A Globular Cluster (GC) is a group of stars, typically in the shape of a sphere, that is bound to a Galaxy and orbits its center. They may be found above or below the Galactic Plane, but within the galaxy's presumed dark-matter halo.
Globular clusters and Open Clusters are generally thought to be stars formed over a short period of time, making the stars approximately the same age (Coeval). However, clusters have shown a very slight division in the plotted Main-Sequence line on globular cluster H-R Diagrams, and the observation of spectral signs of Aluminum and Sodium suggest a second generation of stars as these are elements likely to be synthesized by short-lived first-generation stars (O-Type Star and A-Type Star) in their giant phases.
Current aging methods (e.g., using the Turn-off Point) of Milky Way-globular clusters show them to be quite old, some on the order of 13 billion years, with ~10 billion years common. Examples of younger globular clusters can be found in the Magellanic Clouds.
Color-Magnitude Diagram (CMD)
Velocity Dispersion (σ)
Globular Cluster Luminosity Function (GCLF)
H-R Diagram (HRD)
Palomar Globular Clusters (Pal)
Sagittarius Dwarf Elliptical Galaxy (Sgr dE)
Star Formation History (SFH)
Star Formation Rate (SFR)
Stellar Age Determination
Turn-off Point (TO)