= Solution
A <spiral galaxy> has a rotating stellar/gas disk with spiral structure, central bulge, extended halo and <Globular cluster>[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 arm>[spiral arms]. Very old stars are prominent in <Globular cluster>[globular clusters], halo and bulge, though the disk also contains old populations. These are distributions with overlapping ages, not disjoint bins.
\Image[/past-exam-of-the-short-options-paper-shared-by-prelims-and-parts-a-and-b-of-the-physics-course-of-the-university-of-oxford/2016-stellar-evolution-and-galaxy.png]
{title=Schematic stellar evolutionary regions and spiral-galaxy components used in the solutions}
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