Galaxies today fall roughly into two categories: elliptically-shaped collections of reddish, old stars that formed predominantly during a period early in the history of the universe, and spiral shaped objects dominated by blue, young stars. The Milky Way is an example of the latter, a spiral galaxy actively making new stars. In order to understand the growth of galaxies over cosmic time and the past star formation history of the universe, astronomers study the population of old stars in distant ellipticals from earlier epochs, stars which in turn formed at an even early time. Star formation produces supernovae which enrich their environments with elements, including the diagnostic element magnesium. Measuring the amount of magnesium (relative to iron) in a galaxy thus helps to fix the strength and duration of prior episodes of star formation.