FACULTY OF TECHNOLOGY AND SOCIETY | Dissertation defence
Dissertation Defence – Shilpa Bijavara Seshashayana

The universe began as a singularity: an unimaginably dense and hot state where our usual ideas of space and time no longer apply. Then the universe expanded and cooled; that moment of rapid expansion is known as the Big Bang. In its earliest moments, the universe probably contained roughly equal amounts of matter and antimatter. If they had remained perfectly balanced, they would have annihilated each other, leaving mostly radiation behind. With this mass-annihilation, there would be no stars, no planets and no life. However, something tipped the balance, leaving behind a thin residue after the annihilations ended. This excess of matter was extremely small, almost negligible, yet it was sufficient. Everything we see today is built from that faint imbalance. While its origin remains unknown, several compelling ideas have been proposed, with some linking it to electroweak symmetry breaking and others pointing to new physics beyond the Standard Model. But this leftover matter formed everything we see today, including galaxies, stars, atoms, people and even human-made structures such as the International Space Station.

On a clear, moonless night far from city lights, you can see a quiet reminder of this origin story. A pale, glowing band stretches across the sky, dense with stars. That is the Milky Way, our home galaxy, one small piece of a universe that has been evolving ever since that first expansion. The Milky Way did not appear fully formed. It has been growing and changing for over 10 billion years. Gas collected and cooled to form stars. These stars then shaped their surroundings by blowing material outwards through stellar winds, and in some cases ended their lives in supernova explosions. This stirred, enriched and recycled the galaxy's gas, creating new generations of stars. In order to understand how the Milky Way became what it is today, we need to find clues that can survive this constant cycle of birth, death and mixing. This is where stars become cosmic chemical time capsules. When a star forms, it locks the chemical composition of its birth cloud into its atmosphere. Even if the star later drifts across the galaxy, this chemical fingerprint usually remains largely unchanged, like a fossil record that can still be read.

In this thesis, I use open clusters -- groups of stars born together from the same cloud as powerful indicators of how the Milky Way formed its disc. As the stars in a cluster share the same age and starting material, they preserve a clear chemical signature, akin to a birth certificate. This makes them extremely useful for reconstructing the past of the Milky Way. Many individual Milky Way stars have traveled far from their place of formation and have mixed histories that can obscure the patterns we are trying to discern. Clusters, in contrast, keep the signal clear. I use these stellar families as time-stamped snapshots of our galaxy. Each cluster captures the composition of the interstellar gas at the moment and location of its formation. By comparing clusters spanning a wide range of ages and distances from the galactic center, I can transform a static chemical map of the Milky Way's disc into an evolving narrative. This enables me to trace the formation of stars in different regions and the gradual development of the Milky Way into the spiral galaxy we see today.

As the entire story is written in elements, I measure carbon, nitrogen, oxygen, and fluorine, as well as rock-forming elements such as sodium, magnesium, aluminium, silicon, sulfur, potassium, calcium, and titanium, which build planets. I also track metals such as iron, nickel, copper and zinc, as well as rarer heavy elements such as yttrium, cerium, neodymium and ytterbium. These elements are not all produced in the same way. Some are produced early on by massive stars, some are forged in supernova explosions, and some are released later by bloated, aging asymptotic giant branch stars. Each element acts as a clue from a different cosmic factory, carrying information about different stages in the evolution of the galaxy. While any single element can hint at part of the Milky Way’s history, a wide set of elements can reveal a more complete picture of how the galaxy formed stars. In this sense, our work is like the archaeology of cosmic matter, using chemical evidence to infer events that we cannot observe directly.

The elements found in these stars are the same ingredients that would later become planets and life. These elements were enriched by earlier generations of stars and continually stirred by the Milky Way’s cycling of gas. By tracing these signatures across space and time, this thesis establishes a link between detailed abundance patterns and fundamental questions about the origin of matter, the formation of our home galaxy, and how the Milky Way can inform us about the evolution of similar galaxies. Ultimately, this thesis is a part of the origin story of both the Milky Way and ourselves, tracing a path from stardust to people.