Introductory Astronomy: Stellar Populations

Stars are generally classed into two main groups called Population Iand Population II. The stars of the two populations are very similar. They all burn elements through fusion in the same way and follow the same evolution sequence. However, there are important differences in their characteristics which distinguish the two groups; one of the main differences lies in the metal content of the stars in each group. (Remember that astronomers consider "metals" to be any elements besides hydrogen and helium!).

Population I stars are metal rich stars; they contain about 2-3 percent metals. They are found in the disk of the Galaxy. They travel on circular orbits about the center of the Galaxy and generally remain in the plane of the Galaxy as they orbit. (The older Population I stars are found farther out of the plane than the younger stars.) Population I stars are relatively young stars which have formed within the last few billion years. Extreme Population I stars (the most metal rich stars) are found only in the spiral arms; these are the youngest stars. Intermediate Population I stars (like the Sun) are located through the disk. They are slightly less metal rich.

Population II stars are metal poor stars; they contain about 0.1 percent metals. They are found in the spherical component of the Galaxy (the halo and the bulge). They have randomly tipped, elliptical orbits which can plunge through the disk of the Galaxy and which take some of them (the halo stars) to large distances from the center. They are relatively old stars, with ages ranging from 2 - 14 billion years. Extreme Population II stars (the most metal poor) are found in the halo and the globular clusters; these are the oldest stars. Intermediate Population II stars are located in the bulge. They are slightly more metal rich than the extreme Population II stars, but less metal rich than the intermediate Population I stars.

The differences in metalicity and age between the two Populations suggests that the Population II stars formed early during the formation of the Galaxy. At this time, the Galaxy would have contained gas that was nearly pure hydrogen and helium. As these stars evolved, they produced metals through fusion. Stellar winds and flashes (as well as the violent deaths of massive stars) would carry these metals into the galaxy. Thus, stars that formed at later times would be born from gas which had a higher metal content and would be more metal rich even before fusion began. As more stars evolve, the metal enrichment of the Galaxy (and the stars which form in it) would continue to increase.