Two broad classes of star s and stellar groupings, whose members differ primarily in age, chemical composition, and location in galaxies .
They were distinguished and named by Walter Baade (18931960). Population I consists of younger stars, clusters, and associations. These occur in and near the arms of the 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 billion15 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 .