Final answer:
Galactic disk-population stars, or Population I, are younger, metal-rich, and have nearly circular orbits in the disk, while the spheroidal population, or Population II, consists of older, metal-poor stars with elliptical orbits stretching into the halo.
Step-by-step explanation:
The characteristics that distinguish a galaxy's disk-population of stars, referred to as Population I, from its spheroidal population, referred to as Population II, are based on chemical composition, age, location, and orbital motions.
Population I stars are younger, found in the galactic disk, are metal-rich, and take nearly circular orbits around the galactic center. These stars are often present in spiral arms and include bright supergiants and main-sequence stars, which are associated with young open star clusters and areas rich in interstellar matter and molecular clouds.
On the other hand, Population II stars are older, have fewer heavy elements, and are found in the halo as well as the disk, with many following eccentric elliptical orbits that can take them far above or below the galactic plane.
These include stars in globular clusters and those surrounded by planetary nebulae, such as RR Lyrae variable stars. The differences in chemical composition and orbital motion are indicators of the different stages in the evolution and formation of the galaxy.