M5 is a globular star cluster (GC) located about 24.5 kly away in the halo region of the Milky Way.  It has a mass of about 860,000 suns [1].  It's 100,000's of stars are gravitationally bound together, all receding from us at 54 km/sec, while simultaneously spinning about M5's rotation axis at a peak speed of about 3 km/sec, which slightly flattens its shape perpendicular to the rotation axis [2].

Many GCs contain a primordial population of stars such as red giants formed when the Milky Way was first created.  GCs also contain a second population of stars that may be explained by collisions or mergers between the primordial stars due to the very high density of stars found in GCs.  Some of those second population stars are known as "collisional blue stragglers" [3].  

GCs may be used to study the history of galactic formation.  Computer models present the formation of GCs in the dense gas clouds in the disks of early galaxies similar to ongoing open star cluster formation that is seen today.  Gravity tides from galactic arms and gas clouds, star deaths, and close star flybys move stars outwards causing the GC to loose stars as they age.  As early galaxies merged to form larger ones, the GCs are thrown out of the galactic disks they formed in and into the halo that surrounds the disk of the resulting larger merged galaxy.  When a GC moves into the far less dense galactic halo region, the rate of GC star loss slows down preserving the larger GCs as seen today [4].  Stellar chemistry and evolution help provide age information for GCs and of the other stars in their progenitor galaxy, which in-turn provides the history of galaxy mergers in the Milky Way [5][6].

[1] Boyles, J.; et al. (Nov. 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal, 742
[2] Lanzoni, B.; et al. (Jul. 2018), "The Strong Rotation of M5 (NGC 5904) as Seen from the MIKiS Survey of Galactic Globular Clusters", The Astrophysical Journal, 861
[3] Kravtsov, V.; et al. (2022), "Stellar collisions in globular clusters: the origin of multiple stellar populations",  MNRAS arXiv:2203.02893v2
[4] Kruijssen, J.; et al. (Jul. 2019), "The E-MOSAICS project: tracing galaxy formation and assembly with the age-metallicity distribution of globular clusters", Monthly Notices of the Royal Astronomical Society, 486, 3 
[5] Kruijssen, J.; et al. (Jul. 2019), "The formation and assembly history of the Milky Way revealed by its globular cluster population", Monthly Notices of the Royal Astronomical Society, 486, 3 
[6] Kruijssen, J. (Jul. 2021), "Relics of a Distant Past", Sky & Telescope, p.14