Globular star clusters have always been the delight of amateur and professional astronomers alike. These wondrous swarms of ancient stars are impressive sights in almost any telescope. Because globulars are more numerous in the direction of the Milky Way’s center, located towards Sagittarius and Scorpius, summer nights are the best time for globular hunting. Let us pay a visit to one!
The huge, loose cluster M4 is only 1.3° west of brilliant Antares, the fiery orange heart of the Scorpion. The “M” stands for Charles Messier, and anything that made this 18th-century observer’s list is an easy target for newcomers to astronomy. All the telescopes Messier used were quite small; not even one had as much light grasp as a good 4-inch telescope of today.
Such an instrument at 70x will show many of M4’s individual stars, some of which seem to be arranged in a central bar that runs nearly north-south. 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 globular cluster, the longer one stares, the more patterns seem to emerge.
At nearly 7,000 light years away, M4 is considered by some authorities to be the closest globular cluster to Earth. (Others believe that NGC 6397 in Ara is slightly closer.) At this distance, the cluster’s apparent size of 26 arcminutes corresponds to a diameter of about 40 light years. M4 lies toward the Galactic center, within roughly 2,000 light years of the Galactic central plane, so that interstellar material in the disk of our Galaxy blocks out some of its light and makes it dimmer (by a few magnitudes) than it would otherwise appear.
Astronomers classify M4 as a loose globular cluster of class IX. Globulars fall into classifications designated I to XII. A globular with a classification of I has the highest stellar density at its core. XII represents a homogenous globular with no increase in star concentration toward the center.
Finder map – field width 15°, stars to magnitude +8.5.