Population I stars (also known as the disk population) are relatively metal rich, containing between 2% and 3% metals. These stars are found in the disk of a galaxy and remain approximately in place as they orbit the galaxy. Typically, these stars are relatively young, having formed in the last few billion years. Extreme Population I stars (the most metal rich) are found only in the spiral arms. Intermediate Population I stars (like the Sun) are located throughout the disk and are slightly less metal rich.

The difference in metal content and age between Population I and Population II stars suggests that Population II stars formed early during the formation of the galaxy, when the galaxy contained nearly pure hydrogen and helium. As the Population II stars evolved, they produce metals through fusion. Stellar winds and novae carried these metals into the galaxy. Hence, the younger stars would be metal rich before fusion began.