Messier 4 — Globular Cluster in Scorpius

M4 is the closest globular cluster to Earth, at a distance of only about 7,200 light-years. Located in the constellation Scorpius, it lies just 1.3 degrees west of the brilliant red star Antares. M4 contains roughly 100,000 stars and spans about 75 light-years across. Unlike many globulars, it has a relatively loose structure with a concentration class IX. A distinctive feature is a prominent bar-like structure of stars running through its core, visible in small telescopes.

Very easy to find, sitting just 1.3 degrees west of Antares. At magnitude 5.6, it is visible to the naked eye from dark sites. Binoculars show a large, bright, somewhat loose glow. Even a small 3-inch telescope resolves individual stars across the cluster. The characteristic central bar of 11th-magnitude stars is visible in 4-inch or larger telescopes at around 100x. Best observed from June through August when Scorpius rides high. From mid-northern latitudes it stays rather low, so a clear southern horizon helps.

Discovered by Philippe Loys de Cheseaux in 1746 and independently cataloged by Charles Messier in 1764. It was one of the first globular clusters in which individual stars were resolved. In 1987, a millisecond pulsar was discovered in M4 — spinning over 300 times per second.