M4

"Messier 4 or M4 (also known as NGC 6121 or the Spider Globular Cluster) is a globular cluster in the constellation of Scorpius.It was the first globular cluster in which individual stars were resolved. It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright star Antares.

M4 is approximately 6,000 light-years away, making it the closest globular cluster to the Solar System. It has an estimated age of 12.2 billion years.

Measuring the metallicity strongly suggests this cluster hosts two distinct stellar populations, differing by age. Thus the cluster probably saw two main cycles or phases of star formation." (Wikipedia)