Messier 28 — Globular Cluster in Sagittarius

M28 is a globular cluster in the constellation Sagittarius, located about 17,900 light-years from Earth. It spans roughly 60 light-years in diameter and contains several hundred thousand stars. The cluster has a concentration class IV, meaning it is moderately concentrated toward its center. M28 is notable for being one of the first globular clusters in which a millisecond pulsar was discovered — PSR B1821-24, spinning at 327 rotations per second. The cluster is estimated to be about 12 billion years old and lies near the galactic center, meaning its light passes through considerable interstellar dust.

Located about 1 degree northwest of Lambda Sagittarii (Kaus Borealis), the top star of the Teapot asterism. In binoculars, M28 appears as a small, fuzzy ball of light. A 4-inch telescope at 100x shows a grainy, unresolved glow with a brighter core. An 8-inch telescope begins to resolve individual stars at the cluster's edges, though the core remains dense and unresolved. Its proximity to the spectacular M22 means M28 is often passed over, but it rewards patient observation. Best viewed from July through September when Sagittarius is highest.

Discovered by Charles Messier on July 27, 1764. William Herschel first resolved it into stars in 1786. In 1987, M28 became one of the first globular clusters in which a millisecond pulsar was detected, making it significant for high-energy astrophysics. The cluster has since been found to host at least 12 pulsars.