Stellar Populations

Like most initial classifications, the division of stars into Population I and Population II stars is a bit of an oversimplification. Astronomers now classify stars into five distinct populations based on how strongly they exhibit the Population I or II characteristics. These populations are: Extreme Population I, Older Population I, Disk Population II, Intermediate Population II, and Halo Population II. Astronomers are currently arguing over whether these groups represent distinct populations, or a gradual blending from Population I properties to Population II properties.

There are also some exceptions to the rule that young Population I stars have heavy elements while older Population II stars do not. For example, the Magellanic Clouds, irregular companion galaxies to the Milky Way, are Population I stars with few heavy elements. The core of the Milky Way also contains Population II stars that do contain heavy elements. Why? Most likely, in the core of the galaxy there was a very rapid generation of very massive stars. They are gone now, but they enriched the core with heavy elements very quickly. The high concentration of stars near the core contributed to this process. The opposite occurred in the Magellanic Clouds. Star formation proceeded so slowly, that there was not an early generation of massive stars to produce heavy elements.