A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.
The first known globular cluster, now called M22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer.However, given the small aperture of early telescopes, individual stars within a globular cluster were not resolved until Charles Messier observed M4 in 1764. (To learn more, click here).
Credits: NASA, ESA, STScI and A. Sarajedini (University of Florida)