Clusters of Clusters: Globular Pairings
Globulars provide rich pickings for deep-sky hunters
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.