• Distinct categories of stars found in our own and other galaxies. Population I stars are the younger, being found in spiral arms of galaxies, and are comparatively rich in heavy elements formed in supernova explosions. Population II stars are older and occur preferentially in galactic haloes rather than in spiral arms. They have far lower abundances of heavy elements, by at least tenfold, since the universe contained far fewer such atoms when they formed. There is also a postulated Population III, a hypothetical group of stars in the early universe which would have produced some of the heavy elements we see today.