The distribution of the chemical elements in our galaxy serves as a “fossil record” of its evolutionary history

Our galaxy is a highly evolved entity. Not merely a random assortment of stars, like so many grains of sand on a beach, it is an elegant structure that shows both order and complexity. We know that the Milky Way is a spiral disk galaxy, similar to many others we see in the sky. This surprisingly beautiful shape is so common among galaxies that the universe almost seems to delight in building them. The end product is especially remarkable in the light of what is believed to be the starting point: nebulous blobs of gas. How the universe made the Milky Way from such simple beginnings is not altogether clear. The task of unraveling this mystery has been cast to astronomers, such as myself, who attempt to construct models of the Galaxy’s evolution based on its present appearance. These models need to account for not only the largescale gravitational forces involved in assembling the Galaxy, but also the chemical composition of its primary components, the stars. It turns out that the chemistry of the stars holds clues to how the Galaxy was made and how it has changed through time. The gas blobs that evolved into the Milky Way consisted merely of hydrogen and helium (and a smattering of lithium), the elements that were created in the Big Bang. All the other elements were literally created by the stars. Unlike the medieval alchemists, the stars can actually transmute one element into another—they are prodigious chemical factories. Nevertheless, even today hydrogen and helium make up about 98 percent of the normal matter in the universe. It’s the distribution of the elements that make up the final 2 percent that makes all the difference to studies of galactic evolution. The most recent models of our galaxy’s chemical evolution actually need to incorporate many other observed properties as constraints. These include the density of gas in various parts of the disk, the rate at which stars are born and die, refined measures of the Sun’s chemical composition, and the rate at which the elements are produced by the stars, among many others. Astronomers love constraints because without them a model is little more than hand-waving conjecture. The tricky part is coming up with a successful model that incorporates as many constraints as possible. Although the development of new technologies has improved the quality of the observations and so refined the constraints on astronomers’ models, we are still far from a complete understanding of our galaxy’s evolution. Like our galaxy, the field itself is still evolving. Here I provide an overview of how astronomers attempt to uncover our galaxy’s past, and I introduce a new model that accounts for some of the most recent observations.