Globular Cluster Terzan 5 (Terzan 5) is a globular cluster in the Milky Way Galaxy. It was discovered in 1968 by French astronomer Paul Emile Victor Théodore Gérard. Terzan 5 is located in the constellation Sagittarius, about 18,000 light-years from Earth.
With an apparent magnitude of 11.6, Terzan 5 is one of the brightest globular clusters visible to amateur astronomers. It has a core radius of 3.2 arcminutes and a tidal radius of 53 arcminutes. The cluster contains over 200,000 stars and has a mass of approximately 2×10^6 solar masses.
Terzan 5 is unusual in that it contains two distinct populations of stars: a metal-rich population with [Fe/H] = +0.4 and a metal-poor population with [Fe/H] = -1.8. This dual metallicity structure is thought to be due to the accretion of another globular cluster or dwarf galaxy by Terzan 5 in its early history.
The age of Terzan 5 is estimated to be 12 billion years, making it one of the oldest known globular clusters in our Galaxy. Its large age and dual metallicity structure make it an important object for studies of stellar evolution and chemical enrichment in galaxies like our own Milky Way.