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…continued

Clusters of Clusters: Globular Pairings
by Alan Whitman

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In and Around the Teapot

Sagittarius Teapot
The Sagittarius Milky Way region contains globular clusters by the score. Indeed, seven of the 29 globulars listed in the Messier catalog are found within this constellation's borders. Click on the diagram to see a detailed view.
Image courtesy Dennis di Cicco. Sky & Telescope illustration.
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

NGC 6522 and 6528
NGC 6528 and NGC 6522 form one of the sky's closest pairings of globular clusters. Located near the 3.6-magnitude star gamma Sagittarii, they are separated by only 16'. Click on the image for a closer look at the two clusters.
Courtesy Digital Sky Survey.
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."

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