Sketch by Michael Vlasov
M4 lies about 7,200 light years away, and is one of the closest globular clusters. It displays a bar-like structure that runs through its center. The bar consists of 11th magnitude stars, is 2.5′ long and runs at a position angle of 12°. The cluster is obscured by interstellar matter, and so is dimmed greatly. Deep photography reveals a diameter of 36, equating to 75 light years, whereas its visual diameter has been estimated at 14′. At Class IX, it is one of the most open of globular clusters, with a half-mass radius of 3.65′ or 8 light years.
M4 was discovered by De Chéseaux in 1745-46 and subsequently catalogued by Messier in 1764 who resolved it into stars. It was the only one he could resolve, calling it a “cluster of very small stars”, and so was the first globular ever resolved into stars.