Metallicity is the proportion of a star or gas cloud made of elements heavier than hydrogen and helium, which astronomers loosely call metals. Since the Big Bang produced almost only the two lightest elements, everything heavier was manufactured later inside stars and scattered by their deaths. A star's metallicity therefore records how much cosmic chemical processing had occurred before it formed, making it a rough clock for stellar generations. The oldest stars in the galactic halo are strikingly metal-poor, and the theoretical first generation had essentially none.