Galactic Disk, Bulge & Halo: Structures & Vocabulary

The Milky Way is not a uniform swarm of stars. It contains a thin star-forming disk, a thicker older stellar disk, a central bar and bulge, and an extended stellar and dark-matter halo. These components differ in shape, rotation, age, metallicity, and gas content.

Different components reflect different formation histories. Dissipative gas settles into a rotating disk, while mergers and dynamical heating can build thicker and more spheroidal populations. The bar redistributes angular momentum, and the dark halo provides much of the gravitational potential at large radii.

Star counts, distances, proper motions, spectroscopy, gas mapping, and globular-cluster orbits reveal distinct spatial and kinematic populations. Infrared observations penetrate dust toward the center, while Gaia maps stellar motions across the disk and halo.

Disk, bulge, and halo are distinguished by geometry, age, metallicity, gas content, and stellar motion. The thin disk rotates rapidly and contains cold gas; halo stars occupy hotter, more eccentric orbits.