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.
