The Universe contains trillions of galaxies, and each galaxy is home to as many as a few hundred billion stars. To understand the Universe, it is necessary to study its building blocks, in the same way one studies atoms to understand the properties of a material. In the case of the Universe, it is crucial to understand the formation and evolution of galaxies. The study of galaxy formation traces back to the classic work of Eggen, Lynden-Bell & Sandage  about 50 years ago, but even to this day, we still do not have a complete picture of how galaxies form and evolve. Many details remain to be explored.
Observing other galaxies besides our own (the Milky Way) has given us important insights in addressing the question “where do we come from, and where are we going?” and allows us to make myriad inferences on the evolution of the Universe. However, our home galaxy is the only one for which we can obtain an enormous wealth of information on the stars it contains: the other galaxies are simply too far away to observe star-by-star.
Before we move on, one might ask, if we understand the formation of the Milky Way, does it tell us anything about the other galaxies? The answer is, Yes! About half of the stars in the present day Universe live in galaxies that are similar to the Milky Way . In other words, the Milky Way is a good model for a typical galaxy found in the Universe. Understanding the formation of the Milky Way will serve as a solid basis for our understanding of external galaxies.
In the past two decades, technological advances have given us the remarkable ability to study the three dimensional spatial locations, velocities and elemental abundances of each star in the Milky Way, where the elemental abundances refer to the amount of each element in the star. As we will explore in this article, understanding the properties of stars holds the key to deciphering the mystery of Milky Way’s history, and it is still the gold standard for studying galactic evolution in modern astronomy.
Who are the intruders?
We now believe that galaxies merged to form the Milky Way. To get to its size today, scientists think the Milky Way might have cannibalized many of its satellite galaxies in the past (see video). In fact, some satellite galaxies and stars from those galaxies are currently being absorbed by the Milky Way. The question is, when we look at the sky today, how can we know which stars were incorporated from satellite galaxies?
A realistic computer simulation illustrating the merging of two galaxies. Credits: Patrik Jonsson, Greg Novak & Joel Primack.
To answer this question, we must understand how heavy elements (those whose atoms contain more than two protons) were created in our Universe. Shortly after the Big Bang, the Universe was almost entirely made up of the elements hydrogen (which has one proton) and helium (two protons). In the process of carrying out nuclear fusion to power themselves, the hot cores of stars serve as perfect laboratories to fuse lighter elements into heavier elements such as oxygen and iron. As some of you might already know, when massive stars die, they go out with a bang! They explode! This explosion, known as a supernova, releases the heavy elements from the stellar core, at the same time producing even heavier elements owing to the extreme condition of the explosion. This explosion pollutes the gas in the galaxy (or enriches, depends on your point of view, since gold is produced in this process too!). Therefore, when subsequent generations of stars form from the debris of dead ones, they carry the fingerprint of the previous explosions (Figure 1).
Figure 1. A schematic illustration of stars forming from the debris of supernovae. (A) Formation of earlier generations of stars. (B) These stars evolve and eventually explode as supernovae in (C). (D) Subsequent generations of stars form from the debris of the previous explosions.
With this in mind, by studying the richness in heavy elements of each star, we can chemically determine the age of each star in the Milky Way — stars formed closer to the present will have more heavy elements in them as they were formed from gas that participated in a longer period of the Galactic enrichment history. The enrichment depends on the size of galaxies. Smaller satellite galaxies form less stars and therefore were impacted by fewer occurrences of supernovae. This breadth and diversity of elemental patterns is unique to each satellite galaxy and is determined by its history of star formation. To answer our initial question in this section, this elemental composition of stars is a tool for distinguishing between stars that are part of the infall debris of satellite galaxies and those that are native to the Milky Way . On top of that, among those that are native to the Milky Way, the composition also informs us on which epoch the stars were formed in the Milky Way.
Where is the dark matter?
According to our current theory of cosmology, all galaxies are embedded in big spheres of dark matter, also known as the dark matter halo, in astronomical jargon (Figure 2). We call dark matter as such because this substance does not emit electromagnetic radiation (light, infrared, etc) nor interact with the normal matter that we see (stars, galaxies) in any way except through gravity. While the famous Large Hadron Collider in Geneva is trying to create dark matter in the laboratory and conclusively prove its existence, astronomers have been indirectly studying dark matter for decades by investigating the gravitational tugs from dark matter on visible objects like stars.
Figure 2. Schematic illustration of the Milky Way embedded in a dark matter halo. Figure is not drawn to scale.
As an analogy, even if the Sun were to stop producing light, we could still figure out its existence and work out its mass by looking at how planets move around it due to its gravitational pull. The planets, in this case, work as dynamic tracers of the Sun. In the same way, by investigating how stars move around the Milky Way, we can infer the mass, the shape and even the distribution of the dark matter around the Milky Way . However, unlike the orbital movement of planets around our Sun, which is concentrated in the center of the solar system, the stars are actually embedded in the dark matter halo.
Exciting results in near future
Figure 3. Schematic illustration of the Milky Way overlaid with the survey region (region shaded in pink and red) covered by the GAIA. Courtesy Luri X. and Robin A.
Next year, the European Space Agency will launch the flagship space telescope GAIA. GAIA will be used to gather extensive data on the spatial locations and velocities of a billion stars in the Milky Way (Figure 3). In addition, many other instruments, including the GALAH survey which started this year in Australia, is currently observing the elemental abundances of one million stars. In less than five to ten years time, GAIA, GALAH and other surveys promise a flood of information that will put all of our current theories to the test, and possibly revolutionize our understanding of the formation of our home galaxy. There may be no better time than now to join science in this expedition of “Galactic archeology”.
Yuan-Sen Ting is an astronomy graduate student at Harvard University.
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Link of interest:
To learn more on how astronomers determine the elemental abundances of stars, you might want to explore an interactive online software module that the author developed: https://www.cfa.harvard.edu/~yuan-sen.ting/lyman_alpha.html.