Introduction to Galactic Astronomy

Introduction to Galactic Astronomy
Key concepts, terminology, and foundational ideas for studying our Milky Way and other galaxies.

1. What Is Galactic Astronomy?

• Definition: The branch of astronomy that investigates the structure, composition, dynamics, and evolution of galaxies—especially the Milky Way.
• Scope:
  • Stellar populations and star clusters
  • Interstellar medium (gas, dust, magnetic fields)
  • Galactic dynamics and dark matter
  • Galactic formation history and future evolution

2. The Milky Way – Our Home Galaxy

3. Basic Galactic Structure

1. Central Bulge / Bar

   • Dense, spheroidal region; contains older, metal‑rich stars.
   • The bar (≈3 kpc) drives gas inflow toward the center, fueling star formation and the central supermassive black hole.

2. Thin Disk

   • Height ≈300 pc, radius ≈15 kpc.
   • Hosts most of the Galaxy’s gas, young massive stars, and open clusters.
   • Exhibits differential rotation (≈220 km s⁻¹ at the Sun’s radius).

3. Thick Disk

   • Height ≈1 kpc, older, more metal‑poor stars.
   • Thought to be formed by early mergers or heating of the thin disk.

4. Stellar Halo

   • Spherical, low‑density component extending >50 kpc.
   • Dominated by old, metal‑poor stars and globular clusters; retains signatures of past accretion events.

5. Dark Matter Halo

   • Extends to several hundred kiloparsecs.
   • Provides the majority of the Galaxy’s mass; inferred from rotation curves and satellite dynamics.

4. Galactic Coordinates

5. Stellar Populations