A star cluster is a large group of stars that share a common origin, having formed from the same giant molecular cloud of gas and dust. These stellar groups remain gravitationally bound as they travel together through a galaxy. Astronomers classify these associations into two primary types based on their structural, compositional, and evolutionary differences.
Open Clusters: Young and Loose Associations
Open clusters are the younger and less dense of the two major cluster types, typically containing a few hundred to a few thousand stars. They are characterized by a loose, often irregular structure, which gives them a somewhat transparent appearance. The stars within these clusters are only weakly bound by gravity, meaning they tend to disperse over astronomical time scales.
Most open clusters are relatively young, typically less than a few hundred million years old. They are found exclusively within the thin, rotating disk and spiral arms of a galaxy, which are regions rich in the gas and dust necessary for new star formation. Because of their youth and location, the member stars are considered metal-rich, classifying them as Population I stars. Over time, the gravitational pull of the galactic disk and encounters with giant molecular clouds cause the clusters to slowly dissipate, scattering their stars into the main body of the galaxy.
Globular Clusters: Ancient, Tightly Bound Systems
Globular clusters are densely packed, massive, and ancient systems, often containing tens of thousands to over a million stars. The name “globular” refers to their appearance, as the strong mutual gravitational attraction forces them into a highly concentrated, symmetrical spherical shape. This immense density results in stars being much closer together in the core than in the solar neighborhood.
These clusters are among the oldest structures in the universe, with ages often measured in billions of years. Globular clusters are primarily located in the galactic halo, forming a vast, spherical cloud that surrounds the main disk and central bulge of the galaxy. Their isolation from the disruptive gravitational forces within the galactic disk contributes to their extreme longevity and stability.
Distinctions in Formation and Galactic Placement
The fundamental differences in appearance and population stem from the distinct conditions under which each cluster type formed. Open clusters form from the metal-rich gas clouds currently cycling within the galaxy’s spiral arms. This active formation environment supplies the raw material for new, chemically rich stars.
Globular clusters are thought to have formed very early in the universe, before the galaxy had fully condensed into a flattened disk structure. They originated from gas that had not yet been enriched by generations of supernovae, making their stars metal-poor, a characteristic of Population II stars. This chemical difference is a key indicator of their age and formation environment.
The location of the clusters also dictates their eventual fate and orbital paths. Open clusters follow relatively circular orbits, remaining confined to the plane of the disk where they are constantly subjected to tidal forces that pull them apart. Conversely, globular clusters travel in highly elliptical, sometimes retrograde, orbits that plunge them through the galactic halo and bulge, often taking them far outside the main plane of the galaxy. Their tremendous mass and isolated positions allow them to remain gravitationally cohesive for nearly the entire lifespan of the galaxy.