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Edge-on, the bright thin galactic disk forms a narrow band, with a central bulge protruding above and below it and a thinner gas-and-dust lane along the midplane. Young stars and H II regions occupy a vertical scale of a few tenths of a kiloparsec or less; a broader old thick disk has scale of order a kiloparsec. These are approximate scale heights, not sharp boundaries. The old stellar halo and globular clusters form a much larger, low-surface-brightness spheroidal component, extending to many tens of kiloparsecs or more. The dark matter halo is inferred dynamically rather than directly visible in such a sketch. The preceding figure is a structural diagram, not an external photograph or a reconstruction of a particular survey.

A spiral galaxy has a rotating stellar/gas disk with spiral structure, central bulge, extended halo and globular clusters, embedded in a still larger dark-matter distribution. For the Milky Way useful rough scales are a stellar-disk diameter of a few tens of kiloparsecs, a thin-disk thickness of a few hundred parsecs, a bulge scale of kiloparsecs and a halo extending tens to hundreds of kiloparsecs depending on tracer. Young stars follow gas-rich star-forming disk regions and spiral arms. Very old stars are prominent in globular clusters, halo and bulge, though the disk also contains old populations. These are distributions with overlapping ages, not disjoint bins.

A star cluster is a gravitationally associated group of stars formed in a relatively compact region. Open clusters and globular clusters provide populations whose members have broadly related formation histories, making them useful laboratories for stellar evolution. Membership is tested using parallax, proper motion and, where available, radial velocity, rather than apparent proximity on the sky alone. A cluster's Hertzsprung-Russell diagram then compares stars of different masses at a shared distance; its main-sequence turnoff helps estimate stellar age. Space-based astrometry sharpens both membership selection and these evolutionary sequences, although some clusters contain multiple stellar populations.^[ref]