Messier 25 — Open Cluster in Sagittarius

M25 is a bright, scattered open star cluster in the constellation Sagittarius, located about 2,000 light-years from Earth. It contains roughly 86 stars spread across about 19 light-years of space. At magnitude 4.6 with an apparent diameter of about 32 arcminutes, it is a conspicuous naked-eye object from dark sites. The cluster is estimated to be about 90 million years old. M25 contains the classical Cepheid variable star U Sagittarii, which varies between magnitudes 6.3 and 7.1 over a period of 6.745 days.

Located about 3.5 degrees east-northeast of M24. At magnitude 4.6, it is visible to the naked eye under dark skies as a hazy patch. Binoculars show a beautiful scattered group of bright stars. A telescope at 25-50x gives the best view, framing the cluster with its bright blue-white and yellow-orange stars. Look for the Cepheid variable U Sagittarii — one of the brighter cluster members that noticeably changes brightness over about a week. Best observed from July through September.

Discovered by Philippe Loys de Cheseaux before 1745. Charles Messier cataloged it in 1764. Interestingly, M25 was accidentally omitted from the NGC catalog, so it has no NGC number — a rare distinction for a Messier object. The Cepheid variable U Sagittarii was discovered in the cluster by J. Schmidt in 1866.