Right Ascension: 16h 17.0m (2000.0)
Declination: -22d 59m (2000.0)
Distance: 27,400 light years
Apparent Magnitude: 7.3
Apparent Size: 8.9'
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"The richest and most condensed mass of stars which the firmament can offer," wrote William Herschel in 1785. M80 shines at 7th magnitude and is located about halfway between Antares and Beta Scorpii. Under ideal conditions an 8-inch may begin to resolve some of the 14th-magnitude stars, but resolution is hindered by the cluster's high density. It is thought to be about 25,000 light-years away.
NGC 6121 (M4)
Constellation:
Scorpius
Right Ascension: 16h 23.6m (2000.0)
Declination: -26d 32m (2000.0)
Distance: 6,800 light years
Apparent Magnitude: 5.6
Apparent Size: 26.3'
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This cluster is easily located a little less than 11/2 ' west of Antares. At times the naked eye can just catch sight of it. The French observer Charles Messier described it as a "cluster of very small stars," which is an appropriate description for its appearance through a modern 3-inch telescope. William Herschel called attention to a curious "ridge of 8 or 10 pretty bright stars" that runs from the cluster's center toward the northeast edge.
NGC 6205 (M13)
Constellation:
Hercules
Right Ascension: 16h 41.7m (2000.0)
Declination: 36d 28m (2000.0)
Distance: 22,800 light years
Apparent Magnitude: 5.8
Apparent Size: 16.6'
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Discovered by Halley in 1714, M13 is the finest globular north of the celestial equator. Under dark skies it can be seen with the naked eye on the western edge of the Hercules Keystone. A 4-inch scope will reveal some of its individual stars, and a 16-inch causes the cluster to explode into a globe of innumerable stars. Look carefully and you may see some star chains at the outer edge that have been noted by such observers as John Herschel and Lord Rosse. M13 is estimated to be about 23,000 light years away and contains over a million suns.
NGC 6218 (M12)
Constellation:
Ophiuchus
Right Ascension: 16h 47.2m (2000.0)
Declination: -1d 57m (2000.0)
Distance: 17,600 light years
Apparent Magnitude: 6.7
Apparent Size: 14.5'
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Messier discovered this cluster in 1764, and amateurs should have a fairly easy time separating the individual stars because of their low space density. A 10-inch scope will show stars across the whole disk. English observer Admiral
Smyth noted several bright "spots" within the group, and Lord Rosse's telescope in Ireland revealed a possible "spiral arrangement" of the stars.
NGC 6354 (M10)
Constellation:
Ophiuchus
Right Ascension: 16h 57.1m (2000.0)
Declination: -4d 6m (2000.0)>
Distance: 13,400 light years
Apparent Magnitude: 6.6
Apparent Size: 15.1'
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Located about 31/2' southeast of M12, this cluster is easy to find just west of the 5th-magnitude star 30 Ophiuchi. It is about 7th magnitude and nearly the same brightness as its neighbor, yet it is more difficult to resolve because of its tighter structure. An 8-inch reflector reveals stars around the edge of M10, but the nucleus remains an indefinite blaze.
NGC 6266 (M62)
Constellation:
Ophiuchus
Right Ascension: 17h 1.2m (2000.0)
Declination: -30d 7m (2000.0)
Distance: 21,500 light years
Apparent Magnitude: 6.5
Apparent Size: 14.1'
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This cluster is located on the border of Scorpius and Ophiuchus, and its 7th-magnitude glow is visible in finderscopes. It is difficult to resolve M62 with an 8-inch telescope even though the individual stars are listed as 11th magnitude. Many observers have commented on the cluster's asymmetric appearance due to a distinctive northward bulge.
NGC 6341 (M92)
Constellation:
Hercules
Right Ascension: 17h 17.1m (2000.0)
Declination: 43d 8m (2000.0)
Distance: 26,400 light years
Apparent Magnitude: 6.4
Apparent Size: 11.2'
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This is one of the unsung glories of the heavens. It is said this cluster is often overlooked because observers favor nearby M13. What a shame! Binoculars show M92 as a fuzzy patch, while a 6-inch scope will resolve its edges. Larger apertures unravel its splendor and may reveal the dark patches that Lord Rosse saw with his gigantic 72-inch speculum-metal reflector in the 19th century.
NGC 6397
Constellation:
Ara
Right Ascension: 17h 40.7(2000.0)
Declination: -53d 40m (2000.0)
Distance: 7,200 light years
Apparent Magnitude: 5.6
Apparent Size: 25.7'
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The obscure southern constellation Ara is home to this globular, which would certainly be better known if it were not so poorly placed for viewing from the Northern Hemisphere. Resolution is possible with 6-inch and possibly even smaller telescopes. The group is estimated to be only 7,200 light-years away and thus one of the closest globulars to Earth.
NGC 6453
Constellation:
Scorpius
Right Ascension: 17h 50.51(2000.0)
Declination: -34d 35m (2000.0)
Distance: light years
Apparent Magnitude: 9.9
Apparent Size: 3.5'
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The glorious open star cluster M7 in the tail of Scorpius is best viewed with binoculars. A careful scan of the area with a telescope, however, will
reveal NGC 6453 just 3degrees, northwest of the second brightest star in M7. It was first seen by John Herschel in 1837. Only 3.5' in diameter and 10th magnitude, this cluster will be a challenge for observers using small telescopes.
NGC 6541
Constellation:
Corona Australis
Right Ascension: 18h 08.0 (2000.0)
Declination: -43d 42m (2000.0)
Distance: light years
Apparent Magnitude: 6.6
Apparent Size: 13.1'
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This cluster is another victim of poor location as seen from the Northern Hemisphere. Located in Corona Australis, NGC 6541 shines at 7th magnitude and is one of the brightest globulars in the sky. It measures 13 ' across, and some of its 13th-magnitude stars with an 8-inch reflector and moderate power can be seen easily.
NGC 6656 (M22)
Constellation:
Sagittarius
Right Ascension: 18h 36.4m (2000.0)
Declination: -23d 54m (2000.0)
Distance: 10,100 light years
Apparent Magnitude: 5.1
Apparent Size: 24.0'
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This globular ranks as one of the heaven's showpieces. Located about 2.5 degrees northeast of Lambda Sagittarii, M22 resembles M13 in both size and structure. An 8-inch scope will display many of the cluster's 11th-magnitude stars right across a fairly loose core. M22 is not perfectly round, and its east-west bulge can even be glimpsed in 20x80 binoculars. M22 probably contains more than 500,000 stars. In 1918 astronomers at Mount Wilson Observatory counted 75,000 individual star images on a photograph made with the observatory's 60-inch reflector.
NGC 6779 (M56)
Constellation:
Lyra
Right Ascension: 19h 16.6m (2000.0)
Declination: 30d 11m (2000.0)
Distance: 31,600 light years
Apparent Magnitude: 8.3
Apparent Size: 7.1'
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This cluster is located on a line between Gamma Lyrae and Beta Cygni (Albireo). It shows as a round, nebulous, 8th-magnitude glow in binoculars and finderscopes. A few individual stars are visible with a 6-inch aperture, and a 12-inch will resolve the core. The cluster was discovered by Messier in 1779 and is estimated to be about 31,000 light-years away.
NGC 6809 (M55)
Constellation:
Sagittarius
Right Ascension: 19h 40.0m (2000.0)
Declination: -30d 51m (2000.0)
Distance: 16,600 light years
Apparent Magnitude: 6.3
Apparent Size: 19.0'
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This is the least condensed globular in Messier's catalogue and also in this listing. There are,reports of it being resolved with a 3-inch telescope, but this would seem a formidable task from mid-northern latitudes. Low magnification with an 8- or 10-inch aperture separates many stars sprinkled across its nucleus. Such moderate-size telescopes give the impression that M55 is very poorly populated. But most of its stars are 17th magnitude or fainter and not visible in average amateur instruments.
NGC 6864 (M75)
Constellation:
Sagittarius
Right Ascension: 20h 6.1m (2000.0)
Declination: -21d 55m (2000.0)
Distance: 57,700 light years
Apparent Magnitude: 8.5
Apparent Size: 6.0'
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Contrasting with M55, this cluster is a very condensed class I object. It can be resolved in only the largest of backyard telescopes. Messier though he had glimpsed several of the cluster's individual stars, but this seems quite impossible. Most likely he saw the 12th-magnitude foreground stars that are unrelated to M75, but are seen superimposed on it.
NGC 7078 (M15)
Constellation:
Pegasus
Right Ascension: 21h 30.0m (2000.0)
Declination: 12d 10m (2000.0)
Distance: 32,600 light years
Apparent Magnitude: 6.2
Apparent Size: 12.3'
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This is a magnificent group located about 4' northwest of Epsilon Pegasi. Its 6.2-magnitude glow can be seen with binoculars, and a 6-inch scope should show some stars at its edges even though the cluster is tightly structured. There's a tiny, 1" diameter, 13th magnitude planetary nebula, K648, associated with the cluster, but I do not know of any amateur ever having observed this object.
NGC 7089 (M2)
Constellation:
Aquarius
Right Ascension: 21h 33.5m (2000.0)
Declination: -0d 49m (2000.0)
Distance: 36,200 light years
Apparent Magnitude: 6.5
Apparent Size: 13'
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Binoculars reveal M2 as a not-quite-stellar "star" located about 9' west of Alpha Aquarii. A 4-inch scope will show some of its stars, but a 12-inch is needed for complete resolution. Like some other globulars, it does not appear perfectly round. It is estimated to be about 37,000 light-years away and contains no fewer than 100,000 stars.
NGC 1904 (M79)
Constellation:
Lepus
Right Ascension: 5h 24.5m (2000.0)
Declination: -24d 33m (2000.0)
Distance: 41,100 light years
Apparent Magnitude: 7.7
Apparent Size: 8.7'
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This cluster is nestled in the winter constellation Lepus the Hare. It is a tight swarm of stars first noticed by Charles Messier's fellow countryman and rival comet hunter Pierre Mechain in 1780. Neither of the Frenchmen recognized it as a star cluser however, only as a diffuse glow. A modern 6- to 8-inch telescope will show a mottled surface, as if on the verge of resolution. A 10-inch aperture should show some of the 14th-magnitude stars.
NGC 2419
Constellation:
Lynx
Right Ascension: 7h 38.1m (2000.0)
Declination: +38d 53m (2000.0)
Distance: 300,000 light years
Apparent Magnitude: 10.4
Apparent Size: 4.1'
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Visually, this object in Lynx would seem a poor choice to be included in a list of the "finest" globulars. Due to a lack of nearby bright stars, NGC 2419 can prove difficult to find. Furthermore, once located it shines dimly with a total brightness equal only to the light of a single 10th-magnitude star.
Even the largest backyard telescopes show it as little more than an undefinable glow. Why then is it interesting? When we look at NGC 2419 our gaze is traveling across some 300,000 light-years, well beyond the limits of most other globulars. Indeed, NGC 2419 lies farther away than the Magellanic Clouds, and it has thus been dubbed the "intergalactic wanderer."
NGC 4590 (M68)
Constellation:
Hydra
Right Ascension: 12h 39.5m (2000.0)
Declination: -26d 45m (2000.0)
Distance: 32,300 light years
Apparent Magnitude: 7.8
Apparent Size: 12.0'
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In the spring sky M68 can be found riding on the back of Hydra, but the brightest stars in the area belong to Corvus. Once located, the cluster can be partially resolved with a 6-inch telescope. Studies suggest that M68 contains over 100,000 stars and is about 31,000 light-years distant.
NGC 5024 (M53)
Constellation:
Coma Berenices
Right Ascension: 13h 12.9m (2000.0)
Declination: 18d 10m (2000.0)
Distance: 56,400 light years
Apparent Magnitude: 7.6
Apparent Size: 12.6'
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This globular lies amid the intergalactic forest in the springtime sky. It was discovered in 1775 by German astronomer Johann Bode who described it as "round and pretty lively." A 6-inch telescope will begin to resolve stars at the cluster's edges, though none of the stars is brighter than 12th magnitude. While in this area try looking for the fainter, 10th-magnitude globular NGC 5053 just to the southeast. It is about the same apparent size as M53.
NGC 5272 (M3)
Constellation:
Canes Venatici
Right Ascension: 13h 42.2m (2000.0)
Declination: 28d 23m (2000.0)
Distance: 30,600 light years
Apparent Magnitude: 6.2
Apparent Size: 16.2'
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Perhaps the finest globular in the northern spring sky, M3 was discovered by Messier in 1764, who only saw it as "nebula without star." Today's 6-inch scopes will reveal a moderately compressed nucleus surrounded by hundreds of stars. A 12-inch will show stars across its entire face. Many observers, including the father-son team of William and John Herschel, have noted star chains formed by the cluster's outer suns.
NGC 5904 (M5)
Constellation:
Serpens
Right Ascension: 15h 18.6m (2000.0)
Declination: 2d 05m (2000.0)
Distance: 22,800 light years
Apparent Magnitude: 5.6
Apparent Size: 17.4'
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This is another fine object for amateur telescopes, in the same class as the Hercules cluster M13. On a clear dark night, M5 can be seen with the naked eye about 8' southwest of Alpha Serpentis. A 4-inch instrument, and possibly even a 3-inch, will reveal M5's true nature.
NGC 5139 (Omega Centauri)
Constellation:
Centaurus
Right Ascension: 13h 26.8m (2000.0)
Declination: -47d 29m (2000.0)
Distance: 17,000 light years
Apparent Magnitude: 3.7
Apparent Size: 36.3'
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This is without question the grandest globular in our skies and deserves mention here even if it can't be viewed from mid-northern latitudes. Like NGC 104, it is visible as a 4th-magnitude "star" to the unaided eye and was included in Ptolemy's star catalogue over 1,800 years ago. During the 17th century the cluster received a star designation, Omega. In 1677 Edmond Halley first recognized that it was a cluster.
Unfortunately, Omega Centauri never sits very high above the southern horizon. From latitude 40degrees the cluster culminates at less than 3". Some of its stars could even be seen in 11 x 80 binoculars, and several were suspected in 7 x 50's. The cluster is also distinctly oval. Like most galactic globular clusters, the stellar population of Omega Centauri identifies it as one of the oldest objects associated with the Milky Way, indeed its age is comparable to that of the Universe itself. The cluster contains a large number of RR Lyrae variable stars which enable its distance to be determined as 17,000 light years.
Globulars are among the finest celestial sights. It is always interesting to compare their appearance in telescopes of different apertures. What is the smallest aperture that will resolve M13, or show NGC 6453? Can you spot the tenuous star chains of M3, or the central ridge of M4? Exploring these massive star systems will provide hours of enjoyment throughout the year and memories to last a lifetime.
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