Two broad classes of star s and stellar groupings, whose members differ primarily in age, chemical composition, and location in
They were distinguished and named by Walter Baade (1893–1960). Population I consists of younger stars, clusters, and associations. These occur in and near the arms of the {{link=Milky Way Galaxy">Milky Way Galaxy and other spiral galaxies and have been detected in irregular galaxies (such as the Magellanic Cloud s). Population I objects are thought to have originated from interstellar gas that underwent various processes, including supernova explosions, which enriched their constituent matter with heavier elements. Population II consists of older (generally 1 billion–15 billion years old) stars and clusters, presumably formed from interstellar gas clouds that emerged very early in a galaxy's history. Consisting almost entirely of hydrogen and helium, they are found in the galactic halo s of spiral galaxies, in globular cluster s, and, in large numbers, in elliptical galaxies. Astronomers sometimes refer to a Population III as the very first generation of stars to emerge after the big bang .