Globular clusters are collections of hundreds of thousands to over a million stars, gravitationally bound into compact, nearly spherical systems. They represent some of the oldest and most densely packed stellar populations in the Milky Way, having formed shortly after the universe began. Studying these ancient star systems helps astronomers reconstruct the Milky Way’s formation and evolution.
Defining Globular Clusters
Globular clusters are distinguished by their extreme age, typically ranging between 10 and 13 billion years. They formed early in the cosmos, before interstellar gas was heavily enriched with heavier elements. Consequently, the stars within these clusters exhibit low metallicity, containing far less of the elements heavier than hydrogen and helium compared to younger stars like the Sun.
These stellar populations are densely concentrated, reaching up to a million members within a few dozen light-years. The stellar density in the core can be hundreds to a thousand times higher than the density of stars surrounding the Sun. The stars are tightly held by collective gravity, maintaining a spheroidal shape. Unlike younger open clusters in the galactic disk, globular clusters contain no significant gas or dust.
The Galactic Distribution of Clusters
The majority of the Milky Way’s more than 150 known globular clusters are found in the Galactic Halo, a large, diffuse, spherical volume surrounding the disk of the galaxy. This distribution is spheroidal and centered on the galactic core, not confined to the flat plane where the spiral arms reside. The clusters in the halo orbit the galaxy’s center on highly elongated paths, sometimes extending far beyond the visible disk.
A second, smaller concentration of clusters is found within the Galactic Bulge, the dense central region of the Milky Way. These clusters are often difficult to observe because thick dust and gas within the galactic plane obscure the view toward the center. Globular clusters are absent from the Galactic Disk itself, the region containing the galaxy’s spiral arms and the majority of its young stars and gas clouds.
The distribution of these star systems allowed astronomers to determine the true structure of our galaxy. In 1918, Harlow Shapley mapped the positions of globular clusters and noticed their asymmetrical arrangement. He reasoned that if these clusters formed a spherical system around the galaxy’s center, the center must be located far from the Sun, toward the constellation Sagittarius. This discovery placed the Sun about two-thirds of the way out from the core, fundamentally changing the understanding of the Milky Way’s scale.
Formation and Origins
The two primary locations of globular clusters—the diffuse halo and the central bulge—reflect two distinct formation pathways. The oldest and most metal-poor clusters are linked to the formation of the Galactic Halo itself. These clusters are thought to have formed in situ, arising during the initial, rapid collapse of the primordial gas cloud that eventually became the Milky Way.
A competing theory suggests that many clusters were accreted from smaller systems, supported by orbital and chemical evidence. In this scenario, numerous smaller dwarf galaxies orbiting the Milky Way were gravitationally shredded and absorbed. The globular clusters belonging to those dwarf galaxies survived the merger and became integrated into the Milky Way’s halo.
The chemical composition of globular clusters provides evidence for these dual origins. Clusters with the lowest metallicity are found far out in the halo and possess distinct orbital paths, consistent with them being remnants of cannibalized satellite galaxies. Conversely, a population of more metal-rich clusters, often concentrated toward the bulge, is associated with the material that formed the Milky Way’s disk, suggesting they formed within the main body of the galaxy. These ancient star systems are the surviving building blocks of the Milky Way.