Stellar Populations:
====================

* Distinct subsystems in galaxies, characterized by the:

 - Locus and morphology
 - Age
 - Dynamics 
 - Chemical composition, ISM

... and presumably by the different formation histories.

* The old view (Baade 1944):

  Pop. I = Young, disk, metal-rich, rotational support
  Pop. II = Old, bulge/halo, metal-poor(...), pressure support

* A more modern view:

 - The young/middle-aged, thin disk (e.g., the Solar neighborhood, 
   open clusters, star-forming regions, etc.):

	Ages 0 - few Gy, fast roatation (~ 200 km/s), 
	low velocity dispersion (~ 30 km/s), high
	metallicity (~ Solar), scale height ~ 200 pc.

 - The old, thick disk (incl. metal-rich glob.cl.?):

	Age ~ Galactic age, slower roatation (~ 100 km/s),
	larger velocity dispersion (~ 60 km/s), lower
	metallicity, scale height ~ 1.5 kpc.

 - The Galactic Bulge:

	Age = Galactic age, slow roatation (if any), 
	high velocity dispersion (~ 120 km/s), very
	high metallicity (> Solar) near the center,
	Re ~ 2 - 3 kpc.

 ... which may or may not tie with ...

 - The halo (stellar, incl. the metal-poor glob. cl.):

	Age = Galactic age, slow rotation ( < 50 km/s,
	maybe 0), high velocity dispersion (~ 120 km/s), 
	low metallicity (~ 0.1 - 0.001 Solar).

 - And the dark halo:

	Composition and most properties unknown.  
	Density profile close to an isothermal sphere.