Gives clues to the formation of the Galaxy. A Galaxy is a large assembly of stars, gas, and dust held together by gravity. A galaxy is bound together by the mutual gravitational attraction of all of its parts. All orbit in the same general direction.
Orbit speeds similar at a given radius.
Wide ranges of orbital speeds. No Blue M-S stars Random elliptical orbits in all directions. Supernova explosions enrich the interstellar medium with metals. The next generation of stars form out of the metal-enriched interstellar gas. Successive generations get more metal rich.