The Milky Way

Our Home Galaxy

  • The Milky Way (MW) is the name given to the faint band of light visible in the night sky.
  • This light is the sum of billions of stars comprising our home galaxy.
  • Today, we understand that the Milky Way is a flattened, rotating disk of stars, gas and dust, about 100,000 light years in diameter. The density of stars increases towards the center of the Galaxy.
  • The Milky Way appears as a band of light because stars are concentrated within the flattened disk. Since the solar system is also located within the disk, from our perspective more stars are visible looking through the disk then are seen looking away from the disk.
  • The nature and size of the Galaxy, as well as our location within this stellar system were finally appreciated in the early 20th century when:

The locations of the globular clusters were mapped out, indicating that the galactic center was located thousands of light years distant, towards the constellation Sagittarius

Distances to globular clusters and the Andromeda nebula were determined using the newly discovered period-luminosity relation for certain variable stars. The derived distances proved that the Milky Way was an “island universe” of stars, similar to other nebulae seen all around the sky.

  • The MW is classified as a spiral galaxy, containing about 200billion stars.
  • Our solar system is located about 24,000 light years (~8 kiloparsecs) from the center – NOT at the center of the Galaxy.
  • Our Galaxy can be subdivided into several major physical components:

Disk – a thin (a few thousand light years) layer containing stars, most of the interstellar gas, and dust, rotating around the central regions. Star formation occurs in the disk, where stars of allages can be found.

Spiral arms – spiral-shaped regions within the disk in which star formation, O and B stars, gas, and dust clouds are especially concentrated.

Bulge – a slightly flattened, spherical region surrounding the galactic nucleus, containing mostly old, cool, red stars. No star formation occurs in the bulge.

Nucleus – the inner few hundred light years of the Galaxy, where the star density peaks, possibly harboring a massive black hole

Halo – a spherical region encompassing the entire Galaxy, comprised of very old stars, probably the oldest stars in the Galaxy

Globular Cluster System – about 150 old star clusters, each containing up to a million stars. These clusters orbit around the center of the galaxy.

  • The Galaxy began forming about 10-12 billion years, within a few billion years of the Big Bang. The bulge and globular clusters formed first, followed by the disk. Star formation continues within the disk to the present day.