Article 10 The Formation and Evolution of the Milky Way The distribution of the chemical elements in our galaxy serves as a “fossil record” of its evolutionary history Cristina Chiappini Our galaxy is a highly evolved entity. Not merely a ran- born and die, refined measures of the Sun’s chemical dom assortment of stars, like so many grains of sand on a composition, and the rate at which the elements are pro- beach, it is an elegant structure that shows both order and duced by the stars, among many others. Astronomers complexity. We know that the Milky Way is a spiral disk love constraints because without them a model is little galaxy, similar to many others we see in the sky. This sur- more than hand-waving conjecture. The tricky part is prisingly beautiful shape is so common among galaxies coming up with a successful model that incorporates as that the universe almost seems to delight in building many constraints as possible. them. The end product is especially remarkable in the Although the development of new technologies has light of what is believed to be the starting point: nebulous improved the quality of the observations and so refined blobs of gas. How the universe made the Milky Way from the constraints on astronomers’ models, we are still far such simple beginnings is not altogether clear. The task of from a complete understanding of our galaxy’s evolution. unraveling this mystery has been cast to astronomers, Like our galaxy, the field itself is still evolving. Here I pro- such as myself, who attempt to construct models of the vide an overview of how astronomers attempt to uncover Galaxy’s evolution based on its present appearance. our galaxy’s past, and I introduce a new model that ac- These models need to account for not only the large- counts for some of the most recent observations. scale gravitational forces involved in assembling the Gal- axy, but also the chemical composition of its primary The Anatomy Lesson components, the stars. It turns out that the chemistry of As we arbitrarily divide the human body into a torso with the stars holds clues to how the Galaxy was made and a head and limbs, so we can conceptually separate the how it has changed through time. The gas blobs that Galaxy into various components. The flying-saucer evolved into the Milky Way consisted merely of hydro- shape—consisting of the central bulge and the spiral gen and helium (and a smattering of lithium), the ele- disk—is only the most obvious part of the Galaxy (Figure ments that were created in the Big Bang. All the other 2). The spiral disk itself can be subdivided into a thin disk, elements were literally created by the stars. Unlike the which rises about 1,000 light-years above and below the medieval alchemists, the stars can actually transmute one galactic mid-plane, and a thick disk, which extends to element into another—they are prodigious chemical fac- about 3,500 light-years on either side of the plane. The rel- tories. Nevertheless, even today hydrogen and helium ative flatness of our galaxy is evident when one considers make up about 98 percent of the normal matter in the uni- that the galactic disk is generally thought to be about verse. It’s the distribution of the elements that make up 120,000 light-years across. Our sun resides in the thin disk the final 2 percent that makes all the difference to studies about 28,000 light-years from the galactic center. of galactic evolution. Not seen in any photograph of a spiral galaxy is the The most recent models of our galaxy’s chemical evo- spherical halo that completely surrounds the disk and the lution actually need to incorporate many other observed bulge. This is partly because the vast bulk of the halo con- properties as constraints. These include the density of gas sists of dark matter—material of unknown composition in various parts of the disk, the rate at which stars are that cannot be seen, but whose presence is deduced by its 1 Article 10. The Formation and Evolution of the Milky Way Figure 1. Milky Way look-alike galaxies NGC 1232 (above) and NGC arate stellar populations that compose the central bulge (reddish 891 (below) offer a glimpse of how our galaxy would appear if viewed yellow) and the spiral disk (blue). (NGC 1232 image courtesy of the from a distance of several million light-years. The different colors European Southern Observatory.) evident in the face-on view of NGC 1232 indicate the existence of sep- strong gravitational influence. The halo does have a stel- ceived as a cylinder centered on the Sun, with a radius of lar component, often referred to as the stellar halo, but it is 3,000 light-years and “infinite” height (so it includes simply too dim to be seen as a distinct structure. Lying parts of the thick disk and the halo). Of course, we can’t within the halo, however, are structures that can indeed view the stars at the most extreme distances in this cylin- be seen in the telescope: spheroidal collections of stars der. Even so, we do get a closer view of some thick-disk known as globular clusters. About 200 globular clusters are stars and halo stars because some of them happen to be known, and they appear to be some of the oldest objects passing through the thin disk during our era as they or- in the Galaxy. bit the Galaxy. It took many decades of careful study to tease apart the In fact, the different orbits of the stars, their kinematic various regions of the Milky Way, and the process of dis- properties, provide a crucial distinction between stars secting out fine-scale subregions continues even today. that belong to different regions of the Galaxy (Figure 3). One of the reasons it’s so difficult is that we cannot mea- The kinematics of a star is defined by three velocity com- sure the properties of all the stars in the Galaxy—they are ponents: its rotational velocity (V) in the direction of ga- simply too far away. For the most part astronomers can lactic rotation, its vertical velocity (W) perpendicular to only give close scrutiny to stars in the solar neighbor- the galactic plane and its radial velocity (U) away from hood. In evolutionary models this region is generally con- the galactic center. So, for example, stars in the thin disk 2 ANNUAL EDITIONS they came to be. An analysis of the two populations re- vealed that population I stars tend to be relatively rich in elements heavier than helium—which astronomers refer to as metals—whereas population II stars, especially those in the halo and the globular clusters, are relatively poor in metals. A star’s metallicity is determined by spectroscopic measures of its surface, and it is thought to represent the chemical composition of the gas cloud that collapsed to form the star. (Astronomers can also measure the metal- licity of ambient gas clouds directly.) In general, the me- tallicity of a star is defined by the abundance of iron (Fe) compared with its hydrogen (H) abundance. This relation is normalized to the solar abundance on a logarithmic scale: [Fe/H] = log (Fe/H)–log (Fe/H)Sun The most metal-poor star ever observed in our galaxy is located in the halo. It is old and has a metallicity [Fe/H] of about -4.0, or about 10,000 times less than the Sun! That it happens to be an ancient star is not a coincidence. When it was born the stellar chemical factories were only just beginning to start operation, so there was simply not a great abundance of metals that could be incorporated into the star. As a general rule [Fe/H] increases with time so old objects are more metal poor than young ones. How stars make “metals” is now reasonably well un- MALIN/IAC/RCO derstood. For the most part they are formed by a chain of Figure 1 (continued) NGC 891. The edge-on view of NGC 891 reveals fusion of lighter elements. There are actually several as- the extreme flatness of spiral-disk galaxies, which typically have an as- trophysical processes involved, each of which forms a dif- pect ratio of about 1:30. The flying-saucer shape of a spiral galaxy such (Figure 4) as the Milky Way and its distinct stellar populations can be explained ferent assortment of elements . How and when by theoretical models of its formation and chemical evolution. these processes take place is largely dependent on the mass of a star. such as the Sun, have a small vertical velocity and tend to The lightest stars, some having merely one-tenth the stay in the galactic plane, whereas thick-disk stars have mass of our sun, live the longest, potentially for many bil- slightly larger vertical velocities, and halo stars tend to lions of years. In contrast, the heaviest stars, weighing up have the largest vertical velocities (and almost no rota- to 150 solar masses, have comparatively brief lives, on tional velocity). A star’s kinematic properties are one of the order of a few million years. The distinction is crucial the ways that astronomers can recognize an interloper because it is primarily at the end of its life that a star from another part of the Galaxy. makes its contribution of newly synthesized elements to As it happens, the stars in the halo and the disk differ the Galaxy.