Globular star clusters have always been among my favorite deep-sky objects. These wondrous swarms of ancient suns are impressive sights in almost any telescope.
Because globulars are more numerous in the direction of the Milky Way's center, located in Sagittarius, summer nights (in the Northern Hemisphere; winter nights in the Southern) are the best time for globular hunting. In fact, these clusters are so plentiful in this part of the sky that one can view several at once! Let's pay a visit to a few close couples.
The Scorpion's Heart
Our first stop is in Scorpius, where we find the pair of M4 and NGC 6144 separated by almost exactly 1°. The huge, loose cluster M4 is only 1.3° west of brilliant Antares. Despite this star's glare, Walter Scott Houston was able to discern M4 without optical aid while visiting Central America. At a distance of only 7,000 light-years, it is probably the nearest of all globular clusters and therefore one of the easiest to resolve — even a small telescope will show a few of the cluster's individual stars. Look for a string of 11th-magnitude stars forming a striking bar across the center of the cluster. This feature is most prominent in an 8-inch telescope, and in larger instruments it appears enmeshed in a multitude of fainter stars. As with almost any cluster, the longer one stares, the more patterns seem to emerge. While observing with a 17½-inch Dobsonian reflector under the dark skies of the Texas Star Party, I noted many curved lines of stars around M4's margins and a Y-shaped group northeast of the central bar.
Hiding between M4 and Antares is the diminutive 9th-magnitude globular NGC 6144. Clean optics, which minimize scattered light from nearby Antares, help when viewing this globular. Even if you haven't seen it with a telescope, you're probably familiar with pictures of it. NGC 6144 appears in most photographs that show the rich and colorful complex of bright and dark nebulae that begins on the cluster's northern edge and extends northward to the multiple star Rho (ρ) Ophiuchi. My only view of NGC 6144's peppering of 14th-magnitude stars was with a 24-inch Cassegrain reflector in Prince George, British Columbia, at latitude 54° north. As it is always twilight from that latitude when Scorpius culminates, the view I had could probably be easily matched by a 10-inch telescope at a more southerly location.
Three in One
Moving north and west into Ophiuchus, we come across a trio of globulars: M9, NGC 6342, and NGC 6356. As Sky & Telescope associate editor Gary Seronik comments, "These are three of my favorite globulars because I can view all of them with my 8-inch f/6 Newtonian reflector and a low-power eyepiece. To the best of my knowledge, these are the only three globulars close enough together to fit into a single field with this scope."
My own Meade 8-inch f/6 Newtonian could not resolve M9 at 174x
on an excellent night, though it did have a granular appearance. While its distance is comparable to the well-known globular M13 in Hercules, the stars of M9 are
considerably dimmed by dust in the plane of the Milky Way — some of which can be seen as the ¾°-long dark nebula Barnard 64. This nebulosity wraps around the western side of the cluster, extending from ½° northwest of
M9 southward to the cluster's western edge before it broadens just south of the globular.
A little more than a degree south-southeast of M9 is NGC 6342.
At 116x with my 8-inch scope this small 10th-magnitude cluster showed a slight brightening toward its center. I was able to find it with my Astroscan, a 4¼-inch wide-field reflector telescope, using only 34x.
The third member of this trio, NGC 6356, is 1.3° northeast of M9. It's a condensed object with a bright center. The distance to NGC 6356 is estimated at 48,000 light-years, which makes it the most distant object on our tour and puts it about 20,000 light-years farther away than its two neighbors. Yet it can be viewed with a small telescope! As a youth, I saw it with my 2.4-inch refractor, guided by a Walter Scott Houston column in the July 1962 issue of Sky & Telescope. At that time Houston's column primarily featured clusters, galaxies, and nebulae from the Messier catalog; NGC objects were a rare exception. I still remember thinking that this was pretty hot stuff — finding an NGC globular!
In and Around the Teapot
About ½° northwest of the star marking the tip of the Sagittarius Teapot's spout, Gamma (γ) Sagittarii, we find an exceptionally close pair of globulars: NGC 6522 and NGC 6528. They are separated by a mere 16 arcminutes — close enough to be seen together in the field of a medium-power eyepiece. NGC 6528 was barely visible at 64x through my Astroscan, but NGC 6522 was an easy catch. Neither cluster showed any resolution at 100x in the 17½-inch scope at the Texas Star Party.
There are perhaps only 1,000 light-years separating this pair of clusters — they not only appear close together in the sky; they may actually be close neighbors in space. It has been said that the sky of a planet located inside a globular cluster would be filled with so many
brilliant stars that there would be no true night and perhaps nothing known of the universe beyond the cluster itself. However, I can't help but think that the great luminous globe of NGC 6522 must surely be prominent for any inhabitants within NGC 6528.
Finishing off our tour, we come to one of the night sky's true showpieces. For most Northern Hemisphere observers, M22 is the most spectacular globular visible, being both larger and brighter than the better-known M13 — little wonder, considering it is only half as far away! This glorious cluster is easily resolved. Even my trusty 2.4-inch refractor showed a grainy texture at 117x during superb seeing. Moderate apertures produce unforgettable views of swarms of stars arrayed across the elliptically shaped cluster's glowing core. At 174x my 16-inch Meade Newtonian revealed two chevron-shaped dark lanes pointing south. Does anyone else see these patterns?
M22 is accompanied by 9th-magnitude NGC 6642, 1.1° to the west-northwest. Given the spectacular appearance of M22, it's little wonder that NGC 6642 receives so little attention. Halifax Royal Astronomical Society of Canada member Daryl Dewolfe observed this cluster with his 5.7-inch Ceravolo Maksutov-Newtonian, noting, "At 83x there appeared to be a faint chain of stars leading to the globular."
Although we have reached the end of our tour, there are plenty of other pairs and chains of globulars in this region. In fact, M22 and NGC 6642 represent just one side of a compact trapezium of globulars that includes M28 and NGC 6638. If you find yourself under a moonless sky, perhaps at a star party, locate a spot with a good southern horizon and enjoy an evening with some of summer's clusters of globulars!
|Name||Const.||R.A. (2000.0)||Dec. (2000.0)||Diam.||Mag.||Concen.|
|NGC 6144||Sco||16h 27.3m||-26° 02'||17'||9.0||II|
|M9||Oph||17h 19.2m||-18° 31'||11'||7.8||VIII|
|NGC 6342||Oph||17h 21.2m||-19° 35'||5'||9.5||IV|
|NGC 6356||Oph||17h 23.6m||-17° 49'||8'||8.2||II|
|NGC 6522||Sgr||18h 03.6m||-30° 02'||7'||9.9||VIII|
|NGC 6528||Sgr||18h 04.8m||-30° 03'||5'||9.6||VIII|
|MGC 6642||Sgr||18h 31.9m||-23° 29'||9'||8.9||V?|
|M22||Sgr||18h 36.4m||-23° 54'||33'||5.2||VIII|
Note: the diameter (Diam.) and magnitude (Mag.) must be considered together when judging the difficulty of detecting any type of deep-sky object, but with globular clusters there is also a third important parameter — the Shapley-Sawyer concentration class (Concen.). Class I globulars are the most concentrated and have a high surface brightness, while Class XII are low-surface-brightness loose clusters.