Pop I and Pop II
In the left panel we make the distinction between two populations of stars: Population I and Population II.
These are commonly termed "Pop I" and "Pop II", respectively. Pop II stars are older and metal poor (recall that
astronomers call anything heavier than hydrogen and helium a "metal"); Pop I stars are younger and contain more
metals. We shall have more to say about Pop I and Pop II later when we discuss the Milky Way
galaxy in Chapter 23.