Stellar Populations

These different populations can be understood in the context of stellar evolution. When the universe formed in the big bang, only hydrogen and helium were made. The heavy elements were made later in the cores of stars. Therefore, the older Population II stars are deficient in heavy elements, while the younger Population I stars contain heavy elements that were made by the massive first generation stars. Massive stars manufacture and recycle heavy elements. These stars are blue giants and supergiants most of their lives, which are very short by stellar standards. These stars are the brightest blue stars in a group of Population I stars. In a group of older Population II stars, these stars have finished their life cycles, so the brightest stars are the red giants formed near the end of stellar life cycles. The distribution of these different populations is then related to the evolution of the galaxy.

Stellar population studies help us understand stellar evolution, evolution of the galaxy, and the history of the universe.