We believe that our Galaxy consists of four main components: the disk, the central bulge, a spherical halo, and an outer halo.
The disk is a thin flattened system containing stars of many different types and ages, gas (predominantly hydrogen), and dust, orbiting around the galactic centre. The disk has a spiral structure, with our Solar System lying in one of the spiral arms.
The galactic bulge in the central part of the disk has an ellipsoidal shape and contains mainly old stars. It is thought that a massive black-hole lies at its centre.
The disk and the bulge are surrounded by a spherical halo of old stars, about 140 globular clusters (large groups of old stars with a common origin) and a few small and faint satellite dwarf galaxies.
The entire system is thought to be embedded in a massive halo of dark matter of unknown composition and poorly known spatial distribution.