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.