Galactocentric coordinates
Definition: A position system whose origin is the Galactic center, commonly expressed by cylindrical \(R,\phi,z\) or Cartesian axes.
Why it matters here: It lets us describe Milky Way structure without confusing the Sun's off-center viewpoint with the Galaxy's intrinsic geometry.
Relationship: The coordinates turn heliocentric distance and sky direction into the \(R\) and \(z\) used by density models.
Limitation or common confusion: The transformation depends on adopted solar position and orientation; it is not a direct observable.
Thin disk
Definition: The flattened, dynamically cold stellar and gaseous component concentrated near the Galactic mid-plane.
Why it matters here: It contains much of the current star formation and supplies the baseline population for the vertical-density calculation.
Relationship: Its characteristic scale height enters \(\rho(z)=\rho_0e^{-|z|/h}\) and differs by tracer age.
Limitation or common confusion: Thin disk is a population model, not a sharply bounded physical sheet.
Thick disk
Definition: A vertically extended, kinematically hotter disk population with older stars and distinct abundance trends.
Why it matters here: Contrasting it with the thin disk shows why one exponential cannot represent every stellar sample.
Relationship: A two-component density model sums exponentials with different scale heights and normalizations.
Limitation or common confusion: It should not be identified by height alone because disk populations overlap in chemistry and motion.
Bulge
Definition: The centrally concentrated stellar component occupying the inner few kiloparsecs, including a barred or box-peanut structure.
Why it matters here: It is one of the major components that a Galactic mass and light model must separate.
Relationship: Bulge star counts and infrared surface brightness constrain central density but overlap the disk along many sightlines.
Limitation or common confusion: Bulge does not mean a simple spherical classical bulge.
Stellar halo
Definition: A diffuse, approximately spheroidal population of old, metal-poor stars extending far beyond the disk.
Why it matters here: It provides tracers of the Galaxy's outer structure and assembly while contributing little of the total stellar light.
Relationship: Halo density is inferred from selected tracers such as RR Lyrae and relates to, but is not the same as, the dark-matter halo.
Limitation or common confusion: Stellar halo and dark-matter halo are distinct components with different observables.