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Population I indicates the stellar population that has the higher metallicity. Within Andromeda and the Milky Way, metallicity correlates with some other stellar characteristics, such as peculiar velocity, and stellar population classes have been devised to label two distinct groupings, Population I and Population II. The two terms are also used for other galaxies that show such a distinction.
Population I stars (Pop I stars or metal-rich stars), in addition to their substantial metallicity, generally have more circular orbits, are generally located in spiral arms, and show a lot of blue color, a sign of young stars. They are interpreted as the more recent stars, the substantial blue color because they still include hot, short-lifetime stars which dominate the produced electromagnetic radiation (an O-type star can be as bright as a million Sun-like stars).
The class with less metallicity, Population II, includes stars in the bulge, that have less circular orbits, and that don't show the blue. They are interpreted as an older generation, and their early stars are long gone, leaving less blue. It is presumed that the Population I stars formed from gas that included some that was expelled by the short-lived among the Population II stars.