Galaxies and Galactic Structure
Core concepts
Know Hubble's morphological classification, the Milky Way's components (thin and thick disk, bulge, stellar halo, dark-matter halo), rotation curves, and the Tully-Fisher and Faber-Jackson relations. Understand spiral density waves, the role of mergers in ellipticals, active galactic nuclei and their unification, and the metallicity and kinematics that separate populations.
Traps
Do not equate morphological type with evolutionary stage; the Hubble sequence is descriptive. Rotation curves staying flat at large radii imply dark matter or modified gravity, not Keplerian decline. Distinguish the bulge from the halo in both age and metallicity. Remember that the Milky Way's center lies about 8 kpc away and that the Sun orbits at roughly 220 km/s.
Worked example
A galaxy's rotation curve is flat at 200 km/s out to 30 kpc. Enclosed mass M = v^2 r / G. With v = 2×10^5 m/s and r ≈ 9.3×10^20 m, M ≈ (4×10^10)(9.3×10^20)/(6.67×10^-11) ≈ 5.6×10^41 kg, about 2.8×10^11 solar masses. Visible starlight accounts for far less, which is the classic dark-matter argument. Rule of thumb: flat rotation means M ∝ r.
Rule of thumb
Flat rotation curve → enclosed mass grows linearly with radius.