The invisible, non-luminous components of the Milky Way are often collectively described as the "dark matter halo" (sometimes "Dark Halo)", since they are instances of the poorly understood phenomenon of dark matter. The Dark Halo is described as an enormous spheroid with a complex internal structure, far larger and more massive than the bright spiral that it encloses (Freeman & Bland-Hawthorn 2002). It appears to contain relatively familiar objects, including black holes and white dwarf stars, as well as long, intricate filaments of novel forms of matter, which are characterized as "non-baryonic" to distinguish them from ordinary "baryonic" matter.

The spiral Disk of the Milky Way has an overall diameter of about 30 kiloparsecs (100,000 light years), although associated hydrogen clouds extend much farther (Levine et al. 2006). The Disk's thickness is substantially less, ranging from 300 to 600 parsecs (1,000 to 2,000 light years). The Galactic Core bulges above and below the plane of disk rotation for a thickness of more than 1 kiloparsec (3,260 light years). Recent investigations establish that the central Bulge is elongated, as in a barred spiral galaxy, rather than circular, as in a classic spiral (Benjamin et al. 2005). The length of the bar is probably between 8 and 10 kiloparsecs (26,000-32,000 light years). Two major spiral arms extend from the ends of the bar. Sol, our sun, is located about 8.5 kiloparsecs (28,000 light years) from the Galactic Center along the edge of a secondary formation known as the Orion Arm or Orion Spur.

In the neighborhood of our Solar System, stars are separated by an average distance of 1.5 to 3 parsecs (5 to 10 light years). In the Trapezium Cluster, one of the densest star-forming regions of the Orion Arm, a cubic light year of space contains about 700 stars (Lada & Lada 2003). The core of the Milky Way is orders of magnitude more crowded, such that a single cubic parsec can contain hundreds of thousands of stars.

The gravitational center of the Galaxy is occupied by a compact object known as Sagittarius A*, so named because it is located in the direction of the constellation Sagittarius. This object is widely believed to be a supermassive black hole (Melia & Falcke 2001) that is churning its surrounding population of stars in a maelstrom of destruction. (See, e.g., this Chandra X-Ray Observatory image with accompanying explanation; see also Professor Barbara Ryden's online lecture notes.) Along with the perils of the gravitational whirlpool around this black hole, the storm of stellar radiation in the Galactic Core would be inimical to any known life forms or technologies. Most other spiral galaxies – perhaps all of them – contain a similar central black hole, suggesting that these gravitational singularities may be the mechanism through which spiral structures are created (King 2003).

All text is copyright Raymond Harris 2006-2008. Image credits appear in the accompanying caption.