Disk, bulge, bar, and halo structure
The Milky Way is a barred spiral galaxy roughly 100,000 light-years in diameter. Its prominent disk contains spiral arms where star formation concentrates; the disk is thin and dense near the center, flaring to lower density at the outer edges. At the galactic center sits a bulge of old stars and a supermassive black hole (Sagittarius A*, four million solar masses). A central bar of stars connects the inner bulge to the spiral arms, a feature detected through infrared observations penetrating the dust.
Enclosing the disk is a spherical halo of dark matter and globular clusters (dense star clusters containing millions of ancient stars). This halo extends 300,000+ light-years from the galactic center, dwarfing the visible disk and containing the majority of the Milky Way's mass.
Our location in the galactic structure
The Solar System orbits 26,000 light-years from the galactic center, in the Orion Arm (or Local Spur), a minor spiral arm between the larger Perseus and Sagittarius arms. This outer location was fortunate: the radiation environment is less intense, the density of stellar close approaches is lower, and the time available for complex chemistry is longer. Had Earth formed in the galactic bulge, intense radiation and frequent stellar encounters would likely have sterilized any biosphere.
Our orbital period (one galactic year) is roughly 230-250 million years. Since the Sun formed 4.6 billion years ago, it has completed fewer than 20 orbits around the galaxy.