Every “normal” star is a ball of hydrogen and helium, the simplest of all chemical elements. All of the hydrogen and most of the helium were forged in the Big Bang. Stars also contain smatterings of heavier elements, which are known as metals. These elements were forged by the stars themselves. As the stars die, they expel some of these elements into space, where they can be incorporated into new stars. So each generation of stars contains a higher proportion of metals — everything from carbon and oxygen to iron and lead. Based on the amount of metals, astronomers divide stars into two broad categories — Population I and Population II. The Sun belongs to Population I. These stars contain fairly high percentages of metals — about two percent in the case of the Sun. These stars formed fairly recently — after earlier generations had pumped the heavy elements into space. The earliest generations yet seen form Population II. These stars have a much smaller percentage of metals, which means
