Galactic Disk, Bulge & Halo: Physical Mechanisms

The thin disk contains most of the Galaxy's cold gas, dust, spiral structure, and young stars. The thick disk is vertically broader and generally older. The bulge is a dense central stellar component associated with the Galactic bar, while the halo contains old stars, globular clusters, stellar streams, and most of the Galaxy's dark matter.

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.

Gas dissipates energy and settles into rotating disks, while mergers and dynamical heating build thicker or spheroidal populations. Bars redistribute angular momentum through the inner Galaxy.